The inhibition by alloxanthine of oxidation of xanthine by xanthine oxidase is characterized by a prolonged transient phase. Kinetic data accord with a mechanism that involves rapid formation of a reduced enzyme-alloxanthine complex that subsequently undergoes a relatively slow-reversible reaction. In this scheme the slowly formed complex cannot be fully reoxidized by oxygen. From the Ki value for the dissociation of alloxanthine from the rapidly formed complex (1.15 microM) and values of 0.37 min-1 and 0.011 min-1 for the forward and reverse rate constants of the slow reaction, an overall inhibition constant for alloxanthine of 35 nM was calculated. A molybdenum (V) e.p.r. signal from the slowly formed reduced enzyme-alloxanthine complex is described. The rate of appearance of this new signal is consistent with this assignment. The signal (the “Alloxanthine signal”) was simulated with g1 2,0269, g2 1,9593, g3 11.9444 and shows indications of hyperfine coupling to nitrogen. Similarities between it and the Very Rapid signal are discussed. Close structural analogies between the catalytic intermediate represented by the Very Rapid signal and the inhibitor complex represented by the Alloxanthine signal are suggested.

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