Hepatic content of S-adenosylmethionine, S-adenosylhomocysteine and glutathione in rats receiving treatments modulating methyl donor availability

Because of evidence linking methyl group deficiency and increased tumor formation in experimental animals, we explored other possible methods of producing a methyl group deficiency. Rats fed a low methionine diet lacking choline (MCD) were injected intraperitoneally daily for 3 wk with large doses of nicotinamide. Hepatic levels of lipids were elevated, S-adenosylmethionine (SAM) levels and the SAM:S-adenosylhomocysteine (SAH) ratio were decreased, and SAH level was not consistently changed. In livers of rats fed the MCD diet without folate (MCFD), lipids were also elevated and SAM reduced as compared to MCD-fed rats. In rats fed the MCD diet plus a methionine (Met) supplement (MCD + Met), hepatic SAM levels and the SAM:SAH ratio were higher and lipid levels lower than in MCD-fed rats, indicating that the MCD diet is marginally deficient in methyl donor groups. The injection of nicotinamide or the removal of folate from the MCD diet increased the severity of methyl donor deficiency, as shown by lower hepatic SAM levels and higher hepatic lipid levels. Hepatic glutathione levels were similar in MCD- and MCFD-fed rats and were lower than in rats fed the methionine-supplemented MCD diet or injected with nicotinamide.     

Role of S-adenosylmethionine, folate, and betaine in the treatment of alcoholic liver disease: summary of a symposium

This report is a summary of a symposium on the role of S-adenosylmethionine (SAM), betaine, and folate in the treatment of alcoholic liver disease (ALD), which was organized by the National Institute on Alcohol Abuse and Alcoholism in collaboration with the Office of Dietary Supplements and the National Center for Complementary and Alternative Medicine of the National Institutes of Health (Bethesda, MD) and held on 3 October 2005. SAM supplementation may attenuate ALD by decreasing oxidative stress through the up-regulation of glutathione synthesis, reducing inflammation via the down-regulation of tumor necrosis factor-alpha and the up-regulation of interleukin-10 synthesis, increasing the ratio of SAM to S-adenosylhomocysteine (SAH), and inhibiting the apoptosis of normal hepatocytes and stimulating the apoptosis of liver cancer cells. Folate deficiency may accelerate or promote ALD by increasing hepatic homocysteine and SAH concentrations; decreasing hepatic SAM and glutathione concentrations and the SAM-SAH ratio; increasing cytochrome P4502E1 activation and lipid peroxidation; up-regulating endoplasmic reticulum stress markers, including sterol regulatory element-binding protein-1, and proapoptotic gene caspase-12; and decreasing global DNA methylation. Betaine may attenuate ALD by increasing the synthesis of SAM and, eventually, glutathione, decreasing the hepatic concentrations of homocysteine and SAH, and increasing the SAM-SAH ratio, which can trigger a cascade of events that lead to the activation of phosphatidylethanolamine methyltransferase, increased phosphatidylcholine synthesis, and formation of VLDL for the export of triacylglycerol from the liver to the circulation. Additionally, decreased concentrations of homocysteine can down-regulate endoplasmic reticulum stress, which leads to the attenuation of apoptosis and fatty acid synthesis.     

Effects of excessive dietary methionine on oxidative stress and dyslipidemia in chronic ethanol-treated rats

BACKGROUND/OBJECTIVE

The aim of this study was to examine the effect of high dietary methionine (Met) consumption on plasma and hepatic oxidative stress and dyslipidemia in chronic ethanol fed rats.

MATERIALS/METHODS

Male Wistar rats were fed control or ethanol-containing liquid diets supplemented without (E group) or with DL-Met at 0.6% (EM1 group) or 0.8% (EM2 group) for five weeks. Plasma aminothiols, lipids, malondialdehyde (MDA), alanine aminotransferase (ALT), and aspartate aminotransferase were measured. Hepatic folate, S-adenosylmethionine (SAM), and S-adenosylhomocysteine (SAH) were measured.

RESULTS

DL-Met supplementation was found to increase plasma levels of homocysteine (Hcy), triglyceride (TG), total cholesterol (TC), and MDA compared to rats fed ethanol alone and decrease plasma ALT. However, DL-Met supplementation did not significantly change plasma levels of HDL-cholesterol, cysteine, cysteinylglycine, and glutathione. In addition, DL-Met supplementation increased hepatic levels of folate, SAM, SAH, and SAM:SAH ratio. Our data showed that DL-Met supplementation can increase plasma oxidative stress and atherogenic effects by elevating plasma Hcy, TG, and TC in ethanol-fed rats.

CONCLUSION

The present results demonstrate that Met supplementation increases plasma oxidative stress and atherogenic effects by inducing dyslipidemia and hyperhomocysteinemia in ethanol-fed rats.     

Effects of dietary supplementation of high-dose folic acid on biomarkers of methylating reaction in vitamin B12-deficient rats

Folate is generally considered as a safe water-soluble vitamin for supplementation. However, we do not have enough information to confirm the potential effects and safety of folate supplementation and the interaction with vitamin B₁₂ deficiency. It has been hypothesized that a greater methyl group supply could lead to compensation for vitamin B₁₂ deficiency. On this basis, the present study was conducted to examine the effects of high-dose folic acid (FA) supplementation on biomarkers involved in the methionine cycle in vitamin B₁₂-deficient rats. Sprague-Dawley rats were fed diets containing either 0 or 100 µg (daily dietary requirement) vitamin B₁₂/kg diet with either 2 mg (daily dietary requirement) or 100 mg FA/kg diet for six weeks. Vitamin B₁₂-deficiency resulted in increased plasma homocysteine (p<0.01), which was normalized by dietary supplementation of high-dose FA (p<0.01). However, FA supplementation and vitamin B₁₂ deficiency did not alter hepatic and brain S-adenosylmethionine (SAM) and S-adenosylhomocysteine (SAH) concentrations and hepatic DNA methylation. These results indicated that supplementation of high-dose FA improved homocysteinemia in vitamin B₁₂-deficiency but did not change SAM and SAH, the main biomarkers of methylating reaction.     

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.     

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 vitamin B12 deficiency on S-adenosylmethionine metabolism in rats

The effect of vitamin B12 (B12) deficiency on the levels of S-adenosylmethionine (SAM) in tissues and the activities of hepatic methionine synthase, methionine adenosyltransferase and glycine N-methyltransferase were investigated. The striking depression of methionine synthase activity was observed in all rats fed the B12-deficient diets with or without methionine supplementation for 150 days. The SAM level in liver was decreased by B12 deficiency. However, brain SAM level was not affected. The activities of hepatic methionine adenosyltransferase isozymes, alpha-form and beta-form, were decreased by B12 deficiency. Hepatic glycine N-methyltransferase activity in rats fed the low methionine-B12-deficient diet showed a tendency to lower, although the change the activity was not statistically significant, compared with B12-supplemented rats. It is proposed that the fall in the activity of hepatic methionine adenosyltransferase may be one of the causes of the decreased hepatic SAM level in B12-deficient rats.     

Dietary and genetic compromise in folate availability reduces acetylcholine, cognitive performance and increases aggression: Critical role of S-adenosyl methionine

Folate deficiency has been associated with age-related neurodegeneration. One direct consequence of folate deficiency is a decline in the major methyl donor, S-adenosyl methionine (SAM). We demonstrate herein that pro-oxidant stress and dietary folate deficiency decreased levels of acetylcholine and impaired cognitive performance to various degrees in normal adult mice (9–12months of age, adult mice heterozygously lacking 5’,10’-methylene tetrahydrofolate reductase, homozygously lacking apolipoprotein E, or expressing human ApoE2, E3 or E4, and aged (2–2.5 year old) normal mice. Dietary supplementation with SAM in the absence of folate restored acetylcholine levels and cognitive performance to respective levels observed in the presence of folate. Increased aggressive behavior was observed among some but not all genotypes when maintained on the deficient diet, and was eliminated in all cases supplementation with SAM. Folate deficiency decreased levels of choline and N-methyl nicotinamine, while dietary supplementation with SAM increased methylation of nicotinamide to generate N-methyl nicotinamide and restored choline levels within brain tissue. Since N-methyl nicotinamide inhibits choline transport out of the central nervous system, and choline is utilized as an alternative methyl donor, these latter findings suggest that SAM may maintain acetylcholine levels in part by maintaining availability of choline. These findings suggest that dietary supplementation with SAM represents a useful therapeutic approach for age-related neurodegeneration which may augment pharmacological approaches to maintain acetylcholine levels, in particular during dietary or genetic compromise in folate usage.     

13-cis-retinoic acid alters methionine metabolism in rats

The effect of dietary 13-cis-retinoic acid (CRA) on hepatic methionine metabolism was examined in young male rats. Rats were fed a 10% casein diet (controls) or this diet supplemented with L-methionine (10 g/kg diet), with or without the addition of CRA (100 mg/kg diet), for 10 d. Methionine-supplemented rats exhibited 7.3- and 1.7-fold greater concentrations of hepatic S-adenosylmethionine (SAM) and S-adenosylhomocysteine (SAH), respectively, relative to controls, which resulted in a 4.9-fold greater SAM:SAH ratio. Likewise, hepatic methionine and taurine concentrations were 6.9- and 4.3-fold greater, respectively, in methionine-supplemented rats than in controls. The addition of CRA to the methionine-supplemented diet prevented the elevations in the hepatic methionine concentration and the SAM:SAH ratio, whereas taurine levels were greater than in methionine-supplemented rats. In rats pretreated with the methionine-supplemented diet, a reduction in the SAM:SAH ratio occurred within 2 d following the addition of CRA to the methionine-supplemented diet. Rats receiving the methionine-supplemented diet exhibited 9.2- and 3.7-fold greater urinary taurine and inorganic sulfate excretions, respectively, relative to controls. Addition of CRA to the methionine-supplemented diet significantly reduced sulfate excretion by 21%. These findings indicate that dietary CRA has the ability to alter the catabolism of methionine and subsequently influence hepatic transmethylation as reflected by the SAM:SAH ratio.     

Folate exacerbates the effects of ethanol on peripubertal mouse mammary gland development

Alcohol consumption is linked with increased breast cancer risk in women, even at low levels of ingestion. The proposed mechanisms whereby ethanol exerts its effects include decreased folate levels resulting in diminished DNA synthesis and repair, and/or acetaldehyde-generated DNA damage. Based on these proposed mechanisms, we hypothesized that ethanol would have increased deleterious effects during periods of rapid mammary gland epithelial proliferation, such as peripuberty, and that folate deficiency alone might mimic and/or exacerbate the effects of ethanol. To test this hypothesis, weight-matched 28–35 day old CD2F1 female mice were pair-fed liquid diets ±3.2% ethanol, ±0.1% folate for 4 weeks. Folate status was confirmed by assay of liver and kidney tissues. In folate deficient mice, no significant ethanol-induced changes to the mammary gland were observed. Folate replete mice fed ethanol had an increased number of ducts per section, due to an increased number of terminal short branches. Serum estrogen levels were increased by ethanol, but only in folate replete mice. These results demonstrate that folate deficiency alone does not mimic the effects of ethanol, and that folate deficiency in the presence of ethanol blocks proliferative effects of ethanol on the mammary ductal tree.     

DNA stability and genomic methylation status in colonocyte isolated from methyl-donor-deficient rats

Summary Background: Epidemiological studies report an inverse relationship between intake of the B vitamine folic acid and colon cancer. Folate is important for DNA synthesis and repair. Moreover, the production of S-adenosylmethionine (SAM), essential for normal DNA methylation and gene expression, is dependent on folic acid. Folate deficiency may increase the risk of malignant transformation by perturbing these pathways. Aims of the study: The principal aim of this study was to determine the effects of folate deficiency on DNA stability and DNA methylation in rat colonocytes in vivo. As the metabolic pathways of folate and other dietary methyl donors are closely linked, the effects of methionine and choline deficiency were also evaluated. Methods: Male Hooded-Lister rats were fed a diet deficient in folic acid, or in methionine and choline, or in folate, methionne and choline for 10 weeks. DNA strand breakage and misincorporated uracil were determined in isolated colonocytes using alkaline single cell gel electrophoresis. Global DNA methylation was measured in colonic scrapings. Folate was measured in plasma, erythrocyte and liver samples. Results: Methyl donor deficiency induced DNA strand breakage in colonocytes isolated from all experimental groups. Uracil levels in colonocytes DNA remained unchanged compared with controls. DNA methylation was unaffected either by folate and/or methionine and choline depletion. Rats fed a folate-deficient diet had less folate in plasma, red blood cells and liver than controls. Conclusions: Folate and methyl deficiency in vivo primarily afects DNA stability in isolated colonocytes of rats, without affecting overall DNA methylation. Received: 16 February 2000, Accepted: 25 April 2000     

Dietary folate and selenium affect dimethylhydrazine-induced aberrant crypt formation,global DNA methylation and one-carbon metabolism in rats

Several observations suggest a role for DNA methylation in cancer pathogenesis. Although both selenium and folate deficiency have been shown to cause global DNA hypomethylation and increased cancer susceptibility, the nutrients have different effects on one-carbon metabolism. Thus, the purpose of this study was to investigate the interactive effects of dietary selenium and folate. Weanling, Fischer-344 rats (n = 23/diet) were fed diets containing 0 or 2.0 mg selenium (as selenite)/kg and 0 or 2.0 mg folate/kg in a 2 x 2 factorial design. After 3 and 4 wk of a 12-wk experiment, 19 rats/diet were injected intraperitoneally with dimethylhydrazine (DMH, 25 mg/kg) and 4 rats/diet were administered saline. Selenium deficiency decreased (P < 0.05) colonic DNA methylation and the activities of liver DNA methyltransferase and betaine homocysteine methyltransferase and increased plasma glutathione concentrations. Folate deficiency increased (P < 0.05) the number of aberrant crypts per aberrant crypt foci, the concentration of colonic S-adenosylhomocysteine and the activity of liver cystathionine synthase. Selenium and folate interacted (P < 0.0001) to influence one-carbon metabolism and cancer susceptibility such that the number of aberrant crypts and the concentrations of plasma homocysteine and liver S-adenosylhomocysteine were the highest and the concentrations of plasma folate and liver S-adenosylmethionine and the activity of liver methionine synthase were the lowest in rats fed folate-deficient diets and supplemental selenium. These results suggest that selenium deprivation ameliorates some of the effects of folate deficiency, probably by shunting the buildup of homocysteine (as a result of folate deficiency) to glutathione.     

Effects of chronic ethanol and diet treatment on urinary folate excretion and development of folate deficiency in the rat

Because the folate deficiency of chronic alcoholism has been proposed to result from ethanol-induced effects on metabolism or urinary excretion of folate, the present study was designed to evaluate the role of chronic ethanol-induced urinary folate loss on folate homeostasis in the rat. Male Sprague-Dawley rats were fed nutritionally sufficient liquid diets for 12 wk with or without ethanol, folate and sulfonamide. Urinary folate excretion was increased in ethanol-fed rats consuming folate-containing diets, but not in rats fed folate deficient diets. Consumption of folate-deficient diets led to a rapid decrease in urinary folate excretion, suggesting renal adaptation to conserve folate. Tissue and plasma levels of folate were mostly unaffected by ethanol ingestion in rats fed folate-containing diets. Ethanol treatment did not consistently enhance tissue folate depletion in rats fed folate-deficient diets. The results suggest that in rats consuming diets containing high levels of folate, chronic ethanol ingestion increased urinary folate excretion, but not to a sufficient magnitude to consistently affect folate homeostasis.     

Disruption of lipid metabolism in the liver of the pregnant rat fed folate-deficient and methyl donor-deficient diets

The importance of folic acid and the methionine cycle in fetal development is well recognised even though the mechanism has not been established. Since the cycle is active in the maternal liver, poor folate status may modify hepatic metabolism. Pregnant rats were fed diets deficient in folic acid (-F) or in three key methyl donors, folic acid, choline and methionine (-FLMLC) and the maternal liver was analysed on day 21 of gestation. Two-dimensional gel electrophoresis of soluble proteins identified differentially abundant proteins, which could be allocated into nine functional groups. Five involved in metabolic processes, namely, folate/methionine cycle, tyrosine metabolism, protein metabolism, energy metabolism and lipid metabolism, and three in cellular processes, namely, endoplasmic reticulum function, bile production and antioxidant defence. The mRNA for sterol regulatory element-binding protein-1c and acetyl-CoA carboxylase-1 (fatty acid synthesis) were decreased by both -F and -FLMLC diets. The mRNA for PPARalpha and PPARgamma and carnitine palmitoyl transferase (fatty acid oxidation) were increased in the animals fed the -FLMLC diets. Changes in the abundance of proteins associated with intracellular lipid transport suggest that folate deficiency interferes with lipid export. Reduced fatty acid synthesis appeared to prevent steatosis in animals fed the -F diet. Even with increased oxidation, TAG concentrations were approximately three-fold higher in animals fed the -FLMLC diet and were associated with an increase in the relative abundance of proteins associated with oxidative stress. Fetal development may be indirectly affected by these changes in hepatic lipid metabolism.     

Effects of betaine supplementation and choline deficiency on folate deficiency-induced hyperhomocysteinemia in rats

The effect of betaine status on folate deficiency-induced hyperhomocysteinemia was investigated to determine whether folate deficiency impairs homocysteine removal not only by the methionine synthase (MS) pathway but also by the betaine-homocysteine S-methyltransferase (BHMT) pathway. For this purpose, we investigated the effect of dietary supplementation with betaine at a high level (1%) in rats fed a folate-deprived 10% casein diet (10C) and 20% casein diet (20C). We also investigated the effect of choline deprivation on folate deficiency-induced hyperhomocysteinemia in rats fed 20C. Supplementation of folate-deprived 10C and 20C with 1% betaine significantly suppressed folate deprivation-induced hyperhomocysteinemia, but the extent of suppression was partial or limited, especially in rats fed 10C, the suppression of plasma homocysteine increment being 48.5% in rats fed 10C and 69.7% in rats fed 20C. Although betaine supplementation greatly increased hepatic betaine concentration and BHMT activity, these increases did not fully explain why the effect of betaine supplementation was partial or limited. Folate deprivation markedly increased the hepatic concentration of N,N-dimethylglycine (DMG), a known inhibitor of BHMT, and there was a significant positive correlation between hepatic DMG concentration and plasma homocysteine concentration, suggesting that folate deficiency increases hepatic DMG concentration and thereby depresses BHMT reaction, leading to interference with the effect of betaine supplementation. Choline deprivation did not increase plasma homocysteine concentration in rats fed 20C, but it markedly enhanced plasma homocysteine concentration when rats were fed folate-deprived 20C. This indicates that choline deprivation reinforced folate deprivation-induced hyperhomocysteinemia. Increased hepatic DMG concentration was also associated with such an effect. These results support the concept that folate deficiency impairs homocysteine metabolism not only by the MS pathway but also by the BHMT pathway.     

Association between decreased vitamin levels and MTHFR, MTR and MTRR gene polymorphisms as determinants for elevated total homocysteine concentrations in pregnant women

OBJECTIVES: To examine the association between methylenetetrahydrofolate reductase (MTHFR) (C677T and A1298C), methionine synthase (MTR) A2756G and methionine synthase reductase (MTRR) A66G gene polymorphisms and total homocysteine (tHcy), methylmalonic acid (MMA) and S-adenosylmethionine/S-adenosylhomocysteine (SAM/SAH) levels; and to evaluate the potential interactions with folate or cobalamin (Cbl) status. SUBJECTS/METHODS: Two hundred seventy-five healthy women at labor who delivered full-term normal babies. Cbl, folate, tHcy, MMA, SAM and SAH were measured in serum specimens. The genotypes for polymorphisms were determined by PCR-restriction fragment length polymorphism (RFLP). RESULTS: Serum folate, MTHFR 677T allele and MTR 2756AA genotypes were the predictors of tHcy levels in pregnant women. Serum Cbl and creatinine were the predictors of SAM/SAH ratio and MMA levels, respectively. The gene polymorphisms were not determinants for MMA levels and SAM/SAH ratios. Low levels of serum folate were associated with elevated tHcy in pregnant women, independently of the gene polymorphisms. In pregnant women carrying MTHFR 677T allele, or MTHFR 1298AA or MTRR 66AA genotypes, lower Cbl levels were associated with higher levels of tHcy. Lower SAM/SAH ratio was found in MTHFR 677CC or MTRR A2756AA genotypes carriers when Cbl levels were lower than 142 pmol/l. CONCLUSIONS: Serum folate and MTHFR C677T and MTR A2576G gene polymorphisms were the determinants for tHcy levels. The interaction between low levels of serum Cbl and MTHFR (C677T or A1298C) or MTRR A66G gene polymorphisms was associated with increased tHcy.     

Potentiation of arsenic neurotoxicity by folate deprivation: protective role of S-adenosyl methionine

Folate deficiency contributes to a variety of age-related neurological and psychological disorders including amyotrophic lateral sclerosis (ALS). The environmental neurotoxin arsenic has recently been linked with decreased neurofilament (NF) content in peripheral nerve. We examined herein, whether or not folate deprivation potentiated the impact of arsenic on NF dynamics. Arsenic inhibited translocation of NFs into axonal neurites in culture and increased perikaryal NF phosphoepitopes. Folate deprivation potentiated the impact of arsenic on these phenomena. Supplementation with S-adenosyl methionine (SAM) attenuated the impact of folate deprivation on arsenic neurotoxicity, consistent with the decrease in SAM following folate deprivation and the requirement for SAM-mediated methylation for arsenic bioelimination. These findings demonstrate how key nutritional deficiencies can potentiate the impact of enrivonmental neurotoxins.     

Hepatic glycine N-methyltransferase is up-regulated by excess dietary methionine in rats

Glycine N-methyltransferase (GNMT) regulates S-adenosylmethionine (SAM) levels and the ratio of SAM:S-adenosylhomocysteine (SAH). In liver, methionine availability, both from the diet and via the folate-dependent one-carbon pool, modulates GNMT activity to maintain an optimal SAM:SAH ratio. The regulation of GNMT activity is accomplished via posttranslational and allosteric mechanisms. We more closely examined GNMT regulation in various tissues as a function of excess dietary methyl groups. Sprague Dawley rats were fed either a control diet (10% casein plus 0.3% L-methionine) or the control diet supplemented with graded levels (0.5-2%) of L-methionine. Pair-fed control groups of rats were included due to the toxicity associated with high methionine consumption. As expected, the hepatic activity of GNMT was significantly elevated in a dose-dependent fashion after 10 d of feeding the diets containing excess methionine. Moreover, the abundance of hepatic GNMT protein was similarly increased. The kidney had a significant increase in GNMT as a function of dietary methionine, but to a much lesser extent than in the liver. For pancreatic tissue, neither the activity of GNMT nor the abundance of the protein was responsive to excess dietary methionine. These data suggest that additional mechanisms contribute to regulation of GNMT such that synthesis of the protein is greater than its degradation. In addition, methionine-induced regulation of GNMT is dose dependent and appears to be tissue specific, the latter suggesting that the role it plays in the kidney and pancreas may in part differ from its hepatic function.     

Potentiation of arsenic neurotoxicity by folate deprivation: Protective role of S-adenosyl methionine

Folate deficiency contributes to a variety of age-related neurological and psychological disorders including amyotrophic lateral sclerosis (ALS). The environmental neurotoxin arsenic has recently been linked with decreased neurofilament (NF) content in peripheral nerve. We examined herein, whether or not folate deprivation potentiated the impact of arsenic on NF dynamics. Arsenic inhibited translocation of NFs into axonal neurites in culture and increased perikaryal NF phosphoepitopes. Folate deprivation potentiated the impact of arsenic on these phenomena. Supplementation with S-adenosyl methionine (SAM) attenuated the impact of folate deprivation on arsenic neurotoxicity, consistent with the decrease in SAM following folate deprivation and the requirement for SAM-mediated methylation for arsenic bioelimination. These findings demonstrate how key nutritional deficiencies can potentiate the impact of enrivonmental neurotoxins.