| Abstract: | The peroxidase activity of prostaglandin H (PGH) synthase catalyzes reduction of 5-phenyl-4-pentenyl hydroperoxide to 5-phenyl-4-pentenyl alcohol with a turnover number of approximately 8000 mol of 5-phenyl-4-pentenyl hydroperoxide/mol of enzyme/min. The kinetics and products of reaction establish PGH synthase as a classical heme peroxidase with catalytic efficiency similar to horseradish peroxidase. This suggests that the protein of PGH synthase evolved to facilitate peroxide heterolysis by the heme prosthetic group. Comparison of an extensive series of phenols, aromatic amines, beta-dicarbonyls, naturally occurring compounds, and nonsteroidal anti-inflammatory drugs indicates that considerable differences exist in their ability to act as reducing substrates. No correlation is observed between the ability of compounds to support peroxidatic hydroperoxide reduction and to inhibit cyclooxygenase. In addition, the resolved enantiomers of MK-410 and etodolac exhibit dramatic enantiospecific differences in their ability to inhibit cyclooxygenase but are equally potent as peroxidase-reducing substrates. This suggests that there are significant differences in the orientation of compounds at cyclooxygenase inhibitory sites and the peroxidase oxidation site(s). Comparison of 5-phenyl-4-pentenyl hydroperoxide reduction by PGH synthase and horseradish peroxidase reveals considerable differences in reducing substrate specificity. Both the cyclooxygenase and peroxidase activities of PGH synthase inactivate in the presence of low micromolar amounts of hydroperoxides and arachidonic acid. PGH synthase was most sensitive to arachidonic acid, which exhibited an I50 of 0.6 microM in the absence of all protective agents. Inactivation by hydroperoxides requires peroxidase turnover and can be prevented by reducing substrates. The I50 values for inactivation by 15-hydroperoxy-5,8,11,13-eicosatetraenoic acid are 4.0 and 92 microM, respectively, in the absence and presence of 500 microM phenol, a moderately good reducing substrate. The ability of compounds to protect against hydroperoxide-induced inactivation correlates directly with their ability to act as reducing substrates. Hydroquinone, an excellent reducing substrate, protected against hydroperoxide-induced inactivation when present in less than 3-fold molar excess over hydroperoxide. The presence of a highly efficient hydroperoxide-reducing activity appears absolutely essential for protection of the cyclooxygenase capacity of PGH synthase. The peroxidase activity is, therefore, a twin-edged sword, responsible for and protective against hydroperoxide-dependent inactivation of PGH synthase.(ABSTRACT TRUNCATED AT 400 WORDS) |
