Influence of dietary methionine on the metabolism of selenomethionine in rats

To determine the influence of methionine on selenomethionine (SeMet) metabolism, weanling male rats were fed for 8 wk a basal diet marginally deficient in sulfur amino acids, containing 2.0 micrograms selenium (Se)/g as DL-SeMet and supplemented with 0, 0.3, 0.6 or 1.2% DL-methionine. Increased dietary methionine caused decreased selenium deposition in all tissues examined but increased glutathione peroxidase (GSHPx, EC 1.11.1.9) activity in testes, liver and lungs. A positive correlation was found between dietary methionine and the calculated percentage of selenium associated with GSHPx. In a second experiment, 75SeMet was injected into weanling male rats which had been fed the basal diet containing 2.0 micrograms selenium as DL-SeMet with or without the addition of 1.0% methionine. The selenoamino acid content of tissues and the distribution of 75Se in erythrocyte proteins were determined. In comparison to the rats fed the basal diet without added methionine, significantly more 75Se-selenocysteine was found in liver and muscle, more 75Se was found in erythrocyte GSHPx and less 75Se was found in erythrocyte hemoglobin of rats fed 1.0% methionine. These data suggest that methionine diverts SeMet from incorporation into general proteins and enhances its conversion to selenocysteine for specific selenium-requiring proteins, such as GSHPx.