Co-oxidation of diethylstilbestrol and structural analogs by prostaglandin synthase

Structural analogs of diethylstilbestrol (DES) with at least one phenolic hydroxyl group are metabolized by prostaglandin H synthase (PHS) from ram seminal vesicle microsomes (RSVM) in vitro in the presence of arachidonic acid (20:4). U.v. spectroscopy revealed the formation of p-quinoid intermediates in incubations of DES, tetrafluoro-DES and dimethylstilbestrol, and tautomerization of the quinones to the respective dien-compounds which were characterized by h.p.l.c. and GC/MS. Indomethacin inhibits the formation of these metabolites which are identical to the major metabolites formed in incubations with horseradish peroxidase/hydrogen peroxide. Covalent binding to protein was observed in incubations of PHS co-substrates. Notably, formation of reactive intermediates which bind to protein is not limited to DES-analogs which form quinone intermediates: radiolabeled hexestrol and E,E-dienestrol yield protein-bound radioactive products upon incubation with RSVM and 20:4, probably via one electron-oxidation to a phenoxy radical. PHS-catalyzed metabolism of structural analogs of DES is accompanied by a concentration-dependent increase in cyclo-oxygenase activity. The measurement of the 20:4-dependent oxygen uptake rates in vitro can serve as a convenient assay for estrogenic compounds which undergo co-oxidation. At high concentrations, however, DES structural analogs can inhibit rather than stimulate PHS. The PHS-catalyzed formation of reactive intermediates from DES structural analogs and their effect on PHS may be of importance for their biological activity in estrogen target tissues with low mono-oxygenase activity.