Studies on the stoichiometry of estrogen oxidation catalyzed by purified prostaglandin-H-synthase holoenzyme.

Prostaglandin-H-synthase (PHS) is a key enzyme in the biosynthesis of prostaglandins (PGs) from arachidonic acid and can oxidatively metabolize synthetic and steroidal estrogens. To investigate the relationship between estrogen cooxidation and PG synthesis, purified PHS-holoenzyme was incubated with radiolabeled arachidonic acid and various estrogens, namely diethylstilbestrol (DES), estradiol (E2), 2-hydroxyestradiol (2-OHE2), and 2-methoxyestradiol (2-MeOE2). The amount and pattern of PGs synthesized were analyzed by TLC and HPLC, estrogen metabolism was studied by HPLC. All tested compounds increased conversion of arachidonic acid to PG H2-derived prostanoids. A stoichiometric ratio between net estrogen oxidation and net PG H2 formation of approximately 2:1 for monophenolic compounds (2-MeOE2, E2) and of 1:1 for diphenolic estrogens (DES, 2-OHE2) was found, indicating that estrogens are apparently acting as electron donors for the PHS-peroxidase. In contrast, glutathione was not found to provide electrons for the reduction of PGG2 to PGH2, and rather decreased the conversion of arachidonic acid. The results of this in vitro study are discussed with respect to its implications for the in vivo situation.