CHYLOMICRON-INDUCED PROTHROMBIN ACTIVATION AND PLATELET-AGGREGATION

The effects on platelet aggregation of native rat chyle chylomicrons, chylomicron remnants, and chylomicrons that had been preincubated with rat or human EDTA-plasma, serum, whole blood, or pure human prothrombin were examined. The native chyle chylomicrons did not induce platelet aggregation but decreased ADP- and thrombin-induced platelet aggregation and [C-14]serotonin release. Chylomicron remnants also failed to induce platelet aggregation, but they potentiated the aggregation and the [C-14]serotonin release induced by ADP and thrombin. Aggregation, after a lag phase of 15 to 20 minutes, was seen when platelets were incubated with chylomicrons that had been preincubated with plasma and then isolated as the top layer after a single centrifugation at d=1.006. This aggregation was inhibited in a dose-dependent manner by an antiserum against prothrombin that also inhibited thrombin-induced platelet aggregation. After washing by centrifugation the plasma-preincubated chylomicrons did not induce platelet aggregation, but this effect could be restored by adding a small amount of prothrombin, which did not cause aggregation when added alone or together with native chyle chylomicrons. Addition of 2% (vol/vol) plasma, however, induced aggregation when added together with either native chyle chylomicrons or washed preincubated chylomicrons, but not when added alone. Binding of I-125-labeled prothrombin to native chyle chylomicrons was demonstrated by gradient ultracentrifugation. During incubation of washed plasma-preincubated chylomicrons with I-125-prothrombin and platelets, a significant conversion of I-12S-prothrombin to I-125-prethrombin and I-125-thrombin occurred, as demonstrated by autoradiography after separation on sodium dodecyl sulfate-polyacrylamide gel electrophoresis. The interaction between chylomicrons and prothrombin, and possibly other coagulation proteins, thus enhances prothrombin activation in the presence of platelets.