Selenium is an essential nutrient for man, in large part through the crucial part it plays in the mechanism of action of glutathione peroxidase. This enzyme seems to serve an important role in protecting cell membranes from oxidative damage by peroxides through reduction of the peroxide at the expense of glutathione. A less direct and negative role for the enzyme may occur in the etiology of rheumatoid arthritis. There is evidence to link some of the most effective drugs used in treatment of rheumatoid arthritis, including the gold drugs, with potent inhibition of glutathione peroxidase. At present, although the structure of the enzyme from bovine erythrocytes is known, the details of functioning of the selenium moiety, both in its normal enzymatic degradation of peroxides and in its inhibition by arthritis drugs is far from clear. By examining the molecular species and interactions of the selenium and its immediate environment in glutathione peroxidase, both with substrates and inhibitors, a much better understanding of both the functional need for selenium (cf. sulfur) and the mechanism whereby gold drugs and others are effective in arthritis treatment should be obtained. This understanding is the long term goal of this project. It is proposed: 1. To use 77-Se NMR as a very sensitive means of following changes in oxidation state of selenium in glutathione peroxidase during the catalytic cycle and to identify the species involved. 2. To characterize the binding sites on the enzyme for reduced and oxidized glutathione by 1-H NMR difference spectroscopy. 3. To obtain evidence concerning subunit interactions from equivalence of 77-Se resonances and 1-H difference spectra. 4. To investigate in model systems and in glutathione peroxidase the interactions between gold, and other arthritis drugs, and selenocysteamine and its seleninic acid, using 77-Se and 199-Hg NMR. 5. To compare the bovine enzyme with the human, ovine and rat liver enzymes by 1-H NMR difference spectroscopy. 6. To compare the mechanism of action of a synthetic low molecular weight glutathione peroxidase mimic (PZ-51) with that of the enzyme using 1-H and 77-Se NMR.