Publications

A high molecular weight glycoprotein consisting of three disulfide-linked 142,000 molecular weight chains has been isolated from human blood platelets. The glycoprotein, designated thrombospondin, is released by platelets in response to thrombin treatment and is proteolyzed when left in the presence of platelets after liberation. It is relatively insensitive to degradation by thrombin. Thrombospondin is a filamentous protein of dimensions approximately 7 X 70 nm and contains 1.9% neutral sugars, 1.4% amino sugars, 0.7% sialic acid, and no hexuronic acid. Amino acid analysis reveals that the level of cysteine is approximately 260 residues per molecule. Thrombospondin binds to immobilized heparin but is released by 0.45 M sodium chloride. A single band is obtained by isoelectric focusing, indicating a pI of 4.7 as well as a relatively high degree of purity. Degradation of the intact molecule with trypsin yields a stable core particle of molecular weight 210,000 comprised of three 70,000 chains.

Thrombin-induced platelet aggregation and release were investigated in washed platelet suspensions and in suspensions of inert particles in order to evaluate the role of fibrinogen-fibrin transformation in aggregometer tracings. Thrombin (0.25-2.0 U/ml) produced two waves of light transmission increase (LTI) in both platelet and inert particle suspensions containing fibrinogen, and concomittantly aggregates were observed under phase microscopy. Without fibrinogen, thrombin induced rapid release of platelet ADP but failed to cause second wave of LTI. The kinetics of LTI in platelet and inert particle systems were related to both thrombin and fibrinogen concentrations. A rapid second wave of LTI could be produced by direct interaction of thrombin-treated platelets or inert particles with polymerizing fibrin, and was inhibited by sodium sulfite and low pH of 5.1 which prevent fibrin monomer polymerization. No fibrin strands were noted in platelet aggregates fixed at the completion of the second wave of LTI. Apyrase and PGE1 inhibited the rate of first but not that of second wave LTI. The results suggest that the release of platelet ADP induced by thrombin primarily affects the first phase aggregation, and the second phase may result from interaction of thrombin-exposed platelets and polymerizing fibrin. Thus, the blood coagulation mechanism may be directly involved in platelet aggregation.