This application concerns the dynamic aspects of whole-body acid metabolism in adult humans, with reference to the dietary importance of the nutritionally "dispensable" (non-essential) or "conditionally indispensable" amino acids. Our hypothesis is that, despite a constitutive capacity for de novo synthesis, their cellular availability for tissue/organ protein synthesis and other functions in vivo is determined by the nitrogen and amino acid composition of the diet. We also hypothesize that there is a strict dietary need for a preformed source of alpha-amino nitrogen, additional to that from the indispensable amino acids. A major focus here is on sulfur amino acid (SAA) interrelationships. We hypothesize that dietary cystine is a more efficient source as a glutathione (GSH) precursor than methionine. The SAA aims are: (i) to further explore the interrelationships between methionine and cysteine (or procysteine) intakes on methionine/cyst(e)ine and GSH metabolism in healthy adults, including measurement of isotopic enrichments of plasma homocysteine (hcy)and cystathionine; (ii) to measure rates of GSH synthesis using L-2 5-13C oxothiazolidine-4-carboxylic acid (OTZ) as labeled precursor and to use L- 2-13C OTZ as a marker of GSH metabolism; (iii) to study the metabolism of homocysteine, using labeled metaprobes, in relation to SAA intake; (iv) to further refine and improve upon our tracer model of GSH metabolism using measurements of isotopic labeling in glutamyl-cysteine and cysteinyl glycine; (v) to begin to accumulate the data necessary for the eventual construction of a compartmental model of GSH metabolism. With respect to dietary alpha-amino acid nitrogen intake we will (i) use the 24h indicator (13C-leucine) amino acid oxidation technique to evaluate the requirement for a source of alpha-amino N, including an assessment of its role in GSH homeostasis and (ii) use of 15N-homoarginine as a metaprobe for assessing of arginine metabolism. The proposed studies will further establish the quantitative and metabolic role played by the non-specific nitrogen component of the "protein" intake in whole-body protein (nitrogen) and specific amino acid homeostasis.