Photoaffinity receptor antagonist for human platelet thromboxane A2/prostaglandin H2 receptors.

9,11-Dimethylmethano -11,12-methano-16-(3-azido-4-iodophenoxy)-13,14- dihydro-13-aza-15 alpha beta-omega-tetranor TXA2 (I-PTA-PON3) was synthesized and evaluated as a potential photoaffinity probe of the human platelet thromboxane A2/prostaglandin H2 (TXA2/PGH2) receptor. I-PTA-PON3 inhibited the aggregation of washed human platelets induced by the TXA2 mimetic U46619 [(15S)-hydroxy-11 alpha, 9 alpha-(epoxymethano)prosta-5Z,13E-dienoic acid]. Schild analysis of the data revealed a Kd of 9.5 nM and a slope not significantly different from -1. Equilibrium binding studies using [125I]PTA-OH, a TXA2/PGH2 receptor antagonist, showed that I-PTA-PON3 plus photolysis resulted in a 52% reduction in the number of binding sites (1252 +/- 202/platelet) compared to the nonphotolyzed group (2557 +/- 293/platelet) (N = 5, P less than 0.05) with no significant change in the Kd. Repetition of the incubation with I-PTA-PON3 and photolysis a second time resulted in a further 77% (578 +/- 163 binding sites/platelet) reduction in the number of binding sites. Incubation of washed human platelets with I-PTA-PON3 (163 nM) followed by photolysis and removal of the non-covalently bound I-PTA-PON3 resulted in no change in the EC50 value for the TXA2 mimetic, U46619, when compared to controls that were either exposed to I-PTA-PON3 and not photolyzed or exposed only to photolysis. The second photolysis of I-PTA-PON3 resulted in a significant 42% increase in the EC50 value of U46619-induced aggregation compared to the non-photolyzed group (N = 4, P less than 0.05). These results suggest that I-PTA-PON3 is a useful probe for the study of TXA2/PGH2 receptors and that spare TXA2/PGH2 receptors may exist in the platelet.