Prostaglandin H synthase-2 inhibitors interfere with prostaglandin H synthase-1 inhibition by nonsteroidal anti-inflammatory drugs

Ram seminal vesicle microsomes, a rich source of prostaglandin H synthase-l , were incubated with 100 nM of the prostaglandin H synthase-2 inhibitors N -(2-cyclohexyloxy-4-nitrophenyl) methanesulfonamide (NS-398) and 5-bromo-2- (4-fluorophenyl)-3-(4-methylsulfonyl) thiophene (DuP-697) prior to exposure to the prostaglandin H synthase inhibitors aspirin, indomethacin, ibuprofe n or naproxen. Activity of the enzyme was measured by following the convers ion of arachidonic acid to prostaglandin E-2 and prostaglandin F2 alpha. Al though prostaglandin H synthase-l activity was not altered by these concent rations of the prostaglandin H synthase-2 inhibitors, it was found that exp osure to these agents prior to aspirin or indomethacin (irreversible prosta glandin H synthase inhibitors) significantly attenuated the inhibition obta ined by the latter inhibitors. On the other hand, the same concentrations o f the prostaglandin H synthase-2 inhibitors did not interfere with prostagl andin H synthase-l inhibition that was induced by naproxen or ibuprofen (co mpetitive prostaglandin H synthase inhibitors). Attenuation of the indometh acin inhibition of prostaglandin H synthase-1 by prostaglandin H synthase-2 inhibitors was observed only when the microsomes were pre-exposed to DuP-6 97 or NS-398 in the absence, but not in the presence, of arachidonic acid. The effect of DuP-697 was found to be irreversible, however, washing away t he agent reversed the action of NS-398. Similar phenomena have been reporte d by us in bovine aortic endothelial cells and in human dermal fibroblasts. Attenuation of the inhibition by aspirin and indomethacin, without alterin g the enzyme's basal activity or the inhibition induced by ibuprofen or nap roxen may suggest the possibility that the prostaglandin H synthase-2 speci fic inhibitors DuP-697 and NS-398 affect prostaglandin H synthase-l by inte raction with a site different from the enzyme's catalytic site. (C) 2001 El sevier Science B.V. All rights reserved.