Nonequivalence of the flavin adenine dinucleotide moieties of chicken liver xanthine dehydrogenase.

Prior reduction of chicken liver xanthine dehydrogenase with xanthine or NADH leads to accelerated loss of enzymic FAD on exposure to 3 M KI. Reduction with NADH leads to the dissociation of all of the FAD, whereas reduction with xanthine results in the rapid dissociation of only 50% of the FAD. In both cases the xanthine ---f NAD activity is completely abolished. Oxidation of xanthine in the presence of artificial electron acceptors is effected at comparable rates by the two apoproteins. Oxidation of NADH by the partially deflavinated enzyme is generally commensurate with its flavin content, whereas the fully deflavinated enzyme is incapable of oxidizing NADH. Spectrophotometric and electron paramagnetic resonance studies on the apoproteins have shown that the partially deflavinated enzyme is reducible by both xanthine and NADH, whereas the fully deflavinated enzyme is reducible only by xanthine. The results are discussed in the context of the arrangement of the electron carrier components in the active center of the enzyme.