Platelet Activation by 2‐(4‐bromo‐2,3‐dioxobutylthio)Adenosine 5′‐Diphosphate is Mediated by Its Binding to A Putative ADP Receptor, Aggregin

Platelet responses induced by ADP are mediated by a unique P 2T purinergic receptor. Although a variety of ADP analogs, substituted at C2, have been used to delineate pharmacological properties of the ADP‐binding site(s), the identity of the receptor protein has not been firmly established. 2‐(4‐Bromo‐2,3‐dioxobutylthio)‐ADP[2‐BrCH 2 (CO) 2 CH 2 ‐S‐ADP], a well‐characterized ADP analog, has been previously used as an affinity label to examine the structure/function relationship of ADP‐requiring enzymes [Kapetanovic, E., Bailey, J. B. & Colman, R. F. (1985) Biochemistry 24 , 7586–7593]. We found that it induced platelet shape change, aggregation, exposure of fibrinogen binding sites, secretion and mobilization of intracellular calcium, but was less potent than ADP. Under non‐stirring conditions, incubation of platelets with this analog for longer time periods blocked ADP‐induced shape change, aggregation, and the ability of ADP to antagonize the rise in intracellular levels of CAMP induced by iloprost (a prostaglandin I 2 analog). Of a variety of agonists examined, only ADP‐induced aggregation was almost completely inhibited in platelets irreversibly modified by the analog. An autoradiogram of the gel obtained by SDS/PAGE of solubilized platelets modified by the ADP analog followed by reduction of the dioxo group by NaB[ 3 H] 4 showed the presence of a single radiolabeled protein band at 100 kDa. Platelets incubated first with either ADP, ATP, or 2‐methylthio‐ADP were not labeled by 2‐BrCH 2 (CO) 2 CH 2 S‐ADP and NaB[ 3 H] 14 8‐BrCH 2 (CO) 2 CH 2 ‐S‐ADP was previously shown by us to irreversibly antagonize ADP‐induced platelet responses by selectively modifying aggregin. Incubation of platelets with 2‐BrCH 2 (CO) 2 CH 2 S‐ADP completely blocked labeling of aggregin in platelets by 8‐BrCH 2 (CO) 2 CH 2 S‐[ 32 P]ADP. These results show that 2‐BrCH 2 (CO) 2 CH 2 S‐ADP initially interacts reversibly with aggregin (100 kDa), a putative ADP receptor, and induces platelet shape change and aggregation, and at longer periods of incubation reacts irreversibly to block the ability of ADP to antagonize stimulated adenylate cyclase activity. In contrast, 6‐BrCH 2 (CO) 2 CH 2 S‐ADP was found to be a weak and reversible inhibitor of ADP‐induced platelet aggregation. Prior incubation of platelets with the latter analog reduced labeling of aggregin by 8‐BrCH 2 (CO) 2 CH 2 S‐[ 32 P]ADP. Taken together, the results further show that substitution by the BrCH 2 (CO) 2 CH 2 group at the C2 and C8 positions is tolerated, while the presence of a free amino function at the C6 position is essential for its interaction with aggregin.