Use of GSH/GSSG redox to evaluate oxidative stress. This proposal is based upon our long-standing efforts to understand the control and function of the GSH-dependent antioxidant system in humans. The balance between GSH and GSSG is determined by the balance of oxidation due to oxidative stress and the capacity of the NADPH-dependent reductase to reduce GSSG back to GSH. Studies on the interactions of GSH and cysteine pools in human plasma revealed that GSH redox characteristics are suitable for use as an indicator of the balance between oxidative and antioxidant processes (26). In addition, our studies of human cells in culture show that substantial variation in GSH/GSSG redox occurs during the normal life cycle of cells (13, 27-28) and, most central to the present proposal, substantial oxidation can be induced by Cys deficiency (29). These results imply that variation in GSH/GSSG redox may be more than an indicator of oxidative stress, it may be an indicator of the cell survival/cell death balance which is central to tissue homeostasis. With this interpretation, the GSH/GSSG redox reflects underlying tissue health; an oxidized state serves as the platform for toxicity regardless of whether this oxidation is due to a genetic abnormality, nutritional deficiency, inflammation, or chemical exposure. Thus, any chemical exposure that results in GSH oxidation or cysteine utilization should exacerbate effects due to limited cysteine intake. Because a) sulfur amino acid deficiency is associated with oxidation of thiol/disulfide redox, b) sulfur amino acids are required for so many different metabolic processes, and c) so many enzymes are potentially affected by a change in thiol/disulfide redox state, diverse metabolic effects may occur that are not readily predicable from the metabolic profile of cysteine or from the metabolic pathways of chemicals detoxified by cysteine-dependent pathways. Hence, a central question of the current proposal is whether a chemical exposure which increases demand for sulfur SAA interacts with the dietary availability of SAA to produce perturbations of metabolism that can be detected by high resolution 1H-NMR spectroscopy.