The inactivation of methionine synthase in isolated rat hepatocytes by sodium nitroprusside

The restoration of hippocampal nerve de-myelination by methylcobalamin relates with the enzymatic regulation of homocysteine level in a rat model of moderate grade hepatic encephalopathy

This article describes how methylcobalamin (MeCbl) restores nerve myelination in a moderate- grade hepatic encephalopathy (MoHE) model of ammonia neurotoxicity. The comparative profiles of myelin basic protein (MBP), homocysteine (Hcy) and methionine synthase (MS: a MeCbl- dependent enzyme) activity versus nerve myelination status were studied in the hippocampus of the control, the MoHE (developed by administering 100 mg/kg bw thioacetamide i.p. for 10 days) and the MoHE rats treated with MeCbl (500 µg/kg BW i.p.) for 7 days. Compared to those of control rats, the hippocampal CA1 and CA3 regions of the MoHE rats showed significantly lower myelinated areas and MBP immunostaining. This coincided with the deranged myelin layering in TEM images, decreased MBP protein and its transcript levels in hippocampus of MoHE rats. However, all these parameters recovered to normal levels after MeCbl treatment. MeCbl is a cofactor of MS that catalyzes the conversion of Hcy to methionine as a feeder step of methylation reactions. We observed significantly increased serum and hippocampal Hcy levels in MoHE rats, however, these levels were restored to control values with a concordant activation of MS due to MeCbl treatment. A significant recovery in neurobehavioral impairments in the MoHE rats due to MeCbl treatment was also observed. These findings suggest that MoHE pathogenesis is associated with deranged nerve myelination in the hippocampus and that MeCbl treatment is able to restore it mainly by activating MS, a MeCbl-dependent Hcy-metabolizing enzyme.

Design, synthesis and biological activity of N5-substituted tetrahydropteroate analogs as non-classical antifolates against cobalamin-dependent methionine synthase and potential anticancer agents

Cobalamin-dependent methionine synthase (MetH) is involved in the process of tumor cell growth and survival. In this study, a novel series of N⁵-electrophilic substituted tetrahydropteroate analogs without glutamate residue were designed as non-classical antifolates and evaluated for their inhibitory activities against MetH. In addition, the cytotoxicity of target compounds was evaluated in human tumor cell lines. With N⁵-chloracetyl as the optimum group, further structure research on the benzene substituent and on the 2,4-diamino group was also performed. Compound 6c, with IC₅₀ value of 12.1 μM against MetH and 0.16-6.12 μM against five cancer cells, acted as competitive inhibitor of MetH. Flow cytometry studies indicated that compound 6c arrested HL-60 cells in the G₁-phase and then inducted late apoptosis. The molecular docking further explained the structure-activity relationship.

Biochemistry, function, and deficiency of vitamin B12 in Caenorhabditis elegans

Caenorhabditis elegans is a nematode that has been widely used as an animal for investigation of diverse biological phenomena. Vitamin B12 is essential for the growth of this worm, which contains two cobalamin-dependent enzymes, methylmalonyl-CoA mutase and methionine synthase. A full complement of gene homologs encoding the enzymes associated with the mammalian intercellular metabolic processes of vitamin B12 is identified in the genome of C elegans However, this worm has no orthologs of the vitamin B12-binders that participate in human intestinal absorption and blood circulation. When the worm is treated with a vitamin B12-deficient diet for five generations (15 days), it readily develops vitamin B12 deficiency, which induces worm phenotypes (infertility, delayed growth, and shorter lifespan) that resemble the symptoms of mammalian vitamin B12 deficiency. Such phenotypes associated with vitamin B12 deficiency were readily induced in the worm.

Antivitamins for Medicinal Applications

Antivitamins represent a broad class of compounds that counteract the essential effects of vitamins. The symptoms triggered by such antinutritional factors resemble those of vitamin deficiencies, but can be successfully reversed by treating patients with the intact vitamin. Despite being undesirable for healthy organisms, the toxicities of these compounds present considerable interest for biological and medicinal purposes. Indeed, antivitamins played fundamental roles in the development of pioneering antibiotic and antiproliferative drugs, such as prontosil and aminopterin. Their development and optimisation were made possible by the study, throughout the 20th century, of the vitamins' and antivitamins' functions in metabolic processes. However, even with this thorough knowledge, commercialised antivitamin-based drugs are still nowadays limited to antagonists of vitamins B9 and K. The antivitamin field thus still needs to be explored more intensely, in view of the outstanding therapeutic success exhibited by several antivitamin-based medicines. Here we summarise historical achievements and discuss critically recent developments, opportunities and potential limitations of the antivitamin approach, with a special focus on antivitamins K, B9 and B12 .

Mechanistic Studies on the Reaction of Nitrocobalamin with Glutathione: Kinetic evidence for formation of an aquacobalamin intermediate

The essential but also toxic gaseous signaling molecule nitric oxide is scavenged by the reduced vitamin B12 complex cob(II)alamin. The resulting complex, nitroxylcobalamin (NO--Cbl(III)), is rapidly oxidized to nitrocobalamin (NO2Cbl) in the presence of oxygen; however it is unlikely that nitrocobalamin is itself stable in biological systems. Kinetic studies on the reaction between NO2Cbl and the important intracellular antioxidant, glutathione (GSH), are reported. In this study, a reaction pathway is proposed in which the β-axial ligand of NO2Cbl is first substituted by water to give aquacobalamin (H2OCbl+), which then reacts further with GSH to form glutathionylcobalamin (GSCbl). Independent measurements of the four associated rate constants k1, k-1, k2, and k-2 support the proposed mechanism. These findings provide insight into the fundamental mechanism of ligand substitution reactions of cob(III)alamins with inorganic ligands at the β-axial site.

In vivo protein tyrosine nitration in Arabidopsis thaliana

Nitration of tyrosine (Y) residues of proteins is a low abundant post-translational modification that modulates protein function or fate in animal systems. However, very little is known about the in vivo prevalence of this modification and its corresponding targets in plants. Immunoprecipitation, based on an anti-3-nitroY antibody, was performed to pull-down potential in vivo targets of Y nitration in the Arabidopsis thaliana proteome. Further shotgun liquid chromatography-mass spectrometry (LC-MS/MS) proteomic analysis of the immunoprecipitated proteins allowed the identification of 127 proteins. Around 35% of them corresponded to homologues of proteins that have been previously reported to be Y nitrated in other non-plant organisms. Some of the putative in vivo Y-nitrated proteins were further confirmed by western blot with specific antibodies. Furthermore, MALDI-TOF (matrix-assisted laser desorption ionization-time of flight) analysis of protein spots, separated by two-dimensional electrophoresis from immunoprecipitated proteins, led to the identification of seven nitrated peptides corresponding to six different proteins. However, in vivo nitration sites among putative targets could not be identified by MS/MS. Nevertheless, an MS/MS spectrum with 3-aminoY318 instead of the expected 3-nitroY was found for cytosolic glyceraldehyde-3-phosphate dehydrogenase. Reduction of nitroY to aminoY during MS-based proteomic analysis together with the in vivo low abundance of these modifications made the identification of nitration sites difficult. In turn, in vitro nitration of methionine synthase, which was also found in the shotgun proteomic screening, allowed unequivocal identification of a nitration site at Y287.

Redetermination of the X-ray structure of nitroxylcobalamin: base-on nitroxylcobalamin exhibits a remarkably long Co-N(dimethylbenzimidazole) bond distance

The X-ray structures of three new crystals of nitroxylcobalamin (NOCbl) have been determined. Unlike our earlier reported structure in which NOCbl was partially oxidized (L. Hannibal, C. A. Smith, D. W. Jacobsen and N. E. Brasch, Angew. Chem., Int. Ed. 2007, 46, 5140), the O atom of the nitroxyl ligand is located in a single position with a N=O bond distance of 1.12-1.14 Å, consistent with a double bond. The Co-N-O angle is in the 118.9-120.3 Å range. The α-axial Co-N(dimethylbenzimidazole) (Co-NB3) bond distance is a remarkable 2.32-2.35 Å in length, ~0.1 Å longer than that reported for all other cobalamin structures. The change in the Gibbs free energy for the base-on/base-off equilibrium now correlates extremely well with the Co-NB3 bond distance, as observed for other cobalamins.

NMR spectroscopy and molecular modelling studies of nitrosylcobalamin: further evidence that the deprotonated, base-off form is important for nitrosylcobalamin in solution

The structure of nitrosylcobalamin (NOCbl) in solution has been studied by NMR spectroscopy and the 1H and 13C NMR spectra have been assigned. 13C and 31P NMR chemical shifts, the UV-vis spectrum of NOCbl and the observed pKbase-off value of approximately 5.1 for NOCbl provide evidence that a significant fraction of NOCbl is present in the base-off, 5,6-dimethylbenzimidazole (DMB) deprotonated, form in solution. NOE-restrained molecular mechanics modelling of base-on NOCbl gave annealed structures with minor conformational differences in the flexible side chains and the nucleotide loop position compared with the X-ray structure. A molecular dynamics simulation at 300 K showed that DMB remains in close proximity to the alpha face of the corrin in the base-off form of NOCbl. Simulated annealing calculations produced two major conformations of base-off NOCbl. In the first, the DMB is perpendicular to the corrin and its B3 nitrogen is about 3.1 A away from and pointing directly at the metal ion; in the second the DMB is parallel to and tucked beneath the D ring of the corrin.

Commonly used tropical medicinal plants exhibt distinct in vitro antioxidant activities against hepatotoxins in rat liver

Lipid peroxidation in biological systems has been considered as one of the major mechanisms of cell injury in aerobic organisms subjected to oxidation stress. Plants, among other functions, are considered to act as free radical scavengers and as antioxidants. Iron II (Fe(2+)), sodium nitroprusside (SNP) and nitropropionic acid stimulate the production of free radicals and lipid peroxidation. In this study, four commonly used tropical medicinal plants (Kigelia africana, Calotropis procera, Hibiscus sabdariffa and Alchornea cordifolia) were studied (in vitro) for their effects on the formation of thiobarbituric acid reactive substances (TBARS) induced by different pro-oxidants (10 microM FeSO(4), 5 microM -sodium SNP and 2mM 3-nitropropionic acid) in rat liver homogenate. All the pro-oxidants significantly increased (P<0.05) the formation of TBARS, which indicates increased lipid peroxidation in the rat liver (in vitro). However, all the plant extracts statistically (P<0.05) reduced the production of TBARS in a concentration-dependent manner in all the tested pro-oxidant-induced oxidative stresses. Alchornea cordifolia appeared to offer the highest protection. The results of the present study suggest that the use of these plants in the treatment of various diseases, especially liver disease, is probably due to their ability to act as antioxidants.

Inhibition of cobalamin-dependent methionine synthase by substituted benzo-fused heterocycles

The cobalamin-dependent cytosolic enzyme, methionine synthase (EC.2.1.1.13), catalyzes the remethylation of homocysteine to methionine using 5-methyltetrahydrofolate as the methyl donor. The products of this remethylation--methionine and tetrahydrofolate--participate in the active methionine and folate pathways. Impaired methionine synthase activity has been implicated in the pathogenesis of anaemias, cancer and neurological disorders. Although the need for potent and specific inhibitors of methionine synthase has been recognized, there is a lack of such agents. In this study, we designed, synthesized and evaluated the inhibitory activity of a series of substituted benzimidazoles and small benzothiadiazoles. Kinetic analysis revealed that the benzimidazoles act as competitive inhibitors of the rat liver methionine synthase, whilst the most active benzothiadiazole (IC(50) = 80 microm) exhibited characteristics of uncompetitive inhibition. A model of the methyltetrahydrofolate-binding site of the rat liver methionine synthase was constructed; docking experiments were designed to elucidate, in greater detail, the binding mode and reveal structural requirements for the design of inhibitors of methionine synthase. Our results indicate that the potency of the tested compounds is related to a planar region of the inhibitor that can be positioned in the centre of the active site, the presence of a nitro functional group and two or three probable hydrogen-bonding interactions.

Nitric oxide inhibits mammalian methylmalonyl-CoA mutase

Methylmalonyl-CoA mutase is a key enzyme in intermediary metabolism, and children deficient in enzyme activity have severe metabolic acidosis. We found that nitric oxide (NO) inhibits methylmalonyl-CoA mutase activity in rodent cell extracts. The inhibition of enzyme activity occurred within minutes and was not prevented by thiols, suggesting that enzyme inhibition was not occurring via NO reaction with cysteine residues to form nitrosothiol groups. Enzyme inhibition was dependent on the presence of substrate, implying that NO was reacting with cobalamin(II) (Cbl(II)) and/or the deoxyadenosyl radical (.CH(2)-Ado), both of which are generated from the co-factor of the enzyme, 5'-deoxyadenosyl-cobalamin (AdoCbl), on substrate binding. Consistent with this hypothesis was the finding that high micromolar concentrations (> or =600 microm) of oxygen also inhibited enzyme activity. To study the mechanism of NO reaction with AdoCbl, we simulated the enzymatic reaction by photolyzing AdoCbl, and found that even at low NO concentrations, NO reacted with both the generated Cbl(II) and .CH(2)-Ado indicating that NO could effectively compete with the back formation of AdoCbl. Thus, NO inhibition of methylmalonyl-CoA mutase appeared to be from the reaction of NO with both AdoCbl intermediates (Cbl(II) and .CH(2)-Ado) generated during the enzymatic reaction. The inhibition of methylmalonyl-CoA mutase by NO was likely of physiological relevance because a NO donor inhibited enzyme activity in intact cells, and scavenging NO from cells or inhibiting cellular NO synthesis increased methylmalonyl-CoA mutase activity when measured subsequently in cell extracts.

Reactions of nitric oxide with vitamin B12 and its precursor, cobinamide

Despite early claims that nitric oxide does not react with cobalamin under any circumstances, it is now accepted that NO has a high affinity for cobalamin in the 2+ oxidation state [Cbl(II)]. However, it is still the consensus that NO does not react with Cbl(III). We confirmed that NO coordinates to Cbl(II) at all pH values and that Cbl(III) does not react with NO at neutral pH. At low pH, however, Cbl(III) does react with NO by way of a two-step process that also reduces Cbl(III) to Cbl(II). To account for the pH dependence, and because of its intrinsic interest, we also studied reactions of NO with cobinamide [Cbi] in the 2+ and 3+ oxidation states. Both Cbi(II) and Cbi(III) react readily with NO at all pH values. Again, Cbi(III) is reduced during the process of coordinating NO. Compared to cobalamin, cobinamide lacks the tethered 5,6-dimethylbenzamidazolyl moiety bound to the cobalt ion. It may, therefore, be considered a "base-off" form of Cbl. To explain the reaction of Cbl(III) at low pH, we infer that the base-off form of Cbl(III) exists in trace amounts that are rapidly reduced to Cbl(II), which then binds NO efficiently. Base dissociation, we postulate, is the rate-limiting step. Interestingly, Cbi(II) has 100 times greater affinity for NO than does Cbl(II), proving that there is a strong trans effect due to the tethered base in nitrosyl derivatives of both Cbl(II) and Cbl(III). The affinity of Cbi(II) for NO is so high that it is a very efficient NO trap and, consequently, may have important biomedical uses.

Effect of fish oil on LDL oxidation and plasma homocysteine concentrations in health

Oxidation of low-density lipoprotein (LDL) and hyperhomocysteinemia are believed to play a role in therogenesis. Whether n-3 polyunsaturated fatty acids increase LDL susceptibility to oxidation or influence homocysteine (Hcy) metabolism has long been a subject of controversy. In this study, we evaluated the effect of 8 weeks of dietary supplementation with 6 g/day of fish oil (FO; 3 g of n-3 fatty acids) on plasma lipoproteins, in vitro LDL peroxidation, antioxidant status, and plasma Hcy concentrations in 16 normolipidemic subjects. FO rapidly and significantly (P < .01) decreased plasma total and very low density lipoprotein triglyceride concentrations and had no effect on LDL or high-density-lipoprotein cholesterol. The mean lag time before onset of Cu(2+)-induced LDL oxidation, as well as plasma and LDL alpha-tocopherol and beta-carotene concentrations, was unchanged. However, changes in plasma aminothiol concentrations occurred during the study. Specifically, a progressive and significant increase in total Hcy plasma concentrations was observed (13.4% and 20% after 4 and 8 weeks, respectively; P < .01). Total glutathione concentrations were significantly higher after 8 weeks (P < .05). The tHcy increase was not associated with changes in plasma folate or vitamin B(12) concentrations. However, concentrations of plasma nitric oxide metabolites (NO(x) = NO(2) + NO(3)) were significantly higher than at baseline after 8 weeks of FO intake (74%; P < .01). Further, the changes in total Hcy and NO(x) plasma concentrations observed after 8 weeks of FO were found to be significantly correlated (r = .78, P < .001). With this study, we report for the first time the apparent interaction of n-3 fatty acids and nitric oxide on Hcy metabolism.

Electrochemical and spectral studies of the reactions of aquocobalamin with nitric oxide and nitrite ion

Electrochemistry and Raman spectroscopy have shown that aquocob(III)alamin (Cbl(III)) can be reduced by nitric oxide (NO) to form Cbl(II) on an electrode surface. The Cbl(II) formed in this way can bind NO to form nitrosyl-cobalamin, Cbl(II)-NO, which is reduced to form Cbl(I) at about -1.0 V vs a KCl saturated Ag/AgCl reference electrode. In addition, nitrite was found to bind both Cbl(III) and Cbl(II) and a binding constant of 3.5 x 10(2) M(-1) was measured for (NO(2)-Cbl(II))(1-). UV-vis spectrophotometry and mass spectroscopy were used to show that Cbl(I) reduces NO to form Cbl(II)-NO and N(2)O and N(2), and this reaction is involved in the cyclic voltammetry of cobalamin in the presence of excess NO where a catalytic reduction of NO occurs involving the cycling of Cbl(II)-NO/Cbl(I). This redox couple is also involved in the electrochemical catalytic reduction of nitrite. These results can be used to explain a number of physiological effects involving NO interaction in biological systems with added cobalamin or with cobalamin in the methionine synthase enzyme.

Nitric oxide inhibits methionine synthase activity in vivo and disrupts carbon flow through the folate pathway

Many of nitric oxide's biological effects are mediated via NO binding to the iron in heme-containing proteins. Cobalamin (vitamin B(12)) is structurally similar to heme and is a cofactor for methionine synthase, a key enzyme in folate metabolism. NO inhibits methionine synthase activity in vitro, but data concerning NO binding to cobalamin are controversial. We now show spectroscopically that NO reacts with all three valency states of cobalamin and that NO's inhibition of methionine synthase activity most likely involves its reaction with monovalent cobalamin. By following incorporation of the methyl moiety of [(14)C]methyltetrahydrofolic acid into protein, we show that NO inhibits methionine synthase activity in vivo, in cultured mammalian cells. The inhibition of methionine synthase activity disrupted carbon flow through the folate pathway as measured by decreased incorporation of [(14)C]formate into methionine, serine, and purine nucleotides. Homocysteine, but not cysteine, attenuated NO's inhibition of purine synthesis, providing further evidence that NO was acting through methionine synthase inhibition. NO's effect was observed both when NO donors were added to cells and when NO was produced physiologically in co-culture experiments. Treating cells with an NO synthase inhibitor increased formate incorporation into methionine, serine, and purines and methyl-tetrahydrofolate incorporation into protein. Thus, physiological concentrations of NO appear to regulate carbon flow through the folate pathway.

Homocyst(e)ine impairs endocardial endothelial function

Homocyst(e)ine injured vascular endothelium and modulated endothelial-dependent vascular function. Endothelium plays an analogous role in both the vessel and the endocardium. Therefore, we hypothesized that homocyst(e)ine modulated endocardial endothelium (EE) dependent cardiac function. The ex vivo cardiac rings from normal male Wistar-Kyoto rats were prepared. The contractile responses of left and right ventricular rings were measured in an isometric myobath, using different concentrations of CaCl2. The response was higher in the left ventricle than right ventricle and was elevated in endocardium without endothelium. The half effective concentration (EC50) and maximum tension generated by homocyst(e)ine were 106 and 5-fold lower than endothelin (ET) and angiotensin II (AII), respectively. However, in endothelial-denuded endocardium, homocyst(e)ine response was significantly increased (p < 0.005, compared with intact endothelium) and equal to the response to ET and AII. To determine the physiological significance of ET, AII, homocyst(e)ine, and endothelial nitric oxide in EE function, cardiac rings were pretreated with AII (10-10 M) or ET (10-13 M) and then treated with homocyst(e)ine (10-8 M). Results suggested that at these concentrations AII, ET, or homocyst(e)ine alone had no effect on cardiac contraction. However, in the presence of 10-10 M AII or 10-13 M ET, the cardiac contraction to homocyst(e)ine (10-8 M) was significantly enhanced (p < 0.01, compared with without pretreatment) and further increased in the endocardium without endothelium. The pretreatment of cardiac ring with the inhibitor of nitric oxide, Nω-nitro-L-arginine methyl ester (L-NAME), increased contractile response to homocyst(e)ine. These results suggested that homocyst(e)ine impaired EE-dependent cardiac function and acted synergistically with AII and ET in enhancing the cardiac contraction.Key words: endocardial remodeling, homocyst(e)ine, contraction, endothelin, angiotensin, endothelial-derived relaxing factor (EDRF), Nω-nitro-L-arginine methyl ester (L-NAME), endothelial dysfunction, ex vivo cardiac function, heart failure.

Effect of hydrazine upon vitamin B12-dependent methionine synthase activity and the sulphur amino acid pathway in isolated rat hepatocytes

The effect of the industrial chemical, hydrazine (4-12 mM), on methionine synthase (EC 2.1.1.13) activity and levels of the sulphur amino acids homocysteine, cysteine, and taurine as well as GSH were investigated in vitro in isolated rat hepatocyte suspensions and monolayers in order to explain some of the adverse in vivo effects of hydrazine. None of the concentrations of hydrazine were overtly cytotoxic in hepatocyte suspensions (measured as lactate dehydrogenase [LDH] leakage) after 3 hr. However, after 24 hr in culture cells treated with 12 mM, hydrazine showed a significant increase in LDH leakage. Methionine synthase activity was reduced by hydrazine (8 and 12 mM) in suspensions (by 45 and 55%, after 3 hr) and monolayers (12 mM; 65-80% after 24 hr). This was not due to nitric oxide production and the inhibitor of nitric oxide synthase, Nomega-nitro-L-arginine, failed to protect against the hydrazine-induced loss of ATP and GSH and the reduction in urea synthesis at 24 hr. Homocysteine export was increased by 6 mM hydrazine, and total taurine content of treated cells was increased by 12 mM hydrazine. Thus, hydrazine was found to have several important and possibly deleterious effects on some parts of the sulphur amino acid pathway.

Taurine: protective properties against ethanol-induced hepatic steatosis and lipid peroxidation during chronic ethanol consumption in rats

Alcohol was administered chronically to female Sprague Dawley rats in a nutritionally adequate totally liquid diet for 28 days. This resulted in hepatic steatosis and lipid peroxidation. Taurine, when co-administered with alcohol, reduced the hepatic steatosis and completely prevented lipid peroxidation. The protective properties of taurine in preventing fatty liver were also demonstrated histologically. Although alcohol was found not to affect the urinary excretion of taurine (a non-invasive marker of liver damage), levels of serum and liver taurine were markedly raised in animals receiving alcohol + taurine compared to animals given taurine alone. The ethanol-inducible form of cytochrome P-450 (CYP2E1) was significantly induced by alcohol; the activity was significantly lower than controls and barely detectable in animals fed the liquid alcohol diet containing taurine. In addition, alcohol significantly increased homocysteine excretion into urine throughout the 28 day period of ethanol administration; however, taurine did not prevent this increase. There was evidence of slight cholestasis in animals treated with alcohol and alcohol + taurine, as indicated by raised serum bile acids and alkaline phosphatase (ALP). The protective effects of taurine were attributed to the potential of bile acids, especially taurine conjugated bile acids (taurocholic acid) to inhibit the activity of some microsomal enzymes (CYP2E1). These in vivo findings demonstrate for the first time that hepatic steatosis and lipid peroxidation, occurring as a result of chronic alcohol consumption, can be ameliorated by administration of taurine to rats.

The effect of ethanol and its metabolites upon methionine synthase activity in vitro

The association of alcoholism with macrocytic anaemia has lead to investigation of the role of cobalamin-dependent methionine synthase in mediating alcohol toxicity. Several studies have found that long-term ingestion of large quantities of ethanol causes inhibition of liver methionine synthase activity in vivo: however, ethanol has not been found to inhibit the enzyme directly. The effect of ethanol and its breakdown products, acetate and acetaldehyde, on highly purified rat liver methionine synthase was tested in vitro. Enzyme activity was not inhibited by ethanol or acetate. Acetaldehyde was found to inhibit methionine synthase activity, with an apparent IC50 of 2 mM. The reported inhibition by acetaldehyde was found to become irreversible over time. Acetaldehyde-induced inhibition of liver methionine synthase activity is thus proposed as the most likely explanation of the reported in vivo effect of ethanol upon methionine synthase.