Two mechanisms for inhibition of ADP-induced platelet shape change by 5'-p-fluorosulfonylbenzoyladenosine. Conversion to adenosine, and covalent modification at an ADP binding site distinct from that which inhibits adenylate cyclase.

The interaction of ADP with platelets leads to shape change, exposure of fibrinogen binding sites, and aggregation, all of which have been shown to be inhibited by 5'-p-fluorosulfonylbenzoyladenosine (FSBA), an alkylating analogue of adenine nucleotides which binds covalently to a 100-kDa polypeptide in intact platelet membranes (Figures, W. R., Niewiarowski, S., Morinelli, T., Colman, R. F., and Colman, R. W. (1981) J. Biol. Chem. 256, 7789-7795). In plasma, FSBA can break down to adenosine which stimulates adenylate cyclase. To distinguish between direct effects of FSBA and the actions of adenosine, we have used washed platelet suspensions and adenosine deaminase. We studied the effects of FSBA on shape change and cyclic AMP metabolism, and on the binding of 2-methylthio-ADP, which mimics the effects of ADP on cyclic AMP metabolism at concentrations too low to activate platelets. Inhibition of ADP-induced shape change of platelets incubated with FSBA for 2 min in platelet-rich plasma was greatly reduced by adenosine deaminase. In the presence of a phosphodiesterase inhibitor, 100 microM FSBA increased platelet cyclic AMP to the same extent as did 10 microM adenosine. These effects were inhibited by theophylline, an adenosine receptor antagonist, and by adenosine deaminase. Incubation of washed platelets for 60 min with FSBA and adenosine deaminase caused a concentration-dependent inhibition of ADP-induced shape change. Inhibition closely paralleled the covalent incorporation of 3H from tritiated FSBA into platelet membranes. Under these conditions, FSBA did not block inhibition of cyclic AMP accumulation by ADP, nor did it block the binding of 2-methylthio-ADP. We conclude that part of the inhibition of shape change caused by brief exposure to FSBA is due to adenosine, but at longer times shape change is inhibited in association with covalent incorporation of sulfonylbenzoyladenosine. This effect of FSBA is independent of adenosine and occurs at a site distinct from that at which ADP inhibits adenylate cyclase.