[Interaction of DNA Methyltransferase Dnmt3a with Phosphorus Analogs of S-Adenosylmethionine and S-Adenosylhomocysteine]

Enzymatic methyltransferase reactions are of crucial importance for cell metabolism. S-Adenosyl-L-methionine (AdoMet) is a main donor of the methyl group. DNA, RNA, proteins, and low-molecular-weight compounds are substrates of methyltransferases. In mammals, DNA methyltransferase Dnmt3a de novo methylates the C5 position of cytosine residues in CpG sequences in DNA. The methylation pattern is one of the factors that determine the epigenetic regulation of gene expression. Here, interactions with the catalytic domain of Dnmt3a was for the first time studied for phosphonous and phosphonic analogs of AdoMet and S-adenosyl-L-homocysteine (AdoHcy), in which the carboxyl group was substituted for respective phosphorus-containing group. These AdoMet analogs were shown to be substrates of Dnmt3a, and the methylation efficiency was only halved as compared with that of natural AdoMet. Both phosphorus-containing analogs of AdoHcy, which is a natural methyltransferase inhibitor, showed similar inhibitory activities toward Dnmt3a and were approximately four times less active than AdoHcy. The finding that the phosphonous and phosphonic analogs are similar in activity was quite unexpected because the geometry and charge of their phosphorus-containing groups differ substantially. The phosphorus-containing analogs of AdoMet and AdoHcy are discussed as promising tools for investigation of methyltransferases.

Elevated S-adenosylhomocysteine induces adipocyte dysfunction to promote alcohol-associated liver steatosis

It has been previously shown that chronic ethanol administration-induced increase in adipose tissue lipolysis and reduction in the secretion of protective adipokines collectively contribute to alcohol-associated liver disease (ALD) pathogenesis. Further studies have revealed that increased adipose S-adenosylhomocysteine (SAH) levels generate methylation defects that promote lipolysis. Here, we hypothesized that increased intracellular SAH alone causes additional related pathological changes in adipose tissue as seen with alcohol administration. To test this, we used 3-deazaadenosine (DZA), which selectively elevates intracellular SAH levels by blocking its hydrolysis. Fully differentiated 3T3-L1 adipocytes were treated in vitro for 48 h with DZA and analysed for lipolysis, adipokine release and differentiation status. DZA treatment enhanced adipocyte lipolysis, as judged by lower levels of intracellular triglycerides, reduced lipid droplet sizes and higher levels of glycerol and free fatty acids released into the culture medium. These findings coincided with activation of both adipose triglyceride lipase and hormone sensitive lipase. DZA treatment also significantly reduced adipocyte differentiation factors, impaired adiponectin and leptin secretion but increased release of pro-inflammatory cytokines, IL-6, TNF and MCP-1. Together, our results demonstrate that elevation of intracellular SAH alone by DZA treatment of 3T3-L1 adipocytes induces lipolysis and dysregulates adipokine secretion. Selective elevation of intracellular SAH by DZA treatment mimics ethanol's effects and induces adipose dysfunction. We conclude that alcohol-induced elevations in adipose SAH levels contribute to the pathogenesis and progression of ALD.

Procaine and S-Adenosyl-l-Homocysteine Affect the Expression of Genes Related to the Epigenetic Machinery and Change the DNA Methylation Status of In Vitro Cultured Bovine Skin Fibroblasts

Cloning using somatic cell nuclear transfer (SCNT) has many potential applications such as in transgenic and genomic-edited animal production. Abnormal epigenetic reprogramming of somatic cell nuclei is probably the major cause of the low efficiency associated with SCNT. Strategies to alter DNA reprogramming in donor cell nuclei may help improve the cloning efficiency. In the present study, we aimed to characterize the effects of procaine and S-adenosyl-l-homocysteine (SAH) as demethylating agents during the cell culture of bovine skin fibroblasts. We characterized the effects of procaine and SAH on the expression of genes related to the epigenetic machinery, including the DNA methyltransferase 1 (DNMT1), DNA methyltransferase 3 alpha (DNMT3A), DNA methyltransferase 3 beta (DNMT3B), TET1, TET2, TET3, and OCT4 genes, and on DNA methylation levels of bovine skin fibroblasts. We found that DNA methylation levels of satellite I were reduced by SAH (p = 0.0495) and by the combination of SAH and procaine (p = 0.0479) compared with that in the control group. Global DNA methylation levels were lower in cells that were cultivated with both compounds than in control cells (procaine [p = 0.0116], SAH [p = 0.0408], and both [p = 0.0163]). Regarding gene expression, there was a decrease in the DNMT1 transcript levels in cells cultivated with SAH (p = 0.0151) and SAH/procaine (0.0001); a decrease in the DNMT3A transcript levels in cells cultivated with SAH/procaine (p = 0.016); and finally, a decrease in the DNMT3B transcript levels in cells cultivated with procaine (p = 0.0007), SAH (p = 0.0060), and SAH/procaine (p = 0.0021) was found. Higher levels of TET3 transcripts in cells cultivated with procaine (p = 0.0291), SAH (p = 0.0373), and procaine/SAH (p = 0.0013) compared with the control were also found. Regarding the OCT4 gene, no differences were found. Our results showed that the use of procaine and SAH during bovine cell culture was able to alter the epigenetic profile of the cells. This approach may be a useful alternative strategy to improve the efficiency of reprogramming the somatic nuclei after fusion, which in turn will improve the SCNT efficiency.

Characterization of Inhibitor Binding Through Multiple Inhibitor Analysis: A Novel Local Fitting Method

Understanding inhibitor binding modes is a key aspect of drug development. Early in a drug discovery effort these considerations often impact hit finding strategies and hit prioritization. Multiple inhibitor experiments, where enzyme inhibition is measured in the presence of two simultaneously varied inhibitors, can provide valuable information about inhibitor binding. These experiments utilize the inhibitor concentration dependence of the observed combined inhibition to determine the relationship between two compounds. In this way, it can be determined whether two inhibitors bind exclusively, independently, synergistically, or antagonistically. Novel inhibitors can be tested against each other or reference compounds to assist hit classification and characterization of inhibitor binding. In this chapter, we discuss the utility and design of multiple inhibitor experiments and present a new local curve fitting method for analyzing these data utilizing IC50 replots. The IC50 replot method is analogous to that used for determining mechanisms of inhibition with respect to substrate, as originally proposed by Cheng and Prusoff (Cheng and Prusoff Biochem Pharmacol 22: 3099-3108, 1973). The IC50 replot generated by this method reveals distinct patterns that are diagnostic of the nature of the interaction between two inhibitors. Multiple inhibition of the histone methyltransferase EZH2 by EPZ-5687 and the reaction product S-adenosylhomocysteine is presented as an example of the method.

Elevated S-adenosylhomocysteine alters adipocyte functionality with corresponding changes in gene expression and associated epigenetic marks

Maternal deficiencies in micronutrients affecting one-carbon metabolism before and during pregnancy can influence metabolic status and the degree of insulin resistance and obesity of the progeny in adulthood. Notably, maternal and progeny plasma S-adenosylhomocysteine (SAH) levels are both elevated after vitamin deficiency in pregnancy. Therefore, we investigated whether this key one-carbon cycle intermediate directly affects adipocyte differentiation and function. We found that expansion and differentiation of murine 3T3-L1 preadipocytes in the presence of SAH impaired both basal and induced glucose uptake as well as lipolysis compared with untreated controls. SAH did not alter preadipocyte factor 1 (Dlk1) or peroxisome proliferator-activated receptor-γ 2 (Pparγ2) but significantly reduced expression of CAAT enhancer-binding protein-α (Cebpα), Cebpβ, and retinoid x receptor-α (Rxrα) compared with untreated adipocytes. SAH increased Rxrα methylation on a CpG unit (chr2:27,521,057+, chr2:27,521,049+) and CpG residue (chr2:27,521,080+), but not Cebpβ methylation, relative to untreated adipocytes. Trimethylated histone H3-Lys27 occupancy was significantly increased on Cebpα and Rxrα promoters in SAH-treated adipocytes, consistent with the reduction in gene expression. In conclusion, SAH did not affect adipogenesis per se but altered adipocyte functionality through epigenetic mechanisms, such that they exhibited altered glucose disposal and lipolysis. Our findings implicate micronutrient imbalance in subsequent modulation of adipocyte function.

S-adenosyl-homocysteine is a weakly bound inhibitor for a flaviviral methyltransferase

The methyltransferase enzyme (MTase), which catalyzes the transfer of a methyl group from S-adenosyl-methionine (AdoMet) to viral RNA, and generates S-adenosyl-homocysteine (AdoHcy) as a by-product, is essential for the life cycle of many significant human pathogen flaviviruses. Here we investigated inhibition of the flavivirus MTase by several AdoHcy-derivatives. Unexpectedly we found that AdoHcy itself barely inhibits the flavivirus MTase activities, even at high concentrations. AdoHcy was also shown to not inhibit virus growth in cell-culture. Binding studies confirmed that AdoHcy has a much lower binding affinity for the MTase than either the AdoMet co-factor, or the natural AdoMet analog inhibitor sinefungin (SIN). While AdoMet is a positively charged molecule, SIN is similar to AdoHcy in being uncharged, and only has an additional amine group that can make extra electrostatic contacts with the MTase. Molecular Mechanics Poisson-Boltzmann Sovation Area analysis on AdoHcy and SIN binding to the MTase suggests that the stronger binding of SIN may not be directly due to interactions of this amine group, but due to distributed differences in SIN binding resulting from its presence. The results suggest that better MTase inhibitors could be designed by using SIN as a scaffold rather than AdoHcy.

Impaired methylation as a novel mechanism for proteasome suppression in liver cells

The proteasome is a multi-catalytic protein degradation enzyme that is regulated by ethanol-induced oxidative stress; such suppression is attributed to CYP2E1-generated metabolites. However, under certain conditions, it appears that in addition to oxidative stress, other mechanisms are also involved in proteasome regulation. This study investigated whether impaired protein methylation that occurs during exposure of liver cells to ethanol, may contribute to suppression of proteasome activity. We measured the chymotrypsin-like proteasome activity in Huh7CYP cells, hepatocytes, liver cytosols and nuclear extracts or purified 20S proteasome under conditions that maintain or prevent protein methylation. Reduction of proteasome activity of hepatoma cell and hepatocytes by ethanol or tubercidin was prevented by simultaneous treatment with S-adenosylmethionine (SAM). Moreover, the tubercidin-induced decline in proteasome activity occurred in both nuclear and cytosolic fractions. In vitro exposure of cell cytosolic fractions or highly purified 20S proteasome to low SAM:S-adenosylhomocysteine (SAH) ratios in the buffer also suppressed proteasome function, indicating that one or more methyltransferase(s) may be associated with proteasomal subunits. Immunoblotting a purified 20S rabbit red cell proteasome preparation using methyl lysine-specific antibodies revealed a 25kDa proteasome subunit that showed positive reactivity with anti-methyl lysine. This reactivity was modified when 20S proteasome was exposed to differential SAM:SAH ratios. We conclude that impaired methylation of proteasome subunits suppressed proteasome activity in liver cells indicating an additional, yet novel mechanism of proteasome activity regulation by ethanol.

S-adenosyl-N-decyl-aminoethyl, a potent bisubstrate inhibitor of mycobacterium tuberculosis mycolic acid methyltransferases

S-Adenosylmethionine-dependent methyltransferases (AdoMet-MTs) constitute a large family of enzymes specifically transferring a methyl group to a range of biologically active molecules. Mycobacterium tuberculosis produces a set of paralogous AdoMet-MTs responsible for introducing key chemical modifications at defined positions of mycolic acids, which are essential and specific components of the mycobacterial cell envelope. We investigated the inhibition of these mycolic acid methyltransferases (MA-MTs) by structural analogs of the AdoMet cofactor. We found that S-adenosyl-N-decyl-aminoethyl, a molecule in which the amino acid moiety of AdoMet is substituted by a lipid chain, inhibited MA-MTs from Mycobacterium smegmatis and M. tuberculosis strains, both in vitro and in vivo, with IC(50) values in the submicromolar range. By contrast, S-adenosylhomocysteine, the demethylated reaction product, and sinefungin, a general AdoMet-MT inhibitor, did not inhibit MA-MTs. The interaction between Hma (MmaA4), which is strictly required for the biosynthesis of oxygenated mycolic acids in M. tuberculosis, and the three cofactor analogs was investigated by x-ray crystallography. The high resolution crystal structures obtained illustrate the bisubstrate nature of S-adenosyl-N-decyl-aminoethyl and provide insight into its mode of action in the inhibition of MA-MTs. This study has potential implications for the design of new drugs effective against multidrug-resistant and persistent tubercle bacilli.

Characterization of a mimivirus RNA cap guanine-N2 methyltransferase

A 2,2,7-trimethylguanosine (TMG) cap is a signature feature of eukaryal snRNAs, telomerase RNAs, and trans-spliced nematode mRNAs. TMG and 2,7-dimethylguanosine (DMG) caps are also present on mRNAs of two species of alphaviruses (positive strand RNA viruses of the Togaviridae family). It is presently not known how viral mRNAs might acquire a hypermethylated cap. Mimivirus, a giant DNA virus that infects amoeba, encodes many putative enzymes and proteins implicated in RNA transactions, including the synthesis and capping of viral mRNAs and the promotion of cap-dependent translation. Here we report the identification, purification, and characterization of a mimivirus cap-specific guanine-N2 methyltransferase (MimiTgs), a monomeric enzyme that catalyzes a single round of methyl transfer from AdoMet to an m(7)G cap substrate to form a DMG cap product. MimiTgs, is apparently unable to convert a DMG cap to a TMG cap, and is thereby distinguished from the structurally homologous yeast and human Tgs1 enzymes. Nonetheless, we show genetically that MimiTgs is a true ortholog of Saccharomyces cerevisiae Tgs1. Our results hint that DMG caps can satisfy many of the functions of TMG caps in vivo. We speculate that DMG capping of mimivirus mRNAs might favor viral protein synthesis in the infected host.

Synergistic effects of homocysteine, S-adenosylhomocysteine and adenosine on apoptosis in BV-2 murine microglial cells

Homocysteine (Hcy), S-adenosylhomocysteine (SAH) and adenosine (Ado) are methionine metabolism intermediates that may act synergistically in certain disease. In this study, we examined whether HCy, SAH and Ado may synergistically induce neuronal apoptosis of BV-2 microglial cells. We found that an incubation of BV-2 cells with 1 mM Hcy, 1 muM SAH and 100 muM Ado (SAH + Hcy + Ado) led to marked apoptosis of BV-2 cells, as evidenced by several markers of apoptosis. A synergistic effect of SAH + Hcy + Ado on apoptosis (2.55-fold, P < 0.05) was obtained, as calculated using the data of Annexin V-positive cells. This combination markedly induced intracellular levels of reactive oxygen species (ROS) starting at 6 h and significantly decreased the mitochondrial potential starting at 12 h. The combination significantly elevated caspase-9 and caspase-3 activities at 24 and 48 h. The combination also induced hypomethylation (at 24 and 48 h), as indicated by significantly decreased 5-methyldeoxycytidine levels and SAM/SAH ratios. Pre-incubation of cells with alpha-tocopherol (30 muM) reduced the increase of ROS (at 6 h) and significantly restored cell viability (at 24 and 48~h) in the SAH + Hcy + Ado group. Overall, the present study demonstrates that SAH, Hcy and Ado synergistically induce BV-2 apoptosis, possibly by generation of ROS and induction of intracellular hypomethylation.

Contribution of whole blood to the control of plasma asymmetrical dimethylarginine

The endogenous nitric oxide (NO) synthase (NOS) inhibitor asymmetrical dimethylarginine (ADMA) is elevated in many patients and may contribute to the initiation and progression of their disease. While some mechanistic pathways have been identified, tissue-specific contributions to ADMA control remain unclear. We sought to determine if whole blood (WB) could participate in ADMA control ex vivo. Anesthetized male Sprague-Dawley rats underwent exsanguinations, and WB preparations were incubated at 37°C for 5 h. ADMA and symmetrical dimethylarginine were analyzed by high-pressure liquid chromatography. Incubation of lysed red blood cell (RBC) supernatant yielded a significant decrease in ADMA that was blocked by 4124W, a synthetic inhibitor of dimethylarginine dimethylaminohydrolase, the only reported enzyme to hydrolyze ADMA. Hydrolysis of ADMA was diminished by addition of physiologically relevant concentrations of zinc (i.e., 20 μM). Conversely, when rat WB or WB supernatant was incubated at 37°C, it liberated quantities of free ADMA (1–2 μM) that in vivo would likely have pathological consequences. Addition of arginine methyltransferase inhibitors to these incubations did not reduce ADMA release, indicating no dominant role for active protein methylation during these incubations. This ADMA liberation was significantly reduced by addition of protease inhibitors, indicating a dependence on peptide bond hydrolysis. Total ADMA (protein incorporated plus free) was determined by acid hydrolysis and found to be 43.18 ± 4.79 μM in WB with ∼95% of this in RBCs. These ex vivo data demonstrate the potential of blood to control the NO-NOS system by modulating free ADMA.

Asymmetric dimethylarginine (ADMA) accelerates cell senescence

Asymmetric dimethylarginine (ADMA) is an endogenous inhibitor of nitric oxide synthase and its accumulation has been associated with cardiovascular disease. We aimed to investigate the role of ADMA in endothelial cell senescence. Endothelial cells were cultured until the tenth passage. ADMA was replaced every 48 hours starting at the fourth passage. ADMA significantly accelerated senescence-associated beta-galactosidase activity. Additionally, the shortening of telomere length was significantly speeded up and telomerase activity was significantly reduced. This effect was associated with an increase of oxidative stress: both allantoin, a marker of oxygen free radical generation, and intracellular reactive oxygen species increased significantly after ADMA treatment compared with control, whereas nitric oxide synthesis decreased. Furthermore, ADMA-increased oxidative stress was accompanied by a decrease in the activity of dimethylarginine dimethylaminohydrolase, the enzyme that degrades ADMA, which could be prevented by the antioxidant pyrrolidine dithiocarbamate. Exogenous ADMA also stimulated secretion of monocyte chemotactic protein-1 and interleukin-8. Co-incubation with the methyltransferase inhibitor S-adenosylhomocysteine abolished the effects of ADMA. These data suggest that ADMA accelerates senescence, probably via increased oxygen radical formation by inhibiting nitric oxide elaboration. This study provides evidence that modest changes of intracellular ADMA levels are associated with significant effects on slowing down endothelial senescence.

Catalytic properties and kinetic mechanism of human recombinant Lys-9 histone H3 methyltransferase SUV39H1: participation of the chromodomain in enzymatic catalysis

Histone H3 lysine 9 (H3K9) methylation is a major component of gene regulation and chromatin organization. SUV39H1 methylates H3K9 at the pericentric heterochromatin region and participates in the maintenance of genome stability. In this study, a recombinant purified SUV39H1 is used for substrate specificity and steady-state kinetic analysis with peptides representing the un- or dimethylated lysine 9 histone H3 tail or full-length human recombinant H3 (rH3). Recombinant SUV39H1 methylated its substrate via a nonprocessive mechanism. Binding of either peptide or AdoMet first to the enzyme made a catalytically competent binary complex. Product inhibition studies with SUV39H1 showed that S-adenosyl-l-homocysteine is a competitive inhibitor of S-adenosyl-l-methionine and a mixed inhibitor of substrate peptide. Similarly, the methylated peptide was a competitive inhibitor of the unmethylated peptide and a mixed inhibitor of AdoMet, suggesting a random mechanism in a bi-bi reaction for recombinant SUV39H1 in which either substrate can bind to the enzyme first and either product can release first. The turnover numbers (k(cat)) for the H3 tail peptide and rH3 were comparable (12 and 8 h(-)(1), respectively) compared to the value of 1.5 h(-)(1) for an identical dimethylated lysine 9 H3 tail peptide. The Michaelis constant for the methylated peptide (K(m)(pep)) was 13-fold lower compared to that of the unmethylated peptide. The Michaelis constants for AdoMet (K(m)(AdoMet)) were 12 and 6 microM for the unmethylated peptide substrate and rH3, respectively. A reduction in the level of methylation was observed at high concentrations of rH3, implying substrate inhibition. Deletion of the chromodomain or point mutation of the conserved amino acids, W64A or W67A, of SUV39H1 impaired enzyme activity despite the presence of an intact catalytic SET domain. Thus, SUV39H1 utilizes both the chromodomain and the SET domain for catalysis.

The Role of Phospholipid Methylation in 1-Methyl-4-Phenyl-Pyridinium Ion (MPP+)-Induced Neurotoxicity in PC12 Cells

Excessive methylation has been proposed to be involved in the pathogenesis of Parkinson's disease (PD), via mechanisms that involve phospholipid methylation. Meanwhile, 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) was found to stimulate phospholipid methylation via the oxidized metabolite, 1-methyl-4-phenyl-pyridinium (MPP+), in the rat brain and liver tissues. In the present study, we investigated the effect of MPP+ on phosphatidylethanolamine N-methyltransferases (PENMT) and the potential role of this pathway in MPP(+)-induced neurotoxicity using PC12 cells. The results obtained indicate that MPP+ stimulated phosphatidylethanolamine (PTE) methylation to phosphatidylcholine (PTC) and correspondingly increased the formation of lysophosphatidylcholine (lyso-PTC). Moreover, the addition of S-adenosylmethionine (SAM) to the cell culture medium increases MPP(+)-induced cytotoxicity. The incubation of 1mM MPP+ and various concentrations of SAM (0-4 mM) decreased the viability of PC12 cells from 80% with MPP+ alone to 38% viability with 4 mM SAM for 4 days incubation. The data also revealed that the addition of S-adenosylhomocysteine (SAH), a methylation inhibitor, offered significant protection against MPP(+)-induced cytotoxicity, indicating that methylation plays a role in MPP(+)-induced cytotoxicity. Interestingly, lyso-PTC showed similar actions to MPP+ in causing many cytotoxic changes with at least 10 times higher potency. Lyso-PTC induced dopamine release and inhibited dopamine uptake in PC12 cells. Lyso-PTC also caused the inhibition of mitochondrial potential and increased the formation of reactive oxygen species in PC12 cells. These results indicate that phospholipid methylation pathway might be involved in MPP+ neurotoxicity and lyso-PTC might play a role in MPP(+)-induced neurotoxicity.

Homocysteine, a risk factor for atherosclerosis, biphasically changes the endothelial production of kynurenic acid

Increased serum level of homocysteine is an independent risk factor for vascular disease. The effect of DL-homocysteine on the endothelial production of kynurenic acid, an antagonist of alpha7-nicotinic and N-methyl-D-aspartate (NMDA) glutamate receptors, has been evaluated in vitro and in vivo. In rat aortic rings, DL-homocysteine at 40-100 microM enhanced, whereas at >or=400 microM decreased the synthesis of kynurenic acid. S-adenosylhomocysteine mimicked the biphasic action of DL-homocysteine. On the contrary, thiol-containing compounds, L-cysteine and L-methionine, were only inhibiting kynurenic acid production. L-kynurenine uptake blockers, L-phenylalanine and L-leucine, reversed the stimulatory effect of S-adenosylhomocysteine. L-glycine, co-agonist of NMDA receptor, and cis-4-phosphonomethyl-2-piperidine carboxylic acid (CGS 19755), an antagonist of NMDA receptor, have not influenced kynurenic acid formation. In vivo, DL-homocysteine (1.3 mmol, i.p.) increased the level of kynurenic acid in rat serum from 23.7+/-7.1 to 60.7+/-14.2 (15 min, P<0.01) and 55.7+/-13.6 (60 min, P<0.01) pmol/ml, respectively; the endothelial content of kynurenic acid was also increased (51.6+/-5.8 vs. 73.2+/-9.4 fmol/microg of protein; 15 min; P<0.01). DL-homocysteine seems to modulate the production of kynurenic acid both directly and indirectly, possibly following the conversion to S-adenosylhomocysteine. The obtained data suggest a potential contribution of altered formation of kynurenic acid to the endothelial changes induced by hyperhomocysteinemia.

Gene expression analysis in human gastric cancer cell line treated with trichostatin A and S-adenosyl-L-homocysteine using cDNA microarray

Trichostatin A (TSA) and S-adenosyl-L-homocysteine (AdoHcy) have been reported to affect histone modifications. To investigate the effects of two drugs that can reportedly affect chromatin remodeling, we analyzed the gene expression profiles of TSA and AdoHcy in a gastric cancer cell line using 14 K cDNA microarray. The significant analysis of microarray (SAM) identified 98 and 43 differentially expressed genes in TSA and AdoHcy treated sets, respectively, and selected genes were functionally classified. In the gastric cancer cell line, genes related to cell communication, cell growth/maintenance, and morphogenesis were highly expressed with TSA, and genes with cell growth/maintenance, metabolism, oxidoreductase activity were upregulated with AdoHcy. Genes downregulated with TSA included those controlling the cell cycle, cell growth/proliferation, DNA binding, and metabolism, whereas genes involved in calcium signaling, cell growth/proliferation, and metabolism were downregulated with AdoHcy. Furthermore, we identified the genes commonly expressed in both drug treatments. Compared to TSA, AdoHcy did not induce apoptosis in the SNU-16 gastric cancer cell line, and RT-PCR was performed for selective genes to confirm the microarray data. This gene expression profile analysis with TSA and AdoHcy should contribute to a greater understanding of the molecular mechanism of chromatin remodeling and cancer, and provide candidate genes for further studies involving the roles of histone modifications in gastric cancer.

Dual effect of DL-homocysteine and S-adenosylhomocysteine on brain synthesis of the glutamate receptor antagonist, kynurenic acid

Increased serum level of homocysteine, a sulfur-containing amino acid, is considered a risk factor in vascular disorders and in dementias. The effect of homocysteine and metabolically related compounds on brain production of kynurenic acid (KYNA), an endogenous antagonist of glutamate ionotropic receptors, was studied. In rat cortical slices, DL-homocysteine enhanced (0.1-0.5 mM) or inhibited (concentration inducing 50% inhibition [IC50]=6.4 [5.5-7.5] mM) KYNA production. In vivo peripheral application of DL-homocysteine (1.3 mmol/kg intraperitoneally) increased KYNA content (pmol/g tissue) from 8.47 +/- 1.57 to 13.04 +/- 2.86 (P <0.01; 15 min) and 11.4 +/- 1.72 (P <0.01; 60 min) in cortex, and from 4.11 +/- 1.54 to 10.02 +/- 3.08 (P <0.01; 15 min) in rat hippocampus. High concentrations of DL-homocysteine (20 mM) applied via microdialysis probe decreased KYNA levels in rabbit hippocampus; this effect was antagonized partially by an antagonist of group I metabotropic glutamate receptors, LY367385. In vitro, S-adenosylhomocysteine acted similar to but more potently than DL-homocysteine, augmenting KYNA production at 0.03-0.08 mM and reducing it at > or =0.5 mM. The stimulatory effect of S-adenosylhomocysteine was abolished in the presence of the L-kynurenine uptake inhibitors L-leucine and L-phenyloalanine. Neither the N-methyl-D-aspartate (NMDA) antagonist CGS 19755 nor L-glycine influenced DL-homocysteine- and S-adenosylhomocysteine-induced changes of KYNA synthesis in vitro. DL-Homocysteine inhibited the activity of both KYNA biosynthetic enzymes, kynurenine aminotransferases (KATs) I and II, whereas S-adenosylhomocysteine reduced only the activity of KAT II. L-Methionine and L-cysteine, thiol-containing compounds metabolically related to homocysteine, acted only as weak inhibitors, reducing KYNA production in vitro and inhibiting the activity of KAT II (L-cysteine) or KAT I (L-methionine). The present data suggest that DL-homocysteine biphasically modulates KYNA synthesis. This seems to result from conversion of compound to S-adenosylhomocysteine, also acting dually on KYNA formation, and in part from the direct interaction of homocysteine with metabotropic glutamate receptors and KYNA biosynthetic enzymes. It seems probable that hyperhomocystemia-associated brain dysfunction is mediated partially by changes in brain KYNA level.

Encephalitozoon cuniculi mRNA cap (guanine N-7) methyltransferase: methyl acceptor specificity, inhibition BY S-adenosylmethionine analogs, and structure-guided mutational analysis

The Encephalitozoon cuniculi mRNA cap (guanine N-7) methyltransferase Ecm1 has been characterized structurally but not biochemically. Here we show that purified Ecm1 is a monomeric protein that catalyzes methyl transfer from S-adenosylmethionine (AdoMet) to GTP. The reaction is cofactor-independent and optimal at pH 7.5. Ecm1 also methylates GpppA, GDP, and dGTP but not ATP, CTP, UTP, ITP, or m(7)GTP. The affinity of Ecm1 for the cap dinucleotide GpppA (K 0.1 mm) is higher than that for GTP (K(m) 1 mm) or GDP (K(m) 2.4 mm). Methylation of GTP by Ecm1 in the presence of 5 microm AdoMet is inhibited by the reaction product AdoHcy (IC(50) 4 microm) and by substrate analogs sinefungin (IC(50) 1.5 microm), aza-AdoMet (IC(50) 100 microm), and carbocyclic aza-AdoMet (IC(50) 35 microm). The crystal structure of an Ecm1.aza-AdoMet binary complex reveals that the inhibitor occupies the same site as AdoMet. Structure-function analysis of Ecm1 by alanine scanning and conservative substitutions identified functional groups necessary for methyltransferase activity in vivo. Amino acids Lys-54, Asp-70, Asp-78, and Asp-94, which comprise the AdoMet-binding site, and Phe-141, which contacts the cap guanosine, are essential for cap methyltransferase activity in vitro.

Determinants of cofactor binding to DNA methyltransferases: insights from a systematic series of structural variants of S-adenosylhomocysteine

S-Adenosylmethionine (AdoMet) is a commonly used cofactor, second only to ATP in the variety of reactions in which it participates. It is the methyl donor in the majority of methyl transfer reactions, including methylation of DNA, RNA, proteins and small molecules. Almost all structurally characterised methyltransferases share a conserved AdoMet-dependent methyltransferase fold, in which AdoMet is bound in the same orientation. Although potential interactions between the cofactor and methyltransferases have been inferred from crystal structures, there has not been a systematic study of the contributions of each functional group to binding. To explore the binding interaction we synthesised a series of seven analogues of the methyltransferase inhibitor S-adenosylhomocysteine (AdoHcy), each containing a single modification, and tested them for the ability to inhibit methylation by HhaI and HaeIII DNA methyltransferase. Comparison of the Ki values highlights the structural determinants for cofactor binding, and indicates which nucleoside and amino acid functional groups contribute significantly to AdoMet binding. An understanding of the binding of AdoHyc to methyltransferases will greatly assist the design of AdoMet inhibitors.

Endogenous nitric oxide synthesis inhibitor asymmetric dimethyl L-arginine accelerates endothelial cell senescence

OBJECTIVE

Asymmetrical dimethylarginine (ADMA) is an endogenous inhibitor of nitric oxide synthase (NOS), and its accumulation has been associated with cardiovascular disease. We aimed to investigate the role of ADMA in endothelial cell senescence.

METHODS AND RESULTS

Endothelial cells were cultured until the tenth passage. ADMA was replaced every 48 hours starting at the fourth passage. ADMA significantly accelerated senescence associated beta-galactosidase activity. Additionally, the shortening of telomere length was significantly accelerated and the telomerase activity was significantly reduced. This effect was associated with an increase of oxidative stress: allantoin, a marker of oxygen free radical generation, and intracellular reactive oxygen species (ROS) increased significantly after ADMA treatment compared with control, whereas cellular thiol status and NOx synthesis decreased. Furthermore, ADMA-increased oxidative stress was accompanied by a decrease in the activity of dimethylarginine dimethylaminohydrolase (DDAH), the enzyme that degrades ADMA, which could be prevented by the antioxidant pyrrolidine dithiocarbamate. Exogenous ADMA also stimulated secretion of MCP-1 and interleukin-8. Coincubation with the methyltransferase inhibitor S-adenosylhomocysteine abolished the effects of ADMA.

CONCLUSIONS

These data suggest that ADMA accelerates senescence, probably via increased oxygen radical formation by inhibiting nitric oxide elaboration. This study provides evidence that modest changes of intracellular ADMA levels are associated with significant effects on slowing endothelial senescence.

Nucleosides: XI. Synthesis and Antiviral Evaluation of 5'-Alkylthio-5'-deoxy Quinazolinone Nucleoside Derivatives as S-Adenosyl-L-homocysteine Analogs

4-amino-1-(beta-D-ribofuranosyl)quinazolin-2-one (3) was prepared by a direct glycosylation of 4-aminoquinazolin-2-one (7) using the Vorbruggen's silylation method and provided exclusively the beta-anomer. This quinazoline nucleoside and its 2',3'-O-isopropylidene derivative (9) did not undergo the coupling reaction with dialkyl disulfides in the presence of tri-n-butylphosphine unless their 4-amino groups were protected by N,N-dimethylaminomethylidene. This approach provides a viable alternative synthetic route to 5'-alkylthio-5'-deoxy nucleosides.

S-Adenosylhomocysteine Enhances Hydrogen Peroxide-Induced DNA Damage by Inhibition of DNA Repair in Two Cell Lines

It has been proposed that hyperhomocysteinemia may exert its pathogenic effects largely through metabolic accumulation of S-adenosylhomocysteine (SAH), a strong noncompetitive inhibitor of most methyltransferases. Here, we investigated the effects of SAH on H(2)O(2)-induced cellular DNA damage in comparison with the effects of homocysteine (Hcy) in a mouse endothelial cell line and a human intestinal cell line. Cells were preincubated for 2 h with H(2)O(2) (20 microM) followed by incubation with SAH or Hcy for 3 h. DNA strand breakage was determined using comet assay and DNA repair capacity determined using the same assay over time at 1, 2, and 3 h during SAH incubation. In both types of cells, SAH at 0.25-2 microM strongly and dose dependently enhanced H(2)O(2)-dependent DNA damage and inhibited DNA repair, whereas Hcy had a much weaker effect. SAH markedly increased uracil misincorporation, and this effect was also much stronger than that of Hcy. Taken together, our results show that SAH potentiates H(2)O(2)-induced DNA damage in cell cultures through impaired DNA repair capability and suggest that such effects are related to uracil misincorporation. Although the in vivo relevance of our findings is unclear, the biological significance of SAH-mediated detrimental effect, secondary to elevated intracellular Hcy, is an interesting area awaiting further exploration.

The methyl donor S-Adenosylmethionine inhibits active demethylation of DNA: a candidate novel mechanism for the pharmacological effects of S-Adenosylmethionine

S-Adenosylmethionine (AdoMet) is the methyl donor of numerous methylation reactions. The current model is that an increased concentration of AdoMet stimulates DNA methyltransferase reactions, triggering hypermethylation and protecting the genome against global hypomethylation, a hallmark of cancer. Using an assay of active demethylation in HEK 293 cells, we show that AdoMet inhibits active demethylation and expression of an ectopically methylated CMV-GFP (green fluorescent protein) plasmid in a dose-dependent manner. The inhibition of GFP expression is specific to methylated GFP; AdoMet does not inhibit an identical but unmethylated CMV-GFP plasmid. S-Adenosylhomocysteine (AdoHcy), the product of methyltransferase reactions utilizing AdoMet does not inhibit demethylation or expression of CMV-GFP. In vitro, AdoMet but not AdoHcy inhibits methylated DNA-binding protein 2/DNA demethylase as well as endogenous demethylase activity extracted from HEK 293, suggesting that AdoMet directly inhibits demethylase activity, and that the methyl residue on AdoMet is required for its interaction with demethylase. Taken together, our data support an alternative mechanism of action for AdoMet as an inhibitor of intracellular demethylase activity, which results in hypermethylation of DNA.

Kinetic mechanism of the tRNA-modifying enzyme S-adenosylmethionine:tRNA ribosyltransferase-isomerase (QueA)

The bacterial enzyme S-adenosylmethionine:tRNA ribosyltransferase-isomerase (QueA) catalyzes the unprecedented transfer and isomerization of the ribosyl moiety of S-adenosylmethionine (AdoMet) to a modified tRNA nucleoside in the biosynthesis of the hypermodified nucleoside queuosine. The complexity of this reaction makes it a compelling problem in fundamental mechanistic enzymology, and as part of our mechanistic studies of the QueA-catalyzed reaction, we report here the elucidation of the steady-state kinetic mechanism. Bi-substrate kinetic analysis gave initial velocity patterns indicating a sequential mechanism, and provided the following kinetic constants: K (M)(tRNA)= 1.9 +/- 0.7 microM and K (M)(AdoMet)= 98 +/- 5.0 microM. Dead-end inhibition studies with the substrate analogues S-adenosylhomocysteine and sinefungin gave competitive inhibition patterns against AdoMet and noncompetitive patterns against preQ(1)-tRNA(Tyr), with K(i) values of 133 +/- 18 and 4.6 +/- 0.5 microM for sinefungin and S-adenosylhomocysteine, respectively. Product inhibition by adenine was noncompetitive against both substrates under conditions with a subsaturating cosubstrate concentration and uncompetitive against preQ(1)-tRNA(Tyr) when AdoMet was saturating. Inhibition by the tRNA product (oQ-tRNA(Tyr)) was competitive and noncompetitive against the substrates preQ(1)-tRNA(Tyr) and AdoMet, respectively. Inhibition by methionine was uncompetitive versus preQ(1)-tRNA(Tyr), but noncompetitive against AdoMet. However, when methionine inhibition was investigated at high AdoMet concentrations, the pattern was uncompetitive. Taken together, the data are consistent with a fully ordered sequential bi-ter kinetic mechanism in which preQ(1)-tRNA(Tyr) binds first followed by AdoMet, with product release in the order adenine, methionine, and oQ-tRNA. The chemical mechanism that we previously proposed for the QueA-catalyzed reaction [Daoud Kinzie, S., Thern, B., and Iwata-Reuyl, D. (2000) Org. Lett. 2, 1307-1310] is consistent with the constraints imposed by the kinetic mechanism determined here, and we suggest that the magnitude of the inhibition constants for the dead-end inhibitors may provide insight into the catalytic strategy employed by the enzyme.

S-adenosylmethionine transport in Rickettsia prowazekii

Rickettsia prowazekii, the causative agent of epidemic typhus, is an obligate, intracellular, parasitic bacterium that grows within the cytoplasm of eucaryotic host cells. Rickettsiae exploit this intracellular environment by using transport systems for the compounds available in the host cell's cytoplasm. Analysis of the R. prowazekii Madrid E genome sequence revealed the presence of a mutation in the rickettsial metK gene, the gene encoding the enzyme responsible for the synthesis of S-adenosylmethionine (AdoMet). Since AdoMet is required for rickettsial processes, the apparent inability of this strain to synthesize AdoMet suggested the presence of a rickettsial AdoMet transporter. We have confirmed the presence of an AdoMet transporter in the rickettsiae which, to our knowledge, is the first bacterial AdoMet transporter identified. The influx of AdoMet into rickettsiae was a saturable process with a K(T) of 2.3 micro M. Transport was inhibited by S-adenosylethionine and S-adenosylhomocysteine but not by sinfungin or methionine. Transport was also inhibited by 2,4-dinitrophenol, suggesting an energy-linked transport mechanism, and by N-ethylmaleimide. AdoMet transporters with similar properties were also identified in the Breinl strain of R. prowazekii and in Rickettsia typhi. By screening Escherichia coli clone banks for AdoMet transport, the R. prowazekii gene coding for a transporter, RP076 (sam), was identified. AdoMet transport in E. coli containing the R. prowazekii sam gene exhibited kinetics similar to that seen in rickettsiae. The existence of a rickettsial transporter for AdoMet raises intriguing questions concerning the evolutionary relationship between the synthesis and transport of this essential metabolite.

Characterization of gamma-tocopherol methyltransferases from Capsicum annuum L and Arabidopsis thaliana

Tocopherols are essential micronutrients in human and animal nutrition due to their function as lipophilic antioxidants. They are exclusively synthesized by photosynthetic organisms including higher plants. Despite the attributed beneficial health effects and many industrial applications, research on the tocopherol biosynthetic pathway and its regulation in plants is still limited. In the work presented here we performed a detailed biochemical characterization of a gamma-tocopherol methyltransferase (gamma-TMT) from Arabidopsis thaliana and of a gamma-TMT purified from Capsicum annuum fruits, a tissue with high accumulation of tocopherols. The biochemical characteristics of both enzyme preparations were remarkably similar including substrate specificities. Both enzymes converted delta- and gamma- into beta- and alpha-tocopherol, respectively, but beta-tocopherol was not accepted as a substrate, pointing to a specific methylation at the C(5)-position of the tocopherol aromatic head group. A kinetic analysis performed with the Arabidopsis enzyme was consistent with an iso-ordered bi-bi type reaction mechanism. Our results emphasize the role of gamma-TMT in regulating the spectrum of accumulated tocopherols in plants.

S-adenosyl-methionine-induced apoptosis in PC12 cells

Our previous studies showed that S-adenosyl-methionine (SAM) induced Parkinson's disease-like changes in rat. It caused death to dopamine neurons in the substantia nigra, which appeared shrunken and fragmented, indicative of apoptosis-like changes (Charlton and Crowell [1995] Mol. Chem. Neuropathol. 26:269-284; Charlton [1997] Life Sci. 61:495-502). In this study, we investigated whether SAM causes apoptosis in both undifferentiated PC12 (PC12) cells and nerve growth factor (NGF)-differentiated PC12 (D-PC12) cells. S-adenosyl-homocysteine (SAH), the nonmethyl analog of SAM, was also tested. SAM and SAH (1.0 nM to 10.0 microM) caused lactate dehydrogenase (LDH) release from the PC12 cells and D-PC12 cells; cells with morphological changes and fluorescent DNA fragmentation staining were detected among both PC12 cell and D-PC12 cell. Compared with the PC12 cell, the D-PC12 cell, a postmitotic cell, was more sensitive to the toxic effects of SAM or SAH and presented much greater LDH release, suggesting a lethal effect; surprisingly, the amounts of apoptotic cells did not differ significantly between the two kinds of cells. In medium deprived of exogenous methionine, a decline in LDH release was observed in PC12 and D-PC12 cells. Also, lower levels of intracellular SAM and SAH were observed in the methionine-deleted media, which were reversed by the addition of either SAM or SAH. An antivitamin B(12) monoclonal antibody was added to methionine-depleted medium, resulting in deficiency of both endogenous and exogenous methionine, which caused further decreases in LDH release and reduction in the levels of intracellular SAM and SAH. The preliminary data showed different sensitivities to SAM or SAH between PC12 cell and D-PC12 cells, which suggests that PC12 cell may be more stable as a metabolic model. Apoptosis of PC12 cells was also assessed by PARP cleavage detection, Western blot analysis of Bax and Bcl-2 proteins, and DNA laddering on agarose gel electrophoresis. The proapoptoic protein Bax was dominantly expressed, whereas Bcl-2 was slightly down-regulated by SAM. SAH weakly induced the expression of Bax and slightly decreased Bcl-2 levels. The effects of SAM and its analog, SAH, were demonstrated conclusively to induce apoptosis in PC12 cells.

Synthesis of an alpha-aminophosphonate nucleoside as an inhibitor of S-adenosyl-L-homocysteine hydrolase

A phosphonic acid analogue of S-adenosyl-L-homocysteine was prepared by a novel method and the epimeric mixture separated. Preliminary studies indicate that each epimer causes time-dependent inactivation of S-adenosyl-L-homocysteine hydrolase, however each presented distinct kinetic characteristics.

Rapid conversion of tea catechins to monomethylated products by rat liver cytosolic catechol-O-methyltransferase

1. The metabolic O-methylation of several catechol-containing tea polyphenols by rat liver cytosolic catechol-O-methyltransferase (COMT) has been studied. 2. When (-)-epicatechin was used as substrate, its O-merthylation showed dependence on incubation time, cytosolic protein concentration, incubation pH and concentration of S-adenosyl-L-methionine. The O-methylation of increasing concentrations of (-)-epicatechin followed typical Michaelis-Menten kinetics, and the apparent Km and Vmax were 51 microM and 2882 pmol mg protein(-1) min(-1), respectively, at pH 7.4, and were 17 microM and 2093 pmol mg protein(-1) min(-1), respectively, at pH 10.0. 3. Under optimized conditions for in vitro O-methylation, (-)-epicatechin, (+)-epicatechin and (-)-epigallocatechin were rapidly O-methylated by rat liver cytosol. In comparison, (-)-epicatechin gallate and (-)-epigallocatechin gallate vere O-methylated at significantly lower rates under the same reaction conditions. catalysed O-methylation of (-)-epicatechin and (-)-epigallocatechin was inhibited in a concentration-dependent manner by S-adenosyl-L-homocysteine, a demethylated product of S-adenosyl-L-methionine. The IC50 was approximately 10 microM. 5. In summary, the results showed that several catechol-containing tea polyphenols were rapidly O-methylated by rat liver cytosolic COMT. These observations raise the possibility that some of the biological effects of tea polyphenols may be exerted by their O-methylated products or may result from their potential inhibition of the COMT-catalysed O-methylation of endogenous catecholamines and catechol oestrogens.

A dual role for substrate S-adenosyl-L-methionine in the methylation reaction with bacteriophage T4 Dam DNA-[N6-adenine]-methyltransferase

The fluorescence of 2-aminopurine ((2)A)-substituted duplexes (contained in the GATC target site) was investigated by titration with T4 Dam DNA-(N6-adenine)-methyltransferase. With an unmethylated target ((2)A/A duplex) or its methylated derivative ((2)A/(m)A duplex), T4 Dam produced up to a 50-fold increase in fluorescence, consistent with (2)A being flipped out of the DNA helix. Though neither S-adenosyl-L-homocysteine nor sinefungin had any significant effect, addition of substrate S-adenosyl-L-methionine (AdoMet) sharply reduced the Dam-induced fluorescence with these complexes. In contrast, AdoMet had no effect on the fluorescence increase produced with an (2)A/(2)A double-substituted duplex. Since the (2)A/(m)A duplex cannot be methylated, the AdoMet-induced decrease in fluorescence cannot be due to methylation per se. We propose that T4 Dam alone randomly binds to the asymmetric (2)A/A and (2)A/(m)A duplexes, and that AdoMet induces an allosteric T4 Dam conformational change that promotes reorientation of the enzyme to the strand containing the native base. Thus, AdoMet increases enzyme binding-specificity, in addition to serving as the methyl donor. The results of pre-steady-state methylation kinetics are consistent with this model.

Characterization of BseMII, a new type IV restriction-modification system, which recognizes the pentanucleotide sequence 5'-CTCAG(N)(10/8)/

We report the properties of the new BseMII restriction and modification enzymes from Bacillus stearothermophilus Isl 15-111, which recognize the 5'-CTCAG sequence, and the nucleotide sequence of the genes encoding them. The restriction endonuclease R.BseMII makes a staggered cut at the tenth base pair downstream of the recognition sequence on the upper strand, producing a two base 3'-protruding end. Magnesium ions and S:-adenosyl-L-methionine (AdoMet) are required for cleavage. S:-adenosylhomocysteine and sinefungin can replace AdoMet in the cleavage reaction. The BseMII methyltransferase modifies unique adenine residues in both strands of the target sequence 5'-CTCAG-3'/5'-CTGAG-3'. Monomeric R.BseMII in addition to endonucleolytic activity also possesses methyltransferase activity that modifies the A base only within the 5'-CTCAG strand of the target duplex. The deduced amino acid sequence of the restriction endonuclease contains conserved motifs of DNA N6-adenine methylases involved in S-adenosyl-L-methionine binding and catalysis. According to its structure and enzymatic properties, R.BseMII may be regarded as a representative of the type IV restriction endonucleases.

O-Methylation of tea polyphenols catalyzed by human placental cytosolic catechol-O-methyltransferase

In the present study, we evaluated the metabolic O-methylation of several catechol-containing tea polyphenols by human placental catechol-O-methyltransferase (COMT). (-)-Epicatechin, (+)-epicatechin, and (-)-epigallocatechin were good substrates for metabolic O-methylation by placental cytosolic COMT (150-500 pmol/mg of protein/min), but (-)-epicatechin gallate and (-)-epigallocatechin gallate were O-methylated at much lower rates (<50 pmol/mg of protein/min). When (-)-epicatechin was used as substrate, its O-methylation by human placental COMT showed dependence on incubation time, cytosolic protein concentration, incubation pH, and concentration of S-adenosyl-L-methionine (the methyl donor). Analysis of cytosolic COMT from six human term placentas showed that the O-methylation of increasing concentrations of (-)-epicatechin or (-)-epigallocatechin follows typical Michaelis-Menten kinetics, with K(m) and V(max) values of 2.2 to 8.2 microM and 132 to 495 pmol/mg of protein/min for (-)-epicatechin and 3.9 to 6.7 microM and 152 to 310 pmol/mg of protein/min for (-)-epigallocatechin, respectively. Additional analysis revealed that COMT-catalyzed O-methylation of (-)-epicatechin and (-)-epigallocatechin was strongly inhibited in a concentration-dependent manner by S-adenosyl-L-homocysteine (IC(50) = 3.2-5.7 microM), a demethylated product of S-adenosyl-L-methionine. This inhibition by S-adenosyl-L-homocysteine follows a mixed (competitive plus noncompetitive) mechanism of enzyme inhibition. In summary, several catechol-containing tea polyphenols are rapidly O-methylated by human placental cytosolic COMT. This metabolic O-methylation is subject to strong inhibitory regulation by S-adenosyl-L-homocysteine, which is formed in large quantities during the O-methylation of tea polyphenols.

Design and synthesis of mimics of S-adenosyl-L-homocysteine as potential inhibitors of erythromycin methyltransferases

A series of indanotriazine C-ribosides were prepared as SAH mimics, and tested for their ability to inhibit erythromycin resistance methylases Erm AM and Erm C'. A carbocyclic analogue derived from quinic acid was also synthesized and tested.

Recombinant human DNA (cytosine-5) methyltransferase. II. Steady-state kinetics reveal allosteric activation by methylated dna

Initial velocity determinations were conducted with human DNA (cytosine-5) methyltransferase (DNMT1) on unmethylated and hemimethylated DNA templates in order to assess the mechanism of the reaction. Initial velocity data with DNA and S-adenosylmethionine (AdoMet) as variable substrates and product inhibition studies with methylated DNA and S-adenosylhomocysteine (AdoHcy) were obtained and evaluated as double-reciprocal plots. These relationships were linear for plasmid DNA, exon-1 from the imprinted small nuclear ribonucleoprotein-associated polypeptide N, (CGG.CCG)(12), (m(5)CGG. CCG)(12), and (CGG.CCG)(73) but were not linear for (CGG. Cm(5)CG)(12). Inhibition by AdoHcy was apparently competitive versus AdoMet and uncompetitive/noncompetitive versus DNA at </=20 microM AdoMet. Addition of the product (methylated DNA) to unmethylated plasmid DNA increased V(max(app)) resulting in mixed stimulation and inhibition. Velocity equations indicated a two-step mechanism as follows: first, activation of DNMT1 by methylated DNA that bound to an allosteric site, and second, the addition of AdoMet and DNA to the catalytic site. The preference of DNMT1 for hemimethylated DNA may be the result of positive cooperativity of AdoMet binding mediated by allosteric activation by the methylated CG steps. We propose that this activation plays a role in vivo in the regulation of maintenance methylation.

An endogenous proteinacious inhibitor for S-adenosyl-L-methionine-dependent transmethylation reactions; identification of S-adenosylhomocystein as an integral part

A proteinacious inhibitor with a molecular weight of 1,600 Da which inhibits S-adenosyl-L-methionine-dependent transmethylation reactions was purified from porcine liver to homogeneity by procedures including boiling, Sephadex G-25 column chromatography and repeated HPLC. Employing both Nuclear Magnetic Resonance (NMR) and Fast Atom Bombardment-Mass (FAB-Mass) spectroscopy, S-adenosylhomocysteine was conclusively identified as an integral part of the inhibitor. The purified S-adenosylhomocysteine was competitive with S-adenosyl-L-methionine with Ki value of 6.3x10(-6) M towards protein methylase II.

Methyltransferase inhibitor S-adenosyl-L-homocysteine sensitizes human breast carcinoma MCF7 cells and related TNF-resistant derivatives to TNF-mediated cytotoxicity via the ceramide-independent pathway

In this study we investigated the signalling requirements for TNF-induced cytotoxicity modulated by the methyltransferase inhibitor S-adenosyl-L-homocysteine (AdoHcy) using the TNF-sensitive human breast carcinoma MCF7 cells and its established TNF-resistant clones (R-A1 and clone 1001). Our data indicate that inhibition of methylation reactions by adenosine plus homocysteine, which are known to condense within cells to AdoHcy, markedly potentiated TNF-induced cytotoxicity in MCF7 cells and rendered related TNF-resistant variants, TNF-sensitive by a mechanism independent from the ceramide pathway. We demonstrated that the dominant-negative derivative of FADD (FADD-DN) blocked methylation inhibition/TNF-induced cell death. Moreover, TNF-mediated cytotoxicity modulated by AdoHcy was blocked by the ICE-inhibiting peptide z-VAD-fmk, suggesting that an ICE-like protease is required for the methylation inhibition/TNF-inducible death pathway. In conclusion, these results suggest that the methyltransferase inhibitor AdoHcy potentiates TNF-induced cytotoxicity in MCF7 cells and renders TNF-resistant MCF7 clones, TNF-sensitive via the ceramide independent pathway and that FADD and the ICE-like protease are likely necessary components in transducing methylation inhibition/TNF signals for cell death.

Acyl homoserine-lactone quorum-sensing signal generation

Acyl homoserine lactones (acyl-HSLs) are important intercellular signaling molecules used by many bacteria to monitor their population density in quorum-sensing control of gene expression. These signals are synthesized by members of the LuxI family of proteins. To understand the mechanism of acyl-HSL synthesis we have purified the Pseudomonas aeruginosa RhlI protein and analyzed the kinetics of acyl-HSL synthesis by this enzyme. Purified RhlI catalyzes the synthesis of acyl-HSLs from acyl-acyl carrier proteins and S-adenosylmethionine. An analysis of the patterns of product inhibition indicated that RhlI catalyzes signal synthesis by a sequential, ordered reaction mechanism in which S-adenosylmethionine binds to RhlI as the initial step in the enzymatic mechanism. Because pathogenic bacteria such as P. aeruginosa use acyl-HSL signals to regulate virulence genes, an understanding of the mechanism of signal synthesis and identification of inhibitors of signal synthesis has implications for development of quorum sensing-targeted antivirulence molecules.

Role of SAM-dependent thiol methylation in the renal toxicity of several solvents in mice

The role of S-adenosylmethionine (SAM)-dependent thiol methylation in the nephrotoxicity of seven industrial solvents was studied in mice. The seven following solvents were utilized: bromobenzene (BB), styrene (STY), tetrachloroethylene (TTCE), trichloroethylene (TCE), 1,1-dichloroethylene (DCE), 1,2-dichloroethane (DCA) and hexachlorobutadiene (HCB). The experimental model comprised mice pretreated with periodate oxidized adenosine (ADOX) (100 micromol kg(-1) i.p.) 30 min before injection of solvents. In the first 4 h after ADOX treatment, the SAM levels were about fourfold higher than controls for the liver and kidney. The S-adenosylhomocysteine (SAH) levels were increased by factors of 11 and 14 and the SAM/SAH ratios were decreased by factors of 3 and 10 for the liver and kidney, respectively. These results show that ADOX treatment probably induces an inhibition of methyltransferase SAM-dependent in the liver and kidney and thus decreases the methylation capabilities. A single oral administration of BB (500 or 800 mg kg(-1)), TTCE (3500 or 4000 mg kg(-1)), TCE (3000 or 3500 mg kg(-1)) or STY (400 or 600 mg kg(-1)) did not induce renal toxicity, evaluated by the percentage of damaged tubules compared to controls. On the other hand, the three solvents DCE, HCB and DCA were nephrotoxic and the percentage of damaged tubules observed for each solvent was significantly different from the value of <1.8% for controls: 19% and 40% for DCE (130 and 200 mg kg(-1)), 50% and 46% for HCB (80 and 100 mg kg(-1)) and 5.1% and 7.6% for DCA (1000 and 1500 mg kg(-1)). The ADOX treatment in the mice did not modify the renal toxicity of the seven solvents. Thus, their renal toxicity, when it existed, was probably independent of the SAM-dependent thiolmethyltransferase activity in the mice. The results of this study are discussed from two viewpoints. The first concerns the general considerations on inhibition of thiol methyltransferase activities in mice and the second is related to the different solvents that are evoked individually.

Farnesylcysteine methyltransferase activity and Ras protein expression in human stomach tumor tissue

The processing pathway of G-proteins and Ras family proteins includes the isoprenylation of the cysteine residue, followed by proteolysis of three terminal residues and alpha-carboxyl methyl esterification of the cysteine residue. Farnesylcysteine methyltransferase (FCMT) activity is responsible for the methylation reaction which play a role in the membrane attachment of a variety of cellular proteins. Four kinds of Ras protein (c-Ha-ras, c-N-Ras, c-Ki-Ras, pan-Ras) expression were detected in adenocarcinoma of human tissue by immunohistochemical method, and hematoxylin and eosin staining. The level of Ras protein in human stomach tumor tissues was much higher than in normal and peritumoral regions of the same biopsy samples. The FCMT activities of each cellular fractions were high in mitochondrial fraction followed by microsomal fraction, whole homogenate and cytosolic fraction. The inhibitory effect on FCMT activity on stomach tumor tissue was determined after treatment with 0.25 microM of S-adenosyl-L-homocysteine. S-adenosyl-L-homocysteine inhibited FCMT activity from 11.2% to 30.5%. These results suggested that FCMT might be involved in Ras proteins activity.

BplI, a new BcgI-like restriction endonuclease, which recognizes a symmetric sequence

Bcg I and Bcg I-like restriction endonucleases cleave double stranded DNA specifically on both sides of their asymmetric recognition sequences which are interrupted by several ambiguous base pairs. Their heterosubunit structure, bifunctionality and stimulation by AdoMet make them different from other classified restriction enzymes. Here we report on a new Bcg I-like restriction endonuclease, Bpl I from Bacillus pumilus , which in contrast to all other Bcg I-like enzymes, recognizes a symmetric interrupted sequence, and which, like Bcg I, cleaves double stranded DNA upstream and downstream of its recognition sequence (8/13)GAGN5CTC(13/8). Like Bcg I, Bpl I is a bifunctional enzyme revealing both DNA cleavage and methyltransferase activities. There are two polypeptides in the homogeneous preparation of Bpl I with molecular masses of approximately 74 and 37 kDa. The sizes of the Bpl I subunits are close to those of Bcg I, but the proportion 1:1 in the final preparation is different from that of 2:1 in Bcg I. Low activity observed with Mg2+increases >100-fold in the presence of AdoMet. Even with AdoMet though, specific cleavage is incomplete. S -adenosylhomocysteine (AdoHcy) or sinefungin can replace AdoMet in the cleavage reaction. AdoHcy activated Bpl I yields complete cleavage of DNA.

[N-acetylserotonin and hypotensive effect of MAO-A inhibitors]

Vladimir Zinovievich Gorkin's theory of the transformation of catalytic activity of amine oxidases and, therweby, selectivity of amine oxidases, carried over from my personal acquaintance with Vladimir Zinovievich, significantly influenced our studies into the mechanism of MAO-induced stimulation of pineal melatonin biosynthesis from serotonin. We found that this effect depended on the selective inhibition of MAO-A, but not MAO-B, which resulted in the increased formation of N-acetylserotonin (N-AS), the intermediate precursor of melatonin. The hypotensive effect of clorgyline was attenuated by pinealectomy, suggesting that increased N-AS and/or production might contribute to the hypotensive effect of selective MAO-A inhibition. Basal and isoproterenol-induced pineal levels of N-AS, but not melatonin, were lower in 12 week old (hypertensive) than in 4 week old (normotensive) spontaneously hypertensive (SHR) rats. N-AS decreased blood pressure in 12 week old SHR rats. The hypotensive effect of N-AS was augmented by pinealectome and by pretreatment with the S-adenosylhomocysteine, the inhibitor of N-AS conversion into melatonin. Our data demonstrate that hypotensive effect of N-AS is independent of its conversion to melatonin. We suggest that hypotensive effect of selective MAO-A inhibition might depend on the increased formation of N-AS, but not melatonin. The age-associated decrease in N-AS production may contribute to the developing of hypertension in SHR rats, and the age-associated increase of blood pressure in humans. Our data warrant the clinical trial of N-AS for the treatment of essential hypertension.

Pancreatic exocrine secretion is blocked by inhibitors of methylation

A number of early experiments suggested a relationship between methyl group metabolism and the exocrine secretion of the pancreas. These included nutritional studies showing that ethionine, the ethyl analog of methionine which inhibits cellular methylation reactions, is a specific pancreatic toxin. Other studies indicated that protein carboxymethylation might be involved. We now show that in vivo ethionine inhibits amylase secretion from freshly isolated rat pancreatic acini, while in vitro ethionine inhibits amylase secretion from the AR42J pancreatic cell line. S-Adenosylhomocysteine (SAH) is a product inhibitor of all methyltransferase reactions involving S-adenosylmethionine (SAM), and treatments that elevate cellular levels of SAH such as inhibition of S-adenosylhomocysteine hydrolase and the in vitro addition of adenosine and homocysteine result in the inhibition of amylase secretion in both isolated pancreatic acini and AR42J cells. Measurement of SAM and SAH levels in AR42J cells shows that inhibition of secretion is more closely related to elevation of SAH levels than to a decrease in the SAM/SAH ratio. Small G-proteins are carboxymethylated on the C-terminal prenylated cysteine and inhibitors of membrane-associated prenylcysteine methyltransferase, N-acetylfarnesylcysteine, N-acetylgeranylgeranylcysteine, and farnesylthioacetic acid (FTA), block secretion in AR42J cells. N-Acetylgeranylcysteine is not an inhibitor of the methyltransferase and does not inhibit amylase secretion. FTA inhibits membrane-associated prenylcysteine methyltransferase from AR42J cells with a Ki in the 45-69 microm range. These results suggest that a methylation event is needed for pancreatic exocrine secretion which may be the reversible methylation of a G-protein involved in signal transduction or membrane trafficking.

Transport of S-adenosylmethionine in isolated rat liver mitochondria

Mitochondria do not have the enzyme, methionine adenosyltransferase (ATP: L-methionine S-adenosyltransferase, EC 2.5.1.6), necessary for the biosynthesis of S-adenosylmethionine. Nevertheless, about 30% of total hepatic S-adenosylmethionine resides in the mitochondria and radiolabeled S-adenosylmethionine may be isolated from the mitochondria after administration of radiolabeled methionine. This leads to the hypothesis that a carrier-mediated system is responsible for S-adenosylmethionine transport from the cytosol into the mitochondria. We have characterized such a system in isolated rat liver mitochondria. Uptake of S-adenosylmethionine consisted of two components. One component was incorporation of the methyl group into phospholipids as shown by thin-layer chromatography. The second component represented uptake into the mitochondria since addition of excess unlabeled S-adenosylmethionine resulted in efflux of labeled substrate. This countertransport is characteristic of a carrier-mediated transport system. Uptake (corrected for incorporation into phospholipids) was saturable with an apparent Km = 8.9 microM and Vmax = 54.3 pmol x mg protein(-1) x min(-1). Uptake was not inhibited by methionine, adenosine, 5'-methylthioadenosine, carnitine, choline, betaine, quinine, or hemicholinium-3. Uptake was inhibited by sinefungin and by S-adenosylhomocysteine (Ki = 53.4 microM). Uptake of S-adenosylmethionine was not dependent on the electrical potential across the mitochondrial membrane. These results indicate that S-adenosylmethionine is taken up into mitochondria via a specific, carrier-mediated system.

Characterization of brain beta-carboline-2-N-methyltransferase, an enzyme that may play a role in idiopathic Parkinson's disease

The activity of beta-carboline-2-N-methyltransferase results in the formation of neurotoxic N-methylated beta-carbolinium compounds. We have hypothesized that these N-methylated beta-carbolinium cations may contribute to the development of idiopathic Parkinson's disease. This report describes experiments undertaken to optimize assay conditions for bovine brain beta-carboline-2-N-methyltransferase activity. The activity of beta-carboline-2-N-methyltransferase is primarily localized in the cytosol, has a pH optimum of 8.5-9, and obeys Michaelis-Menten kinetics with respect to its substrates, 9-methylnorharman (9-MeNH) and S-adenosyl-L-methionine (SAM). Kinetic constants, KM and Vmax, with respect to 9-MeNH, are 75 microM and 48 pmol/h/mg protein, respectively. The KM for SAM is 81 microM and the Vmax is 53 pmol/h/mg protein. In addition, enzyme activity is inhibited by S-adenosyl-L-homocysteine (SAH) or zinc, and is increased 2-fold in the presence of iron or manganese. Enzyme characterization is a prerequisite to the purification of this N-methyltransferase from bovine brain as well as comparison of its activity in human brain from control and Parkinson's disease individuals.

Essential arginine residues in isoprenylcysteine protein carboxyl methyltransferase

We used specific amino acid modifying reagents to characterize the isoprenylcysteine carboxyl methyltransferase in kidney membranes. The enzyme was inactivated by reagents specific for arginine, histidine, cysteine, and tryptophan residues. Protection by the product and inhibitor S-adenosyl-L-homocysteine was observed for arginine modification by phenylglyoxal and tryptophan modification by N-bromosuccinimide. We focused on modification by phenylglyoxal, a highly specific modifier of arginine residues. The inactivation of methyltransferase by phenylglyoxal follows pseudo-first-order kinetics and the order of the reaction, n, with respect to phenylglyoxal was 1.2. The inactivation increased with the alkalinity of the preincubation medium and was maximal at pH 10. Kinetic analysis showed that the K(m) for S-adenosyl-L-methionine is not significantly affected by treatment with phenylglyoxal but that the Vmax is reduced p-Hydroxyphenylglyoxal, a more hydrophilic derivative of phenylglyoxal, was a less potent inactivator of methyltransferase than phenylglyoxal, suggesting that arginine residues modified are in a hydrophobic environment. The methyltransferase is protected from phenylglyoxal modification by S-adenosyl-L-homocysteine but not S-adenosyl-L-methionine, sinefungin, N-acetyl-S-farnesyl-L-cysteine, or farnesylthioacetate. The arginine residue modified may thus be located either at the active site or at another additional binding site for S-adenosyl-L-homocysteine. These results indicate that arginine residues are essential for the enzymatic activity of isoprenylcysteine carboxyl methyltransferase.

Calmodulin N-methyltransferase. Kinetics, mechanism, and inhibitors

The present study was undertaken to determine kinetic and inhibition parameters and the mechanism of S-adenosyl-L-methionine:calmodulin-L-lysine N6-methyltransferase (EC 2.1.1.60, CLNMT), an enzyme for which calmodulin is a substrate. Partially purified CLNMT isolated from rat testes had a Vmax of 540 pmol/min/mg and Km values for mushroom demethylcalmodulin and S-adenosyl-L-methionine of 230 nM and 2.0 microM, respectively. Kinetic analysis indicated a complex Bi Bi sequential kinetic mechanism for CLNMT where S-adenosyl-L-methionine binds initially and is followed by demethylcalmodulin binding. When the effects of 20 different compounds that are either inhibitors of calmodulin-specific or methylation-specific functions were examined, CLNMT displayed a pattern of inhibition which differs from that seen with calmodulin-activated enzymes. The product of calmodulin methylation, fully trimethylated calmodulin, and nonmethylatable VU-3 calmodulin acted as competitive inhibitors of CLNMT, with Ki values of 310 and 400 nM, respectively. Of the 13 compounds tested, which are inhibitors of calmodulin-dependent cyclic nucleotide phosphodiesterase, only the calmodulin-binding domain from Ca2+/calmodulin-dependent kinase II, melittin, and calmidazolium were effective inhibitors of CLNMT and each exhibited a complex pattern of inhibition with Kis values of 21, 50, and 65 nM, respectively. The only potent methylation-specific inhibitor was S-adenosyl-L-homocysteine, which also displayed a complex pattern of inhibition.

Characterization of prenylcysteine methyltransferase in insulin-secreting cells

Prenylcysteine carboxymethyltransferase, an enzyme involved in the post-translational modification of many signalling proteins, was characterized in insulin-secreting INS-1 cells and normal rat pancreatic islets. The activity of this enzyme was monitored by the methylation of an artificial substrate (a prenylated cysteine analogue) with S-adenosy1[methyl-3H]methionine as methyl donor. More than 95% of the methyltransferase activity was associated with the membranes, and high-salt treatment only partially extracted the enzyme from the membranes. The highest specific activity was in the insulin-granule-enriched 25000 g pellet obtained by differential centrifugation. However, a highly purified insulin-enriched fraction obtained by density centrifugation in Percoll did not exhibit methyltransferase activity. The analyses of marker enzymes for cellular organelles revealed that the methyltransferase was co-localized, with the plasma membrane and probably the endoplasmic reticulum, but not with the mitochondria or lysosomes. Guanosine 5'-[gamma-thio]-triphosphate failed to increase methyltransferase activity directly, although it promotes the methylation of GTP-binding proteins. Mastoparan, Ca2+, cAMP and the protein kinase C activator phorbol 12-myristate 13-acetate did not alter enzyme activity. In addition, methyltransferase activity was not stably modified by stimulation of intact cells using glucose or other agents. However, the carboxymethylation of certain low-molecular-mass G-proteins is increased by glucose stimulation; conversely, treatment of cells with N-acetyl-S-trans,trans-farnesyl-L-cysteine inhibited glucose- and forskolin-induced insulin secretion. These results suggest that the membrane-associated prenylcysteine carboxymethyltransferase may be constitutively active and that the methylation of target proteins in vivo is regulated by the access of these proteins to the methyltransferase, as well as by their active (GTP-liganded) configuration.

Evidence that the catechol 3,4-Dihydroxytamoxifen is a proximate intermediate to the reactive species binding covalently to proteins

Metabolism of tamoxifen by rat and human hepatic microsomal cytochrome P450s (CYPs) forms a reactive intermediate that irreversibly binds to microsomal proteins (C. Mani and D. Kupfer, Cancer Res., 51: 6052-6058, 1991.). The current study examines the nature of the tamoxifen metabolite that is proximate to the reactive intermediate(s). The rate of covalent binding of tamoxifen metabolites, tamoxifen N-oxide, N-desmethyltamoxifen, and tamoxifen N-oxide-epoxide was approximately equal to or less than that of tamoxifen. By contrast, covalent binding of 4-hydroxytamoxifen (4-OH-tam) was 3-5-fold higher than that of tamoxifen, indicating that among the metabolites examined, 4-OH-tam or its metabolite(s) is most proximate to the reactive intermediate(s). Incubation of 4-OH-tam with liver microsomes from PCN-treated rat yielded three detectable metabolites. One was identified as 4-OH-tam N-oxide via its facile reduction back to 4-OH-tam by titanium(III) chloride. Another metabolite of 4-OH-tam, assumed to be 3,4-dihydroxytamoxifen (3,4-di-OH-tam) catechol, was demonstrated by its monomethylation with [3H]S-adenosyl-L-methionine ([3H]SAM) in presence of endogenous catechol-O-methyltransferase. Monomethylated catechol from 4-OH-tam was formed at a 3-4-fold higher rate than from tamoxifen. It was reasoned that if the catechol is most proximate metabolite to the reactive intermediate, then its methylation would reduce the formation of the reactive intermediate and result in lower rate of covalent binding. In fact, addition of radioinert SAM to incubations of tamoxifen inhibited covalent binding by 17-23%. By contrast, inclusion of 1.0 mM S-adenosyl-L-homocysteine, a potent inhibitor of catechol-O-methyltransferase-mediated methylation of 3,4-di-OH-tam, essentially overcame the inhibition of the covalent binding by SAM. Additionally, ascorbic acid and glutathione, inhibitors of covalent binding of tamoxifen, produced an elevation of methylated catechol. These findings collectively indicate that 3,4-di-OH-tam is proximate to the ultimate reactive intermediate that results in covalent binding to microsomal proteins.

Inhibition of catechol O-methyltransferase-catalyzed O-methylation of 2- and 4-hydroxyestradiol by quercetin. Possible role in estradiol-induced tumorigenesis

Catecholestrogens have been postulated to mediate the induction of kidney tumors by estradiol in male Syrian hamsters. In this study, we examined the mechanism of inhibition by quercetin of the catechol O-methyltransferase-catalyzed O-methylation of catecholestrogens as a basis for the previously reported enhancement of estradiol-induced tumorigenesis by this flavonoid. In hamsters treated with 50 micrograms of [6,7-3H]estradiol, quercetin increased concentrations of 2- and 4-hydroxyestradiol in kidney by 80 and 59%, respectively. In animals treated with two 10-mg estradiol implants, quercetin also decreased by 63-65% the urinary excretion of 2- and 4-hydroxyestradiol monomethyl ethers. Taken together, these results demonstrate the in vivo inhibition of the O-methylation of catecholestrogens by quercetin. S-Adenosyl-L-homocysteine, produced by the methylation of catecholestrogens, noncompetitively inhibited the O-methylation of 2- and 4-hydroxyestradiol by hamster kidney cytosolic catechol O-methyltransferase (IC50 approximately 10-14 microM). Due to the rapid O-methylation of quercetin itself, quercetin decreased renal concentrations of S-adenosyl-L-methionine by approximately 25% in control or estradiol-treated hamsters and increased concentrations of S-adenosyl-L-homocysteine by 5-15 nmol/g of wet tissue, which was estimated to cause a 30-70% inhibition of the enzymatic O-methylation of catecholestrogens. Quercetin or fisetin (a structural analog) inhibited the O-methylation of 2- and 4-hydroxyestradiol by a competitive plus noncompetitive mechanism (IC50 approximately 2-5 microM). These results suggest that the in vivo O-methylation of catecholestrogens is inhibited more by S-adenosyl-L-homocysteine than by quercetin. The accumulation of 2- and 4-hydroxyestradiol during co-administration of estradiol and quercetin may enhance metabolic redox cycling of catecholestrogens and thus estradiol-induced kidney tumorigenesis.

Mono- and dimethylation of arsenic in rat liver cytosol in vitro

Production of methylarsonate and dimethylarsinate from radiolabelled [73 As]arsenite and [73 As]arsenate was examined in an assay system that contained cytosol prepared from a 20% homogenate (w/v) of livers from 8- 10-week-old male Fischer 344 rats. After a 60-min incubation at 37 degrees C with added S-adenosylmethionine and glutathione, up to 50% of carrier-free [73As]arsenite and about 15% of carrier-free [73As]arsenate were methylated. Incubation of cytosol at 100% degrees C for 1 min before addition to the assay system completely abolished methylation of arsenite. Production of methylarsonate increased in proportion to the arsenite concentration in the assay system; however, 50 microM arsenite inhibited production of dimethylarsinate. Methylarsonate production from carrier-free [73-As]arsenite was not dependent on addition of exogenous S-adenosylmethionine to the assay system. Addition of 0.1 mM S-adenosylmethionine maximized dimethylarsinate production. Addition of 0.1 or 1.0 mM S-adenosylhomocysteine decreased methylation of arsenite, especially dimethylarsinate production. Omission of glutathione from the assay system nearly abolished the methylation of arsenite. Addition of exogenous glutathione to the assay system (up to 20 mM) decreased protein binding of arsenic and increased the production of methylarsonate and dimethylarsinate. The effects of sodium selenite, mercuric chloride, EDTA, p-anisic acid and 2,3-dichloro-alpha-methylbenzylamine on the methylation of arsenite were determined. Addition of 10 microM selenite to the assay system nearly abolished the formation of either methylated species. Addition of 1 or 10 microM mercuric chloride inhibited dimethylarsinate production in a concentration-dependent manner but had little effect on methylarsonate yield. Addition of 10 mM EDTA to the assay system inhibited formation of both methylated metabolites, suggesting that an endogenous divalent cation might be involved in enzymatic methylation of arsenic. Neither p-anisic acid, an inhibitor of cytosolic methyltransferases, nor 2,3-dichloro-alpha-methylbenzylamine, an inhibitor of microsomal methyltransferases, inhibited the conversion of inorganic arsenic to mono- or dimethylated metabolites.