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PubMed2003

Methionine sulfoxide reduction in mammals: characterization of methionine-R-sulfoxide reductases.

Methionine residues in proteins are susceptible to oxidation by reactive oxygen species, but can be repaired via reduction of the resulting methionine sulfoxides by methionine-S-sulfoxide reductase (MsrA) and methionine-R-sulfoxide reductase (MsrB). However, the identity of all methionine sulfoxide reductases involved, their cellular locations and relative contributions to the overall pathway are poorly understood. Here, we describe a methionine-R-sulfoxide reduction system in mammals, in which two MsrB homologues were previously described. We found that human and mouse genomes possess three MsrB genes and characterized their protein products, designated MsrB1, MsrB2, and MsrB3. MsrB1 (Selenoprotein R) was present in the cytosol and nucleus and exhibited the highest methionine-R-sulfoxide reductase activity because of the presence of selenocysteine (Sec) in its active site. Other mammalian MsrBs contained cysteine in place of Sec and were less catalytically efficient. MsrB2 (CBS-1) resided in mitochondria. It had high affinity for methionine-R-sulfoxide, but was inhibited by higher concentrations of the substrate. The human MsrB3 gene gave rise to two protein forms, MsrB3A and MsrB3B. These were generated by alternative splicing that introduced contrasting N-terminal and C-terminal signals, such that MsrB3A was targeted to the endoplasmic reticulum and MsrB3B to mitochondria. We found that only mitochondrial forms of mammalian MsrBs (MsrB2 and MsrB3B) could compensate for MsrA and MsrB deficiency in yeast. All mammalian MsrBs belonged to a group of zinc-containing proteins. The multiplicity of MsrBs contrasted with the presence of a single mammalian MsrA gene as well as with the occurrence of single MsrA and MsrB genes in yeast, fruit flies, and nematodes. The data suggested that different cellular compartments in mammals maintain a system for repair of oxidized methionine residues and that this function is tuned in enzyme- and stereo-specific manner.

Alternative Splicing↗

PubMed1987

The normal pituitary examined with positron emission tomography and (methyl-11C)-L-methionine and (methyl-11C)-D-methionine.

Four patients with radiologically normal pituitary gland were examined with positron emission tomography after the administration of (methyl-11C)-L-methionine. On a following day the examination was repeated with (methyl-11C)-D-methionine. The accumulation rate of L-methionine in the pituitary was measured, giving a value that was about twice that of normal brain tissue. The accumulation rate of D-methionine in the pituitary was almost a factor of 10 lower than that of L-methionine. In the normal brain tissue that ratio was 2.3. The study clearly indicates that the methionine uptake in the pituitary is stereospecific. 11C-D-methionine is freely distributed in the tissue without entrapment, whereas 11C-L-methionine is irreversibly bound. It is concluded that PET with 11C-L-methionine can be used to study amino acid utilization in the pituitary.

Adult↗

PubMed2006

Minimum methionine requirement and cysteine sparing of methionine in healthy school-age children.

BACKGROUND: Cysteine can provide a portion of the sulfur amino acid requirement in adults. Whether this is true in children-and, if so, to what extent-is not known. OBJECTIVES: The objectives were to determine minimum methionine requirements in healthy, school-age children when excess cysteine is provided and to subsequently determine the cysteine-sparing effect by comparing these methionine requirements with those determined previously in the same children when no cysteine was provided. DESIGN: Six healthy, school-age children randomly received graded intakes of methionine (0, 2.5, 5, 7.5, 10, and 15 mg . kg(-1) . d(-1)) along with 21 mg cysteine . kg(-1) . d(-1) in the diet. The mean methionine requirement was determined by using a biphasic linear regression crossover analysis of measurements of the rate of appearance of (13)CO(2) in the breath (F(13)CO(2)), which identified a breakpoint at the minimal F(13)CO(2) in response to graded levels of methionine intake. RESULTS: The mean and population-safe minimum methionine requirements, in the presence of excess dietary cysteine, were found to be 5.8 and 7.3 mg . kg(-1) . d(-1), respectively. The mean and population-safe (upper 95% CI) methionine requirements, in the absence of dietary cysteine, were previously determined to be 12.9 and 17.2 mg . kg(-1) . d(-1), respectively. These values represent a cysteine-sparing effect of 55% and 58% in comparison with mean and population-safe methionine requirements, respectively. CONCLUSION: Excess intake of dietary cysteine results in the reduction in the requirements for methionine to a minimum obligatory requirement level.

Breath Tests↗

PubMed1982

Methionine metabolism in rainbow trout fed diets of differing methionine and cystine content.

The metabolism of methionine was studied in rainbow trout fed diets containing different levels of methionine and cystine. Growth data indicated that methionine requirement was between 0.5 and 1% dry diet in the absence of dietary cystine, but 0.5% was adequate when dietary cystine was 2%. In fish fed diets deficient in sulfur-containing amino acids, elevated hepatic activities of glutathione reductase were found, whereas glutathione peroxidase and glutathione levels were unaffected. Plasma and liver concentrations (18 hours after feeding) of methionine and cystine were affected by dietary methionine, but dietary cystine had little effect. Cystine appeared to be converted to taurine in the liver. In fish injected intraperitoneally with [14COOH]- and [14CH3]methionine, over 24 hours the carboxyl group was oxidized more than the methyl group and more was incorporated into protein. However, much more of the methyl group was incorporated into the lipid fraction. The results suggest the operation of the transsulfuration pathway of methionine catabolism. Oxidation of methionine was related to its concentration in the tissues, and little affected by dietary cystine. A 28-day experiment on the metabolism of injected [14COOH]methionine showed that its turnover was slow, and much of the radioactivity was associated with protein.

Amino Acids↗

PubMed1998

Peptide-bound methionine as methionine sources for protein accretion and cell proliferation in primary cultures of ovine skeletal muscle.

Ruminants have high concentrations of peptide-bound amino acids in the circulation. Earlier studies in our laboratory showed that a myogenic cell line (C2C12) developed from mouse skeletal muscle and a mammary epithelial cell line (MAC-T) developed from bovine mammary epithelial tissues are able to utilize peptides as amino acid sources. In this study, primary cultures of ovine myogenic satellite cells were evaluated for their ability to use peptide-bound methionine as a source of methionine for protein accretion and cell proliferation. The basal medium contained methionine-free Dulbecco's modified Eagle's medium supplemented with 6% desalted fetal bovine serum. Treatment media included the basal medium supplemented with no methionine, methionine or one of 22 methionine-containing peptides. No protein or DNA accretion was observed in the presence of basal medium alone. Growth responses to all of the peptides were obtained, with protein and DNA accretion ranging from 49 to 107% and from 45 to 144% of the corresponding methionine response, respectively. These results indicate that the ovine myogenic satellite cells possess the ability to utilize methionine-containing peptides as methionine sources for protein accretion and cell proliferation.

Animals↗

PubMed1999

Ruminal escape, gastrointestinal absorption, and response of serum methionine to supplementation of liquid methionine hydroxy analog in dairy cows.

Availability of liquid methionine hydroxy analog [D,L-2-hydroxy-4-(methylthio)-butanoic acid] was evaluated in two experiments using four cannulated lactating dairy cows. The first experiment was designed as a 4 x 4 Latin square. Each cow received a pulse dose of 0, 30, 60, or 90 g of the methionine analog in the rumen each day for 10 d. Duodenal samples were collected at 16, 20, and 24 h after dosing for the last 5 d and pooled. The methionine analog was not detected in duodenal contents because it passed rapidly from the rumen relative to the sampling protocol. In the second experiment, cows were offered 90 g of the methionine analog and 600 ml of Cr-EDTA (3.5 g of Cr) mixed with ground corn for a period of 20 min after which any remains of the treatment were placed in the rumen. The concentration of the analog peaked in ruminal and duodenal fluid at 1 and 3 h, respectively. Based on the fractional rate constants for ruminal and duodenal disappearance of the methionine analog and passage of the liquid, it was determined that 50.0 +/- 2.8% of the methionine analog escaped ruminal degradation and became available for intestinal absorption (44.6 +/- 5.7%) or was absorbed from the omasum (5.4 +/- 3.3%). Serum methionine concentration peaked 6 h after analog dosing at a level that was three times the predose level, indicating that the methionine analog that escaped ruminal degradation was absorbed and metabolized to methionine.

Animals↗

PubMed1981

Effectiveness of the free acid of methionine hydroxy analogue as a methionine supplement in broiler diets.

A liquid methionine supplement known as methionine hydroxy analogue-free acid (MHA-FA) was evaluated as a source of methionine in the diet of young broiler chicks. Diets calculated to be deficient in methionine were supplemented with MHA-FA and performance compared to that obtained when comparable diets were supplemented with molar equivalents of methionine from L-methionine, DL-methionine, or the calcium salt of methionine hydroxy analogue. Performance of chicks fed the test product was equal to that obtained on the other methionine products. The use of MHA-FA might allow for a more even distribution in the diet, easier handling, and a lower manufacturing cost with resultant savings in the cost of producing poultry and eggs.

Animal Feed↗

PubMed1986

Effect of growth factors and methionine on glycolysis and methionine transport in rat fibroblasts and fibroblasts transfected with myc and ras genes.

The inhibition of glycolysis in tumor cells by methionine requires that the cells be incubated with methionine for several hours in the presence of serum. We now show that in the case of confluent rat-1 fibroblasts transfected with the ras gene the serum can be substituted by insulin and insulin-like growth factor I or II. No other growth factor tested was effective. In subconfluent ras cells additional growth factors (transferrin and high density lipoproteins) were required for maximal inhibition of glycolysis by methionine. Exploration of the mechanism of action of methionine revealed that the accumulation of [35S]methionine into rat-1 fibroblasts was only marginally increased by insulin. We propose that methionine inhibits an adenosine triphosphatase activity because addition of low concentrations of Nonidet P-40 greatly enhanced glycolysis even in the presence of methionine, suggesting that it did not affect the glycolytic enzymes directly. Methionine also affected growth both in monolayer and soft agar. Rat-1 fibroblasts transfected with the ras gene were markedly more sensitive to methionine than cells transfected with the myc gene.

Animals↗

PubMed1976