Inactivation of Platelet Factor 4 in Plasma

Some factors, influencing the stability of platelet factor 4 (PF‐4) and of protamine in human plasma were investigated. PF‐4 and protamine were slowly inactivated in normal citrated plasma, but were stable when equivalent amounts of heparin were present. Plasmin enhanced the speed of inactivation. Low plasmin concentrations inactivated PF‐4 before fibrinogenolysis could be detected by a prolonged clotting time. Thus, inactivation of PF‐4, detectable in a heparin‐thrombin time system, seems to be a fairly sensitive indicator of plasmin actvity. Inactivation of PF‐4 was prevented by addition of protease inhibitors (Trasylol and Soy Bean Trypsin inhibitor) and is therefore most likely due to enzymatic degradation. The possibility that spontaneous inactivation of PF‐4 in normal plasma is due to traces of plasmin is discussed.