Thrombin binding to platelets defines functional receptors: inhibition of thrombin-induced platelet activation by catalytically-inactivated thrombin.
It has been widely questioned as to whether the observed binding of a-thrombin to intact platelets defines receptors coupled to signal transduction or merely thrombin binding sites. We have now shown that at α-thrombin concentrations sufficient to induce a full shape change response without aggregation (0.1 nM), PPACK-thrombin (that is, α-thrombin treated with the irreversible active site inhibitor D-phenylalanyl-L-prolyl-L-arginine chloromethylketone) dose-dependently inhibits platelet shape change (IC(50)∼70 nM), the concomitant increases in [Ca(2+)Ii (IC(50)∼75 nM) and ATP secretion (IC(50)∼50 nM). Since PPACK-thrombin competes fully in the binding of a-thrombin to high, moderate and low affinity sites on intact platelets, these results show that this binding defines functional receptors coupled to platelet activation.
Cell Biology Department, American Red Cross, Rockville, MD, 20855, USA.
N J
NJ
Greco
N N
NN
Tandon
G A
GA
Jamieson