The Mechanism of Conversion of Xanthine Dehydrogenase to Xanthine Oxidase

Summary

Xanthine dehydrogenase and xanthine oxidase are complex metalloflavoproteins that represent alternate forms of the same gene product. The cDNAs encoding the enzymes have been cloned from several sources, and structural information is becoming available. Using purified enzyme, comparative analyses between the two forms were attempted by spectroscopic and kinetics methods. The most significant difference between the two forms is the protein conformation around flavin adenine dinucleotide (FAD), which changes the redox potential of the flavin and the reactivity of FAD with the electron acceptors, nicotinamide adenine dinucleotide (NAD) and molecular oxygen. The flavin semiquinone is thermodynamically stable in xanthine dehydrogenase but is unstable in xanthine oxidase. Detailed analyses by stopped-flow techniques suggest that the flavin semiquinone reacts with oxygen to form superoxide anion while the fully reduced flavin reacts to form hydrogen peroxide. Although xanthine dehydrogenase can produce greater amounts of superoxide anion than xanthine oxidase during xanthine oxygen reaction, it seems not to be physiologically significant in the cell, where excess NAD exists under normal conditions.