Interrelations between plasma homocysteine and intracellular S-adenosylhomocysteine

Longitudinal Analysis of One-Carbon Metabolism-Related Metabolites in Maternal and Cord Blood of Japanese Pregnant Women

One-carbon metabolism (OCM) is a complex and interconnected network that undergoes drastic changes during pregnancy. In this study, we investigated the longitudinal distribution of OCM-related metabolites in maternal and cord blood and explored their relationships. Additionally, we conducted cross-sectional analyses to examine the interrelationships among these metabolites. This study included 146 healthy pregnant women who participated in the Chiba Study of Mother and Child Health. Maternal blood samples were collected during early pregnancy, late pregnancy, and delivery, along with cord blood samples. We analyzed 18 OCM-related metabolites in serum using stable isotope dilution liquid chromatography/tandem mass spectrometry. We found that serum S-adenosylmethionine (SAM) concentrations in maternal blood remained stable throughout pregnancy. Conversely, S-adenosylhomocysteine (SAH) concentrations increased, and the total homocysteine/total cysteine ratio significantly increased with advancing gestational age. The betaine/dimethylglycine ratio was negatively correlated with total homocysteine in maternal blood for all sampling periods, and this correlation strengthened with advances in gestational age. Most OCM-related metabolites measured in this study showed significant positive correlations between maternal blood at delivery and cord blood. These findings suggest that maternal OCM status may impact fetal development and indicate the need for comprehensive and longitudinal evaluations of OCM during pregnancy.

Zebrafish cox17 modulates primitive erythropoiesis via regulation of mitochondrial metabolism to facilitate hypoxia tolerance

Cox17 is required in the assembly of mitochondrial intermembrane space (IMS) and Cu metallization of cytochrome C oxidase (CcO) in mitochondria as well as Cu homeostasis in cells. Cox deficiency is associated with hematopoietic diseases such as tubulopathy and leukodystrophy, but whether and how cox17 functions in hematopoiesis are still unknown. Here, we report the effects of zebrafish cox17 deficiency on primitive erythropoiesis, mitochondrial metabolism, and hypoxia tolerance. Cox17^-/- larvae were sensitive to hypoxia stress, with reduced primitive erythropoiesis. Meanwhile, cox17^-/- mutants showed a significant reduction in the expression of pivotal transcriptional regulators in erythropoiesis, such as scl, lmo2, and gata1a at 14 h post fertilization (hpf), with expression remaining downregulated for scl but upregulated for lmo2 and gata1a at 24 hpf. Mechanistically, cox17^-/- mutants showed impaired mitochondrial metabolism, coupled with a significant decrease in the mitochondrial membrane potential, ATP and SAM content, and the ratio of SAM and SAH. Additionally, disrupting mitochondrial metabolism in wild type (WT) larvae treated with carbonyl cyanide 3-chlorophenylhydrazone (CCCP) could mimic the primitive erythropoiesis defects observed in cox17^-/- mutants. Moreover, cox17^-/- mutants exhibited significantly downregulated WNT signaling and upregulated ER stress, with a significant reduction of beta-Catenin in gata1a⁺ cells and of binding enrichment in both scl and lmo2 promoters of the WNT transcriptional factor TCF4. This is the first report on the novel linkage of cox17 deficiency with defective primitive erythropoiesis and reduced hypoxia tolerance. This study has shed light on the potential mechanism by which Cox deficiency, especially cox17 deficiency, induces Cu homeostasis imbalance, leading to hematopoietic diseases.

Co-inhibition of CD73 and ADORA2B Improves Long-Term Cigarette Smoke Induced Lung Injury

Adenosine (ADO) involvement in lung injury depends on the activation of its receptors. The ADO A_2A receptor (ADORA2A) and A_2B receptor (ADORA2B) are best described to have both tissue-protective and tissue-destructive processes. However, no approach has been effective in delineating the mechanism(s) involved with ADO shifting from its tissue-protective to tissue-destructive properties in chronic airway injury. Using cigarette smoke (CS) as our model of injury, we chronically exposed Nuli-1 cells to 5% CS extract (CSE) for 3 years establishing a long-term CSE exposure model (LTC). We found significant morphological changes, decreased proliferation, and migration resulting in impaired airway wound closure in LTC. Further investigations showed that long-term CSE exposure upregulates CD73 and ADORA2B expression, increases ADO production, inhibits PKC alpha activity and p-ERK signaling pathway. Knocking down ADORA2B and/or CD73 in LTC activates PKC alpha and increases p-ERK signaling. Knocking down both showed better improvement in wound repair than either alone. In vivo experiments also showed that double knockout CD73 and ADORA2B remarkably improved CS-induced lung injury by activating PKC alpha, reducing the inflammatory cell number in bronchoalveolar lavage fluid and the production of inflammatory mediator IL-6, inhibiting the fibrosis-like lesions and decreasing collagen deposition surrounding bronchioles. Collectively, long-term CSE exposure upregulates CD73 expression and increases ADO production, which promotes low affinity ADORA2B activation and subsequent diminution of PKC alpha activity and ERK signaling pathway, and inhibition of airway wound repair. Moreover, the data suggesting ADORA2B and CD73 as potential therapeutic targets may be more efficacious in improving chronic CS lung diseases and impaired wound repair.

Association of MAD1L1 polymorphism (rs871925) with prenatal famine exposure and schizophrenia in a Chinese population: A case-control study

Schizophrenia (SCZ) is a complex, frequently disabling psychiatric disorder. Prenatal exposure to famine, an environmental factor, plays a significant role in the cause of SCZ. We used DNA methylation related sites to analyze their association with prenatal famine exposure and SCZ risk in a Northeast Han Chinese population. A total of 967 subjects (446 patients with SCZ/521 health controls) were recruited. Five single-nucleotide polymorphisms (rs2300149 in ITIH1, rs2675956 in NGEF, rs3758543 in NT5C2, rs7003288 in NA, and rs871925 in MAD1L1) were selected and genotyped. Genotype distribution and allele frequency analysis indicated that rs871925 was significantly associated with SCZ. We also found a significant association between prenatal exposure to famine and rs871925 in the recessive model in the health control group. The generalized multifactor dimensionality reduction analysis suggested a five-locus interaction model association with the risk of developing SCZ. Our data suggested that MAD1L1 rs871925 was associated with prenatal famine exposure and SCZ susceptibility in a Northeast Han Chinese population.

Homocysteine levels associate with subtle changes in leukocyte DNA methylation: an epigenome-wide analysis

AIM

Homocysteine (Hcy) is a sensitive marker of one-carbon metabolism. Higher Hcy levels have been associated with global DNA hypomethylation. We investigated the association between plasma Hcy and epigenome-wide DNA methylation in leukocytes.

METHODS

Methylation was measured using Illumina 450 k arrays in 2035 individuals from six cohorts. Hcy-associated differentially methylated positions and regions were identified using meta-analysis.

RESULTS

Three differentially methylated positions cg21607669 (SLC27A1), cg26382848 (AJUBA) and cg10701000 (KCNMA1) at chromosome 19, 14 and 10, respectively, were significantly associated with Hcy. In addition, we identified 68 Hcy-associated differentially methylated regions, the most significant of which was a 1.8-kb spanning domain (TNXB/ATF6B) at chromosome 6.

CONCLUSION

We identified novel epigenetic loci associated with Hcy levels, of which specific role needs to be further validated.

Hyperhomocysteinemia results from and promotes hepatocellular carcinoma via CYP450 metabolism by CYP2J2 DNA methylation

Hyperhomocysteinemia (HHcy) can result from liver disease or dysfunction and further alters intracellular lipid metabolism. Cytochrome P450 (CYP) arachidonic acid epoxygenases are expressed in human cancers and promote cancer metastasis. This study explored the interaction of Hcy and CYP450 metabolism in hepatocellular carcinoma (HCC). The levels of 4-epoxyeicosatrienoic acid (EET) isomers and their generative enzyme CYP2J2 level as well as intracellular Hcy level were higher in 42 cases of HCC than in paired non-tumor tissue. Elevated Hcy-decreased DNA methylation on SP1/AP1 binding motifs and enhancement on the CYP2J2 promoter via ERK1/2 signaling was essential for CYP2J2 upregulation and EET metabolism. Increased Hcy level enhanced the neoplastic cellular phenotype, which was reversed by CYP2J2 knockdown in vitro. Furthermore, tumor growth and size as well as patterns of CYP2J2 expression and DNA demethylation were increased with HHcy in mice induced orthotopically by 2% (wt/wt) L-methionine with or without folate deficiency. Moreover, the effect was attenuated by shRNA knockdown of CYP2J2. Thus, HHcy results from but can also promote hepatocarcingenesis via CYP450-EET metabolism by crosstalk of DNA demethylation of CYP2J2 and ERK1/2 signaling.

Chronic Alcohol Exposure Differentially Alters One-Carbon Metabolism in Rat Liver and Brain

BACKGROUND

Epigenetic mechanisms such as DNA methylation play an important role in regulating the pathophysiology of alcoholism. Chronic alcohol exposure leads to behavioral changes as well as decreased expression of genes associated with synaptic plasticity. In the liver, it has been documented that chronic alcohol exposure impairs methionine synthase (Ms) activity leading to a decrease in S-adenosyl methionine/S-adenosyl homocysteine (SAM/SAH) ratio which results in DNA hypomethylation; however, it is not known whether similar alterations of SAM and SAH levels are also produced in brain.

METHODS

Male adult Sprague Dawley rats were fed chronically with Lieber-DeCarli ethanol (EtOH) (9% v/v) or control diet. The EtOH-diet-fed rats were withdrawn for 0 and 24 hours. The cerebellum and liver tissues were dissected and used to investigate changes in one-carbon metabolism, SAM, and SAH levels.

RESULTS

We found that chronic EtOH exposure decreased SAM levels, SAM/SAH ratio, Ms, methylene tetrahydrofolate reductase, and betaine homocysteine methyltransferase (Bhmt) expression and increased methionine adenosyltransferase-2b (Mat2b) but not Mat2a expression in the liver. In contrast, chronic EtOH exposure decreased SAH levels, increased SAM/SAH ratio and the expression of Mat2a and S-adenosyl homocysteine hydrolase, while the levels of SAM or Bhmt expression in cerebellum remained unaltered. However, in both liver and cerebellum, chronic EtOH exposure decreased the expression of Ms and increased Mat2b expression. All chronic EtOH-induced changes of one-carbon metabolism in cerebellum, but not liver, returned to near-normal levels during EtOH withdrawal.

CONCLUSIONS

These results indicate a decreased "methylation index" in liver and an increased "methylation index" in cerebellum. The opposing changes of the "methylation index" suggest altered DNA methylation in liver and cerebellum, thus implicating one-carbon metabolism in the pathophysiology of alcoholism.

Stable Isotope Dilution Assays for Clinical Analyses of Folates and Other One-Carbon Metabolites: Application to Folate-Deficiency Studies

Folate deficiency is generally accepted as a potential direct or indirect risk factor for diseases including spina bifida, coronary heart diseases, malfunctions of the central nervous system, and cancer. The direct inclusion of folates in the methylation cycle, including the remethylation of homocysteine and regeneration of S-adenosylmethionine, underlines the importance of these vitamins and other components of one-carbon metabolism. Therefore, the aim of the present study was to develop a multiple stable isotope dilution assay (SIDA) for the respective analytes in plasma and tissue samples to allow for a closer look at the interaction between a severe folate deficiency and local folate status, as well as further interactions with circulating S-adenosylmethionine, S-adenosylhomocysteine, and homocysteine. The analytical methods were based on SIDAs coupled with liquid chromatography-tandem mass spectrometry (LC-MS/MS) analysis using the deuterated folates [2H4]-5-methyltetrahydrofolic acid, [2H4]-5-formyltetrahydrofolic acid, [2H4]-tetrahydrofolic acid, [2H4]-10-formylfolic acid, and [2H4]-folic acid and the deuterated one-carbon metabolites [2H4]-homocysteine, [2H4]-S-adenosylhomocysteine, and [2H3]-S-adenosylmethionine as internal standards. Three analytical methods have been developed for the analysis of homocysteine, S-adenosylmethionine, S-adenosylhomocysteine, and six folate vitamers. Validation data for the analysis of C1-metabolites in plasma and tissue samples or folate analysis in tissue samples revealed excellent sensitivity, precision, and recovery for all analytes studied. The miniaturized methods using sample volumes as low as 50 μL and weighed portions of 5-25 mg will allow the assessment of the status of folates and additional biomarkers of impaired one-carbon metabolism during folate deficiency.

Homocysteine and DNA methylation: a review of animal and human literature

Homocysteine (Hcy) is a sulfur-containing non-protein forming amino acid, which is synthesized from methionine as an important intermediate in the one-carbon pathway. High concentrations of Hcy in a condition called hyperhomocysteinemia (HHcy) are an independent risk factor for several disorders including cardiovascular diseases and osteoporotic fractures. Since Hcy is produced as a byproduct of the methyltransferase reaction, alteration in DNA methylation is studied as one of the underlying mechanisms of HHcy-associated disorders. In animal models, elevated Hcy concentrations are induced either by diet (high methionine, low B-vitamins, or both), gene knockouts (Mthfr, Cbs, Mtrr or Mtr) or combination of both to investigate their effects on DNA methylation or its markers. In humans, most of the literature involves case-control studies concerning patients. The focus of this review is to study existing literature on HHcy and its role in relation to DNA methylation. Apart from this, a few studies investigated the effect of Hcy-lowering trials on restoring DNA methylation patterns, by giving a folic acid or B-vitamin supplemented diet. These studies which were conducted in animal models as well as humans were included in this review.

Maternal choline modifies fetal liver copper, gene expression, DNA methylation, and neonatal growth in the tx-j mouse model of Wilson disease

Maternal diet can affect fetal gene expression through epigenetic mechanisms. Wilson disease (WD), which is caused by autosomal recessive mutations in ATP7B encoding a biliary copper transporter, is characterized by excessive hepatic copper accumulation, but variability in disease severity. We tested the hypothesis that gestational supply of dietary methyl groups modifies fetal DNA methylation and expression of genes involved in methionine and lipid metabolism that are impaired prior to hepatic steatosis in the toxic milk (tx-j) mouse model of WD. Female C3H control and tx-j mice were fed control (choline 8 mmol/Kg of diet) or choline-supplemented (choline 36 mmol/Kg of diet) diets for 2 weeks throughout mating and pregnancy to gestation day 17. A second group of C3H females, half of which were used to cross foster tx-j pups, received the same diet treatments that extended during lactation to 21 d postpartum. Compared with C3H, fetal tx-j livers had significantly lower copper concentrations and significantly lower transcript levels of Cyclin D1 and genes related to methionine and lipid metabolism. Maternal choline supplementation prevented the transcriptional deficits in fetal tx-j liver for multiple genes related to cell growth and metabolism. Global DNA methylation was increased by 17% in tx-j fetal livers after maternal choline treatment (P<0.05). Maternal dietary choline rescued the lower body weight of 21 d tx-j mice. Our results suggest that WD pathogenesis is modified by maternal in utero factors, including dietary choline.

Simultaneous analysis of catechol-O-methyl transferase activity, S-adenosylhomocysteine and adenosine

Novel HPLC method utilizing UV-detection was developed to analyse catechol-O-methyltransferase (COMT) products, vanillic acid and isovanillic acid, S-adenosylhomocysteine (SAH) and adenosine formed from dihydroxybenzoic acid and S-adenosyl-L-methionine (SAM) by incubation of the rat tissues. Entacapone, a COMT inhibitor, prevented the formation of SAH only partially in the striatal homogenate whereas in the kidney homogenate the increase of SAH was prevented by entacapone. In conclusion, this method was reliable, rapid and simple. COMT seemed to be partially responsible on the SAM utilizing methylations in the striatal homogenates while in the high COMT activity tissue, COMT was the main SAH producing methyltransferase.

Effects of S-adenosylhomocysteine and homocysteine on DNA damage and cell cytotoxicity in murine hepatic and microglia cell lines

Limited research has been performed on S-adenosylhomocysteine (SAH) or homocysteine (Hcy)-evoked cell damage in hepatic and neuronal cells. In this study, we assessed effects of SAH or Hcy on cell cytotoxicity and DNA damage in hepatic and neuronal cells and attempted to find the underlying mechanism. Cell cytotoxicity and DNA damage were evaluated in murine hepatic cells (BNL CL.2 cell line) and microglia cells (BV-2 cell line) with SAH or Hcy treatment for 48 h. The influences of SAH or Hcy on lipid peroxidation and DNA methylation were also measured in both cell lines. SAH (5-20 microM) or Hcy (1-5 mM) dose dependently inhibited cell cytotoxicity and enhanced DNA damage in both types of cells. Furthermore, SAH treatment markedly increased intracellular SAH levels and DNA hypomethylation, whereas Hcy caused minimal effects on these two parameters at much higher concentrations. Hcy significantly induced lipid peroxidation, but not SAH. The present results show that SAH might cause cellular DNA damage in hepatic and microglia cells by DNA hypomethylation, resulting in irreversible DNA damage and increased cell cytotoxicity. In addition, higher Hcy could induce cellular DNA damage through increased lipid peroxidation and DNA hypomethylation. We suggest that SAH is a better marker of cell damage than Hcy in hepatic and microglia cells.

Protein carboxyl methylation and the biochemistry of memory

Bacterial chemotaxis is mediated by two reversible protein modification chemistries: phosphorylation and carboxyl methylation. Attractants bind to membrane chemoreceptors that control the activity of a protein kinase which acts in turn to control flagellar motor activity. Coordinate changes in receptor carboxyl methylation provide a negative feedback mechanism that serves a memory function. Protein carboxyl methylation might play an analogous role in the nervous system. Two protein carboxyl methyltransferases serve to regulate signal transduction pathways in eukaryotic cells. One is highly expressed in the Purkinje layer of the cerebellum where it methyl esterifies prenylated cysteine residues at the carboxyl-termini of Ras-related and heterotrimeric G-proteins. The other is abundant throughout the brain where it methylates the carboxyl-terminus of protein phosphatase 2A. The phosphatase methyltransferase and the protein methylesterase that reverses phosphatase methylation are structurally related to the corresponding bacterial chemotaxis methylating and demethylating enzymes. Recent results indicate that deficiencies in phosphatase methylation play an important role in the etiology of Alzheimer's disease.

Relationship of impairment induced by intracellular S-adenosylhomocysteine accumulation with DNA methylation in human umbilical vein endothelial cells treated with 3-deazaadenosine

The aim of this study was to estimate the relationship of endothelial dysfunction induced by intracellular S-adenosylhomocysteine (SAH) accumulation and DNA methylation in human umbilical vein endothelial cells (HUVEC). The isolated HUVEC were incubated with 3-deazaadenosine (DZA) to induce experimental intracellular SAH accumulation. The impairment of HUVEC function was assessed by changes in morphology and proliferative ability. The expression of DNA methyltransferase-1 (DNMT1) and the atherosclerosis related genes [oestrogen receptor-alpha (ER-alpha), extracellular superoxide dismutase (EC-SOD) and monocyte chemoattractant protein-1 (MCP-1)] were analysed using quantitative real-time PCR. Global DNA methylated status was measured using the cytosine extension assay. The methylated patterns of ER-alpha, EC-SOD and MCP-1 genes were determined with methylation-specific PCR. We found that DZA administration increased intracellular SAH levels progressively and simultaneously decreased Hcy content in medium. Moreover, the supplementation induced HUVEC apoptosis, inhibited proliferation ability and DNMT1 mRNA expression (P < 0.05) and furthermore reduced global DNA methylation status (P < 0.05). Correlation analysis showed the presence of a negative correlation between intracellular SAH concentration, proliferative ability, and expression of ER-alpha, EC-SOD, and DNMT1 (r = -0.89, -0.86, -0.92 and -0.88 respectively, P < 0.001); and a positive correlation with MCP-1 expression and DNA [(3)H]-dCTP incorporation (r = 0.89 and 0.93 respectively, P < 0.001). Our results showed that endothelial dysfunction induced by intracellular SAH accumulation is mediated by regulating the expression of atherosclerosis related genes in HUVEC, which is not related with gene promoter methylated patterns, but may be associated with altered global DNA hypomethylated status. These findings suggest that SAH can act as the potential molecular biological marker in the promotion of atherogenesis.

Body mass index is an important determinant of methylation biomarkers in women of reproductive ages

B vitamin deficiencies lead to moderate hyperhomocysteinemia, which has been associated with health and disease. However, concomitant derangements in cellular methylation, reflected by altered plasma S-adenosylmethionine (SAM) or S-adenosylhomocysteine (SAH) concentrations, may be the primary cause. Therefore, we identified determinants of homocysteine, SAM, and SAH concentrations in 336 women, aged 20-48 y, as part of a large study focusing on risk factors for reproductive disorders. Blood was obtained to determine plasma SAM, SAH, and total homocysteine (tHcy), serum vitamin B-12 and folate, RBC folate concentrations, and the related single nucleotide polymorphisms 5,10-methylenetetrahydrofolate reductase (MTHFR) 677C > T and 1298A > C, methionine synthase reductase (MTRR) 66A > G, and nicotinamide N-methyltransferase IVS1-151G > A. Questionnaires provided information on demographics, lifestyles, and nutrient intakes. Correlation coefficients were calculated and multivariable associations were assessed with a general linear model. Serum folate was positively correlated with SAM concentrations (r = 0.159; P = 0.004). Folate and vitamin B-12 were not correlated with SAH concentrations or the SAM:SAH ratio but were inversely correlated with tHcy concentrations (serum folate r = -0.324; RBC folate r = -0.294; vitamin B-12 r = -0.307; P < 0.01). From the multivariable analysis, BMI was the strongest determinant of SAM (standardized beta = 19.145; P < 0.001) and SAH concentrations (standardized beta = 3.241; P = 0.010). MTHFR 677TT (standardized beta = 0.195; P = 0.001), B vitamin supplement use (standardized beta = -0.156; P < 0.001) and dietary protein intake (standardized beta = -0.011; P < 0.001) were the strongest determinants of tHcy concentrations. Thus, the determinants of SAM and SAH differ from those of tHcy concentrations. Given that BMI was a strong determinant of SAM concentrations, it should be included in future studies on cellular methylation.

S-Adenosylhomocysteine promotes the invasion of C6 glioma cells via increased secretion of matrix metalloproteinase-2 in murine microglial BV2 cells

S-Adenosylhomocysteine (SAH) is a risk factor for many diseases, including tumor progression and neurodegenerative disease. In this study, we examined the hypothesis that SAH may indirectly enhance the invasion of C6 glioma cells by induction of matrix metalloproteinase-2 (MMP-2) secreted from the murine microglia BV2 cells. We obtained conditioned medium (CM) by incubating BV2 cells with SAH (1-50nM) for 24 h. We found that the SAH-containing CM (SAH-BV2-CM) strongly enhanced the invasiveness of C6 glioma cells and that this effect increased with increasing concentrations of SAH in the SAH-BV2-CM. The effect of CM could be attributed to its MMP-2 activity, as a result of increased protein and messenger RNA expression of MMP-2 in BV2 cells induced by SAH. In BV2 cells treated with SAH, the binding abilities of nuclear factor-kappa B (NF-kappaB) and stimulatory protein-1 (Sp1) to the MMP-2 promoter were increased, whereas the level of NF-kappaB inhibitor was decreased. In addition, SAH significantly increased the phosphorylation of extracellular signal-regulated kinase (ERK) and phosphatidylinositol-3-kinase/serine/threonine protein kinase (or protein kinase B) (PI3K/Akt) proteins but did not affect that of c-Jun NH2-terminal kinase or p38. Pretreatment of BV2 cells with an inhibitor specific for ERK (U0126) markedly abated the expression of ERK and MMP-2. Furthermore, SAH significantly and dose dependently decreased tissue inhibitor of metalloproteinase-2 (TIMP-2) in BV2 cells. Thus, SAH may induce the invasiveness of C6 glioma cells by decreased TIMP-2 expression and increased MMP-2 expression in BV2 cells. The latter effect is likely mediated through the ERK and PI3K/Akt pathways, with increased binding activities of NF-kappaB and Sp1 to the MMP-2 gene promoter.

Inherited disorders in the conversion of methionine to homocysteine

During the last decade much important new information relating to the metabolic pathway from methionine to homocysteine has been gained. Interest has been stimulated by the discovery of two novel disorders, glycine N-methyltransferase deficiency and S-adenosylhomocysteine hydrolase deficiency. Another disorder in this pathway, methionine adenosyltransferase deficiency, has been increasingly detected, thanks to the expansion of newborn screening programmes by tandem mass spectrometry technology. These significant steps allow important insight into the pathogenesis of these three disorders, as well as into the mechanisms of damage to various organs (liver, brain, muscle) and point to the relevance of these disorders for crucial biological processes such as methylation, transsulfuration or carcinogenesis in mammals, the pathogenesis of numerous pathological conditions, in particular those associated with hyperhomocysteinaemia, the action and possible toxicity of some drugs or consequences of nutritional variations. This review summarizes current knowledge of three inherited disorders in this metabolic pathway and draws attention to their much broader significance for human health and understanding of important biological processes.

Methylation status in healthy subjects with normal and high serum folate concentration

OBJECTIVE

We assessed the impact of high serum folate concentration on erythrocyte S-adenosylmethionine (SAM), S-adenosylhomocysteine (SAH) concentrations, SAM/SAH ratio, CpG methylation levels across the promoter region of the extracellular superoxide dismutase (ec-SOD) gene, and ec-SOD activity in healthy men.

METHODS

Serum folate levels were measured in 111 subjects who were categorized in quintiles according to their folate status. Subjects located at the lowest, middle, and upper quintiles were selected for assessment of SAM and SAH by high-performance liquid chromatography, C677T genotype of the 5,10-methylenetetrahydrofolate reductase (MTHFR) gene, ec-SOD methylation of CpG sites in lymphocytes genomic DNA by bisulfate treatment, and ec-SOD activity by a chemical assay.

RESULTS

Sixteen subjects were in the lowest serum folate quintile (<23.6 nmol/L), 17 in the middle (>34-<42 nmol/L), and 14 in the highest (>45nmol/L). SAM concentration was higher in the upper than in the middle and lowest quintiles (5.57 +/- 1.58, 2.52 +/- 0.97, 2.29 +/- 1.2 micromol/L; P < 0.0001). SAH concentration was higher in the upper compared with the lowest quintile (0.76 +/- 0.24 versus 0.52 +/- 0.23 micromol/L, P < 0.001). There were no differences in the SAM/SAH ratio, ec-SOD activity, methylation status of CpG sites of the ec-SOD gene, and TMTHFR C677T genotype between groups.

CONCLUSION

Serum folate concentrations in the highest quintile among healthy humans are associated with increased erythrocyte SAM and SAH concentrations, but not with SAM/SAH ratio or with methylation levels of CpG sites across the promoter region of the ec-SOD gene. Further research is required to determine if these findings are beneficial or harmful.

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.

Synergistic effects of S-adenosylhomocysteine and homocysteine on DNA damage in a murine microglial cell line

BACKGROUND

Homocysteine (Hcy) and S-adenosylhomocysteine (SAH) are 2 major metabolites of methionine. However, little is known about their interactions in human diseases.

METHODS

We determined the interaction of Hcy with SAH on DNA damage (measured as comet formation) and DNA hypomethylation (assayed as 5-methyldeoxycytidine, 5-mdc) in BV-2 cells (immortalized murine microglia).

RESULTS

Hcy at 100 micromol/l and SAH at 4 micromol/l alone caused little DNA strand breaks, whereas 100 micromol/l Hcy in combination with 0.5 to 4 micromol/l SAH led to marked DNA damage and uracil misincorporation. The combination of 100 micromol/l Hcy with 4 micromol/l SAH (SAH+Hcy) significantly increased intracellular H(2)O(2), and the DNA damage induced by SAH+Hcy was strongly inhibited by addition of superoxide dismutase, catalase or desferrioxamine, suggesting the involvement of reactive oxygen species. DNA damage induced by SAH+Hcy may also involve DNA hypomethylation (i.e., decreased %5-mdc) because of the high correlation between them. The effects induced by SAH+Hcy were specific to SAH but not to Hcy because they were markedly decreased by replacing SAH with adenosine (4.0 micromol/l) but was not affected by replacing Hcy with cysteine (100 micromol/l).

CONCLUSION

SAH in combination with Hcy can cause synergistic DNA damage in BV-2 cells. It remains to be seen whether some of the Hcy-related diseases may be caused by a collaborative action of Hcy with SAH.

S-adenosylmethionine prevents chronic alcohol-induced mitochondrial dysfunction in the rat liver

An early event that occurs in response to alcohol consumption is mitochondrial dysfunction, which is evident in changes to the mitochondrial proteome, respiration defects, and mitochondrial DNA (mtDNA) damage. S-adenosylmethionine (SAM) has emerged as a potential therapeutic for treating alcoholic liver disease through mechanisms that appear to involve decreases in oxidative stress and proinflammatory cytokine production as well as the alleviation of steatosis. Because mitochondria are a source of reactive oxygen/nitrogen species and a target for oxidative damage, we tested the hypothesis that SAM treatment during alcohol exposure preserves organelle function. Mitochondria were isolated from livers of rats fed control and ethanol diets with and without SAM for 5 wk. Alcohol feeding caused a significant decrease in state 3 respiration and the respiratory control ratio, whereas SAM administration prevented these alcohol-mediated defects and preserved hepatic SAM levels. SAM treatment prevented alcohol-associated increases in mitochondrial superoxide production, mtDNA damage, and inducible nitric oxide synthase induction, without a significant lessening of steatosis. Accompanying these indexes of oxidant damage, SAM prevented alcohol-mediated losses in cytochrome c oxidase subunits as shown using blue native PAGE proteomics and immunoblot analysis, which resulted in partial preservation of complex IV activity. SAM treatment attenuated the upregulation of the mitochondrial stress chaperone prohibitin. Although SAM supplementation did not alleviate steatosis by itself, SAM prevented several key alcohol-mediated defects to the mitochondria genome and proteome that contribute to the bioenergetic defect in the liver after alcohol consumption. These findings reveal new molecular targets through which SAM may work to alleviate one critical component of alcohol-induced liver injury: mitochondria dysfunction.

Aβ peptides as one of the crucial volume transmission signals in the trophic units and their interactions with homocysteine. Physiological implications and relevance for Alzheimer’s disease

Amyloid peptides (Abeta) can operate as volume transmission (VT) signals since they are continuously released from cells of the central nervous system and diffuse in the extra-cellular space of the brain. They have both regulatory and trophic functions on cellular networks. In agreement with Abeta regulatory actions on glial-neuronal networks, the present paper reports new findings demonstrating that intrastriatal injections of Abeta peptides reduce striatal tyrosine hydroxylase, increase striatal GFAP immunoreactivities and lower pain threshold in experimental rats. Furthermore, it has been demonstrated that exogenous homocysteine (Hcy) binds Abeta(1-40) favouring its beta-sheet conformation both in vitro and in vivo and hence the formation of beta-fibrils and development of neurotoxicity. Thus, the hypothesis is discussed that Abeta peptides represent crucial VT-signals in the brain and their action is altered by dysmetabolic signals such as high Hcy extra-cellular levels, known to be an important risk factor for Alzheimer's disease.

Intracellular Levels of S-Adenosylhomocysteine but Not Homocysteine Are Highly Correlated to the Expression of nm23-H1 and the Level of 5-Methyldeoxycytidine in Human Hepatoma Cells With Different Invasion Activities

Cellular methylation imbalance is associated with tumor progression, hepatic cancer, and cardiovascular disease. S-Adenosylhomocysteine (SAH) is an inhibitor of cellular methyltransferases, and increasing evidence suggests that SAH rather than homocysteine (Hcy) plays a crucial role in mediating these disorders related to methylation imbalance. The anti-metastatic gene nm23-H1 was recently identified in murine and human cancer lines, and the expressions of nm23-H1 mRNA and protein have been shown to be useful tumor invasion markers. We investigated the relationships of tumor cell invasion activities with the intracellular levels of SAH and Hcy and the level of DNA methylation (measured as the cellular content of 5-methyldeoxycytidine, 5-mdc) in four hepatocarcinoma cell lines (Sk-Hep1, J5, Hep-G2, Hep-3B) and one normal liver cell line (Chang's liver cells) with different invasion activities (Sk-Hep1 > J5 > Hep-G2 = Hep-3B > Chang's liver cells). We found that the intracellular level of SAH was the highest in SK-Hep1 cells and was correlated with the invasion activities (r = 0.75, P = 0.008), whereas the level of intracellular Hcy was the highest in Chang's liver cells and was not significantly correlated with the invasion activities of these cell lines (r = 0.24, P = 0.38). The levels of 5-mdc increased with decreasing invasion activities of these cell lines (r = 0.82, P = 0.002), that is, the order of DNA hypomethylation in these cell lines was Sk-Hep1 > J5 > Hep-G2 = Hep-3B > Chang's liver cells, because the lower levels of 5-mdc% represent the higher DNA hypomethylation. Thus, our results demonstrate that SAH rather than Hcy is associated with invasion activities of hepatoma cells, and they suggest that SAH may play an important role in the invasion activities through DNA hypomethylation.

S-adenosylhomocysteine hydrolase from the archaeon Pyrococcus furiosus: biochemical characterization and analysis of protein structure by comparative molecular modeling

S-adenosylhomocysteine hydrolase (AdoHcyHD) is an ubiquitous enzyme that catalyzes the breakdown of S-adenosylhomocysteine, a powerful inhibitor of most transmethylation reactions, to adenosine and L-homocysteine. AdoHcyHD from the hyperthermophilic archaeon Pyrococcus furiosus (PfAdoHcyHD) was cloned, expressed in Escherichia coli, and purified. The enzyme is thermoactive with an optimum temperature of 95 degrees C, and thermostable retaining 100% residual activity after 1 h at 90 degrees C and showing an apparent melting temperature of 98 degrees C. The enzyme is a homotetramer of 190 kDa and contains four cysteine residues per subunit. Thiol groups are not involved in the catalytic process whereas disulfide bond(s) could be present since incubation with 0.8 M dithiothreitol reduces enzyme activity. Multiple sequence alignment of hyperthermophilic AdoHcyHD reveals the presence of two cysteine residues in the N-terminus of the enzyme conserved only in members of Pyrococcus species, and shows that hyperthermophilic AdoHcyHD lack eight C-terminal residues, thought to be important for structural and functional properties of the eukaryotic enzyme. The homology-modeled structure of PfAdoHcyHD shows that Trp220, Tyr181, Tyr184, and Leu185 of each subunit and Ile244 from a different subunit form a network of hydrophobic and aromatic interactions in the central channel formed at the subunits interface. These contacts partially replace the interactions of the C-terminal tail of the eukaryotic enzyme required for tetramer stability. Moreover, Cys221 and Lys245 substitute for Thr430 and Lys426, respectively, of the human enzyme in NAD-binding. Interestingly, all these residues are fairly well conserved in hyperthermophilic AdoHcyHDs but not in mesophilic ones, thus suggesting a common adaptation mechanism at high temperatures.

Increase in S-Adenosylhomocysteine Content and Its Effect on the S-Adenosylhomocysteine Hydrolase Activity under Transient High Plasma Homocysteine Levels in Rats

The objective of this study was to examine how transient high plasma homocysteine (Hcy) levels affect the metabolism of Hcy, the activity and expression of S-adenosylhomocysteine (SAH) hydrolase which catalyzes both SAH hydrolysis and SAH synthesis. Wistar ST rats (males) were cannulated in the right jugular vein for intravenous infusion of physiological saline or DL-Hcy solutions (15 and 30 mg/mL) for 1 h at 1.1 mL/h/rat. The content of S-adenosylmethionine (SAM), SAH-synthetic activity of SAH hydrolase and the expression of SAH hydrolase mRNA in liver extracts showed no significant difference in the Hcy infused groups as compared to the Control group. On the other hand, the contents of hepatic SAH in the Hcy infused groups were dose-dependent and significantly higher than that of the Control group. Thus, this study showed that hepatic SAH increased without any increase in the SAH-synthetic activity and the expression of SAH hydrolase mRNA under transient high plasma Hcy levels after intravenous infusion of Hcy.

Role of elevated S-adenosylhomocysteine in rat hepatocyte apoptosis: protection by betaine

Previous studies from our laboratory have shown that ethanol consumption results in an increase in hepatocellular S-adenosylhomocysteine levels. Because S-adenosylhomocysteine is a potent inhibitor of methylation reactions, we propose that increased intracellular S-adenosylhomocysteine levels could be a major contributor to ethanol-induced pathologies. To test this hypothesis, hepatocytes isolated from rat livers were grown on collagen-coated plates in Williams' medium E containing 5% FCS and exposed to varying concentrations of adenosine in order to increase intracellular S-adenosylhomocysteine levels. We observed increases in caspase-3 activity following exposure to adenosine. This increase in caspase activity correlated with increases in intracellular S-adenosylhomocysteine levels and DNA hypoploidy. The adenosine-induced changes could be significantly attenuated by betaine administration. The mechanism of betaine action appeared to be via the methylation reaction catalyzed by betaine-homocysteine-methyltransferase. To conclude, our results indicate that the elevation of S-adenosylhomocysteine levels in the liver by ethanol is a major factor in altering methylation reactions and in increasing apoptosis in the liver. We conclude that ethanol-induced alteration in methionine metabolic pathways may play a crucial role in the pathologies associated with alcoholic liver injury and that betaine administration may have beneficial therapeutic effects.

Intracellular S-adenosylhomocysteine increased levels are associated with DNA hypomethylation in HUVEC

Hyperhomocysteinemia is a risk factor for atherosclerosis and vascular disease; however, the mechanism underlying this association remains poorly understood. Increased levels of intracellular S-adenosylhomocysteine (AdoHcy), secondary to homocysteine-mediated reversal of the AdoHcy hydrolase reaction, have been associated with reduced DNA methylation patterns and pointed as responsible for the hyperhomocysteinemia-related endothelial dysfunction. Methylation is an epigenetic feature of genomic DNA, which leads to alterations in gene expression. So far, the effect of intracellular AdoHcy accumulation on DNA methylation patterns has not yet been fully substantiated by experimental evidence. The present study was designed to evaluate, in cultured endothelial cells, the effect of AdoHcy accumulation on genomic global DNA methylation status. Experimental intracellular accumulation of AdoHcy was induced by adenosine-2,3-dialdehyde (ADA), an inhibitor of AdoHcy hydrolase. Increased concentrations of inhibitor were tested, and unsupplemented medium incubations were used as controls. Cytosolic and nuclear fractions were obtained from trypsinized cells after 72 h of incubation. Total homocysteine concentration was quantified (culture medium and cytosolic fractions) by high-performance liquid chromatography (HPLC). S-Adenosylmethionine and AdoHcy concentrations were measured (cytosolic fractions) by stable-isotope dilution LC-tandem mass spectrometry method. Genomic DNA was obtained from the nuclear fraction, and global DNA methylation status was evaluated by the cytosine extension assay. The results showed that supplementation of the culture medium with ADA had no cytotoxic effect and increased the intracellular AdoHcy concentration in a dose-dependent manner. A significant negative correlation was observed between intracellular AdoHcy and genomic DNA methylation status. These findings strongly point to the importance of AdoHcy as a pivotal biomarker of genomic DNA methylation status.

Disturbed homocysteine and methionine cycle intermediates S-adenosylhomocysteine and S-adenosylmethionine are related to degree of renal insufficiency in type 2 diabetes

BACKGROUND

Diabetic nephropathy is a common complication in patients with type 2 diabetes that may increase atherothrombotic risk. Hyperhomocysteinemia (HHcy) further increases the risk in those patients. We studied concentrations of total homocysteine (tHcy) and its related metabolites S-adenosylmethionine (AdoMet) and S-adenosylhomocysteine (AdoHcy) in relation to B-vitamin status and renal function in patients with type 2 diabetes who developed diabetic nephropathy.

METHODS

The study included 93 patients with renal failure and type 2 diabetes. Chronic kidney disease was classified into four subgroups according to the National Kidney Foundation based on glomerular filtration rate plus pathologic abnormalities or markers of kidney damage.

RESULTS

Serum or plasma concentrations of the metabolites increased significantly with worsening of renal function, whereas serum concentrations of the B vitamins (folate, vitamins B12 and B6) did not differ appreciably between the groups. Moreover, plasma concentrations of AdoHcy and AdoMet were markedly increased in patients with kidney failure compared with those in stage 2 (median AdoHcy, 112.7 vs 10.5 nmol/L; median AdoMet, 162.0 vs 80.0 nmol/L). The AdoMet/AdoHcy ratio was more than 80% lower in patients with renal failure compared with stage 2. Vitamin B12 was a significant determinant of concentrations of AdoMet, tHcy, methylmalonic acid (MMA), and cystathionine.

CONCLUSIONS

Increased plasma concentrations of tHcy and methionine cycle intermediates (AdoMet, AdoHcy) are related to disturbed renal function in patients with type 2 diabetes. Vitamin B12 and/or folate are significant predictors of tHcy, cystathionine, MMA, and AdoMet. The effect of therapeutic doses of the B vitamins on AdoMet, AdoHcy, and their ratio should be tested in renal patients.

Evaluation of chemical and diastereoisomeric stability of S-adenosylmethionine in aqueous solution by capillary electrophoresis

Capillary electrophoresis was used for monitoring the stability of S-adenosylmethionine in aqueous solution under different conditions of storage and incubation used for "in vitro" and "in vivo" experiments, by evaluating both the entity of degradation and the possibility of epimerization at the sulfonium group. The determination of S,S-S-adenosylmethionine in presence of its R,S-epimer and degradation products was performed in uncoated capillary of 50 microm ID using 150 mM sodium phosphate buffer at pH 2.5. The analyses were performed in short or long-end injection modes depending if a fast monitoring of the degradation products or the evaluation of the diastereoisomeric ratio were carried out, respectively. In the long-end injection mode the baseline separation of S-adenosylmethionine diastereoisomeric forms and degradation products was obtained in less than 10 min with efficiency values in the range of 172,520-311,439 number of theoretical plates per meter. The results showed that freezing was the optimum storage mode for S-adenosylmethionine aqueous solutions preserving from degradation and diastereoisomeric ratio alterations. Under incubation conditions at 38 degrees C during 14 days period S-adenosylmethionine showed a strong degradation and the formation of three main increasing degradation products. After 7 and 14 days only the 52% and 32% of the initial drug concentration were available and the active S,S-S-adenosylmethionine form was the most affected.

A comparison of the effects of betaine and S-adenosylmethionine on ethanol-induced changes in methionine metabolism and steatosis in rat hepatocytes

Previous studies showed that chronic ethanol administration alters methionine metabolism in the liver, resulting in increased intracellular S-adenosylhomocysteine (SAH) levels and increased homocysteine release into the plasma. We showed further that these changes appear to be reversed by betaine administration. This study compared the effects of betaine and S-adenosylmethionine (SAM), another methylating agent, on ethanol-induced changes of methionine metabolism and hepatic steatosis. Wistar rats were fed ethanol or control Lieber-Decarli liquid diet for 4 wk and metabolites of the methionine cycle were measured in isolated hepatocytes. Hepatocytes from ethanol-fed rats had a 50% lower intracellular SAM:SAH ratio and almost 2-fold greater homocysteine release into the media compared with controls. Supplementation of betaine or SAM in the incubation media increased this ratio in hepatocytes from both control and ethanol-fed rats and attenuated the ethanol-induced increased hepatocellular triglyceride levels by approximately 20%. On the other hand, only betaine prevented the increase in generation of homocysteine in the incubation media under basal and methionine-loaded conditions. SAM can correct only the ratio and the methylation defects and may in fact be detrimental after prolonged use because of its propensity to increase homocysteine release. Both SAM and betaine are effective in increasing the SAM:SAH ratio in hepatocytes and in attenuating hepatic steatosis; however, only betaine can effectively methylate homocysteine and prevent increased homocysteine release by the liver.

Homocysteine effect on protein degradation rates

OBJECTIVES

To show the effect of homocysteine (Hcy) on the degradation rates of proteins.

DESIGN AND METHODS

Degradation rates of short-lived proteins in neutrophils were measured in in vivo human model of elevated plasma Hcy and lower vitamin status and in animal model of Hcy added in vitro to rat neutrophils.

RESULTS

In the human study, we found significant coefficients of correlation between plasma total homocysteine (tHcy) and the degradation rates of 21 protein fractions. In the animal model, Hcy significantly increased degradation rates of 57 protein fractions.

CONCLUSIONS

The increase in protein degradation rates, induced by Hcy, may provide a clue to our understanding of the mechanism of Hcy detrimental effects. Hcy may amplify the specific effect of cellular solutes on protein conformation, thereby monitor protein degradation rates to control enzyme activity. Consequently, the cell may lose its ability to maintain an efficient control of some crucial metabolic pathways, possibly leading to atherogenesis.

Role of TNF-alpha in ethanol-induced hyperhomocysteinemia and murine alcoholic liver injury

We previously reported a link between ethanol-induced elevation of homocysteine, endoplasmic reticulum (ER) stress, and alcoholic liver injury in the murine model of intragastric ethanol feeding. We studied the role of TNFalpha in this setting by using TNFR1 knockout mice (C57 BL/6). There was a 7.4-fold increase of homocysteine in wild-type and a 6-fold increase in TNFR1 knockout mice with intragastric alcohol exposure for 4 weeks. Plasma TNFalpha increased in the wild-type (18.4 +/- 3.3 pg/mL vs. 8.4 +/- 1.3 pg/mL (control)) and in the knockouts (12.9 +/- 1.4 pg/mL vs. 7.2 +/- 1.6 pg/mL (control)). Similar extent of fatty liver was observed in both types. Increased ALT was observed in both groups. Necroinflammatory foci were increased significantly in ethanol-fed knockouts but not to the same extent as in the ethanol-fed wild type. Increase of hepatic apoptosis and reduction of S-adenosyl-L-methionine was detected in both types of animals fed ethanol. ER stress demonstrated by RT-PCR of mRNA of selective ER stress markers GRP78, CHOP, and SREBP1 was increased equivalently in both types of mice. Betaine administration decreased ER stress in conjunction with attenuation of the elevated plasma homocysteine in both types of animals. Betaine increased hepatic S-adenosyl-L-methionine by 28 fold in the knockouts and by 24-fold in wild type. In conclusion, TNFalpha makes a moderate contribution to the ALT elevation, necroinflammation, apoptosis, a small contribution to the fatty liver and no contribution to hyperhomocysteinemia and ER stress in intragastric alcohol fed mice.

S-adenosylhomocysteine hydrolase deficiency in a human: a genetic disorder of methionine metabolism

We report studies of a Croatian boy, a proven case of human S-adenosylhomocysteine (AdoHcy) hydrolase deficiency. Psychomotor development was slow until his fifth month; thereafter, virtually absent until treatment was started. He had marked hypotonia with elevated serum creatine kinase and transaminases, prolonged prothrombin time and low albumin. Electron microscopy of muscle showed numerous abnormal myelin figures; liver biopsy showed mild hepatitis with sparse rough endoplasmic reticulum. Brain MRI at 12.7 months revealed white matter atrophy and abnormally slow myelination. Hypermethioninemia was present in the initial metabolic study at age 8 months, and persisted (up to 784 microM) without tyrosine elevation. Plasma total homocysteine was very slightly elevated for an infant to 14.5-15.9 microM. In plasma, S-adenosylmethionine was 30-fold and AdoHcy 150-fold elevated. Activity of AdoHcy hydrolase was approximately equal to 3% of control in liver and was 5-10% of the control values in red blood cells and cultured fibroblasts. We found no evidence of a soluble inhibitor of the enzyme in extracts of the patient's cultured fibroblasts. Additional pretreatment abnormalities in plasma included low concentrations of phosphatidylcholine and choline, with elevations of guanidinoacetate, betaine, dimethylglycine, and cystathionine. Leukocyte DNA was hypermethylated. Gene analysis revealed two mutations in exon 4: a maternally derived stop codon, and a paternally derived missense mutation. We discuss reasons for biochemical abnormalities and pathophysiological aspects of AdoHcy hydrolase deficiency.

Quantification of serum and urinary S-adenosylmethionine and S-adenosylhomocysteine by stable-isotope-dilution liquid chromatography-mass spectrometry

BACKGROUND

We have developed an assay that uses stable-isotope-dilution liquid chromatography-mass spectrometry to assess S-adenosylmethionine (SAM) and S-adenosylhomocysteine (SAH) in body fluids to investigate the relationship of these metabolites to hyperhomocysteinemia.

METHODS

Commercially obtained SAM (D(3) methyl) and (13)C(5)-SAH uniformly labeled in the adenosyl moiety, which was synthesized using S-adenosylhomocysteine hydrolase, were added to samples followed by perchloric acid protein precipitation, C(18) chromatography, and analysis by liquid chromatography-mass spectrometry with quantification by comparison of the areas of internal standard and endogenous peaks.

RESULTS

Estimates of intraassay imprecision (CV) were 5% and 17% for SAM and SAH, respectively (n = 10). SAM decreased and SAH increased in serum and plasma samples at both 4 degrees C and room temperature over 80 h. SAM and SAH were unstable in samples stored longer than 2 years at -20 degrees C. In 48 volunteers, the estimated reference intervals [from mean (2 SD) of log-transformed data] for serum SAM and SAH were 71-168 and 8-26 nmol/L, respectively. Fractional excretion of SAM was higher than that of SAH, and the urinary SAM:SAH ratio was much higher than the serum or erythrocyte SAM:SAH ratios.

CONCLUSIONS

Stable-isotope-dilution liquid chromatography-mass spectrometry can be used to quantify SAM and SAH in biological fluids and tissues. Sample handling and storage must be stringently controlled for any epidemiologic or clinical use of such assays.

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.

Betaine lowers elevated s-adenosylhomocysteine levels in hepatocytes from ethanol-fed rats

Previous studies showed that chronic ethanol administration inhibits methionine synthase activity, resulting in impaired homocysteine remethylation to form methionine. This defect in homocysteine remethylation was shown to increase plasma homocysteine and to interfere with the production of hepatic S-adenosylmethionine (SAM) in ethanol-fed rats. These changes were shown to be reversed by the administration of betaine, an alternative methylating agent. This study was undertaken to determine additional effects of ethanol on methionine metabolism and their functional consequences. The influences of methionine loading and betaine supplementation were also evaluated. Adult Wistar rats were fed ethanol or a control Lieber-DeCarli liquid diet for 4 wk, and metabolites of the methionine cycle were measured in vitro in isolated hepatocytes under basal and methionine-supplemented conditions. S-Adenosylhomocysteine (SAH) concentrations were elevated in hepatocytes isolated from ethanol-fed rats compared with controls and in hepatocytes from both groups when supplemented with methionine. The addition of betaine to the methionine-supplemented incubation media reduced the elevated SAH levels. The decrease in the intracellular SAH:SAM ratio due to ethanol consumption inhibited the activity of the liver-specific SAM-dependent methyltransferase, phosphatidylethanolamine methyltransferase. Our data indicate that betaine, by remethylating homocysteine and removing SAH, overcomes the detrimental effects of ethanol consumption on methionine metabolism and may be effective in correcting methylation defects and treating liver diseases.

Effect of hyperhomocysteinemia on plasma or tissue adenosine levels and renal function

BACKGROUND

Hyperhomocysteinemia (hHcys) is considered an independent risk factor of cardiovascular diseases. Recent studies in our laboratory have shown that hHcys produced glomerular dysfunction and sclerosis independently of hypertension. However, the mechanism mediating these pathogenic effects of homocysteine (Hcys) is poorly understood. Because Hcys and adenosine (Ado) are simultaneously produced via hydrolysis of S-adenosylhomocysteine (SAH), we hypothesized that hHcys may produce its pathogenic effects by decrease in plasma or tissue Ado concentrations.

METHODS AND RESULTS

L-Hcys (1.5 micromol/min per kilogram) was infused intravenously for 60 minutes to produce acute hHcys in Sprague-Dawley rats. Plasma Hcys levels increased from 6.7+/-0.4 to 14.7+/-0.5 micromol/L, but Ado decreased from 141.7+/-15.1 to 52.4+/-6.8 nmol/L in these rats with acute hHcys. This hHcys-induced reduction of Ado was also observed in the kidney dialysate. In rats with chronic hHcys, plasma Ado levels were also significantly decreased. By kinetic analysis of the enzyme activities, decrease in renal Ado levels in hHcys was shown to be associated with inhibition of SAH hydrolase but not 5'-nucleotidase. Functionally, intravenous infusion of Hcys was found to decrease renal blood flow, glomerular filtration rate, and sodium and water excretion, which could be blocked by the Ado receptor antagonist 8-SPT.

CONCLUSIONS

These results strongly suggest that hHcys decreases plasma and tissue Ado concentrations associated with inhibition of SAH hydrolase. Decrease in plasma and tissue Ado may be an important mechanism mediating the pathogenic effects of Hcys.

Quantification of plasma S-adenosylmethionine and S-adenosylhomocysteine as their fluorescent 1,N(6)-etheno derivatives: an adaptation of previously described methodology

A simplified reversed phase HPLC system for the detection of fluorescent 1,N(6)-etheno derivatives of SAM (S-adenosylmethionine) and S-adenosylhomocysteine (SAH) is described. The most important changes from the previously reported method are a shorter derivatization reaction time, the use of a solid-phase extraction resulting in an increase of the method's sensitivity, and the use of only one chromatographic system to separate SAM and SAH (in which the use of an ion-pairing reagent in the mobile phase is avoided). The linearity of the method was established, and the intra-assay coefficients of variation were 10.4 and 4.7% for SAM and SAH, respectively. Normal plasma values (n=8), evaluated with the present methodology, were, for SAM and SAH, respectively, 57+/-12 and 28+/-3 nM (mean+/-SD).

Impaired homocysteine metabolism and atherothrombotic disease

Based on recent retrospective, prospective, and experimental studies, mild to moderate elevation of fasting or postmethionine-load plasma homocysteine is accepted as an independent risk factor for cardiovascular disease and thrombosis in both men and women. Hyperhomocysteinemia results from an inhibition of the remethylation pathway or from an inhibition or a saturation of the transsulfuration pathway of homocysteine metabolism. The involvement of a high dietary intake of methionine-rich animal proteins has not yet been investigated and cannot be ruled out. However, folate deficiency, either associated or not associated with the thermolabile mutation of the N(5,10)-methylenetetrahydrofolate reductase, and vitamin B(6) deficiency, perhaps associated with cystathionine beta-synthase defects or with methionine excess, are believed to be major determinants of the increased risk of cardiovascular disease related to hyperhomocysteinemia. Recent experimental studies have suggested that moderately elevated homocysteine levels are a causal risk factor for atherothrombotic disease because they affect both the vascular wall structure and the blood coagulation system. The oxidant stress that results from impaired homocysteine metabolism, which modifies the intracellular redox status, might play a central role in the molecular mechanisms underlying moderate hyperhomocysteinemia-mediated vascular disorders. Because folate supplementation can efficiently reduce plasma homocysteine levels, both in the fasting state and after methionine loading, results from further prospective cohort studies and from on-going interventional trials will determine whether homocysteine-lowering therapies can contribute to the prevention and reduction of cardiovascular risk. Additionally, these studies will provide unequivocal arguments for the independent and causal relationship between hyperhomocysteinemia and atherothrombotic disease.

Pathogenesis of Hyperhomocysteinemia-New Insights

Mild to moderately elevated levels of homocysteine (Hey) in plasma, denoted as hyperhomocysteinemia, is emerging as a prevalent and strong risk factor for atherosclerotic vascular disease in coronary, cerebral and peripheral vessels, as well as for arterial and venous thromboembolism. Despite its clinical significance, the molecular mechanism of homocysteine's effects is not yet clearly defined. Most of the effects of homocysteine that have been demonstrated in vitro, affecting endothelial function have been attributed to the oxidant reactivity of this molecule, which is shown to affect the vasoregulatory and thrombotic/fibrinolytic function of endothelium. However, the relevance of these observations to the clinical situations is questionable, since excessively high concentrations of homocysteine are used in most of the experiments. We have observed that homocysteine, at physiologically relevant concentrations, specifically induces the expression of tissue factor by monocytes, and a non-specific redox effect is not involved. Tissue factor expression by monocytes is mediated by increased intracellular concentrations of the metabolic intermediate, S-adenosylhomocysteine, which is a potent inhibitor of methyl transferases. These studies suggest that tissue factor expression by circulating monocytes by intracellular perturbations may be a plausible mechanism by which homocysteine may induce thrombosis.