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.