Xanthine Oxidase VI. INFLUENCE OF pH ON SUBSTRATE SPEClFICITY*

Milk xanthine oxidase exhibits a broad substrate specificity that includes purines, aldehydes, and pteridines (1). The mechanism of substrate oxidation, particularly that of purines, has been the subject of several studies (2-4). The present report concerns the observation of a new class of substrates, quaternary heterocyclic compounds, which are oxidized at pH values above 9.6. A partial list of substrates that are oxidized at pH 7.8 and 10.8 is given in Table I. The data to be presented indicate that all substrates are oxidized at the same active center and binding site and support the view that quaternary compounds become substrates owing to the titration of a group on the enzyme that normally prevents their approach to the substrate-binding site. These findings should provide additional information on which to base a reaction mechanism, and they will be useful in attempts to establish the nature of the substrate-binding site.