Xanthine Oxidase. Vii. Inhibition by Amino Group Reagents.

In the previous paper (1) milk xanthine oxidase was reported to oxidize quaternary, heterocyclie compounds only at pH values above 9.6, whereas purines and aldehydes are most rapidly oxidized at neutral pH. This specificity change was attributed to an alteration in the substrate-binding site brought about by the titration of a group or groups with a pK, value of about 10.7. This suggestion prompted an investigation of the effects of amino group-reactive agents on xanthine oxidase in the hope that evidence could be found for the existence of a single such group at or near the substrate-binding site. The observation that the enzyme is inhibited by reaction of its amino groups with 2,4-dinitrofluorobenzene, 2,4,6-trinitrobenzenesulfonate, and benzaldehyde; the effects of these inhibitors; their mechanism of action; and some of the properties of inhibited xanthine oxidase comprise the subject of this report.