Inhibition of cyclooxygenase activity and platelet aggregation by epoxyeicosatrienoic acids. Influence of stereochemistry.

Certain epoxyeicosatrienoic acids (EETs) that were not cyclooxygenase substrates were effective cyclooxygenase inhibitors. Both (+/-)-14,15-cis-EET and (+/-)-8,9-cis-EET inhibited purified enzyme at concentrations from 1 to 50 microM; (+/-)-11,12-cis-EET was ineffective at concentrations below 100 microM. For the case of 14,15-cis-EET, only the (14R,15S)-stereoisomer was active. Other isomers including (14S,15R)-cis-EET, (14R,15R)-trans-EET, (14S,15S)-trans-EET, and the erythro and threo vicinal 14,15-diols were inactive. In addition to their effects on isolated enzyme preparations, cyclooxygenase activity in platelet suspensions, reflected by thromboxane B2 formation, was also inhibited by (14R,15S)-cis-EET and (+/-)-8,9-cis-EET but not by the other isomers. Thus potency and stereospecificity requirements were maintained for cyclooxygenase within intact platelets. Unlike the stereospecific inhibition of the cyclooxygenase enzyme, platelet aggregation induced by arachidonic acid was inhibited by all EET isomers at concentrations from 1 to 10 microM with no evident stereospecificity. Inhibition of aggregation was not uniformly associated with inhibition of thromboxane B2 formation; ordinarily, these two parameters correlate closely. This dissociation was not maintained for another biochemical process involved in platelet activation. For instance, there was a uniform correlation between inhibition of phosphorylation of a 40-kDa platelet protein and inhibition of aggregation. Our results suggest that effects of EET may originate from either stereospecific or nonspecific mechanisms. Definition of such mechanisms may be important to appreciate any physiological relevance of these substances.