Platelet activation by thrombin in the absence of the high-affinity thrombin receptor

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Abstract
The receptor status of the moderate-affinity platelet binding site for .alpha.-thrombin has been established by treating platelets with Serratia marcescens protease under conditions causing cleavage of 95-97% glycoprotein Ib (2.5 .mu.g for 30 min). High-affinity binding was lost under these conditions, but the platelets continued to show moderate-affinity binding (Kd1 = 16 .+-. 5 nM; 930 .+-. 300 sites/platelet) and low-affinity binding (Kd2 = 4.6 .+-. 3 .mu.M; 170 000 .+-. 90 000 sites/platelet), in good agreement with the values previously obtained for moderate- and low-affinity binding in intact platelets [Harmon, J.T., and Jamieson, G. A. (1986) J. Biol. Chem. 261, 15928-15933]. Platelets treated with Serratia protease under these conditions were about 4-fold less sensitive to activation by .alpha.-thrombin, as measured by serotonin secretion. Crossover studies with analogues showed that binding of .alpha.-thrombin was competable by both D-phenyl-alanyl-L-prolyl-L-arginine chloromethyl ketone treated thrombin and N.alpha.-p-tosyl-L-lysine chloromethyl ketone treated thrombin, and both analogues were capable of inhibiting activation of Serratia-proteolyzed platelets by .alpha.-thrombin. These studies establish that the moderate-affinity platelet binding site for .alpha.-thrombin is a receptor, occupancy of which is required for platelet activation in the absence of the high-affinity receptor.