Target size analysis of prostaglandin endoperoxide synthase. Radiation inactivation of both cyclooxygenase and peroxidase correlated with the monomer of 72 kDa.

To determine the size of the functional catalytic unit of prostaglandin endoperoxide (prostaglandin H) synthase, radiation inactivation experiments were performed. Both microsomes from ovine seminal vesicles and purified enzyme were irradiated with 10 MeV electrons. The enzymic activities of prostaglandin H synthase, cyclooxygenase and peroxidase, showed mono-exponential inactivation curves dependent on radiation dose, indicating molecular masses of approximately 72 kDa. The enzyme in microsomes, in its native environment, as well as in its purified state after solubilisation with nonionic detergent showed identical molecular masses. The results clearly demonstrate that the monomer of the enzyme with an apparent molecular mass of 72 kDa (SDS/PAGE) is the functional unit for catalysis of both activities. Hence the two active sites of cyclooxygenase and peroxidase reside on the same polypeptide chain.