A platelet aggregating substance contained in xanthine oxidase preparation from cow's milk.

Superoxide-generating system (xanthine-xanthine oxidase system) induced the platelet aggregation and the release of 3H-serotonin from washed human platelet. However, the aggregation was induced even when xanthine substrate was excluded from the system or when the activity of xanthine oxidase was completely abolished by the addition of 1.6 mM allopurinol to the system. Xanthine oxidase preparation from cow's milk was chromatographed on Sephadex G-200 column, and it was found that the platelet aggregating substance was separated from xanthine oxidase activity by this procedure. The aggregating activity of the substance was destroyed by boiling it for 3 min. Its molecular weight was estimated to be about 17,000. The levels of malondialdehyde and thromboxane B2 production in the platelets were increased during the aggregating 9 and 4 times, respectively, compared to those of resting cells.