Remethylation and transsulfuration of methionine in cirrhosis: studies with L-[H3-methyl-1-C]methionine

Disturbances of the methionine cycle may result in liver injury. Patients with alcohol-induced liver disease often exhibit hypermethioninemia and a delayed clearance (CL) of methionine, but the extent to which transsulfuration and remethylation pathways of the cyclic methionine metabolism are affected is unknown. Methionine turnover was determined in 7 healthy volunteers and 6 patients with alcohol-induced cirrhosis after oral administration of 2 mg/kg [(2)H(3)-methyl-1-(13)C]methionine, which permitted us to follow transsulfuration by its decarboxylation to (13)CO(2) and remethylation by replacement of the labeled methyl group by an unlabeled one. Basal plasma concentrations of endogenous methionine (50 +/- 5 vs. 25 +/- 2 micromol/L, mean +/- SEM, P <.001) were significantly higher in patients with cirrhosis and its CL was significantly decreased (774 +/- 103 vs. 2,050 +/- 141 mL/min, P <.001). Methionine turnover amounted to 42 +/- 4 vs. 27 +/- 3 micromol/kg/h (P <.05) in controls and patients with cirrhosis, respectively. The fraction of administered methionine undergoing remethylation was lower in patients with cirrhosis (7.6 +/- 1.5 vs. 14.1 +/- 1.1%, P <.005). However, because of the larger pool of circulating methionine, the total flux of methionine through the remethylation pathway was similar in both groups. A significantly lower fraction of the administered dose appeared in the form of (13)CO(2) in breath in patients with cirrhosis (2.2 +/- 0.4 vs. 11.0 +/- 0.8%, P <.001). In conclusion, the data indicate that the liver with cirrhosis compensates for a decreased activity of remethylating enzymes by operating at higher concentrations of methionine. In contrast, transsulfuration is impaired in patients with alcohol-induced cirrhosis such that an assessment of transsulfuration by a simple breath test may provide a clinically useful estimate of hepatic function.     

Effects of polymorphisms of methionine synthase and methionine synthase reductase on total plasma homocysteine in the NHLBI Family Heart Study

The metabolism of homocysteine requires contributions of several enzymes and vitamin cofactors. Earlier studies identified a common polymorphism of methylenetetrahydrofolate reductase that was associated with mild hyperhomocysteinemia. Common variants of two other enzymes involved in homocysteine metabolism, methionine synthase and methionine synthase reductase, have also been identified. Methionine synthase catalyzes the remethylation of homocysteine to form methionine and methionine synthase reductase is required for the reductive activation of the cobalamin-dependent methionine synthase. The methionine synthase gene (MTR) mutation is an A to G substitution, 2756A-->G, which converts an aspartate to a glycine codon. The methionine synthase reductase gene (MTRR) mutation is an A to G substitution, 66A-->G, that converts an isoleucine to a methionine residue. To determine if these polymorphisms were associated with mild hyperhomocysteinemia, we investigated subjects from two of the NHLBI Family Heart Study field centers, Framingham and Utah. Total plasma homocysteine concentrations were determined after an overnight fast and after a 4-h methionine load test. MTR and MTRR genotype data were available for 677 and 562 subjects, respectively. The geometric mean fasting homocysteine was unrelated to the MTR or MTRR genotype categories (AA, AG, GG). After a methionine load, a weak positive association was observed between change in homocysteine after a methionine load and the number of mutant MTR alleles (P-trend=0.04), but this association was not statistically significant according to the overall F-statistic (P=0.12). There was no significant interaction between MTR and MTRR genotype or between these genotypes and any of the vitamins with respect to homocysteine concentrations. This study provides no evidence that these common MTR and MTRR mutations are associated with alterations in plasma homocysteine.     

Methionine sulfoxide reductase A is a stereospecific methionine oxidase

Methionine sulfoxide reductase A (MsrA) catalyzes the reduction of methionine sulfoxide to methionine and is specific for the S epimer of methionine sulfoxide. The enzyme participates in defense against oxidative stresses by reducing methionine sulfoxide residues in proteins back to methionine. Because oxidation of methionine residues is reversible, this covalent modification could also function as a mechanism for cellular regulation, provided there exists a stereospecific methionine oxidase. We show that MsrA itself is a stereospecific methionine oxidase, producing S-methionine sulfoxide as its product. MsrA catalyzes its own autooxidation as well as oxidation of free methionine and methionine residues in peptides and proteins. When functioning as a reductase, MsrA fully reverses the oxidations which it catalyzes.     

Ethanol feeding of micropigs alters methionine metabolism and increases hepatocellular apoptosis and proliferation

Chronic alcoholism is associated with increased cancer risk that may be related to ethanol-induced alterations in methionine and deoxynucleotide metabolism. These metabolic relationships were studied in micropigs fed diets for 12 months that contained 40% ethanol or cornstarch control with adequate folate. Ethanol feeding altered methionine metabolism without changing mean terminal liver folate levels. After initial equilibration to diet, ethanol feeding significantly increased monthly serum homocysteine levels while reducing serum methionine levels over the time course of the experiment. After 12 months, hepatic methionine synthase activity and the ratio of S-adenosylmethionine (SAM) to S-adenosylhomocysteine (SAH) were significantly reduced in ethanol-fed animals, whereas the ratio of liver deoxyuridine triphosphate (dUTP) to deoxythymidine triphosphate (dTTP) was increased and correlated inversely with methionine synthase activity. These findings were associated with increased frequency of hepatocytes with apoptotic bodies and positivity for proliferating cell nuclear antigen (PCNA) in livers from ethanol-fed minipigs. These studies suggest that chronic ethanol feeding perturbs methionine metabolism by impairment of methionine synthase activity, resulting in deoxynucleoside triphosphate (dNTP) imbalance, increased apoptosis, and regenerative proliferation. These biochemical alterations may provide a promoting environment for carcinogenesis during long-term ethanol exposure. (Hepatology 1996 Mar;23(3):497-505)     

High in vivo rates of methionine biosynthesis in transformed human and malignant rat cells auxotrophic for methionine.

Unlike normal cells, malignant rat and two simian virus 40-transformed human cell lines can neither grow nor survive in B12-and folate-supplemented media in which methionine is replaced by homocysteine. Yet three lines of evidence indicate that the malignant and transformed cells synthesize large amounts of methionine endogenously through the reaction catalyzed by 5-methyltetrahydropteroyl-L-glutamate; L-homocysteine S-methyltransferase (EC 2.1.1.13). (1) The activities of this methyltransferase were comparable in extracts of malignant and normal cells. (2) The uptake of radioactive label from [5-14C]methyltetrahydropteroyl-L-glutamic acid (5-Me-H4PteGlu) was at least as great in the malignant cells as in the normals and was nearly totally dependent on the addition of homocysteine, the methyl acceptor; furthermore, 59-84% of the label incorporated by cells was recovered as methionine.     

The atherogenic effect of excess methionine intake

Methionine is the precursor of homocysteine, a sulfur amino acid intermediate in the methylation and transsulfuration pathways. Elevated plasma homocysteine (hyperhomocysteinemia) is associated with occlusive vascular disease. Whether homocysteine per se or a coincident metabolic abnormality causes vascular disease is still an open question. Animals with genetic hyperhomocysteinemia have so far not displayed atheromatous lesions. However, when methionine-rich diets are used to induce hyperhomocysteinemia, vascular pathology is often observed. Such studies have not distinguished the effects of excess dietary methionine from those of hyperhomocysteinemia. We fed apolipoprotein E-deficient mice with experimental diets designed to achieve three conditions: (i) high methionine intake with normal blood homocysteine; (ii) high methionine intake with B vitamin deficiency and hyperhomocysteinemia; and (iii) normal methionine intake with B vitamin deficiency and hyperhomocysteinemia. Mice fed methionine-rich diets had significant atheromatous pathology in the aortic arch even with normal plasma homocysteine levels, whereas mice fed B vitamin-deficient diets developed severe hyperhomocysteinemia without any increase in vascular pathology. Our findings suggest that moderate increases in methionine intake are atherogenic in susceptible mice. Although homocysteine may contribute to the effect of methionine, high plasma homocysteine was not independently atherogenic in this model. Some product of excess methionine metabolism rather than high plasma homocysteine per se may underlie the association of homocysteine with vascular disease.     

Influences of supplementary dietary tungsten on methionine metabolism in rabbits fed a low-cholesterol plus methionine diet

Hyperhomocysteinemia results from an impaired methionine metabolism. Sulfite oxidase, which is an important enzyme in methionine metabolism, contains molybdenum. In contrast, tungsten has a molybdenum-antagonistic effect. Thus, we hypothesized that dietary tungsten may decrease plasma homocysteine levels and influence methionine metabolism. Male New Zealand White rabbits (n=15) were fed a low-cholesterol basal diet and then placed on three different diets: 0.1% cholesterol (Chol), Chol plus 1% methionine (Met), and Chol plus Met plus 0.1% tungsten (W). The animals received these diets for 20 weeks. Biochemical tests of blood and urine were performed. Plasma homocysteine levels were significantly lower in the Chol+Met+W group than in the Chol+Met group. Plasma levels of total cholesterol, triglyceride, lipid peroxide, and urinary 24-h taurine concentrations were higher in the Chol + Met + W group than in the Chol + Met group. In comparison, concentrations of 2, 3-diphosphoglycerate (2, 3-DPG), reduced glutathione (GSH) in erythrocytes, and urinary 24-h SO4(2) were lower in the Chol+Met+W group than in the Chol+Met group. From these results, tungsten could be expected to exhibit an antiatherogenic effect. Conversely, it may have effects on atherogenic factors. Thus, tungsten may play a number of roles in the methionine metabolism.     

Distinct effects of folate and choline deficiency on plasma kinetics of methionine and homocysteine in rats

Both folate and betaine, a choline metabolite, play essential roles in the remethylation of homocysteine to methionine. We have studied the effects of folate and choline deficiency on the plasma kinetics of methionine, especially remethylation of homocysteine to methionine, by means of stable isotope methodology. After a bolus intravenous administration of [(2)H(7)]methionine (5 mg/kg body weight) into the rats fed with folate-, choline-, folate + choline-deficient or control diets, the plasma concentrations of [(2)H(7)]methionine, demethylated [(2)H(4)]homocysteine, and remethylated [(2)H(4)]methionine were determined simultaneously with endogenous methionine and homocysteine by gas chromatography-mass spectrometry-selected ion monitoring. The total plasma clearance of [(2)H(7)]methionine was not significantly different among groups, suggesting that the formation of [(2)H(4)]homocysteine from [(2)H(7)]methionine was not influenced by deficiencies of folate and choline. The area under concentration-time curve of [(2)H(4)]homocysteine significantly increased in the folate- and folate + choline-deficient group as compared with the control, but not in the choline-deficient group. The time profile of plasma concentrations of [(2)H(4)]methionine in the folate-deficient group was the same as the control group, whereas the appearance of [(2)H(4)]methionine in plasma was delayed in the choline- and folate + choline-deficient group. These results suggested plasma levels of remethylated methionine were influenced by choline deficiency rather than folate deficiency.     

Altered methionine metabolism in long living Ames dwarf mice

Ames dwarf mice (df/df) are deficient in growth hormone, prolactin, and thyroid-stimulating hormone and live significantly longer than their normal siblings. In the current study, we found that the hormone deficiencies affect methionine metabolism. We previously reported that the dwarf mice exhibit enzyme activities and levels that combat oxidative stress more efficiently than those of normal mice. Moreover, methionine or metabolites of methionine are involved in antioxidative processes. Thus, we performed an experiment that compared various parameters of methionine metabolism between 18-month old male dwarf (N=6) and wild type (N=5) mice. The specific activity of liver methionine adenosyltransferase (MAT) was significantly elevated (205%, p<0.0001) in the dwarf mice, as were cystathionine synthase (50%, p<0.01), cystathionase (83%, p<0.001), and glycine N-methyltransferase (GNMT, 91%, p<0.001) activities. Even though the activities of MAT and GNMT were elevated, the concentration of liver S-adenosylmethionine was decreased (24%, p<0.001) and S-adenosylhomocysteine increased (113%, p<0.001) in the dwarf mice. These data indicate that dwarf mice, compared to wild type mice, have a markedly different metabolism of methionine. Altered methionine metabolism may partially explain earlier reports indicating less oxidative damage to proteins in dwarf mice. Taken together, the data suggest that methionine metabolism may play a role in oxidative defense in the dwarf mouse and should be studied as a potential mechanism of extended lifespan.     

Altered methionine metabolism in streptozotocin-diabetic rats

Summary We examined the origin of hypermethioninaemia in streptozotocin-diabetic rats. In rats administered streptozotocin over a range from 55 to 75 mg/kg, the dose of drug injected correlated directly with the plasma methionine concentration and inversely with the plasma insulin level. Although insulin administration prevented hypermethioninaemia in streptozotocin-diabetic rats, discontinuing insulin treatment resulted in a time-dependent increase in the plasma methionine level. Plasma methionine concentration was, however, normal in insulin-deprived BB Wistar rats despite severe hyperglycaemia. Thus, although insulin deficiency may be a contributing factor, it does not cause hypermethioninaemia independent of other drug-related effects. Administering a loading dose of methionine (100 mg/kg) indicated that streptozotocin-diabetic rats have a reduced metabolic capacity. Since dietary intake is the primary source of methionine, it is likely that hyperphagia combined with limited disposal produces hypermethioninaemia. Methionine is the most toxic amino-acid; therefore, metabolic studies using the streptozotocin model of insulin deficiency must be interpreted with caution.     

Enzymatic reduction of protein-bound methionine sulfoxide.

An enzyme that catalyzes the reduction of methionine sulfoxide residues in ribosomal protein L12 has been partially purified from Escherichia coli extracts. Methionine sulfoxide present in oxidize [Met]enkephalin is also reduced by the purified enzyme. The enzyme is different from a previously reported E. coli enzyme that catalyzes the reduction of methionine sulfoxide to methionine [Ejiri, S. I., Weissbach, H. & Brot, N. (1980) Anal. Biochem. 102, 393--398]. Extracts of rat tissues, Euglena gracilis, Tetrahymena pyriformis, HeLa cells, and spinach also can catalyze the reduction of methionine sulfoxide residues in protein.     

The effects of oral methionine and homocysteine on endothelial function

BACKGROUND—Raised homocysteine is a risk factor for vascular disease. Homocysteine is formed from methionine, and dietary manipulation of homocysteine in primates and humans with oral methionine is associated with endothelial dysfunction. A cause-effect relation has not been clearly established.
AIM—To study the effect of oral methionine and then oral homocysteine on endothelial function.
METHODS—22 healthy adults were recruited for two randomised crossover studies, each containing 11 subjects. Endothelial function was determined by measuring forearm blood flow in response to intra-arterial infusion of acetylcholine (endothelium dependent) and sodium nitroprusside (endothelium independent). Subjects received methionine or placebo (study 1), or homocysteine or placebo (study 2). Methionine and homocysteine were determined at baseline and t = 4 hours. Endothelial function was determined at four hours. The responses to the vasoactive substances are expressed as the area under the curve of change in forearm blood flow from baseline.
RESULTS—Study 1: plasma methionine and homocysteine concentrations increased significantly versus placebo. The increases were associated with a reduction of endothelium dependent responses (mean (95% confidence interval), arbitrary units), from 48.8 (95% CI 36.4 to 61.2) to 29.9 (95% CI 18.0 to 41.1), p < 0.04; endothelium independent responses were unchanged. Study 2: homocysteine concentration increased significantly while methionine remained unchanged. Endothelium dependent responses were reduced from 34.6 (95% CI 20.6 to 48.6) to 22.8 (95% CI 12.0 to 33.6), p < 0.03.
CONCLUSIONS—Homocysteine and not methionine is responsible for the changes in endothelial function. This supports the hypothesis that homocysteine promotes atherosclerosis by inducing endothelial dysfunction.


Keywords: homocysteine; methionine; endothelial function; plethysmography     

A genetic model of methionine restriction extends Drosophila health- and lifespan

Loss of metabolic homeostasis is a hallmark of aging and is characterized by dramatic metabolic reprogramming. To analyze how the fate of labeled methionine is altered during aging, we applied ¹³C5-Methionine labeling to Drosophila and demonstrated significant changes in the activity of different branches of the methionine metabolism as flies age. We further tested whether targeted degradation of methionine metabolism components would “reset” methionine metabolism flux and extend the fly lifespan. Specifically, we created transgenic flies with inducible expression of Methioninase, a bacterial enzyme capable of degrading methionine and revealed methionine requirements for normal maintenance of lifespan. We also demonstrated that microbiota-derived methionine is an alternative and important source in addition to food-derived methionine. In this genetic model of methionine restriction (MetR), we also demonstrate that either whole-body or tissue-specific Methioninase expression can dramatically extend Drosophila health- and lifespan and exerts physiological effects associated with MetR. Interestingly, while previous dietary MetR extended lifespan in flies only in low amino acid conditions, MetR from Methioninase expression extends lifespan independently of amino acid levels in the food. Finally, because impairment of the methionine metabolism has been previously associated with the development of Alzheimer’s disease, we compared methionine metabolism reprogramming between aging flies and a Drosophila model relevant to Alzheimer’s disease, and found that overexpression of human Tau caused methionine metabolism flux reprogramming similar to the changes found in aged flies. Altogether, our study highlights Methioninase as a potential agent for health- and lifespan extension.

Aging is the primary risk factor for many major human pathologies (1). Age-dependent metabolic reprogramming has been noted in different organisms (2), including worms (3), mice (4), and humans (5–7). Moreover, we have previously demonstrated that metabolism in general, and methionine metabolism in particular, is perturbed during aging in Drosophila (8). In addition, methionine metabolism is altered in the tissues of several long-lived species, such as naked mole rats (9), in long-lived mutants, such as flies selected for delayed reproductive senescence (8, 10), and in long-lived Ames mice (11).Although alterations in levels of specific metabolites suggest that the activity of the methionine metabolism pathway is affected, it is difficult to determine if the flux via methionine metabolism is up- or down-regulated. For example, deficiencies of enzymes involved in methionine metabolism (MAT, CBS, GNMT, AHCY) lead to methionine and homocysteine elevation (hypermethioninemias and hyperhomocysteinemias), and elevated levels of methionine and homocysteine reflect a disruption of flux in methionine metabolism (12). Additionally, metabolite changes do not explain how the flux is reprogrammed between different branches of methionine metabolism, as it is possible that the absolute levels of metabolites do not change if metabolites get processed via different routes.Impairment of methionine metabolism flux results in the accumulation of detrimental metabolites belonging to the methionine cycle, such as S-adenosylhomocysteine (SAH) and homocysteine, and leads to different pathological manifestations (13). Lifespan can be extended by up-regulating the clearance of these metabolites via activation of methionine metabolism flux. In fact, methionine restriction (MetR) extends lifespan in yeast, flies, rodents, and human diploid fibroblasts (14–17) and exerts beneficial effects on metabolic health and inflammatory responses (18–20). Moreover, lifespan in worms and flies can be extended through manipulation of different enzymes, either belonging to the methionine metabolism pathway or those that affect the levels of methionine metabolism metabolites. Some of these manipulations include the overexpression of Cbs (21) and Gnmt (22, 23), and the down-regulation of SamS (24) and AhcyL1/AhcyL2 (8). In addition, manipulation of methionine metabolism in just one tissue is sufficient for lifespan extension (8, 21, 22).Restoring age-reprogrammed activity of methionine metabolism can be an attractive option for the extension of health- and lifespan. However, studies relating to MetR have the following challenges: 1) MetR does not decrease levels of methionine equally across different organs; 2) it is impossible to study tissue-specific effects of MetR via manipulations of methionine levels in the food; and 3) the activation of methionine metabolism enzymes is difficult to achieve due to a lack of small-molecule activators. To solve these problems, we created transgenic flies carrying the enzyme Methioninase, which allows for the rapid, inducible, and tissue-specific degradation of methionine.l-Methionine α-deamino-γ-mercaptomethane-lyase (Methioninase) is a bacterial enzyme that is capable of degrading methionine to ammonia, α-ketobutyrate, and methanthiol. Because methionine dependency was attributed to various cancers, recombinant Methioninase (rMetase) has been tested in various cancer models in vitro and in vivo (25, 26). Methioninase has also entered several clinical trials in humans (26–28). Based on this, Methioninase may provide an alternative option to dietary MetR, since it can be expressed in a tissue-specific manner, and its recombinant form can be used in humans.To understand how methionine metabolism is reprogrammed with age or neurodegeneration, we supplemented flies with a labeled ¹³C5-methionine tracer and estimated its fate between different branches of methionine metabolism. We further created transgenic flies with inducible expression of Methioninase (genetic MetR) and performed metabolomics profiling to demonstrate that the effect was similar to the effects of MetR caused by the depletion of methionine from fly food (dietary MetR). It has been previously shown that dietary MetR extends the lifespan in flies only in low amino acid conditions (15). We demonstrate that either whole-body or tissue-specific expression of Methioninase (genetic MetR) can extend Drosophila lifespan without lowering levels of amino acids in the food. Altogether, our studies offer a strategy for restoring age-dependent defects related to impaired methionine metabolism that has strong potential for lifespan extension and treatment of neurodegenerative diseases in humans.     

A new cytokine： the possible effect pathway of methionine enkephalin

AIM: To investigate experimentally the effects of methionine enkephalin on signal transduction of mouse myeloma NS-1 cells. METHODS: The antigen determinate of delta opioid receptor was designed in this lab and the polypeptide fragment of antigen determinate with 12 amino acids residues was synthesized. Monoclonal antibody against this peptide fragment was prepared. Proliferation of Mouse NS-1 cells treated with methionine enkephalin of 1 x 10(-6) mol x L(-1) was observed. The activities of protein kinase A (PKA) and protein kinase C (PKC) were measured and thereby the mechanism of effect of methionine enkephalin was postulated. RESULTS: The results demonstrated that methionine enkephalin could enhance the proliferation of NS-1 cells and the effect of methionine enkephalin could be particularly blocked by monoclonal antibody. The activity of PKA was increased in both cytosol and cell membrane. With reference to PKC, the intracellular activity of PKC in NS-1 cells was elevated at 1 x 10(-7) mol x L(-1) and then declined gradually as the concentration of methionine enkephalin was raised. The effects of methionine enkephalin might be reversed by both naloxone and monoclonal antibody. CONCLUSION: Coupled with the findings, it indicates that the signal transduction systems via PKA and PKC are involved in the effects of methionine enkephalin by binding with the traditional opioid receptors,and therefore resulting in different biological effects.     

Impairment of methionine sulfoxide reductase during UV irradiation and photoaging

During chronic UV irradiation, which is part of the skin aging process, proteins are damaged by reactive oxygen species resulting in the accumulation of oxidatively modified protein. UV irradiation generates irreversible oxidation of the side chains of certain amino acids resulting in the formation of carbonyl groups on proteins. Nevertheless, certain amino acid oxidation products such as methionine sulfoxide can be reversed back to their reduced form within proteins by specific repair enzymes, the methionine sulfoxide reductases A and B. Using quantitative confocal microscopy, the amount of methionine sulfoxide reductase A was found significantly lower in sun-exposed skin as compared to sun-protected skin. Due to the importance of the methionine sulfoxide reductase system in the maintenance of protein structure and function during aging and conditions of oxidative stress, the fate of this system was investigated after UVA irradiation of human normal keratinocytes. When keratinocytes are exposed to 15 J/cm(2) UVA, methionine sulfoxide reductase activity and content are decreased, indicating that the methionine sulfoxide reductase system is a sensitive target for UV-induced inactivation.     

One- and two-electron oxidations of methionine by peroxynitrite.

Peroxynitrite is stable, but its acid, HOONO, either rearranges to form nitrate or oxidizes nearby biomolecules. We report here the reactions of HOONO with methionine and the methionine analog 2-keto-4-thiomethylbutanoic acid (KTBA). These oxidations proceed by two competing mechanisms. The first yields the sulfoxide; the second-order rate constants, k2, for this process for methionine and KTBA are 181 +/- 8 and 277 +/- 11 M-1.s-1, respectively, at pH 7.4 and 25 degrees C. In the second mechanism, methionine or KTBA undergoes a one-electron oxidation that ultimately gives ethylene. We propose that the one-electron oxidant is an activated form of peroxynitrous acid, HOONO*, that is formed in a steady state mechanism. The ratios of the second-order rate constants for the ethylene-producing reaction (k*2) and the first-order rate constant to produce nitric acid (kN) for methionine and KTBA, k*2/kN, are 1250 +/- 290 and 6230 +/- 1390 M-1, respectively. Both ceric and peroxydisulfate ions also oxidize KTBA to ethylene, confirming a one-electron transfer mechanism. The yields of neither MetSO nor ethylene are affected by several hydroxyl radical scavengers, suggesting that a unimolecular homolysis of HOONO to HO. and .NO2 is not involved in these reactions. HOONO* gives hydroxyl radical-like products from various substrates but displays more selectivity than does the hydroxyl radical; thus, HOONO* is incompletely trapped by typical HO. scavengers. However, a mechanism involving dissociation of HOONO* to caged radicals cannot be ruled out at this time.     

Abnormal hepatic methionine and glutathione metabolism in patients with alcoholic hepatitis

BACKGROUND: Abnormal methionine metabolism occurs in animals fed ethanol and in end-stage cirrhotic patients. Expected consequences of these abnormalities include reduced hepatic S-adenosylmethionine and glutathione (GSH) levels, impaired transmethylation, and reduced homocysteine catabolism, resulting in the often-observed hyperhomocystinemia in cirrhotic patients. These parameters have not been examined simultaneously in patients with less advanced alcoholic liver disease. METHODS: Six patients hospitalized for alcoholic hepatitis were studied. Plasma was analyzed for homocysteine, methionine, and GSH levels. Liver biopsies diagnosed acute alcoholic hepatitis and underlying fibrosis. Liver specimens were processed for messenger RNA (mRNA) levels and various metabolites and were compared with those of six normal controls. RESULTS: Three patients had cirrhosis, and three had only portal fibrosis. Plasma levels of homocysteine and methionine were increased in two of the three patients with cirrhosis but not in the patients with fibrosis. All patients had markedly lower plasma GSH levels (mean +/- SD: 0.27 +/- 0.19 microM, which is at least 10-fold lower than the normal range). Hepatic S-adenosylmethionine levels were reduced by 50%, whereas methionine, GSH, and cysteine levels were reduced by 70-80%. The mRNA levels of most enzymes involved in methionine metabolism and GSH synthesis were decreased, whereas albumin expression was unchanged. Despite the well known induction of cytochrome P450 2E1 in chronic alcoholics, its mRNA levels were nearly 70% lower in these patients. CONCLUSIONS: In alcoholic hepatitis, abnormal hepatic gene expression in methionine and GSH metabolism occurs and often contributes to decreased hepatic methionine, S-adenosylmethionine, cysteine, and GSH levels. It may be important to replenish these thiols in patients hospitalized with alcoholic hepatitis.     

Defective methionine metabolism in cirrhosis: relation to severity of liver disease.

A block in the transsulfuration pathway has previously been suggested in cirrhosis on the basis of increased fasting methionine concentrations, decreased methionine elimination and low levels of methionine end products. To date, methionine elimination has never been studied under controlled steady-state conditions, and the relation of the severity of liver disease to impaired methionine metabolism has not been clarified. We measured methionine plasma clearance in 6 control subjects and in 12 patients with cirrhosis during steady-state conditions obtained by a primed, continuous methionine infusion. In the presence of high-normal fasting methionine concentrations (range = 14 to 69 mumol.L-1 in controls and 26 to 151 mumol.L-1 in cirrhotic patients), methionine plasma clearance was reduced in cirrhotic patients (2.25 +/- S.D. 0.43 ml.sec-1 vs. 2.86 +/- S.D. 0.43 ml.sec-1 in controls; p less than 0.05), whereas methionine half-life was increased (282 +/- 90 min vs. 187 +/- 25 min in controls; p less than 0.05). Fasting methionine significantly correlated with methionine clearance. The infused methionine was not degraded to urea to any significant extent in cirrhotic patients, whereas a threefold increase in urinary urea nitrogen excretion rate was observed in controls. Similarly, taurine concentrations significantly increased both in plasma and in the urine in controls but not in cirrhotic patients. In cirrhotic patients methionine plasma clearance significantly correlated with galactose elimination capacity (r = 0.818) and with the Child-Pugh score (rs = -0.795). The study supports a major role of impaired liver cell function in the reduced metabolism of methionine and decreased formation of methionine end products that occur in cirrhosis.(ABSTRACT TRUNCATED AT 250 WORDS)     

甲硫氨酸诱发高同型半胱氨酸血症对内皮细胞的损伤作用

目的　观察甲硫氨酸诱发高同型半胱氨酸血症对内皮细胞的损伤作用。方法　将 18只新西兰白兔 ,随机分为甲硫氨酸组 (9只 )和对照组 (9只 )。分别喂予添加 3%甲硫氨酸的饲料和普通颗粒饲料 ,第 16周末 ,取血和留取主动脉进行指标测定。结果　甲硫氨酸组血清甲硫氨酸和同型半胱氨酸浓度显著高于对照组 (P <0 .0 1) ,血浆内皮素和血管性血友病因子浓度显著高于对照组 (P <0 .0 1) ,而血浆一氧化氮含量和动脉一氧化氮合酶活性显著低于对照组 (P <0 .0 1) ,主动脉尚未剥脱的和明显损伤的内皮细胞血管性血友病因子表达均明显少于对照组。结论　短期喂食甲硫氨酸诱发高同型半胱氨酸血症 ,即使尚未引起明显的内膜增厚和脂质沉着 ,对血管内皮细胞的功能和结构也有显著的损伤作用。     

Selective oxidation of cysteine and methionine in normal and senile cataractous lenses.

The oxidation state of methionine and cysteine in normal and cataractous lenses is reported. In young lenses no oxidation was detected in any protein fraction examined. Only the intrinsic membrane fraction and membrane-related components showed evidence of oxidation in old (60-65 years of age) normal lenses. However, in a similar age group, with the development of cataract, progressive, dramatic changes were observed. With severe cataracts, 60% or more of the methionine in membrane-associated components was found in the methionine sulfoxide form, and methionine sulfone was observed in one case. Most of the cysteine was found oxidized to either the disulfide form or putative cysteic acid. Mixed disulfides with glutathione were observed. Oxidative changes in soluble components as illustrated by alpha-crystallin occurred more gradually. The data clearly support the viewpoint that extensive oxidation of lens proteins occurs with cataract and that it begins at the lens fiber membrane.