Similarities in the optical spectra of prostaglandin H synthase during its cyclooxygenase and peroxidase reactions

The spectral behavior of the enzyme prostaglandin H synthase was studied in the Soret region under conditions that permitted comparison of enzyme intermediates involved in peroxidase and cyclooxygenase activities. First, the peroxidase activity was examined. The enzyme's spectral behavior upon reacting with 5-phenyl-pent-4-enyl-1-hydroperoxide was different depending on the presence or absence of the reducing substrate, phenol. In the reaction of prostaglandin H synthase with the peroxide in the absence of phenol, formation of the enzyme intermediate compound I is observed followed by partial conversion to compound II and then by enzyme bleaching. In the reaction with both peroxide and phenol the absorbance decreases and a steady-state spectrum is observed which is a mixture of native enzyme and compound II. The steady state is followed by an increase in absorbance back to that of the native enzyme with no bleaching. The difference can be explained by the reactivity of phenol as a reducing substrate with the prostaglandin H synthase intermediate compounds. Cyclooxygenase activity with arachidonic acid could not be examined in the absence of diethyldithiocarbamate because extensive bleaching occurred. In the presence of diethyldithiocarbamate, enzyme spectral behavior similar to that seen in the reaction of the peroxide and phenol was observed. The similarity of the spectra strongly suggests that the enzyme intermediates involved in both the peroxidase and cyclooxygenase reactions are the same.Key words: prostaglandin H synthase, cyclooxygenase activity, peroxidase activity, arachidonic acid, compounds I and II of PGH synthase.