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It was stated in previous papers (Avis, Bergel & Bray, 1955; Avis, Bergel, Bray, James & Shooter, 1956; Avis, Bergel & Bray, 1956) that the enzymie activity of purified xanthine oxidase from cow's milk, like that of other enzymes, deteriorates in aqueous solution under a variety ofmild conditions. Since then we recorded in a preliminary communication (Bergel & Bray, 1956) the stabilizing effects of sodium salicylate on this activity and mentioned briefly during a symposium the inactivation of the enzyme by certain metals (Bergel & Bray, 1958). We have now extended our studies of loss and preservation of the catalytic power of xanthine oxidase in order to establish optimum conditions for storage and utilization of the enzyme in biological experiments (cf. Bergel, Bray, Haddow & Lewin, 1957; Haddow, de Lamirande, Bergel, Bray & Gilbert, 1958; Bergel, 1959). These studies should also help in elucidating more rapidly the chemical nature of the enzyme and in its separation from various closely related inactive compounds. In this paper we demonstrate that at least three inactivation mechanisms, namely interaction with metals, photo-oxidation and gross denaturation, are operative, and that some of the losses of activity can be counteracted by a number of chemicals.

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