Methionine metabolism in mammals

Redox status and protein binding of plasma homocysteine and other aminothiols in patients with early-onset peripheral vascular disease. Homocysteine and peripheral vascular disease

Elevated total homocysteine (Hcy) in plasma is an independent risk factor for early-onset vascular disease in the coronary, cerebral, and peripheral arteries. Different forms of Hcy, and their relation to other aminothiols in plasma, have not been investigated in patients with vascular disease. We therefore investigated 65 patients (35 men and 30 women) operated on for peripheral arterial disease at < 50 years of age and 65 age- and sex-matched control subjects. Total, reduced, oxidized, and protein-bound Hcy, cysteine (Cys), and cysteinylglycine (CysGly) were measured 0 to 11 years (mean, 6 years) postoperatively, in the fasting state, and after a standard methionine loading dose that caused a transient increase in reduced, oxidized, and protein-bound Hcy. All forms of Hcy and Cys, except reduced Cys, were higher in fasting patients than fasting control subjects. A similar difference between the groups was observed after methionine loading. The levels of most Hcy forms both during fasting and after methionine loading were related to smoking, but multivariate analysis showed that the difference between patients and control subjects could not be explained by smoking alone. Notably, reduced Cys and the reduced/total ratio for Cys were significantly higher in control subjects than in patients, both during fasting and after methionine loading. In both groups, the redox status and protein binding of the various aminothiols in plasma were interactive, as demonstrated by positive correlations between their reduced/total ratios and by a decrease in protein-bound Cys when protein-bound Hcy was elevated during methionine loading.(ABSTRACT TRUNCATED AT 250 WORDS)

Liver transplantation in two cases of propionic acidaemia

Orthotopic liver transplantation (OLT) was performed in two patients with propionic acidaemia, a 7-year-old boy and a 9-year-old girl, diagnosed with a severe neonatal form with high risk of metabolic decompensation. In both cases the metabolic liver functions recovered within the 12 postoperative hours; no clinical symptoms of propionic acid toxicity, metabolic acidosis, severe hyperammonaemia, hyperglycinaemia or haematological abnormalities were observed. In both cases insulin-dependent diabetes mellitus occurred early after OLT (persisting in the boy's case). Severe post-transplantation complications were observed (acute rejection and CMV infection in both patients) which did not trigger metabolic decompensation. The boy developed chronic rejection and vanishing bile duct syndrome due to incomplete hepatic arterial thrombosis. He required permanent in-patient care with chronic hyperammonaemia and neurological sequelae involving the basal ganglia and died 15 months after OLT. The girl left hospital after 2 months and is presently leading a normal life with almost no dietary protein restriction (40 g protein per day). Urinary urea excretion and daily protein intake increased after liver transplantation. Propionyl- and tiglylglycine disappeared immediately after OLT. Urinary methylcitrate and 3-hydroxypropionate remained at concentrations corresponding to those before OLT. However, the total of all characteristic metabolites of organic acid analysis was reduced to 50-60% of the values before OLT in both patients. Propionylcarnitine was still detected at significant concentrations. Plasma odd-chain fatty acid concentrations decreased continuously after OLT only in the girl's case. Tissue of both transplanted livers showed increased odd-chain fatty acid concentrations 9 and 15 months after OLT, respectively, in both patients. We consider that at present OLT should only be performed in severe forms of propionic acidaemia.

Maternal hyperhomocysteinemia: a risk factor for neural-tube defects?

The maternal vitamin status, especially of folate, is involved in the pathogenesis of neural-tube defects (NTDs). Maternal folate administration can prevent these malformations. The precise metabolic mechanism of the beneficial effect of folate is unclear. In this study we focus on homocysteine accumulation, which may derive from abnormalities of metabolism of folate, vitamin B12, and vitamin B6. We studied nonpregnant women, 41 of whom had given birth to infants with NTDs and 50 control women who previously had normal offspring. The determinations included the plasma total homocysteine both in the fasting state and 6 hours after the ingestion of a methionine load. In addition, we measured the fasting blood levels of folate, vitamin B12, and vitamin B6. The mean values for both basal homocysteine and homocysteine following a methionine load were significantly increased in the group of women who previously had infants with NTDs. In nine of these subjects and two controls, the values after methionine ingestion exceeded the mean control by more than 2 standard deviations. Cystathionine synthase levels in skin fibroblasts derived from these methionine-intolerant women were within the normal range. Our findings suggest a disorder in the remethylation of homocysteine to methionine due to an acquired (ie, nutritional) or inherited derangement of folate or vitamin B12 metabolism. Increased homocysteine levels can be normalized by administration of vitamin B6 or folate. Therefore, we suggest that the prevention of NTDs by periconceptional folate administration may effectively correct a mild to moderate hyperhomocysteinemia.

Homocyst(e)ine and arterial occlusive diseases

Homocysteine is a thiol-containing amino acid resulting from demethylation of methionine. The free and protein-bound forms of the amino acid and derived disulfides are called homocyst(e)ine [H(e)]. Multiple studies have shown elevated H(e) levels in patients with coronary, cerebrovascular, or peripheral arterial diseases; this association is frequent and independent of most other risk factors for atherosclerosis. In the 1993 Frontiers in Medicine Symposium investigators discussed the genetic, physiological, nutritional, and pharmacological mechanisms involved in the regulation of plasma H(e), the association of H(e) with arterial occlusive diseases, and the relationships of H(e) with nitric oxide and haemostasis. High plasma H(e) levels usually can be reversed with vitamin supplements. Whether vitamin supplements will affect the evolution of arterial occlusive diseases needs to be established in prospective, placebo-controlled, randomized, clinical trials.

Prevention of neural tube defects by and toxicity of L-homocysteine in cultured postimplantation rat embryos

Mild hyperhomocysteinemia is frequently observed in mothers who gave birth to a child with a neural tube defect (NTD). In a previous study we showed L-homocysteine was embryotoxic to gestational day 10 (GD10) rat embryos in culture, however, no NTDs were observed. We therefore investigated the effect of L-homocysteine on the development of neural plate stage (GD9.5) rat embryos. Other objectives of this study were investigation into whether the embryotoxicity of L-homocysteine could be attenuated by compounds related to its metabolism and clarification of the mechanism of L-homocysteine embryotoxicity. In GD9.5 rat embryos L-homocysteine was not toxic at 1- and 2-mM concentrations. Rather at these concentrations it promoted development of the rat embryos in serum that without supplementation caused NTDs in the embryos. L-Methionine had the same preventive effect at even lower concentrations, but folinic acid (1 mM) did not improve embryonic development. N5-Methyltetrahydrofolate (5-CH3-THF) (100 microM), L-serine (6 mM), and L-methionine (6 and 12 mM) attenuated the embryotoxicity of L-homocysteine (6 mM) in GD10 rat embryos. Vitamin B12 (10 microM) completely abolished the embryotoxicity of L-homocysteine, which was shown to be mediated by catalysis of the spontaneous oxidation of L-homocysteine to the less toxic L-homocystine. In GD11 rat embryos, both L- and D-homocysteine were readily taken up when added to the culture (3 mM) and increased embryonic S-adenosylhomocysteine (SAH) levels 14- and 3-fold, respectively. This difference was shown to be caused by the stereospecific preference of SAH hydrolase. We propose the basis for L-homocysteine embryotoxicity is an inhibition of transmethylation reactions by increased embryonic SAH levels.

Phenytoin treatment and folate supplementation affect concentrations of folates in tissues of cobalamin-deficient rats

To further define the mechanism of interaction among phenytoin, folates and cobalamins in rats, we studied the effect of phenytoin (60 mg/(kg.day)) with or without folic acid supplementation and with or without cobalamin deficiency, as well as the effect of supplementing with folic acid (200 mg/kg diet) with or without a cobalamin deficiency, on the tissue concentrations of folates and phenytoin (determined respectively by HPLC and fluorescence polarization). The major tissues (liver, intestinal mucosae, blood and brain) were studied. A folic acid overload (estimated at about 2 mg/day) increased folate levels in the liver, the intestinal mucosae and blood, while there was no effect on cerebral levels. Phenytoin had no significant effect on folate tissue concentration. The major finding was that a folic acid overload caused a considerable decrease in the hepatic and cerebral concentrations of phenytoin. This decrease could be responsible for the increased frequency of epileptic fits in patients treated with this anticonvulsant drug when combined with a folic acid supplement. Concentration changes of the drug in cobalamin-replete or -deficient rats, with or without folic acid supplementation, suggest that the interaction between the anticonvulsant and the two vitamins (folates and cobalamins) occurs at the level of transmethylation reactions.

Study on the presence of homocysteine in ovarian follicular fluid

OBJECTIVE

To study the presence of homocysteine, methionine and the vitamins folate, B12, and B6 in human ovarian follicular fluid (FF).

DESIGN

Measurement of homocysteine, methionine, folate, and vitamins B12 and B6 in ovarian FF and blood.

SETTING

Academic Department of Obstetrics and Gynecology at St. Radboud Hospital, Nijmegen, The Netherlands.

PARTICIPANTS

Fourteen healthy women undergoing an IVF program.

RESULTS

Detectable amounts of homocysteine and methionine were found in FF. Homocysteine concentrations were similar to those in serum. Methionine concentrations proved to be slightly but significantly lower than in corresponding serum samples. Concentrations of vitamins B12 and B6 were significantly lower in FF than in serum, whereas folate concentrations were not significantly different. A statistically significant correlation between corresponding serum and FF concentrations of homocysteine, folate, and vitamin B12 could be established.

CONCLUSIONS

These data support the hypothesis that the ovum might be exposed to high homocysteine or low methionine concentrations, or both, and a lack of vitamins, which might be important in fertilization and early embryogenesis.

Serum betaine, N,N-dimethylglycine and N-methylglycine levels in patients with cobalamin and folate deficiency and related inborn errors of metabolism

Homocysteine and 5-CH3-tetrahydrofolate (5-CH3-THF) are converted to methionine and THF by the CH3-cobalamin (CH3-Cbl)-dependent enzyme methionine synthase. Serum homocysteine levels are elevated in more than 95% of patients with Cbl or folate deficiency and in patients with inborn errors involving the synthesis of 5-CH3-THF or CH3-Cbl. Homocysteine and betaine are converted to methionine and N,N-dimethylglycine by betaine-homocysteine methyltransferase. It requires neither Cbl nor folate, although N,N-dimethylglycine is converted to N-methylglycine and then to glycine in reactions that both involve the formation of 5,10-CH2-THF from THF. Large amounts of betaine are often given orally to patients with inborn errors, even though little is known about its metabolism in normal subjects or these patients. Thus we developed new gas chromatographic-mass spectrometric assays for serum betaine, N,N-dimethylglycine, and N-methylglycine. In 60 blood donors, we found ranges for normal serum of 17.6 to 73.3, 1.42 to 5.27, and 0.60 to 2.67 mumol/L for the three metabolites, respectively, which were normal in the majority of 50 patients with Cbl deficiency, none of whom had increased levels of N-methylglycine. In 25 patients with folate deficiency, serum betaine level was normal in most, but 76% and 60% had elevations of N,N-dimethylglycine and N-methylglycine levels that ranged as high as 343 and 43.2 mumol/L, respectively. All of seven patients on betaine therapy for inborn errors had high values for betaine (167 to 3,900 mumol/L), N,N-dimethylglycine (15.1 to 250 mumol/L), and N-methylglycine (2.93 to 49.3 mumol/L). Serum total homocysteine levels remained very high at 47.2 to 156 mumol/L (normal, 5.4 to 16.2). In patients with cbl C and cbl D mutations, methionine levels remained low or low-normal at 8.3 to 15.6 mumol/L (normal, 13.3 to 42.7) despite betaine treatment. We conclude that (1) betaine levels are maintained in most patients with Cbl and folate deficiency; (2) levels of N,N-dimethylglycine and N-methylglycine are increased in most patients with folate deficiency; and (3) betaine therapy is relatively ineffective in patients with defective synthesis of CH3-Cbl.

The effect of ethanol on one-carbon metabolism: increased methionine catabolism and lipotrope methyl-group wastage

Deficiency of choline and methionine produces hepatic steatosis similar to that seen with ethanol, and supplementation with these lipotropes can prevent ethanol-induced fatty liver. These effects are thought to occur through alterations in membrane phospholipid metabolism, but the mechanism whereby this occurs and the precise nature of the changes brought about by ethanol in the interactions of choline and methionine metabolism remain unclear. Through the known effects on hepatic glutathione (which requires as a precursor a product of methionine catabolism), ethanol might affect hepatic one-carbon metabolism, which requires the participation of both methionine and choline in the transfer of methyl groups. This has been investigated with a radiorespirometric technique to assess the in vivo oxidation of the methyl groups of lipotropes and their intermediates in ethnaol- and control-fed rats. Enzyme activities of one-carbon transfer reactions and the hepatic levels of methionine and alpha-aminobutyrate, an end product of methionine catabolism, have been measured. The effect of ethanol feeding on hepatic S-adenosylmethionine and S-adenosylhomocysteine has also been assessed. Ethanol increases the oxidation to carbon dioxide of the methyl group of methionine by a factor of 2.9 (p = 0.002) and produces a 3.6-fold (p = 0.0001) accumulation of alpha-aminobutyrate, indicating a marked increase in methionine catabolism. Hepatic methionine levels are unchanged by ethanol, however, and this may be explained by a dramatic increase in the turnover of the methyl groups of choline and betaine in response to ethanol (times 3.6 and 4.2, respectively, p < 0.003), suggesting greatly increased use of the choline oxidation pathway to remethylate homocysteine through betaine homocysteine methyltransferase.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of antagonists of polyamine metabolism on African trypanosomes

This review outlines the metabolism of polyamines in African trypanosomes and summarizes evidence to indicate that trypanosome polyamine metabolism differs in several important aspects from that of the mammalian host. These differences relate to the halflife, turnover, substrate specificity and regulation of enzymes within the mainstream of polyamine synthesis and the related pathway of transmethylation. The common denominator for the uniqueness of parasite polyamine metabolism concerns S-adenosylmethionine (AdoMet) whose synthesis is unregulated and, upon accumulating in the cell, appears to result in abnormally high transmethylation activity. Similarly, the catabolism of the AdoMet product of polyamine synthesis, methylthioadenosine, is governed by a phosphorylase having broad substrate specificity, and which, if presented with substrate analogs, can generate cytotoxic metabolites.

Hyperhomocysteinemia in patients operated for lower extremity ischaemia below the age of 50--effect of smoking and extent of disease

Moderate hyperhomocysteinemia may be a risk factor for atherosclerotic peripheral vascular disease (PVD). In order to develop PVD at an early age risk factors are more strongly expressed and hyperhomocysteinemia may be one such factor. Homocysteine is derived from methionine and is metabolised by cystathionine-synthase to cystathionine or remethylated to methionine. Cystathionine-synthase activity is dependent on vitamin B6 while the remethylation of homocysteine is dependent on vitamin B12 and folate. The present study analyses homocysteine in patients operated on for lower extremity ischaemia before the age of 50. Homocysteine before and after loading with methionine, vitamin B6, B12 and folate were measured at follow-up. The patients were compared to age- and sex-matched controls. Significantly more patients than controls had hyperhomocysteinemia, 16/58 vs. 4/65, defined as fasting total homocysteine above 18.6 mumol/l. Loading with methionine did not further discriminate between patients and controls. Smoking patients had higher levels of homocysteine than non-smoking patients or smoking and non-smoking controls. Smoking patients also had lower levels of vitamin B6. When comparing patients with suprainguinal, infrainguinal and multilevel disease the highest homocysteine levels were seen in the latter group. Also, in this group smoking patients had higher homocysteine levels. Multivariate analysis revealed that homocysteine was associated with low levels of vitamin B12, folate and smoking. Smoking therefore seems to be connected to increased homocysteine levels in patients with early development of atherosclerosis, partly explained by decreased levels of B6, B12 and folate.

Hepatic one-carbon metabolism in early folate deficiency in rats

Glycine N-methyltransferase (GNMT) is inhibited by 5-methyltetrahydrofolate polyglutamate in vitro. It is believed to play a regulatory role in the synthesis de novo of methyl groups. We have used the amino-acid-defined diet of Walzem and Clifford [(1988) J. Nutr. 118, 1089-1096] to determine whether folate deficiency in vivo would affect GNMT activity, as predicted by the studies in vitro. Weanling male rats were fed on the folate-deficient diet or a folate-supplemented diet pair-fed to the deficient group. A third group was fed on the folate-supplemented diet ad libitum. Development of folate deficiency rapidly resulted in decreased levels of S-adenosylmethionine (SAM) and elevation of S-adenosylhomocysteine (SAH). The ratios of SAM to SAH were 1.8, 2.7 and 1.5 in the deficient group for weeks 2, 3 and 4 of the experiment, and the values were 9.7, 7.1 and 8.9 for the pair-fed control group and 10.3, 8.8 and 8.0 for the control group ad libitum fed. The activity of GNMT was significantly higher in the deficient group than in either of the two control groups at each time period. This was not due to increased amounts of GNMT protein, but reflected an increase in specific enzyme activity. Levels of folate in both the cytosol and mitochondria were severely lowered after only 2 weeks on the diet. The distribution of folate coenzymes was also affected by the deficiency, which resulted in a marked increase in the percentage of tetrahydrofolate polyglutamates in both cytosol and mitochondria and a very large decrease in cytosolic 5-methyltetrahydrofolate. The increased GNMT activity is therefore consistent with decreased folate levels and decreased inhibition of enzyme activity.

Characterization of betaine efflux from rat liver mitochondria

In order to investigate the control of endogenous betaine supply to the cytoplasmic enzyme betaine-homocysteine methyltransferase, it was necessary to understand how betaine synthesized within the mitochondrial matrix is transported across the mitochondrial inner membrane. Mitochondria were loaded with radiolabelled betaine and efflux was measured in a medium at physiological ionic strength. Efflux of radiolabelled betaine occurred continuously with time. The efflux rate was unaffected by the presence or absence of a source of energy except at high membrane potentials, where betaine efflux rate increased 2-3-fold. Titration of the membrane potential demonstrated a non-ohmic relationship between betaine efflux rate and membrane potential. The rate of betaine efflux was proportional to the matrix betaine concentration up to 9 mM. Efflux was unaffected by addition of analogues of betaine and known mitochondrial transport inhibitors. N-Ethylmaleimide did inhibit efflux by 50%, but evidence suggested that the effect was non-specific. The lack of saturability or other evidence for a transport system suggests that betaine escapes from mitochondria by simple diffusion. The relative diffusion rates of glycine, sarcosine, dimethylglycine and betaine suggest that increasing the degree of N-methylation lowers diffusion rate.

Methionine decreases the embryotoxicity of sodium valproate in the rat: in vivo and in vitro observations

Methionine provided in the drinking water of pregnant rats injected with sodium valproate reduced the frequency of resorptions but did not improve embryo growth. Rats drinking methionine supplemented water had approximately twice the level of serum-free methionine and consumed only one-half the volume of water of controls. Using whole rat embryo cultures, the simultaneous addition of methionine and sodium valproate to the medium provided no protection from neural tube defects, nor did the addition of methionine to a medium of serum obtained from rats previously dosed with sodium valproate. However, protection from the teratogenic effects of sodium valproate was afforded by methionine when the culture medium was sera from rats consuming methionine and was particularly striking when embryos for culture were taken from pregnant rats that had been consuming methionine. These observations along with those of others indicated the importance of dietary and culture media methionine levels in evaluating experimental and regulatory teratology studies and suggested the possibility that methionine may play an important role in human teratology where multifactorial causes have been implicated in problems such as neural tube closure defects.

Rat cystathionine beta-synthase: expression of four alternatively spliced isoforms in transfected cultured cells

The gene for rat cystathionine beta-synthase consists of 17 exons. Its transcripts are alternatively spliced, forming four distinct mRNA species. Type III consists of exons 1 through 12, 14, 15, and 17; type I also contains exon 16. The open reading frame of type IV spans exons 1-->13; type II, 3-->13. We cloned the corresponding cDNAs into appropriate expression vectors and inserted the constructs into Escherichia coli (I, III, and IV) and Chinese hamster (CH) cells (I through IV); all sequences were transcribed and translated. Catalytic activity was observed only for types I and III in lysates of transfected CH cells and transformed E. coli. The catalytic and kinetic properties of I and III were identical despite their structural difference (exon 16). Both isoforms exhibited 6 mM Km constants for homocysteine which were reduced approximately eightfold by AdoMet; this elucidates the mechanism by which AdoMet regulates synthase activity. The four isoforms were differentially degraded by transfected cultured cells. Type III (t1/2 = 18 h) was degraded at 1/3 the rate of type I (t1/2 = 6 h); thus the 14 amino acid residues encoded by exon 16 appear to enhance degradation of CBS. The half-lives of both types II and IV were markedly shorter (ca. 1 h). Western blots comparing rat liver to lysates from transfected CH cells revealed that hepatocytes express both isoforms. Type III was predominant, as predicted by its longer half-life and more abundant mRNA. PCR analysis of cDNA from various tissues revealed that type III mRNA was preferred in liver, kidney, and heart; equal amounts of I and III were found in brain.

Homocysteine catabolism: levels of 3 enzymes in cultured human vascular endothelium and their relevance to vascular disease

Elevated plasma homocysteine enhances the risk of thrombosis and premature arteriosclerosis. We have assessed the activity of the 3 prime enzymes of homocysteine metabolism in cultured human venous endothelial cells, in a study of their possible protective roles. In cells from 4 individuals, cultured in Dulbecco's modified Eagle medium, the mean activity +/- S.D. of cystathionine beta-synthase (nmol of product/h per mg of cell protein, at 37 degrees C) was 3.58 +/- 3.11 at pH 8.6. The assay used was our newly developed amino acid analyser-based procedure. The activity of 5-methyltetrahydrofolate:homocysteine methyltransferase at pH 7.4 was 4.12 +/- 1.25 and betaine:homocysteine methyltransferase (BHMT) was undetectable (< 1.4 nmol/h per mg protein). Cells were also cultured in a medium aimed at stimulating methionine biosynthesis, containing methionine-deficient Dulbecco's modified Eagle medium to which L-homocystine (100 mumol/l) and methylcobalamin (1 mumol/l) had been added. In these cells 5-methyltetrahydrofolate:homocysteine methyltransferase activity increased to 7.95 +/- 1.45, P < 0.001, there was a non-significant decrease in cystathionine beta-synthase activity to 2.16 +/- 1.52 and BHMT activity was still undetectable. These cells were more resistant to in vitro homocysteine-induced detachment than were cells from the same line cultured in Dulbecco's modified Eagle medium alone. Our findings establish that human endothelial cells express 2 of the 3 primary enzymes of homocysteine catabolism. They suggest that persons who are deficient in cystathionine beta-synthase or 5-methyltetrahydrofolate:homocysteine methyltransferase activity may not only develop homocysteinemia, but also have vascular endothelium which is more susceptible to damage by homocysteine than persons with normal enzyme levels.

Brain protein synthesis in the conscious rat using L-[35S]methionine: relationship of methionine specific activity between plasma and precursor compartment and evaluation of methionine metabolic pathways

The method previously developed for the measurement of rates of methionine incorporation into brain proteins assumed that methionine derived from protein degradation did not recycle into the precursor pool for protein synthesis and that the metabolism of methionine via the transmethylation pathway was negligible. To evaluate the degree of recycling, we have compared, under steady-state conditions, the specific activity of L-[35S] methionine in the tRNA-bound pool to that of plasma. The relative contribution of methionine from protein degradation to the precursor pool was 26%. Under the same conditions, the relative rate of methionine flux into the transmethylation cycle was estimated to be 10% of the rate of methionine incorporation into brain proteins. These results indicate the following: (a) there is significant recycling of unlabeled methionine derived from protein degradation in brain; and (b) the metabolism of methionine is directed mainly towards protein synthesis. At normal plasma amino acid levels, methionine is the amino acid which, to date, presents the lowest degree of dilution in the precursor pool for protein synthesis. L-[35S]-Methionine, therefore, presents radiobiochemical properties required to measure, with minimal underestimation, rates of brain protein synthesis in vivo.

The effects of vitamins B12, B6, and folate on blood homocysteine levels

The interaction between plasma homocysteine levels and vitamins B6, B12, and folate is an exciting field and one that has gathered great momentum over the past few years, with the recognition that homocysteine probably plays an important role in occlusive vascular disease. Our understanding in this field is greatly advanced compared to just a few years ago. There are a number of important issues, however, that will need to be addressed in the future if we are to develop a sufficient knowledge base to effectively minimize the risk of occlusive vascular disease ascribable to hyperhomocysteinemia. These include (1) definitive evidence that homocysteine is the actual agent that mediates accelerated occlusive vascular disease and the mechanism by which this occurs; (2) an understanding of what constitutes a pathologic elevation in homocysteine (is there a threshold concentration in the plasma below which no vascular injury occurs? is the peak concentration achieved the critical determinant of injury, or is the area under the curve, or some other feature, more important?); (3) understanding what synergies might exist by adding B6 or B12 to a regimen of folate supplementation (what doses are most appropriate? will toxologic issues limit the utility of supplementation?); and (4) determining the circumstances where reduction of plasma homocysteine will retard or reverse the process of occlusive vascular disease.

How is rat liver S-adenosylmethionine synthetase regulated?

The in vivo regulation of S-adenosylmethionine synthetase, a key enzyme in methionine metabolism, is so far unknown. The enzyme activity has been shown to be modulated by glutathione and the oxidation state of its sulfhydryl groups. Analysis of the protein sequence has revealed the presence of putative phosphorylation sites. A mixed regulatory mechanism combining phosphorylation and the oxido/reduction of sulfhydryl groups is proposed. The role of glutathione in this mechanism is also discussed.

Persistent hypermethioninaemia with dominant inheritance

A clinically benign form of persistent hypermethioninaemia with probable dominant inheritance was demonstrated in three generations of one family. Plasma methionine concentrations were between 87 and 475 mumol/L (normal mean 26 mumol/L; range 10-40 mumol/L); urinary methionine and homocystine concentrations were normal. Plasma homocystine, cystathionine, cystine and tyrosine were virtually normal. The concentrations in serum and urine of metabolites formed by the methionine transamination pathway were normal or moderately elevated. Methionine loading of two affected family members revealed a diminished ability to catabolize methionine, but the activities of methionine adenosyltransferase and cystathionine beta-synthase were not decreased in fibroblasts from four affected family members. Fibroblast methylenetetrahydrofolate reductase activity and its inhibition by S-adenosylmethionine were also normal, indicating normal regulation of N5-methyltetrahydrofolate-dependent homocysteine remethylation. Serum folate concentrations were not increased. The findings in this family differ from those previously described for known defects of methionine degradation. Since the hepatic and fibroblast isoenzymes of methionine adenosyltransferase differ in their genetic control, this family's biochemical findings appear consistent with a mutation in the structural gene for the hepatic methionine adenosyltransferase isoenzyme.

A nonradioactive assay for N5-methyltetrahydrofolate-homocysteine methyltransferase (methionine synthase) based on o-phthaldialdehyde derivatization of methionine and fluorescence detection

The enzyme N5-methyltetrahydrofolate-homocysteine methyltransferase (methionine synthase, EC 2.1.1.13) catalyzes the conversion of homocysteine to methionine in the presence of a reducing system. N5-Methyltetrahydrofolate serves as a methyl donor in this reaction. An assay for the enzyme is described, which is based on methionine quantitation by o-phthaldialdehyde (OPA) derivatization and reversed-phase liquid chromatography. The enzymatic reaction is linear for at least 120 min under reducing conditions (125 mM 2-mercaptoethanol) and running the assay below an oil layer. This reducing system does not interfere with formation of the methionine-OPA adduct, which is separated from interfering compounds and an internal standard (norvaline) by a mobile phase adjusted to pH 5.0. The inclusion of internal standard increases the precision of the assay and corrects for the variable fluorescence yield due to occasional inaccurate pH adjustment before the derivatization step. Norvaline was suitable for this purpose because it elutes close to methionine and is not a natural amino acid present in biological extracts. This nonradioactive assay for methionine synthase was evaluated by comparison with a conventional method based on isolation of radioactive methionine by anion-exchange chromatography and by determination of enzyme activity in extract from cultured cells and liver.

Inhibition of glutathione synthesis in the liver leads to S-adenosyl-L-methionine synthetase reduction

The hepatic levels of glutathione in rats treated with buthionine sulfoximine (4 mmol/kg), an inhibitor of glutathione synthesis, were 72.5% +/- 4.9% of those determined in control animals. This decrease in glutathione concentration was prevented by the administration of glutathione monoethyl ester (7.5 mmol/kg). S-Adenosyl-L-methionine-synthetase activity in the liver of rats treated with buthionine sulfoximine was 39.4% +/- 6.5% of that determined in control animals. Again, glutathione monoethyl ester prevented the effect of buthionine sulfoximine on S-adenosyl-L-methionine-synthetase activity. There was a close correlation (r = 0.936) between the hepatic levels of glutathione and S-adenosyl-L-methionine-synthetase activity. The hepatic concentration of S-adenosyl-L-methionine in buthionine sulfoximine-treated animals was 59.7% +/- 3.7% of that measured in control rats. Contrasting with the protective effects mentioned above, glutathione monoester had no preventive action on buthionine sulfoximine-induced S-adenosyl-L-methionine depletion. Electron microscopic examination of liver samples of rats after buthionine sulfoximine administration showed evidence of liver degeneration, which was attenuated by glutathione monoethyl ester treatment. Glutathione (7.5 mmol/kg) treatment was less effective than glutathione monoethyl ester in attenuating buthionine sulfoximine effects on hepatic S-adenosyl-L-methionine metabolism and morphology. The reduction of S-adenosyl-L-methionine-synthetase activity observed after treatment with buthionine sulfoximine and its prevention by glutathione monoethyl ester, as well as the correlation between the activity of this enzyme and glutathione levels, indicate that glutathione plays an important role in maintaining S-adenosyl-L-methionine-synthetase activity in the liver.

Role of 5'-deoxy-5'-methylthioadenosine in growth of several microbial B12 requirers

5'-Deoxy-5'-Methylthioadenosine (MTA) figures in cellular methionine and polyamine syntheses. It replaces B12 for growth of the chrysomonad protozoan Poteriochromonas malhamensis at a ratio of MTA:B12 of approximatly 10,000,000:1 (by weight). MTA does not replace B12 for other B12-requirers, e.g.: Euglena gracilis, Lactobacillus leichmannii, and Escherichia coli 113-3. The methionine synergism for P. malhamensis growth is also negated when B12 activity is annulled by alkali treatment; MTA is not inactivated by such treatment. The growth promoting activity of various deoxynucloesides and deoxynucloetides for P. malhamensis and other B12-requirers is reported here due to contamination by cobalamins. Ethionine antagonizes the growth-enhancing effect of MTA, methionine, and B12, individually and collectively -evidence that MTA plays a role in supplying methionine for P. malhamensis growth. MTA concentrations in body fluids and mammalian tissues are too low to interfere with the use of P. malhamensis for estimating only metabolically active B12.