Evidence for Two Distinct Forms of Fatty Acid Cyclooxygenase in Brain

Abstract

The enzymatic metabolism of [14C]arachidonic acid (AA) was studied with microsomes prepared from rabbit medulla. Prostaglandin [PG] E2 levels, measured either by radiochemistry or radioimmunoassay, rose rapidly and abruptly plateaued within 5 min, while PGF2.alpha. levels continued to rise for 30 min. The rapid termination of PGE2 biosynthesis was not the result of limited cofactor, substrate or product feedback inhibition, nor was it due to PGE2-9-ketoreductase activity. Inhibition of the PGH2 .fwdarw. PGE2 isomerase by AA or its metabolites could not explain the abrupt halt in PGE2 biosynthesis. Proof for 2 separate cyclooxygenases comes from observations that a preincubation of the brain microsomes with unlabeled AA eliminated PGE2 biosynthesis while PGF2.alpha. production continued. Further evidence to suggest 2 cyclooxygenases in brain is derived from the observation that indomethacin inhibited PGE2 production at concentrations that did not affect PGF2.alpha. biosynthesis. One fatty acid cyclooxygenase appears to be closely associated with PGH2 .fwdarw. PGE2 isomerase and readily undergoes autodestruction, and the 2nd cyclooxygenase is associated with a PGH2 .fwdarw. PGF2.alpha. reductase and is somewhat resistant to arachidonate-induced destruction and to nonsteroidal antiinflammatory agents.