Prostaglandin endoperoxide E2 isomerase is dissociated from prostaglandin endoperoxide synthetase in the renal cortex

The rate of arachidonic acid metabolism by prostaglandin (PG) endoperoxide synthetase by the rabbit renal cortex (approximately 200 pmol/mg of protein/min) is very slow compared to medulla (approximately 2000 pmol/mg/min). However, by using PGH2 as a substrate and limiting reaction times, we were able to directly measure the endoperoxide-dependent PGE2 isomerase and found that both the cortex and medulla possess high levels of this activity (approximately pmol/mg/min). The PG endoperoxide E2 isomerase is dependent on reduced glutathione, but not cysteine, and is inactivated by p-hydroxymercuribenzoate or boiling. Thus, the renal medulla appears to exhibit an efficient coupling of cyclooxygenase and PG endoperoxide E2 isomerase, whereas the cortex has a vast excess of endoperoxide-dependent enzyme.