A platelet derived inhibitor of plasma prekallikrein activation

Platelet products released by ADP mediated platelet aggregation or by repeated freeze-thawing of platelet-rich-plasma caused inhibition of the dextran sulphate induced activation of prekallikrein, measured amidolytically, in subsequently prepared platelet-poor-plasma. Platelet products did not inhibit the amidolytic activity of contact activated-plasma. Addition of antiserum to platelet factor-4 (PF4) neutralized the inhibitory effect of platelet products towards prekallikrein activation. When platelet released products were fractionated by heparin affinity chromatography, 93% of the PF4 applied and 86% of the inhibitory activity of the starting material was recovered in the high-affinity fraction.