ADP RECEPTOR-INDUCED ACTIVATION OF GUANINE-NUCLEOTIDE-BINDING PROTEINS IN HUMAN PLATELET MEMBRANES

ADP receptor-regulated binding of the labeled GTP analog, guanosine 5'-O-(3-[S-35]thiotriphosphate) ([S-35]GTP[gamma-S]), to guanine-nucleotide-binding proteins (G proteins) was studied in human platelet membranes. The potent ADP receptor agonist, 2-methyl-thio-adenosine 5'-diphosphate (2MeSADP), a non-hydrolyzable analog of ADP, increased the binding of [S-35]GTP[gamma-S] without apparent lag phase. Under optimal conditions, i.e. in the presence of GDP (1 - 10-mu-M), 2MeSADP increased the binding up to about threefold, with half-maximal and maximal increase observed at 10 nM and 1-mu-M 2MeSADP, respectively. ADP itself increased the binding of [S-35]GTP[gamma-S] by maximally about twofold, with half-maximal increase occurring at 0.1-mu-M ADP. The agonist-induced stimulation was competitively antagonized by the ADP receptor(s) antagonist, (1S)-adenosine 5'-O-(1-thiotriphosphate) {(Sp)-ATP[alpha-S]}. Other platelet receptor agonists known to act through receptors coupled to G proteins also increased binding of [S-35]GTP[gamma-S] in human platelet membranes, but without being inhibited by (Sp)-ATP[alpha-S]. The data presented indicate that the platelet ADP receptor(s) can interact with and efficiently activate G proteins, the nature of which remains to be identified.