S-protein (S) is a newly discovered 80,000 MW plosma glycoprotein. It functions as an inhibitor of the membrane attack complex of complement. We now wish to report that S also functions as thrombin protecting factor in coagulation; S forms a reversible complex with thrombin which is more resistant to inactivation by antithrombin III (AT III) than thrombin alone. An S-thrombin complex and on S-throm-bin-AT III complex were formed in clotted plasma and with isolated proteins as demonstrated by two dimensional Immunoelectrophoresis. Functional studies measuring the esterolytic or clotting activity of thrombin showed that S in the presence and absence of heparin decreased the rate of inactivation of thrombin by AT III. Similar results were observed using plasma. For example, in the presence of 0.04 u/ml heparin and 1.6 u/ml thrombin, the thrombin time of plasma depleted in S was 150 sec. as opposed to 15 sec. when the plasma was reconstituted with purified S. That this effect of S was due to a decreased inactivation of thrombin by AT III was demonstrated directly by SDS-PAGE analysis of plasma containing 125l-thrombin. In the presence of S the rate of formation of the 95,000 dalton 125I-thrombin-AT III complex was markedly decreased compared to the rate of complex formation in the S-depleted plasma. These data suggest that S may modulate the interactions of thrombin and AT III.
Selective blockade of membrane attack complex formation during simulated extracorporeal circulation inhibits platelet but not leukocyte activation