Spectral intermediates of prostaglandin hydroperoxidase

Microsomes from ram seminal vesicles or purified prostaglandin H synthase supplemented with either arachidonic acid or prostaglandin G2 formed an unstable spectral intermediate with maxima at 430 nm, 525 nm and 555 nm and minima at 410 nm, 490 nm and 630 nm. At -15 degrees C the band at 430 nm disappeared within 4 min whereas the trough at 410 nm increased three fold. At higher temperatures (10-37 degrees C) spectral complex formation and decay were observed in less than 1 s. An apparent KS-value of about 3 microM was determined for the titration of purified prostaglandin synthase with prostaglandin G2 at -20 degrees C. Substrates for cooxidation reactions of prostaglandin synthase such as phenol, hydroquinone and reduced glutathione as well as the peroxidase inhibitors cyanide and azide inhibited the prostaglandin G2-induced spectral complex formation. The oxene donor iodosobenzene and hydrogen peroxide formed a spectral intermediate analogous to the complex observed with prostaglandin G2 or arachidonic acid in ram seminal vesicle microsomes as well as with the purified prostaglandin synthase. These results are interpreted as the formation of a ferryl-oxo complex (FeO)3+ of the heme of prostaglandin synthase with prostaglandin G2 analogous to the formation of compound I of horseradish peroxidase.