The chemistry of xanthine oxidase. 4. The problems of enzyme inactivation and stabilization

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Abstract
The loss of activity of solutions of xanthine oxidase has been studied under a variety of conditions in the presence of Cu salts, p-chloro-mercuribenzoate, H2O2 aad at 37[degree] and 56[degree]. At least 3 mechanisms by which activity is likely to be lost have been distinguished. These mechanisms are heavy-metal inactivation, photo-oxidation and gross denaturation. The effect of reducing ageats (especially ascorbic acid) on inactivation by Cu has been studied and is discussed in relation to the reported inhibitory action of ascorbic acid on xanthine oxidase. Chelating and "complexing" agents protect against metal inactivation. Competitive inhibitors protect against photo-oxidation and gross denaturation. Sodium salicylate (which may function both as an inhibitor and a chelating agent) is a particularly useful protecting agent during manipulation and storage of xanthine oxidase.