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Xanthine dehydrogenase, purified 550-fold from Micrococcus lactilyticus, exhibits a molecular weight of about 250,000 and an absorption spectrum which differs only slightly from that of related molybdo-iron-flavoproteins, and appears to contain 2 g atoms of molybdenum, 8 g atoms of iron, and 2 moles of flavin per mole. The enzyme catalyzes the oxidation of a wide variety of purines and aldehydes. Ferredoxin is the most effective electron acceptor, but oxygen, various dyes, and ferricyanide can also be utilized, as can cytochrome c, but DPN+ cannot. The reductions of ferredoxin and cytochrome c are not dependent upon the presence of molecular oxygen, but may be correlated with the appearance of a stable flavin semiquinone in enzyme which has been reduced by its substrates. The enzyme catalyzes the reversible dismutation of xanthine to hypoxanthine and urate at 60% of the rate for the aerobic oxidation of xanthine. The enzyme is resistant to inhibition by cyanide, and is extremely susceptible to progressive inhibition by methanol. These latter observations strengthen the hypothesis that molybdenum serves as a hydroxyl acceptor and donor in the normal catalytic function of molybdo-iron-flavoproteins. It is also resistant to arsenite, but is progressively inactivated by borate.
Smith et al. (Fri,) studied this question.