NADH OXIDASE ACTIVITY OF HUMAN XANTHINE OXIDOREDUCTASE - GENERATION OF SUPEROXIDE ANION

Human xanthine oxidase was purified from breast milk. The dehydrogenas e form of the enzyme, which predominates in most mammalian tissues, ca talyses the oxidation of NADH by oxygen, generating superoxide anion s ignificantly faster than does the oxidase form. The corresponding form s of bovine enzyme behave very similarly. The steady-state kinetics of NADH oxidation and superoxide production, including inhibition by NAD , by the dehydrogenase forms of both enzymes, are analysed in terms of a model involving two-stage recycling of oxidised enzyme. Established inhibitors of xanthine oxidoreductases (allopurinol oxypurinol, amflu tizole and BOF 4272), which block all other reducing substrates, were ineffective in the case of NADH. Diphenyleneiodonium, on the other han d, was a powerful inhibitor of NADH oxidation. The potential involveme nt of reactive oxygen species arising from NADH oxidation by xanthine oxidoreductase in ischaemia-reperfusion injury and other disease state s, as well as in normal signal transduction, is discusssed.