HEMOPROTEINS ARE DIRECTLY REDUCED AND NOT BY THE SUPEROXIDE PATHWAY DURING XANTHINE OXIDASE-CATALYZED HYDROXYLATION OF PYRAZINOIC ACID

Xanthine oxidase catalyses redox reactions whereby a net exchange of a n oxygen atom between substrate and water occurs with the release of t wo electrons and two protons. This study was aimed at the xanthine oxi dase-catalysed transfer of electrons from pyrazinoic acid to acceptor molecules. UV spectral analyses showed that cytochrome c was stabilise d in its reduced form during pyrazinoic acid oxidation. With xanthine, however, reoxidation of cytochrome c was mediated by hydrogen peroxid e formed during xanthine oxidase catalysis. Rates of nitroblue tetrazo lium (NBT) reduction suggested that superoxide formation in the presen ce of pyrazinoic acid was slow relative to that in the presence of xan thine. Direct reduction of myeloperoxidase haem occurred with xanthine oxidase/pyrazinoic acid under anaerobic conditions, but not with xant hine oxidase/xanthine. In contrast to the physiological (hypo)xanthine /xanthine oxidase system that gives a mixture of mono and divalent red uction of dioxygen, electrons from pyrazinoic acid are transported mai nly to haemoproteins.