Platelet Factor V-Like Coagulant Activity And Catalytic Phospholipid-Like Surface Activity In Clotting And Chromogenic Assays

Abstract

The role of the platelet membrane as a catalytic surface in thrombin generation was studied, in vitro, using normal and F.V-deficient human plasmas. Frozen-and-thawed lysed platelet supernatant from normal or V-deficient donors was used as a reference for physiologic activation, in vitro. Coagulant activities were evaluated by both a clotting test (based on the KAPTT, in which the kaolin suspension was removed prior to recalcification) and by assessing thrombin formation (based on the maximum rate of hydrolysis of S-2238). The degree to which the clotting time was shortened or thrombin formation was enhanced was taken as overall catalytic surface activity. In both tests, log-log plots of the dependent variable as a function of the dilution of a given membrane preparation were linear over several orders magnitude (p >.25). The slopes of such plots were taken as overall catalytic surface effectiveness and were dependent on both the parametric level of F.V and the nature of the membrane. For the same membrane preparation, effectiveness in the clotting test was highly negatively correlated (-r >0.9) with effectiveness in the S-2238 system (i.e., clotting time was inversely proportional to the maximum amount of thrombin generated). The change in effectiveness as a function of the parametric level of F.V was taken as the sensitivity of that surface to F.V. For a given control, both test systems appeared to be equally sensitive to the parametric level of F.V. On the basis of differences in catalytic effectiveness, it was possible to distinguish qualitatively between membrane associated F.V- like activity (platelet factor 1) and phospholipid-like catalytic surface activity (platelet factor 3). Discrete clotting time measurements, like single thrombin determinations, can not adequately describe the catalytic contribution of the platelet membrane in thrombin generation.