96-03591 Massey 1. Technical This investigator has isolated milk xanthine dehydrogenase in a stable state, which can be converted reversibly into the oxidized form, where the preferred electron acceptor is oxygen rather than NAD, as it is with the dehydrogenase form. This work indicates that the major effects of this reversible interconversion, which is brought about by oxidation of spatially adjacent thiol residues in the dehydrogenase to oxidase conversion, involve conformational changes in the protein which affect mainly the flavin redox center. The major thrust of this research is to study in detail the physicochemical properties of the two forms of the enzyme, with the aim of defining the parameters that give the two forms such different catalytic properties. This study involves extensive use of rapid reaction spectrophotometry, flavin replacement studies, catalytic properties of the enzymes with different classes of substrates, and examination of the modes of binding and mechanism of potent inhibitors of the enzymes. 2. Nontechnical The ability of xanthine oxidase to exist in two stable forms with very different catalytic properties makes it a very unusual enzyme. It is widely believed that this interconversion forms the basis of a variety of oxidative stress phenomena, because of the ability of the oxidase form to generate oxygen radicals. In order to assess such hypotheses, it is essential to have a thorough understanding of the properties of the two enzyme forms. This project is focused on such a study.
