Oxidation of phenidone and BW755C by prostaglandin endoperoxide synthetase

1-Phenyl-3-pyrazolidone (phenidone) and 3-amino-1-(m-(trifluoromethyl)-phenyl)-2-pyrazoline (BW755C) are oxidized by the hydroperoxidase component of prostaglandin endoperoxide synthetase and by horseradish peroxidase. The initial oxidation products are radical cations which exhibit visible absorption maxima at 514, 490, and 472 nm (phenidone) and 535, 500, and 488 nm (BW755C). The radical cation of phenidone can be detected by electron paramagnetic resonance spectroscopy as a complex multiline signal centered at g = 2.0039. In addition to being oxidized by peroxidases both compounds are cofactors for the reduction of 15-hydroperoxy-5,8,11,13-eicosatetraenoic acid to 15-hydroxy-5,8,11,13-eicosatetraenoic acid by the hydroperoxidase activity of purified and hematin-reconstituted prostaglandin endoperoxide synthetase. As a consequence of their oxidation by prostaglandin endoperoxide synthetase, phenidone and BW755C inhibit the hydroperoxide-dependent oxidation of phenylbutazone, luminol, diphenylisobenzofuran, epinephrine, and guaiacol by ram seminal vesicle microsomes. The inhibition of phenylbutazone oxidation is competitive and exhibits Ki values of 16 microM (BW755C) and 45 microM (phenidone). BW755C and phenidone stimulate prostaglandin biosynthesis by purified and reconstituted prostaglandin endoperoxide synthetase at concentrations up to 100 microM but inhibit at higher concentrations (I50 values approximately 210 microM and 1180 microM, respectively). The ability of phenidone and BW755C to act as peroxidase reducing cofactors or radical scavengers may contribute to their observed biochemical and pharmacological effects.