Effect of Human Platelets on the Amidolytic Activity of Human Factor XII Activated by Kaolin

Abstract

We have established an assay system for the amidolytic activity of purified human factor XII which is activated only by the negatively charged substance, kaolin, and examined the effect of washed platelets, ADP and serotonin on the amidolytic activity of kaolin-activated factor XII. Kaolin-treated platelets and serotonin of a physiological content in human platelet were found to promote the amidolytic activity of kaolin-activated factor XII, but ADP of a physiological content in human platelet had no such effect. The promoting effect of kaolintreated platelets and serotonin were inhibited by the addition of methysergide. Furthermore, it seems that serotonin affects the amidolytic activity of already activated factor XII by negatively charged substance.