Studies on xanthine oxidase

Abstract

Abstract 1. 1. Histamine counteracts the inhibitory action of excess hypoxanthine or xanthine on xanthine oxidase (xanthine: O2 oxidoreductase, EC 1.2.3.2), but has no effect at low concentrations o substrate. 2. 2. The effect of histamine is more definite at alkaline pH, with decreasing concentration of enzyme, and with increasing duration of incubation. 3. 3. The activity of xanthine oxidase with, or without, histamine is largely influenced by the kind of buffer. The maximum effect of histamine is obtained with dilute solution of potassium phosphate buffer. 4. 4. The effect of histamine is observed practically on all the samples of the enzyme prepared by Ball's method, although its extent varies from batch to batch. Purification of the enzyme raises the rate of activation. Trypsin treatment of the enzyme has no influence on the appearance of the “histamine effect”. 5. 5. An effect similar to that of histamine is produced by l -histidine, KCN and EDTA. A less-marked effect is produced by imidazoleacetic acid, riboflavin, 5-hydroxytryptamine, l -arginine, l -lysine and glycine. 6. 6. It is suggested that histamine may remove metallic ion(s) which are toxic to the enzyme.