This application is based on the overall hypothesis that, although, by definition, non-essential amino acids can be synthesized at high rats by humans, there are circumstances under which. because of inadequate protein and/or non-essential amino acid intake, their availability limits important processes, exemplified by reduced glutathione synthesis. To test this hypothesis, the applicant proposes three main experiments which focus on the regulation of glutathione turnover and synthesis. In the first specific aim variations in the absolute intake of methionine with or without variable supplements of cysteine will be used as a dietary manipulation which, it is hypothesized, will have metabolically significant effects upon glutathione metabolism. In the second specific aim, the applicant proposes to vary total protein intake and the ration of essential to non-essential amino acids and to quantify the impact of these dietary regimes on whole body protein, glycine, and glutathione kinetics. In the third specific aim, the applicant proposes to quantify the effect of a prolonged (5-day fast) on glutathione synthesis and turnover. In the fourth specific aim, the applicant proposes to complete studies of arginine metabolism by demonstrating that dietary ornithine is an inadequate precursor for whole body arginine biosynthesis. The research design will involve relatively prolonged ( > 6 days) periods of adaptation to diets that contain a variety of protein levels and amino acid mixtures followed by stable isotopic tracer infusions. Two novel tracers in particular will be used intrinsically labeled reduced glutathione and 13C 5-oxoproline. The end points of the experiments will be measurements of cysteine, glycine, 5- oxoproline, a glutathione turnover and separate measurements of the incorporation of precursor amino acids into whole blood glutathione. The overall thrust of the proposal is highly significant both to nutrition in general and clinical nutrition in particular. The results in many respects will be unique and will address an area of amino acid nutrition which has been discussed at length in the review literature but has rarely been investigated in a systematic and quantitative way.