Methionine Metabolism In Mammals Research Articles

The metabolism of homocysteine: pathways and regulation.

Two pathways, the methionine cycle and transsulfuration, account for virtually all methionine metabolism in mammals. Every tissue possesses the methionine cycle. Therefore, each can synthesize AdoMet, employ it for transmethylation, hydrolyze AdoHcy, and remethylate homocysteine. Transsulfuration, which occurs only in liver, kidney, small intestine and pancreas, is the means for catabolizing homocysteine. Liver has a unique isoenzyme of MAT that allows the utilization of excess methionine for the continued synthesis of AdoMet. Metabolic regulation is based on the distribution of available homocysteine between remethylation and conversion to cystathionine. The tissue content of the enzymes and their inherent kinetic properties provide the basis for the regulatory mechanism. The effector properties of the metabolites AdoMet, AdoHcy and methylTHF are of particular relevance.

Methionine metabolism in mammals. The methionine-sparing effect of cystine.

Cystine can replace approximately 70% of the dietary requirement for methionine. We used standard enzyme assays, determinations of the hepatic concentrations of metabolites and an in vitro system which simulates the regulatory site formed by the enzymes which utilize homocysteine in this study of the mechanism for this adaptation. A significant alteration in the pattern of hepatic homocysteine metabolism occurs following the substitution of cystine for methionine. The major change is a marked reduction in the synthesis of cystathionine. Decreases in both the level of cystathionine synthase and in the concentration of adenosyl-methionine, a positive effector of the enzyme, explain this finding. Despite significant increases in the hepatic levels of betaine-homocysteine methyltransferase and methyltetrahydrofolate-homocysteine methyltransferase, flow through these reactions remains relatively constant. The betaine enzyme may be essential for efficient methionine conservation. In the absence of choline, cystine cannot replace methionine in an adequate diet limited in the latter amino acid.

Methionine metabolism in mammals. Adaptation to methionine excess.

We conducted a systematic evaluation of the effects of increasing levels of dietary methionine on the metabolites and enzymes of methionine metabolism in rat liver. Significant decreases in hepatic concentrations of betaine and serine occurred when the dietary methionine was raised from 0.3 to 1.0%. We observed increased concentrations of S-adenosylhomocysteine in livers of rats fed 1.5% methionine and of S-adenosylmethionine and methionine only when the diet contained 3.0% methionine. Methionine supplementation resulted in decreased hepatic levels of methyltetrahydrofolate-homocysteine methyltransferase and increased levels of methionine adenosyltransferase, betaine-homocysteine methyltransferase, and cystathionine synthase. We used these data to simulate the regulatory locus formed by the enzymes which metabolize homocysteine in livers of rats fed 0.3% methionine, 1.5% methionine, and 3.0% methionine. In comparison to the model for the 0.3% methionine diet group, the model for the 3.0% methionine animals demonstrates a 12-fold increase in the synthesis of cystathionine, a 150% increase in flow through the betaine reaction, and a 550% increase in total metabolism of homocysteine. The concentrations of substrates and other metabolites are significant determinants of this apparent adaptation.

Methionine metabolism in mammals. Distribution of homocysteine between competing pathways.

Using an in vitro system which contained enzymes, substrates, and other reactants at concentrations which approximated the in vivo conditions in rat liver, we measured the simultaneous product formation by three enzymes which utilize homocysteine. In the control system, 5-methyltetrahydrofolate homocysteine methyltransferase, betaine homocysteine methyltransferase, and cystathionine beta-synthase accounted for 27, 27, and 46%, respectively, of the homocysteine consumed. Subsequent studies demonstrated that the adaptation from a high protein diet to a low protein diet is achieved by a significant increase in betaine homocysteine methyltransferase, and 83% reduction in cystathionine synthase, and a total decrease of 55% in the consumption of homocysteine. S-Adenosylmethionine, by activating cystathionine synthase, contributes significantly to the regulation of the pathway.

Methionine Metabolism in Mammals: Concentration of Metabolites in Rat Tissues

We have evaluated factors which regulate the content of methionine, adenosylmethionine, adenosylhomocysteine, cystine, cysteine and acid-soluble thiols in rat tissues. In liver the concentration of methionine appears relatively insensitive to changes in dietary protein intake. In contrast the hepatic levels of adenosylmethionine, adenosylhomocysteine, cystine, cysteine and soluble thiols increased with augmented dietary protein. The ratio of adenosylmethionine:adenosylhomocysteine approximated 6.0 in livers, brains, kidneys and skeletal muscles from rats fed the stock diet. Independent variation in the concentrations of these two metabolites did occur. However, the ratios in livers of animals maintained on diets with varying casein content equaled or exceeded a value of 5.0. We conclude that the maintenance of the concentration of methionine is the primary result of the various homeostatic mechanisms. In addition, most previous reports have overestimated the tissue content of adenosylhomocysteine.methionine adenosylmethionine adenosylhomocysteine sulfur amino acids transmethylation transsulfuration

Methionine metabolism in mammals: Regulation of methylenetetrahydrofolate reductase content of rat tissues

Methylenetetrahydrofolate reductase was present in all of the 12 rat tissues which we studied. The specific activity of the enzyme was highest in kidney, testes, pancreas, and liver—a pattern similar to that of methyltetrahydrofolate homocysteine methyltransferase. In liver, kidney, brain, and spleen, the specific activity of the reductase declined with age. The protein content of the diet had little effect on the enzyme in liver and brain. Protein restriction resulted in an increased specific activity in kidney and spleen. Treatment with several hormones led to significant and opposite changes in liver and kidney. Hydrocortisone, thyroxine, estradiol, and growth hormone increased the specific activity in liver while decreasing renal levels. Dietary or hormone-induced changes in the activity of methylenetetrahydrofolate reductase did not necessarily parallel the changes in the activity of the homocysteine methyltransferase. Thus it is possible that these exogenous factors may affect the rates of synthesis and of utilization of methyltetrahydrofolate.

Methionine metabolism in mammals: Regulatory effects of S-adenosylhomocysteine

S-Adenosylhomocysteine inhibits betaine-homocysteine methyltransferase. The inhibition is nonlinear, competitive in relation to homocysteine, and noncompetitive in relation to betaine. S-Adenosylhomocysteine activates cystathionine synthase at all concentrations of the substrates, serine and homocysteine. By altering the distribution of homocysteine between these competing pathways, S-adenosylhomocysteine may be significant in the regulation of methionine metabolism in the intact animal.

Methionine metabolism in mammals: The biochemical basis for homocystinuria

Three genetic disorders result in “homo-cystinuria”. Of these, cystathionine synthase deficiency is the most common. Impairment of N 5-methyltetrahydrofolate-homocysteine methyltransferase, due to either deficient coenzyme synthesis (derangement of B 12 metabolism) or to reduced substrate availability (methylenetetrahydrofolate reductase deficiency), is the basis for the other two forms. Analyses of the pathobiochemistry caused by these defects is essential for the development of rational and appropriate therapy. Such studies have the additional benefit of illuminating the pattern of methionine metabolism in the normal human.

Methionine metabolism in mammals: Synthesis of S-adenosylhomocysteine in rat tissues

We have developed a sensitive double-isotope assay to measure the enzymatic synthesis of S-adenosylhomocysteine from adenosine and l-homocysteine. The enzyme is present in all rat tissues except the small intestinal mucosa. In the liver, the enzyme is found in the postmicrosomal supernatant. We studied some of the kinetic properties of the liver enzyme. At high concentration, both substrates inhibit product formation. S-Adenosylmethionine also inhibits the reaction but other metabolites of methiouine do not. The specific activity of the enzyme in liver increases in animals fed a high protein ration. Treatment with hydrocortisone and estradiol also resulted in an increase in the specific activity while administration of thyroxine led to a decrease.

Methionine metabolism in mammals: Kinetic study of betaine-homocysteine methyltransferase

We have studied the kinetic properties of betaine-homocysteine methyltransferase prepared from rat liver. The Michaelis constants for the substrates are K hetaine = 48 μm and K homocysteine = 12 μm. The reaction conforms to the Ordered Bi Bi model. Homocysteine is the first substrate to add to the enzyme and N,N-dimethylglycine is the first product released. In addition to the reaction products, l-cysteine and l-cystine also inhibit the enzyme. These effects of methionine and cyst(e)ine on the betaine-chomocysteine methyltransferase may be significant in the regulation of methionine metabolism in the intact animal.

Methionine metabolism in mammals. Regulation of homocysteine methyltransferases in rat tissue

We studied two mammalian enzymes capable of remethylating homocysteine. Betaine-homocysteine methyltransferase was found only in rat liver while N 5 -methyltetrahydrofolate-homocysteine methyltransferase could be demonstrated in all tissues except small intestinal mucosa. Various hormones significantly affect the specific activity of the enzymes. These effects depend on hormone, enzyme, and tissue. The responses of the two transmethylases to dietary changes differed markedly. Betaine-homocysteine methyltransferase increased with protein and methionine feeding. N 5 -Methyltetrahydrofolate-homocysteine methyltransferase on the other hand increased under conditions that suggested the need for methionine synthesis. We conclude that N 5 -methyltetrahydrofolate-homocysteine methyltransferase activity contributes significantly to the regulation of methionine metabolism in mammals. High protein diets repress the synthesis of this enzyme in liver.

Methionine metabolism in mammals: Effects of age, diet, and hormones on three enzymes of the pathway in rat tissues

A comprehensive survey has been made to determine the effects of age, diet, and hormone administration on the activities of methionine-activating enzyme, cystathionine synthase, and cystathionase in rat tissues. Specific, sensitive assays were employed to measure changes in the levels of these enzymes in liver, kidney, pancreas, and brain. From these studies we concluded that (a) The developmental pattern of each enzyme varied in the several tissues and the patterns of the three enzymes often varied in the same tissue, (b) Many significant, organ-specific changes in the levels of individual enzymes can be effected by dietary and hormone treatments. However, these factors rarely resulted in a parallel change in the three enzymes in one organ or in a single enzyme in all organs, (c) The response of hepatic cystathionine synthase to many of these factors varies markedly from that observed for threonine dehydratase. This provides additional evidence that these two enzyme activities are not properties of a single protein in mammalian liver, (d) There are several possible implications of this study for the treatment of human cystathionine synthase deficiency (homocystinuria).