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PubMed1980

Growth and liver enzyme response in growing rats to graded levels of methionine plus cystine in fortified-barley diets. Response at constant methionine:cystine.

1. Twenty-eight male rats of initial age 27 d were fed on fortified-barley diets for 3 weeks. In all experimental diets, both crude protein (nitrogen x 6.25) and methionine:cystine were constant at 120.0 g/kg dry matter (DM) and 2:1 respectively. The basal diet contained 4.5 g methionine plus cystine/kg DM with L-methionine plus L-cystine (2:1, w/w) added in increments of 0.5 g/kg DM to a final level of 7.0 methionine plus cystine/kg DM. A 'positive-control' diet of barley plus 193.7 g soya-bean meal/kg DM contained 6.0 g methionine plus cystine/kg DM. 2. Weight gain, food conversion efficiency (FCE), urinary urea-N excretion, carcass composition and activities of liver cystathionine synthase (EC 4.2.1.22) and N5-methyltetrahydrofolate-homocysteine-methyltransferase (EC 2.1.1.13) were determined. 3. Weight gain, food consumption, FCE and carcass composition measurements of rats showed either small or no differences between the experimental diets containing 4.5--7.0 g methionine plus cystine/kg DM. For the over-all period, weight gain and FCE of rats receiving the 'positive control' diet were significantly higher than values obtained with rats receiving any of the experimental diets. 4. Cystathionine synthase activity (mumol/mg protein per 60 min; units) increased from 13.38 at 4.5 g dietary methionine plus cystine/kg DM to 18.81 at 5.0 g dietary methionine plus cystine/kg DM. The activity was then inhibited to reach a minimum value of 10.16 units at the 6.0 g/kg DM dietary level. Thereafter the activity increased to a value of 30.00 units at 7.0 g dietary methionine plus cystine/kg DM. 5. The activity of N5-methyltetrahydrofolate-methyltransferase was constant at 0.70--0.74 nmol/mg protein per 60 min between dietary levels of 4.5 and 5.0 g methionine plus cystine/kg DM. The activity then increased to a maximum value of 2.32 nmol/mg protein per 60 min at the 6.0 g/kg DM level. Thereafter the activity decreased, reaching a minimum value of 0.70 nmol/mg protein per 60 min at the 7.0 g methionine plus cystine/kg level. 6. Urinary urea-N excretion decreased significantly from 1.07 g/kg DM intake at the 4.5 g dietary methionine plus cystine/kg DM level to 1.05 g/kg DM at the 5.0 g/kg dietary level, then dropped significantly to a level of 1.01--1.00 g/kg DM intake for the higher levels of dietary methionine plus cystine.

5-Methyltetrahydrofolate-Homocysteine S-Methyltran↗
PubMed1981

Influence of copper supplementation on the relationship between dietary methionine and free plasma methionine.

One-day-old broiler chicks were adapted to a basal, isolated soyprotein-cornstarch diet containing 20% protein, 0.59% methionine and 3,300 kcal metabolizable energy (ME)/kg. They were then fed experimental diets consisting of three levels (0, 500, 1,000 ppm) of copper added to each of the basal diet plus four levels (0, 0.4, 0.8, or 1.5%) of L-methionine for 1 or 4 weeks. Growth was retarded after 1 week by all levels of supplementary methionine, and by 500 or 1,000 ppm excess copper. The plasma concentrations of free methionine, serine, alpha-aminobutyric acid and cystathionine were increased by excess dietary methionine. Excess copper prevented the increase in plasma methionine. After 4 weeks, the plasma methionine concentration and rate of growth of chicks fed 0.4% excess L-methionine did not differ significantly from basal values. The growth retardation caused by 500 ppm excess copper was alleviated by 0.4% supplemental methionine, and the elevations in plasma methionine and liver and spleen iron concentrations observed in chicks fed 1.5% excess methionine were reduced by 1,000 ppm excess copper. The patterns of the relationship between dietary methionine and liver or spleen iron, in the presence or absence of supplementary copper, were similar to those between dietary and free plasma methionine.

Amino Acids↗
PubMed1996

Comparison of the effects of seleno-L-methionine, seleno-DL-methionine, and selenized yeast on reproduction of mallards.

The toxicities of seleno-L-methionine, seleno-DL-methionine, and selenized yeast were compared. Ten pairs of mallards were fed a control diet and 15 pairs were fed diets containing 10 ppm selenium as seleno-DL-methionine, seleno-L-methionine, or selenized yeast. Hatching of fertile eggs was significantly lower for females fed 10 ppm selenium as seleno-DL-methionine (7.6%) and seleno-L-methionine (6.4%) than for controls (41.3%). Survival of ducklings was lower when their parents had been fed 10 ppm selenium as seleno-L-methionine (20.0%) than for controls (98.4%). The number of 6-day-old ducklings produced per female was significantly lower for mallards fed 10 ppm selenium as seleno-DL-methionine (0.47) or selenized yeast (2.67) than for controls (6.10), and was significantly lower for mallards fed seleno-L-methionine (0.13) than for mallards fed selenized yeast. The eighth eggs of females fed the DL or L forms of selenomethionine contained means of 9.2 and 8.9 ppm selenium, wet weight; these means were higher than the mean (6.6 ppm) for females fed selenized yeast. Among embryos that died at 7 days of age or older, the percentage of embryos that were deformed was 1.3% for controls, 24.6% for seleno-DL-methionine, 28.2% for seleno-L-methionine, and 11.0% for selenized yeast. The results suggested that seleno-DL-methionine and seleno-L-methionine were of similar toxicity and were both more toxic than selenium from selenized yeast.

Journal Article↗
PubMed1979

Interactions of Methionine and Selenomethionine with Methionine Adenosyltransferase and Ethylene-generating Systems.

Since selenomethionine appears to be a better precursor of ethylene in senescing flower tissue of Ipomoea tricolor and in indole acetic acid-treated pea stem sections than is methionine (Konze JR, N Schilling, H Kende 1978 Plant Physiol 62: 397-401), we compared the effectiveness of selenomethionine and methionine to participate in reactions which may be connected to ethylene biosynthesis. Evidence is presented that selenomethionine is also a better substrate of methionine adenosyltransferase (ATP: methionine S-adenosyltransferase, EC 2.5.1.6) from I. tricolor, the V(max) for selenomethionine being twice as high as that for methionine. The affinity of the enzyme is higher for methionine than for selenomethionine, however. Methionine added to flower tissue together with selenomethionine inhibits the enhancement of ethylene synthesis by the seleno analog. Likewise, methionine reduces the high, selenomethionine-dependent reaction rates of methionine adenosyltransferase from I. tricolor flower tissue. On the other hand, selenomethionine is less effective as an ethylene precursor than is methionine in model systems involving oxidation by free radicals. It was concluded that activation of methionine by methionine adenosyltransferase and formation of S-adenosylmethionine are more likely to be involved in ethylene biosynthesis than is oxidation of methionine by free radicals.

Journal Article↗
PubMed1984

Methionine and glutamine transport systems in D-methionine utilising revertants of Salmonella typhimurium.

In Salmonella typhimurium, methionine auxotrophs such as metB can use D-methionine as a methionine source. MetP mutations prevent this growth since D-methionine can enter only via the metP high-affinity methionine transport system. D-methionine utilising revertants ( Dmu +) were selected from metB23 metP760 ( HU76 ) following nitrosoguanidine mutagenesis. The properties of two such revertants, HU206 and HU415 , indicated that reversion was not due to backmutation of the metP760 mutation. Genetic analysis indicated that each strain possessed two mutations, designated dmu and gln, in addition to the original metB23 and metP760 mutations. The dmu mutation restores ability to grow on D-methionine, partly restores D- and L-methionine transport activity, and makes the cells particularly sensitive to inhibition by L-glutamine while growing on D but not L-methionine. The growth inhibition by L-glutamine was shown to be caused by competition by L-glutamine for D-methionine transport by the high-affinity methionine system. The gln mutation greatly reduces activity of the high-affinity glutamine transport system. The Dmu + strains are also partly defective in the glutamine low-affinity transport system, possibly because the partially-restored methionine high-affinity system, or a component of tis system, functions in the transport of glutamine by its low-affinity system.

Biological Transport, Active↗
PubMed1999

Mechanism of L-methionine overproduction by Escherichia coli: the replacement of Ser-54 by Asn in the MetJ protein causes the derepression of L-methionine biosynthetic enzymes.

We derived L-methionine-analogue-resistant mutants from Escherichia coli JM109 strain by mutagenesis with N-methyl-N'-nitro-N-nitrosoguanidine and selected the potent L-methionine-overproducing strains by microbioassay using lactic acid bacteria. One of the mutants, strain TN1, produced approximately 910 mg L-methionine/l following the addition of 0.1% yeast extract to fundamental medium containing glucose and ammonium sulfate. The L-methionine biosynthetic enzymes, cystathionine gamma-synthase and cystathionine beta-lyase, of the L-methionine-overproducing mutants were little repressed by L-methionine. To analyse the mechanism of L-methionine overproduction in the mutant strains, the metJ gene coding for the E. coli met repressor, MetJ protein, was cloned and sequenced by the polymerase chain reaction. The same single-amino-acid subsitution (wild-type Ser-->Asn) at position 54 was observed in four independent L-methionine-producing mutants. When the wild-type metJ gene was then introduced into strain TN1 having the mutant metJ gene, the level of enzyme synthesis and the L-methionine productivity in the transformants were found to revert to those of the wild-type. It was therefore considered that only one point mutation in the metJ gene occurred in the L-methionine-producing mutants. These results demonstrate the important role of residue 54 of the MetJ protein in L-methionine overproduction, probably because of the derepression of L-methionine biosynthetic enzymes.

Bacterial Proteins↗
PubMed1990

The effect of excess daily methionine intake on plasma homocysteine after a methionine loading test in humans.

Impaired homocysteine metabolism might be a risk factor for vascular disease. Peroral methionine loading and post-load determination of methionine and homocysteine in plasma has frequently been used for identifying subjects with genetically impaired homocysteine metabolism. However, a methionine-rich diet induces a more efficient homocysteine catabolism in the rat, which suggests that humans on diets with differing methionine content might respond differently to the methionine loading test. To study this we performed methionine loading in six healthy subjects before and after 2 wk of excessive daily methionine intake (300% of normal). On each occasion plasma homocysteine and methionine were measured at several intervals post-load. However, neither the methionine clearance nor the post-load homocysteine concentrations were affected by excess methionine. We conclude that variations in the daily methionine intake will not influence the methionine loading test.

Adult↗
PubMed2001

Pharmacokinetic studies of methionine in rats using deuterium-labeled methionine quantitative assessment of remethylation.

The pharmacokinetics of methionine has been studied in rats by means of stable isotope methodology. After the i.v. bolus injection of [2H7]methionine (5 mg/kg body wt.), the plasma concentrations of [2H7]methionine, demethylated [2H4]homocysteine and remethylated [2H4]methionine were determined simultaneously with endogenous methionine and homocysteine by gas chromatography-mass spectrometry. The half-life for [2H7]methionine were 35.0 +/- 6.9 min. The appearance of the metabolites, [2H4]homocysteine and [2H4]methionine, in the plasma was very rapid. The fraction of [2H7]methionine that remethylated to [2H4]methionine through [2H4]homocysteine were 0.185 +/- 0.028. The administered [2H7]methionine did not influence the plasma levels of endogenous methionine and homocysteine. The present stable isotope methodology has made it possible to evaluate the pharmacokinetics of methionine, including the estimation of remethylation.

Animals↗
PubMed1975

[Determination of metabolism-oriented methionine requirements using 35 S methionine].

Experimental rats (weighing 50-100 gm) received semisynthetic diets containing 8%, 10%, or 12% of crude protein (Soya protein). These were supplemented with graded amounts of L- or DL methionine. After a 5-day feeding period the rats were injected 35S methionine. Subsequently, the levels of urinary 35S excretion were determined over a period of 4 days after methionine injection. The level of urinary 35S excretion was found to be clearly increased if methionine supplementation exceeded the methionine requirements of the animals. Supplementation with 0.15% methionine was just enough for diets containing 8-10% crude protein. 0.2% methionine had to be supplemented to meet the methionine requirements of the animals if the diet contained 12% crude protein. Requirements for the content of sulfur-containing amino acids in the protein were shown to be independent of the protein content of the diet, and were found to vary between 4.4% and 4.7% of the crude protein. The needs for methionine supplementation were independent of the fact whether L methionine or DL methionine was added. It is definite advantage of the present method that methionine demands are determined in close correlation with metabolic processes, including the maintenance metabolism.

Animal Nutritional Physiological Phenomena↗
PubMed1976

Utilization of L-methionine sulfoxide, L-methionine sulfone and cysteic acid by the weanling rat.

Three experiments were conducted to test the ability of weanling rats to utilize the oxidized forms of the sulfur amino acids methionine and cysteine for growth. In the first two experiments, diets were fed which contained graded levels of methionine, methionine sulfoxide and methionine sulfone. The third experiment included a comparison of two dietary levels of cysteine and cysteic acid. The 2 week weight gain and food consumption data indicated that methionine sulfoxide was utilized for growth with only 60% of the efficiency of that achieved by rats fed methionine. Methionine sulfone was not utilized for growth. Analysis of plasma sulfur amino acids showed that the rat has a limited capacity to utilize methionine sulfoxide by effecting its reduction to methionine. Cysteic acid did not support weight gain. This amino acid appeared to be rapidly catabolized to taurine. It was concluded that methionine sulfone and cysteic acid cannot be utilized by the weanling rat. Methionine sulfoxide cannot fully meet the dietary requirement of the rat methionine because of its limited capacity to reduce this amino acid.

Amino Acids↗
PubMed1996

Methionine-containing peptides can be used as methionine sources for protein accretion in cultured C2C12 and MAC-T cells.

Twenty-two methionine-containing di- to octapeptides were evaluated for their ability to be a source of methionine to support protein accretion in C2C12 myogenic and MAC-T bovine mammary epithelial cells. The cell cultures were incubated for 72 h at 37 degrees C in a humidified environment of 90% air: 10% CO2 for C2C12 cells or 95% air: 5% CO2 for MAC-T cells. The basal medium contained methionine-free Dulbecco's modified Eagle's medium and 6% desalted fetal bovine serum. Treatments included basal medium, the basal medium supplemented with one of the 22 methionine-containing peptides, or the basal medium supplemented with free L-methionine. Methionine-containing peptides with the exception of glycylmethionine and prolylmethionine in C2C12 cells were able to support protein accretion with responses ranging from 29.1 to 123.3% of the response of L-methionine. Dipeptides with methionine at the N-terminus promoted greater (P < 0.0001) protein accretion than dipeptides with methionine at the C-terminus. Stimulation of protein accretion by seven pairs of dipeptides with methionine at either the C- or the N-terminus was linearly (P < 0.0001) related to the hydrophobicity of the dipeptides. These results indicate that C2C12 myogenic and MAC-T mammary epithelial cells have the ability to utilize methionine-containing peptides as sources of methionine to support protein accretion.

Amino Acid Sequence↗
PubMed1981

Methionine sulfoxide is transported by high-affinity methionine and glutamine transport systems in Salmonella typhimurium.

Three lines of evidence indicated that methionine sulfoxide is transported by the high-affinity methionine and glutamine transport systems in Salmonella typhimurium. First, methionine-requiring strains (metE) which have mutations affecting both of these transport systems (metP glnP) were unable to use methionine sulfoxide as a source of methionine. These strains could still grow on L-methionine because they possessed a low-affinity system (or systems) which transported L-methionine but not the sulfoxide. A methionine auxotroph with a defect only in the metP system, which was dependent upon the glnP+ system for the transport of methionine sulfoxide, was inhibited by L-glutamine because glutamine inhibited the transport of the sulfoxide by the glnP+ system. Second, a metE metP glnP strain could be transduced at either the metP or glnP genes to restore its ability to grow on methionine sulfoxide. Third, the transport of [14C]methionine sulfoxide was inhibited by methionine and by glutamine in the metP+ glnP+ strain. No transport was detected in the metP glnP double-mutant strain.

Biological Transport↗
PubMed2003

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.

5-Methyltetrahydrofolate-Homocysteine S-Methyltran↗
PubMed2001

Human methionine synthase reductase, a soluble P-450 reductase-like dual flavoprotein, is sufficient for NADPH-dependent methionine synthase activation.

Methionine synthase is a key enzyme in the methionine cycle that catalyzes the transmethylation of homocysteine to methionine in a cobalamin-dependent reaction that utilizes methyltetrahydrofolate as a methyl group donor. Cob(I)alamin, a supernucleophilic form of the cofactor, is an intermediate in this reaction, and its reactivity renders the enzyme susceptible to oxidative inactivation. In bacteria, an NADPH-dependent two-protein system comprising flavodoxin reductase and flavodoxin, transfers electrons during reactivation of methionine synthase. Until recently, the physiological reducing system in mammals was unknown. Identification of mutations in the gene encoding a putative methionine synthase reductase in the cblE class of patients with an isolated functional deficiency of methionine synthase suggested a role for this protein in activation (Leclerc, D., Wilson, A., Dumas, R., Gafuik, C., Song, D., Watkins, D., Heng, H. H. Q., Rommens, J. M., Scherer, S. W., Rosenblatt, D. S., and Gravel, R. A. (1998) Proc. Natl. Acad. Sci. U. S. A. 95, 3059-3064). In this study, we have cloned and expressed the cDNA encoding human methionine synthase reductase and demonstrate that it is sufficient for supporting NADPH-dependent activity of methionine synthase at a level that is comparable with that seen in the in vitro assay that utilizes artificial reductants. Methionine synthase reductase is a soluble, monomeric protein with a molecular mass of 78 kDa. It is a member of the family of dual flavoproteins and is isolated with an equimolar concentration of FAD and FMN. Reduction by NADPH results in the formation of an air stable semiquinone similar to that observed with cytochrome P-450 reductase. Methionine synthase reductase reduces cytochrome c in an NADPH-dependent reaction at a rate (0.44 micromol min(-1) mg(-1) at 25 degrees C) that is comparable with that reported for NR1, a soluble dual flavoprotein of unknown function, but is approximately 100-fold slower than that of P-450 reductase. The K(m) for NADPH is 2.6 +/- 0.5 microm, and the K(act) for methionine synthase reductase is 80.7 +/- 13.7 nm for NADPH-dependent activity of methionine synthase.

5-Methyltetrahydrofolate-Homocysteine S-Methyltran↗