Dietary Protein Level-Dependent Alterations in Urinary Excretion of Thiol Compounds Caused by L-Methionine Supplement in Mice

Abstract

To determine whether alterations in the L-methionine metabolism depend on nutritional conditions such as dietary protein levels, the effects of an L-methionine supplement for a protein-deficient diet on the urinary excretion of low molecular weight thiol compounds were compared to those of the supplement for an adequate protein diet. Although urinary concentrations of L-cysteine and L-homocysteine were increased by the L-methionine supplement irrespective of the dietary protein levels, the levels of increases were higher by the supplement to the protein-deficient diet than to the adequate protein diet. In addition, the urinary concentration of glutathione was increased only by the L-methionine supplement to the protein-deficient diet. However, the L-cysteinylglycine (CysGly) concentration was not affected by the L-methionine supplement to either diet. Thus, increases by the L-methionine supplement in the concentrations of these thiol compounds, except for CysGly, were more remarkable when the dietary protein level was deficient. These results suggest that the metabolism of excess L-methionine could be markedly affected by nutritional conditions, and that the alterations in the metabolism, at least partly, depend on the dietary protein levels.