A novel thrombin enhancement factor in human plasma

Fibrin assembly in human plasma and fibrinogen/albumin mixtures

Magnetic birefringence is used to monitor the kinetics of thrombin-catalyzed fibrin polymerization in model systems of increasing complexity (i.e., fibrinogen solutions, fibrinogen/albumin mixtures, and plasma anticoagulated with citrate) and in plasma containing free calcium which is the physiological condition. The introduction of albumin into fibrinogen solutions shortens the lag period and enhances fiber thickness. The polymerization progress curves are sigmoidal at zero or low albumin concentrations, but at physiological and higher concentrations, they become hyperbola-like from the end of the lag period. High albumin concentration has thus induced a change in the assembly kinetics. The progress curves from plasma in which the cascade is dormant are also hyperbola-like although they round off more quickly because of antithrombin activity. In plasma containing free calcium, thrombin is endogenously produced, and the progress curves are nearly linear; hence, the assembly kinetics are very different from those of the model systems. The curves are not influenced by calcium-dependent cross-linking involving factor XIIIa. The progress curves are also linear when polymerization is induced with Russell's viper venom, which by directly activating factor X circumvents earlier steps in the cascade. This implies that linear polymerization is caused by events posterior to factor X activation and are thus likely to be largely dependent on the functioning of the prothrombinase complex. Addition of thrombin to plasma containing free calcium reduces the lag period. At low exogenous thrombin levels, the polymerization rate is increased, and the progress curves remain linear. However, at higher levels, the curves become more complicated and, paradoxically, full polymerization takes longer.(ABSTRACT TRUNCATED AT 250 WORDS)

Thrombin-reactive polypeptides of platelets may regulate inhibition of thrombin by antithrombin

The central enzyme involved in blood coagulation and activation of platelets is the serine proteinase thrombin. The principal inhibitor of this proteinase in plasma is antithrombin. The mechanism of regulation of the thrombin-antithrombin reaction remains unknown. Two polypeptides of 74 and 55 kDa present on the platelet surface and in plasma are known to specifically enhance the activity of thrombin on different substrates. This study was undertaken to assess the effects of these platelet proteins on thrombin-antithrombin interaction. Direct measurements of residual thrombin activity in mixtures of thrombin and antithrombin, in the presence or absence of the platelet proteins, were made utilizing a specific chromogenic substrate. Under these conditions, when 60% of thrombin activity was inhibited by antithrombin in controls, 100% of enzyme activity was retained in the presence of the platelet proteins. When heparin was used in these assays, the rate of inhibition of thrombin by antithrombin was much more rapid and 62% of thrombin activity remained after 1 min. Under these conditions, the platelet proteins continued to protect thrombin from inactivation with 98% activity remaining at 1 min and 85% activity at 5 min. In contrast, the inhibition of trypsin by antithrombin was not affected by the platelet proteins. Additional studies in platelet aggregation showed that the platelet polypeptides have two effects on thrombin: (i) protection of the enzyme inhibition by antithrombin and (ii) stabilization of thrombin from loss of activity due to aging. The results suggest a novel role for the platelet proteins in hemostasis - regulation of the inhibition of thrombin by antithrombin.