Purification and biochemical characterization of the EcaI DNA methyltransferase

The EcaI GGTNACC-specific DNA-adenine modification methyltransferase has been purified to apparent homogeneity. The active form of the DNA methyltransferase is a single polypeptide. The enzyme has a pH optimum at pH 8.0 and a temperature optimum at 25 degrees C. EcaI DNA methyltransferase transfers one methyl group to the adenine of the recognition site in a single binding event. The Km was 170 nM for DNA and 1.8 microM for the methyl donor S-adenosylmethionine. Methylated DNA is a competitive inhibitor with respect to DNA (Ki = 3.5 nM). The other product of the DNA-methylation reaction, S-adenosylhomocysteine was found to be a competitive inhibitor with respect to S-adenosylmethionine (Ki = 2.7 microM). The S-adenosylmethionine analog sinefungin was shown to be a very strong inhibitor (Ki = 3.5 nM) of the DNA methyltransferase reaction.

Human kidney thiopurine methyltransferase. Photoaffinity labeling with S-adenosyl-L-methionine

Thiopurine methyltransferase (TPMT) catalyzes the S-methylation of heterocyclic and aromatic sulfhydryl compounds such as the thiopurine drug 6-mercaptopurine (6-MP). TPMT activity in human tissue is regulated by a common genetic polymorphism, and "pharmacogenetic" variation in TPMT activity is an important factor in individual differences in thiopurine drug metabolism, toxicity and therapeutic efficacy. Human renal tissue contains two isozymes of TPMT, Peak I and Peak II, that can be separated by ion exchange chromatography. Our experiments were performed to determine whether S-adenosyl-L-methionine (Ado-Met), the methyl donor for the TPMT reaction, could be used as a photoaffinity ligand for these isozymes as one step in the study of the molecular basis for the TPMT genetic polymorphism. When [3H-methyl]Ado-Met and partially purified preparations of either isozyme of human kidney TPMT were exposed to ultraviolet light at 254 nm, followed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis, a 35 kDa protein was the predominant species that was radioactively labeled. The same 35 kDa protein was photoaffinity labeled with [14C-carboxyl]Ado-Met, demonstrating that labeling involved covalent binding of Ado-Met rather than methylation of the protein. TPMT enzymatic activity co-eluted with the 35 kDa protein during sequential DEAE ion exchange, gel filtration and hydroxylapatite chromatography. Inhibitors of TPMT enzymatic activity including S-adenosyl-L-homocysteine, sinefungin, 6-methylmercaptopurine and 3,4-dimethoxy-5-hydroxybenzoic acid inhibited photoaffinity labeling of the 35 kDa protein in preparations of both TPMT Peak I and Peak II isozymes in a concentration-dependent fashion, as did 6-MP, the methyl acceptor substrate for the TPMT reaction. All of these results were compatible with the conclusion that the 35 kDa protein was TPMT. Photoaffinity labeling of TPMT with [3H]Ado-Met should make it possible to purify the enzyme to homogeneity and to study amino acid sequences at or near its active site.

Increased synthesis of polycistronic mRNA associated with increased polyadenylation by vesicular stomatitis virus

Electron microscopy suggested that the mRNA produced in vitro by tsG16(I), a temperature-sensitive mutant of vesicular stomatitis virus, contained an increased proportion of polycistronic mRNAs. Using hybrid selection, we found that the poly(A)+ mRNA synthesized in vitro by tsG16(I) contained approximately two to three times more polycistronic mRNA than did poly(A)+ mRNA synthesized in vitro by the parental wild-type (wt) virus. The increase in polycistronic mRNA occurred at all intergenic junctions examined. In vitro, tsG16(I) has an increased polyadenylation phenotype and a temperature-sensitive transcriptase activity that appear to be due to different mutations. Partial revertants of tsG16(I), which have lost the aberrant polyadenylation phenotype but retain the in vitro thermosensitive transcriptase, produced wt amounts of polycistronic mRNA. This suggested that the increased production of polycistronic mRNA by tsG16(I) may be associated with the increased polyadenylation phenotype of this mutant. These data further support the hypothesis that an increase in size of poly(A) tracts is associated with increased production of polycistronic mRNA.

Induction of G- and R-banding in human chromosomes by the demethylating agent S-adenosyl-L-homocysteine

In this report we describe the procedure of growing human lymphocytes with the demethylating agent S-adenosyl-L-homocysteine (SAH). After this treatment, which is not toxic for cell survival, both R- and G-banding were obtained by new experimental procedures: R-bands have been directly demonstrated with the GC-specific fluorochrome chromomycin A3 without the necessity of any AT-specific counterstaining agent; simultaneous G-banding and active nucleolar organizer regions have been obtained by silver impregnation of chromosomes and subsequent Giemsa staining. These results suggest a possible relationship between local differences in DNA methylation and the determination of the banded chromosome structure.

A possible new class of ribonucleotide reductase from Methanobacterium thermoautotrophicum

The ribonucleotide reductase from the strictly anaerobic methanogen Methanobacterium thermoautotrophicum has been partially purified by ion-exchange and gel-filtration chromatography. Its molecular weight is estimated to be 100,000 by the latter step. Unlike all previously studied ribonucleotide reductases, the enzyme does not employ dithiol compounds such as dithiothreitol as artificial electron donors in in vitro assays. Inhibition of the enzyme by S-adenosylmethionine, oxygen, and azide further distinguishes it from the Escherichia coli anaerobic enzyme, the iron- and manganese-containing, and the adenosylcobalamin-dependent enzymes. Our preliminary results suggest that this enzyme has an activation mechanism different from the known classes of ribonucleotide reductases.

Evidence that the decarboxylation reaction occurs before the first methylation in ubiquinone biosynthesis in rat liver mitochondria

Biosynthesis of ubiquinone-9 was studied by incubating rat liver mitochondria with p-hydroxy[U-14C]benzoate, solanesyl diphosphate and S-adenosyl-L-methionine. When methylation reactions were inhibited by replacing S-adenosyl-L-methionine with S-adenosyl-L-homocysteine, nonaprenyl p-hydroxybenzoate and three other labeled peaks, designated as P1, P2 and P3 according to their retention times on HPLC, were observed. No carboxyl group was present in P1, P2 or P3 because the radioactivities disappeared when p-hydroxy[U-14C]benzoate was replaced by p-hydroxy[carboxyl-14C]benzoate. Compound P2 seemed to be hydroxylated but not methylated because its radioactivity markedly diminished under anaerobic conditions and the radioactivity was not incorporated into the compound from S-adenosyl-L-[methyl-3H]methionine, suggesting that P2 is 6-hydroxynonaprenylphenol. The complete correspondence of the retention times of P2 and chemically synthesized 6-hydroxynonaprenylphenol on HPLC further confirmed this possibility. P2 was a precursor of ubiquinone-9 because the radioactivity of the compound was incorporated into ubiquinone when incubated with mitochondria. The results suggest that the decarboxylation may occur prior to the first methylation in the ubiquinone biosynthesis in rat liver mitochondria, though it has been generally considered that in eukaryotes the first methylation precedes the decarboxylation.

Methylation and demethylation reactions of guanine nucleotide-binding proteins of retinal rod outer segments

Retinal transducin was previously shown to be farnesylated on its gamma subunit. This farnesylation reaction on a cysteine residue near the carboxyl terminus is followed by peptidase cleavage at the cysteine. Thus the modified cysteine becomes the carboxyl terminus. It is shown here that the free carboxyl group can be methylated by an S-adenosyl-L-methionine-dependent methyltransferase associated with the rod outer segment membranes. This process can be inhibited by S-adenosyl-L-homocysteine and sinefungin. Moreover, synthetic N-acetyl-S-farnesyl-L-cysteine, but not N-acetyl-L-cysteine, is a substrate for the enzyme. Rapid demethylation of N-acetyl-S-farnesyl-L-cysteine methyl ester can be observed in the membranes. Transducin is also enzymatically demethylated by the rod outer segment membranes. Moreover, the 23- to 29-kDa small G proteins are methylated and demethylated in this system. These data suggest that methylation/demethylation may play a regulatory role in visual signal transduction.

Calmodulin- and Ca2(+)-insensitive fatty acid methyltransferase from RINm5F cells. Inhibition by trifluoperazine and W7

1. Methylation of endogenous lipids by homogenates of rat insulinoma cells was studied. 2. 3H-methyl groups (38 pmol/mg protein per 10 min) from [3H-methyl]S-adenosyl-L-methionine were incorporated into endogenous lipids, mainly (greater than 80%) into the neutral lipid fraction. 3. The reaction was sensitive to heat, was almost abolished by S-adenosyl-L-homocysteine, but insensitive to the addition of EGTA (5 mM), Ca2+ (5-100 microM) and/or calmodulin (15 microns). 4. At concentrations relevant for calmodulin antagonistic activity strong inhibition by W7 and trifluoperazine (25-100 microM each), but not by CGS 9343B (10 microM), was observed. 5. Calmodulin antagonists of phenothiazine- and sulfonamide-type appear to block the fatty acid methyltransferase in a way unrelated to calmodulin.

The double role of methyl donor and allosteric effector of S-adenosyl-methionine for Dam methylase of E. coli

The turnover of DNA-adenine-methylase of E. coli strongly decreases when the temperature is lowered. This has allowed us to study the binding of Dam methylase on 14 bp DNA fragments at 0 degrees C by gel retardation in the presence of Ado-Met, but without methylation taking place. The enzyme can bind non-specific DNA with low affinity. Binding to the specific sequence occurs in the absence of S-adenosyl-methionine (Ado-Met), but is activated by the presence of the methyl donor. The two competitive inhibitors of Ado-Met, sinefungin and S-adenosyl-homocysteine, can neither activate this binding to DNA by themselves, nor inhibit this activation by Ado-Met. This suggests that Ado-Met could bind to Dam methylase in two different environments. In one of them, it could play the role of an allosteric effector which would reinforce the affinity of the enzyme for the GATC site. The analogues can not compete for such binding. In the other environment Ado-Met would be in the catalytic site and could be exchanged by its analogues. We have also visualized conformational changes in Dam methylase induced by the simultaneous binding of Ado-Met and the specific target sequence of the enzyme, by an anomaly of migration and partial resistance to proteolytic treatment of the ternary complex Ado-Met/Dam methylase/GATC.

Phosphatidylethanolamine methyltransferase and phospholipid methyltransferase activities from Saccharomyces cerevisiae. Enzymological and kinetic properties

In the yeast Saccharomyces cerevisiae, two membrane-associated enzymes catalyze the three-step methylation of phosphatidylethanolamine (PE) to phosphatidylcholine (PC). Phosphatidylethanolamine methyltransferase (PEMT) catalyzes the first methylation reactions (PE----phosphatidylmonomethylethanolamine (PMME] and phospholipid methyltransferase (PLMT) catalyzes the second two methylation reactions (PMME----phosphatidyldimethylethanolamine (PDME)----PC). Using gene disruption mutants of the S. cerevisiae OP13 and CHO2 genes, we independently studied the enzymological properties of microsome-associated PEMT and PLMT, respectively. The enzymological properties of the enzymes differed with respect to their pH optima, cofactor requirements and thermal lability. For the PEMT reactions, the apparent Km values for PE and S-Adenosylmethionine (AdoMet) were 57 microM and 110 microM, respectively. For the PLMT reactions, the apparent Km values for PMME and PDME were 380 microM and 180 microM, respectively. The apparent Km values for AdoMet were 54 microM and 59 microM with PMME and PDME as substrates, respectively. S-Adenosylhomocysteine (AdoHcy) was a competitive inhibitor of PEMT (Ki = 12 microM) and PLMT (Ki = 57 microM and Ki = 54 microM for PMME and PDME, respectively) with respect to AdoMet. AdoHcy was a noncompetitive inhibitor of PEMT (Ki = 160 microM) and PLMT (Ki = 120 microM) with respect to PE and PMME and PDME, respectively.

Binding of Bacillus subtilis ermC' methyltransferase to 23S rRNA

ermC 23S rRNA methyltransferase dimethylates adenine 2085 in Bacillus subtilis 23S rRNA and also regulates its own synthesis by autogenous translational repression. We have characterized the binding of ermC' methyltransferase to 23S rRNA. This protein differs in only five amino acid residues from the ermC product and was chosen for study because of its greater stability and ease of isolation. A filter binding assay was used to study the physical aspects of binding in the absence of methylation. The dissociation equilibrium constant of the binding was found to be 4 x 10(-9) M at 37 degrees C. Kinetic studies of complex formation and dissociation revealed that the kon and koff were 4 x 10(6) M-1 s-1 and 6.8 x 10(-2) s-1 respectively at 16 degrees C. Equilibrium competition experiments showed that the enzyme has varying affinities for a variety of nucleic acids in the order 23S rRNA greater than 16S rRNA greater than M13 DNA, f2 RNA greater than tRNA. One of the end products of methylation, methylated 23S rRNA, had an affinity for the ermC' methyltransferase similar to that of unmethylated 23S rRNA. The binding affinity to 23S rRNA and the kinetics of the interaction were not detectably affected by the presence of AdoMet. The binding of ermC' methyltransferase to 23S rRNA had an unfavorable van't Hoff enthalpy (delta H = +6.2 kcal mol-1) and was driven by entropy (delta S = +56.2 cal mol-1 deg-1). The interaction between the two ligands involved at most two to three ionic pairings, and nonelectrostatic interactions contributed approximately 85% of the binding energy. The structural aspect of the interaction was investigated by probing with dimethyl sulfate, for ermC' methyltransferase dependent protection of 23S rRNA. A region of protection was detected, in the vicinity of the central loop of rRNA domain V and surrounding the site of methylation.

Effects of sinefungin and S-adenosylhomocysteine on DNA and protein methyltransferases from Streptomyces and other bacteria

Sinefungin is a naturally occurring nucleoside isolated from cultures of Streptomyces griseolus and S. incarnatus. It is structurally related to S-adenosyl-methionine (SAM) and S-adenosyl-L-homocysteine (SAH). Its effect and level of action on prokaryotes has not been studied with the same detail as with eukaryotic cells. In this report we describe the effect of sinefungin and SAH on several Streptomyces methyltransferases (DNA and protein MTases) and on other bacterial DNA-MTases. Protein MTases are resistant to sinefungin, whereas DNA-MTases are inhibited. Adenine MTases however, seem more sensitive to this analogue than cytosine MTases.

Revertants of a mutant of vesicular stomatitis virus which has an aberrant polyadenylation activity and a temperature-sensitive transcriptase

tsG16(l), a temperature-sensitive mutant of vesicular stomatitis virus, in vitro has at least three phenotypic differences from its parental wild-type (wt) virus due to mutation of the L gene. It was not known whether (i) the temperature-sensitivity of the transcriptase, (ii) the aberrant polyadenylation phenotype, and (iii) the extent of increased polyadenylation in response to S-adenosylhomocysteine (SAH) were associated with a single mutation. Spontaneous partial revertants were selected from tsG16(I) on the basis of the ability to form plaques at 34.7 degrees (35G16 revertants) or from 35G16 revertants on the basis of the ability to form plaques at 37 degrees (37G16 revertants). All six 35G16 revertants had fully (five) or partially (one) recovered the wt polyadenylation phenotype and the former five had also fully recovered the wt polyadenylation response to SAH. This suggested that a single mutation in tsG16(I) was probably associated with both of these phenotypes and also probably conferred the inability to grow at 34.7 degrees. None of the 35G16 revertants regained the wt phenotype for thermosensitivity of the transcriptase, although both of the 37G16 revertants did. This suggested that in vitro temperature-sensitivity of transcription by tsG16(I) might be due to a mutation different than the one affecting polyadenylation in the absence or presence of SAH.

Disruption of thymidylate synthesis and glycine-serine interconversion by L-methionine and L-homocystine in Raji cells

Excessive concentrations of L-methionine inhibited the folate-dependent de novo synthesis of thymidylic acid (TMP) in Raji cells, demonstrating the usefulness of this cell line for the study of methionine-folate antagonism. The effect was also produced by L-homocystine but not by other amino acids including D-methionine and L-ethionine, suggesting that this effect is exerted by a common intermediate of methionine and homocystine metabolism. L-Methionine, L-homocysteine, S-adenosylmethionine (SAM), and S-adenosylhomocysteine (SAH) are not inhibitors of thymidylate synthase activity. On the other hand the capacity of the cells to incorporate serine 3-carbon and glycine 2-carbon into DNA is impaired by the presence of L-methionine or L-homocystine. Studies with cell-free extracts demonstrated that the glycine cleavage enzyme is inhibited by 45% by L-methionine, L-homocysteine, SAM or SAH. Serine hydroxymethylase on the other hand was slightly stimulated by these sulfur-containing compounds and this stimulation was shown to occur in the intact cell as well. These findings suggest that when levels of L-methionine metabolites are elevated, there is an increase in the use of glycine to maintain the intracellular concentration of serine, which is required for homocysteine detoxification by conversion to cystathionine. The reduction in TMP synthesis caused by excess L-methionine or L-homocystine may result from increased utilization of one-carbon units for serine synthesis.

Alteration of membrane phospholipid methylation by adenosine analogs does not affect T lymphocyte activation

Membrane phospholipid methylation has been described during activation of various immune cells. Moreover recent data indicated modulation of immune cells functions by adenosine. As S-Adenosyl-methionine and S-Adenosyl-homocysteine are adenosine analogs and modulators of transmethylation reactions, the effects of SAH and SAM were investigated on membrane phospholipid methylation and lymphocyte activation. SAM (10(-5) M) was shown to induce the membrane phospholipid methylation as assessed by the 3H-methyl-incorporation in membrane extract. This effect was inhibited by SAH. In contrast SAM and SAH did not affect the phytohemagglutinin-induced proliferative response of peripheral blood mononuclear cells. SAH neither modified the early internalization of membrane CD3 antigens nor did it prevent the late expression of HLA-DR antigens on lymphocytes activated by phytohemagglutinin. These results indicate that in vitro alteration of phospholipid methylation does not affect subsequent steps of human T lymphocyte activation and proliferation.

Effect of toluene on rat synaptosomal phospholipid methylation and membrane fluidity

This study investigated the effects of toluene (1 g/kg, 1 hr, i.p.) on rat synaptosomal phospholipid methylation (PLM), phospholipid composition, and membrane fluidity. Toluene significantly decreased basal PLM (35%) in studies using [3H]methionine [( 3H]Met) as the methyl donor; this was reflected by similar decreases in phosphatidylmonomethylethanolamine (PME) (30%). No effects were observed in either PLM reactions that used [3H]adenosylmethionine [( 3H]AdoMet) as methyl donor, or AdoMet synthetase, suggesting that toluene preferentially affects PLM reactions that derive methyl groups from [3H]Met. Also, toluene decreased synaptosomal phosphatidylethanolamine (PE) (24%), the initial substrate for PLM, and the addition of PE back to PE-depleted synaptosomes restored methyltransferase activity. Agonist-stimulated PLM using norepinephrine (NE) demonstrated that agonist-receptor coupling returned PLM to control values in synaptosomes from toluene-treated rats. NE-stimulated PLM was also blocked by propranolol (PRO), suggesting a role for toluene in receptor-mediated events. Membrane fluidity studies demonstrated that in vivo administration of toluene increased the outer synaptosomal membrane fluidity, whereas in vitro administration of toluene had no effect. Our observations support a positive relationship between increased PLM activity and increased outer, not core, membrane fluidity. These data demonstrate that specific toluene-phospholipid interactions occur in synaptosomes, resulting in altered membrane composition, function and fluidity.

Studies on protein methyltransferase in human cerebrospinal fluid

Protein methyltransferases, rich in most mammalian brains, were studied in human cerebrospinal fluid (CSF). Among several well-characterized groups of methyltransferases, protein methylase I (S-adenosylmethionine:protein-arginine N-methyltransferase, EC 2.1.1.23) was found in significant amounts in human CSF samples. Both myelin basic protein (MBP) -specific and histone-specific protein methylase I activities were observed, the latter being generally higher in most CSF. S-Adenosyl-L-homocysteine, a potent product inhibitor for the methyltransferase, inhibited approximately 90% of MBP-specific protein methylase I activity at a concentration of 1 mM. The optimum pH of the MBP-specific protein methylase I was found to be around 7.2. Identity of exogenously added MBP as the methylated substrate for CSF enzyme was confirmed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. An amino acid analysis of the [methyl-3H]protein hydrolysate showed two major radioactive peaks cochromatographing with monomethyl- and dimethyl (symmetric)-arginine. Human CSF contained relatively high endogenous protein methylase I activity (activity measured without added substrate protein): The endogenous substrate can be immunoprecipitated by antibody raised against calf brain MBP. Finally, CSF from several neurological patients were analyzed for protein methylase I, and the results are presented.

Relationship between intracellular concentration of S-adenosylhomocysteine and inhibition of vaccinia virus replication and inhibition of murine L-929 cell growth

9-(trans-2',trans-3'-Dihydroxycyclopent-4'-enyl)-adenine (compound 1) and -3-deazaadenine (compound 2), which are specific inhibitors of S-adenosylhomocysteine (AdoHcy) hydrolase, were reported earlier by our laboratory (M. Hasobe, J. G. McKee, D. R. Borcherding, and R. T. Borchardt, Antimicrob. Agents Chemother. 31:1849-1851, 1987) to have anti-vaccinia virus activity with reduced murine L-929 cell toxicity compared with the prototype compound neplanocin A. In this study, we showed that the antiviral and cytotoxic effects of compounds 1 and 2 can be related to intracellular concentrations of AdoHey, which are elevated in cells treated with these inhibitors of AdoHcy hydrolase. For example, concentrations of analogs 1 and 2 that produce 50% inhibition of vaccinia virus replication caused only slight elevations in intracellular levels of AdoHcy (from 50 [controls] to 100 to 125 [drug-treated cells] pmol/mg of protein) and elevations in the ratios of AdoHcy/S-adenosylmethionine (from 0.05 to 0.1 [controls] to 0.15 to 0.19 [drug-treated cells]). In contrast to the extreme susceptibility of virus replication to slight elevations in intracellular AdoHcy, cell viability was quite tolerant to higher levels of this metabolite. For example, concentrations of analogs 1 and 2 that produced 50% inhibition of L-929 cell replication caused significant increases in intracellular levels of AdoHcy (to 825 to 950 pmol/mg of protein) and elevations in AdoHcy/S-adenosylmethionine ratios (approximately 1.3). These data make it possible to assign a therapeutic index of 7 to 8 to these compounds on the basis of the comparison of intracellular levels of AdoHcy that caused 50% inhibition of vaccinia virus replication with those that caused 50% inhibition of L-929 cell replication.

The L protein of vesicular stomatitis virus modulates the response of the polyadenylic acid polymerase to S-adenosylhomocysteine

TsG16(I) is a temperature-sensitive (ts) mutant of vesicular stomatitis virus, Indiana serotype, which overproduces polyadenylic acid [poly(A)] in an in vitro transcription system due to a mutation in the L protein. Others have reported that L-S-adenosylhomocysteine (S-Ado-Hcy) causes wild-type (wt) virus to overproduce poly(A) in vitro. The possibility that tsG16(I) constitutively expresses a property induced by S-Ado-Hcy in the case of wt virus was found not to be so since polyadenylation by the mutant was still sensitive to S-Ado-Hcy. Indeed, S-Ado-Hcy caused tsG16(I) to overproduce poly(A) in vitro to a greater extent than its parental wt virus. The increase in polyadenylation observed in response to saturating levels of S-Ado-Hcy differed for tsG16(I), for its parental wt virus and for another wt strain. To characterize which viral protein modulated the polyadenylation response to S-Ado-Hcy, purified virions were fractionated and their phenotypes in homologous and heterologous reconstitution assays were examined. The results indicated that the viral L protein modulated the response in all three stocks of virus. These data provide further evidence to suggest that the L protein of vesicular stomatitis virus plays a role in polyadenylation of the viral mRNA.

Histamine increases phospholipid methylation and H2-receptor-adenylate cyclase coupling in rat brain

Histamine stimulated the enzymatic synthesis of phosphatidylcholine from phosphatidylethanolamine in crude synaptic membranes of rat brain containing the methyl donor S-adenosyl-L-methionine (SAM). In the presence of, but not in the absence of SAM, histamine increased cyclic AMP accumulation at the concentrations that stimulate phospholipid methylation. S-Adenosyl-L-homocysteine, an inhibitor of phospholipid methyltransferases, inhibited histamine-stimulated phospholipid methylation and histamine-induced cyclic AMP accumulation in the presence of SAM in a concentration-dependent manner. Histamine-induced [3H]methyl incorporation into phospholipids exhibited a marked regional heterogeneity in rat brain in the order of cortex greater than medulla oblongata greater than hippocampus greater than striatum greater than midbrain greater than hypothalamus. The regional distribution of histamine-induced cyclic AMP accumulation exactly paralleled histamine-stimulated [3H]methyl incorporation in rat brain. Histamine-induced cyclic AMP accumulation was inhibited by the addition of cimetidine or famotidine, but not by mepyramine or diphenhydramine. The accumulation of cyclic AMP in the presence of SAM was observed by the addition of impromidine or dimaprit, but not by 2-pyridylethylamine. These results indicate that phospholipid methylation is induced by histamine and may participate in H2-receptor-mediated stimulation of adenylate cyclase in rat brain.

Inhibition of phosphoinositide metabolism in human polymorphonuclear leukocytes by S-adenosylhomocysteine

Transmembrane signaling by chemoattractants in leukocytes appears to require activation of phosphoinositide metabolism with subsequent generation of the second messenger substances, inositol(1,4,5)trisphosphate and diacylglycerol. In addition, previous studies have shown that conditions which lead to an intracellular increase in S-adenosylhomocysteine (AdoHcy), a by-product and competitive inhibitor of S-adenosylmethionine-mediated methylation reactions, inhibit all chemoattractant-mediated functions of leukocytes, suggesting that AdoHcy also interferes with chemoattractant transmembrane signaling. In the present study, we determined whether AdoHcy altered the metabolism of phosphoinositides in human polymorphonuclear leukocytes. Treatment of 32P-labeled polymorphonuclear leukocytes with the adenosine deaminase inhibitor, erythro-9-(2-hydroxy-3-nonyl)adenine, plus exogenous adenosine and L-homocysteine thiolactone, conditions which cause an increase in AdoHcy, produced as much as a 37% decrease in the amount of [32P]phosphatidylinositol 4-monophosphate associated with the cells. The formation of inositol bisphosphate was inhibited by as much as 45% by erythro-9-(2-hydroxy-3-nonyl)adenine, adenosine, and L-homocysteine thiolactone suggesting decreased availability of phosphatidylinositol 4-monophosphate. In support of this, AdoHcy, in concentrations ranging from 0.01 to 0.1 mM, inhibited the transfer of gamma-32P from gamma-[32P] ATP to phosphatidylinositol (PtdIns). The inhibition of PtdIns kinase was competitive with an apparent Ki for AdoHcy of 43 microM. Increased intracellular AdoHcy reduced chemoattractant-mediated increases in inositol(1,4,5)trisphosphate formation suggesting abrogation of transmembrane signaling. These findings for the first time demonstrate that AdoHcy is a competitive inhibitor of PtdIns kinase and thus a regulator of the phosphoinositide pathway.

S-adenosyl-L-methionine protects the hippocampal CA1 neurons from the ischemic neuronal death in rat

We investigated the effect of S-adenosyl-L-methionine (SAMe) on the prevention of the delayed neuronal death in rats subjected to transient and brief forebrain ischemia. As the results, SAMe dose-dependently protected the hippocampal CA1 neurons from degeneration and necrosis, whose effect was suppressed by simultaneous administration of S-adenosyl-L-homocysteine, a potent inhibitor in transmethylation. No protective effect was observed in CDP-choline, phosphatidylcholine and L-methionine. Therefore, it is necessary for the prevention of the delayed neuronal death to enhance cerebral SAMe level and to activate transmethylation using SAMe as a methyl donor in postischemic brain.

Catalytic mechanism and inhibition of tRNA (uracil-5-)methyltransferase: evidence for covalent catalysis

tRNA (Ura-5-)methyltransferase catalyzes the transfer of a methyl group from S-adenosylmethionine (AdoMet) to the 5-carbon of a specific Urd residue in tRNA. This results in stoichiometric release of tritium from [5-3H]Urd-labeled substrate tRNA isolated from methyltransferase-deficient Escherichia coli. The enzyme also catalyzes an AdoMet-independent exchange reaction between [5-3H]-Urd-labeled substrate tRNA and protons of water at a rate that is about 1% that of the normal methylation reaction, but with identical stoichiometry. S-Adenosylhomocysteine inhibits the rate of the exchange reaction by 2-3-fold, whereas an analogue having the sulfur of AdoMet replaced by nitrogen accelerates the exchange reaction 9-fold. In the presence (but not absence) of AdoMet, 5-fluorouracil-substituted tRNA (FUra-tRNA) leads to the first-order inactivation of the enzyme. This is accompanied by the formation of a stable covalent complex containing the enzyme, FUra-tRNA, and the methyl group of AdoMet. A mechanism for catalysis is proposed that explains both the 5-H exchange reaction and the inhibition by FUra-tRNA: the enzyme forms a covalent Michael adduct with substrate or inhibitor tRNA by attack of a nucleophilic group of the enzyme at carbon 6 of the pyrimidine residue to be modified. As a result, an anion equivalent is generated at carbon 5 that is sufficiently reactive to be methylated by AdoMet. Preliminary experiments and precedents suggest that the nucleophilic catalyst of the enzyme is a thiol group of cysteine. The potent irreversible inhibition by FUra-tRNA suggests that a mechanism for the "RNA" effects of FUra may also involve irreversible inhibition of RNA-modifying enzymes.

Inhibition of leishmanial DNA synthesis by sinefungin

RNA, DNA and protein biosynthesis were studied in Leishmania donovani and L. tropica promastigotes cultured with or without sinefungin. Thymidine incorporation was significantly impaired by this compound. Neither the uptake of thymidine nor its phosphorylation were inhibited. Furthermore the ratios of deoxyribonucleotide to the corresponding ribonucleotide were not significantly affected by sinefungin. Analysis of the DNA indicates that the inhibition of thymidine incorporation affects mostly nuclear DNA, kDNA being less affected by this drug. No such effect on thymidine incorporation was observed in macrophages, the host cells of these parasites.