1. Two major forms of xanthine oxidase are demonstrated for the mouse. On polyacrylamide-gel electrophoresis the duodenal form migrates faster towards the anode than that of the liver. Both forms also differ in their (NH4)2SO4 precipitation patterns and sucrose-density-gradient molecular-weight determinations. 2. The liver form is fully converted into the duodenal form by incubation at 37°C with 2.5mg of crude trypsin/ml for 1½h, without loss of activity. The trypsin-treated liver form behaves like the normal duodenal form as characterized by electrophoresis, (NH4)2SO4 precipitation patterns, and sucrose-density-gradient molecular-weight determinations. 3. Partial conversion is also brought about by purified trypsin and chymotrypsin, but not with β-carboxypeptidase or lipase. The conversion is inhibited by soya-bean trypsin inhibitor. 4. In embryo mice the duodenal form is similar to the liver form on electrophoresis. 5. These studies indicate, as might be expected, that the duodenal form is a modified version of the liver enzyme, probably caused by proteolytic alteration.

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