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Exposure of human blood platelets to thrombin results in the rapid release of a 420,000 dalton glycoprotein, designated thrombospondin, which is comprised of three polypeptides of equivalent molecular weight which are cross- linked by disulfide bonds. When purified human platelet thrombospondin is exposed to thrombin (4 units/ml) or plasmin (2 units/ml) for prolonged periods of time, proteolytic cleavages occur which result in the removal of 10,000 dalton and 30,000 dalton polypeptides with a concomitant decrease in the apparent molecular weight of the thrombospondin chains (Lawler, J.W. and Slayter, H.S., submitted for publication). In contrast, when the supernatant from thrombin- treated platelets is exposed to additional thrombin (4 units/ml) the 30,000 dalton, but not the 10,000 dalton fragment is released from thrombospondin. Proteolytic release of the 10,000 dalton polypeptide was inhibited by calcium and a nondialyzable component of the supernatant preparations. To further characterize the nondialyzable component, the supernatant was subjected to heparin-Sepharose affinity chromatography. Stepwise elution with varying NaCl concentrations yields a non-affinity flow through peak, a low affinity peak (eluted with 0.45 M NaCl) and a high affinity peak (eluted with 2.0 M NaCl). In addition to thrombospondin, the low affinity peak contains a β-thromboglobulin like protein as judged by RIA, apparent molecular weight on SDS-polyacrylamide gel electrophoresis (7,500-9,000 dalton) and heparin affinity. After dialysis of the low affinity peak against 0.14 M NaCl, 15 mM Tris-HCl (pH 7.6), 2 mM CaCl2 containing 0.02% NaN3, thrombin treatment resulted in the proteolytic release of the 30,000 dalton polypeptide, but not the 10,000 dalton polypeptide, from thrombospondin. These results suggest that calcium and a low molecular weight platelet protein can affect the pattern of thrombin digestion of thrombospondin.

Factors Influencing Thrombin Digestion Of Human Platelet Thrombospondin