Mechanism of inhibition of xanthine oxidase with a new tight binding inhibitor

Abstract

The mechanism of inhibition of milk xanthine oxidase and xanthine dehydrogenase by the tight binding inhibitor, sodium-8-(3-methoxy-4- phenylsulfinylphenyl)pyrazolo[1,5-α]-1,3,5-triazine-4-olate monohydrate (BOF-4272), was studied after separation of the two isomers. The steady state kinetics showed that the inhibition by these compounds was a mixed type. One of the isomers had a K(i) value of 1.2 x 10-9 M and a K(i)' value of 9 x 10-9 M, while the other isomer had a K(i) value of 3 x 10-7 M and a K(i)' value of 9 x 10-6 M. Spectral changes were not observed by mixing either the oxidized or reduced form of the enzyme with BOF-4272. The stopped-flow study and the effects of BOF-4272 on various substrates showed that BOF-4272 bound to the xanthine binding site of the enzyme. K(d) values of the enzyme and one of the isomers, which has a higher affinity for the enzyme, were also found to be 2 x 10-9 M for the active form of the enzyme and 7 x 10-9 M for the desulfo-form using fluorometric titration, and the binding has stoichiometry of 1:1. The inhibitor could not bind to the enzyme when the enzyme was previously treated with oxipurinol.