BINDING OF PROTHROMBIN TO CHYLE CHYLOMICRONS - EFFECTS OF TEMPERATURE AND CALCIUM-IONS, AND ROLE OF SURFACE PHOSPHOLIPIDS

The ability of chyle chylomicrons to bind prothrombin has been studied. Rat chyle chylomicrons were incubated with human I-125-prothrombin and binding was examined by separating the chylomicrons from free I-125-prothrombin by density-gradient ultracentrifugation, and by gel filtration on Sepharose CL-2B. A significant binding of prothrombin to chyle chylomicrons occurred. The complex formation was calcium dependent, and decreased markedly when the temperature was lowered from 37 degrees C to 20 degrees C and when pH was raised above 8. The time course for the binding at 37 degrees C in presence of 2 mmol/L CaCl2 exhibited an initial lag phase at about 10 minutes. Thereafter most of the binding occurred within 30 minutes. Bound prothrombin could not be removed from chyle chylomicrons by treatment with EDTA, suggesting that this binding is not a simple Ca2+ dependent association between prothrombin and chyle chylomicrons. Inclusion of 1% purified human serum albumin caused a 50% decrease in binding, half of which was reversed by increasing the Ca2+ ion concentration. Addition of pancreatic phospholipase A(2) (PLA(2)) in doses sufficient to hydrolyze more than 95% of the phosphatidylethanolamine (PE) and 37% of the phosphatidylcholine (PC) decreased the binding by 50%. Doses of PLA(2) that hydrolyzed 60-80% of PE and 4-10% of the PC decreased the binding by only 7-15%. It is suggested that the binding of prothrombin to chyle chylomicrons is in part mediated by negatively charged phospholipids of the chylomicron surface, although a specific role of the PE could not be demonstrated.