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Allopurinol and other 6-unsubstituted pyrazolo3,4-d-pyrimidines have been shown to reduce all of the oxidation-reduction reactive groups of milk xanthine oxidase. The great inhibitory action of these compounds has been shown to be due to a sort of suicide reaction, in which the product formed by enzymatic hydroxylation in position 6 is joined in complex with a partially reduced from of the enzyme. Several lines of evidence are presented indicating that complex formation is with the enzyme-bound molybdenum in the Mo(IV) state. Studies on the stoichiometry of binding of the product of allopurinol reaction, allo-xanthine(4,6-dihydroxypyrazolo3,4-dpyrimidine), revealed that the amount of allo-xanthine bound was proportional to the percentage of functional active sites in the enzyme preparation; i.e. it serves as an active site label. From these results it is concluded that the best preparations of xanthine oxidase so far obtained contain only 75 to 80% functional active sites, even though they possess their full complement of flavin, molybdenum, and iron-sulfur components. Evidence is presented that the presence of nonfunctional active sites is responsible for several of the slow reactions previously observed with the enzyme.
Massey et al. (Mon,) studied this question.