Prostaglandin E synthase (PGES) catalyzes isomerization of PGH2to PGE2which exhibits potent and various biological activities. Here we report the molecular identification of two distinct glutathione-dependent PGESs, the terminal PGE2-biosynthetic enzymes of the cyclooxygenase (COX) pathway. These PGESs exhibit unique functional coupling with two upstream COX isozymes, COX-1 and COX-2. Cytosolic PGES (cPGES), known as p 23, is constitutively and ubiquitously expressed and predominantly converts COX-1-derived PGH2to PGE2. Microsomal PGES (mPGES), identical to MGST 1-L 1, is an inducible perinuclear enzyme that is functionally linked with COX-2 in marked preference to COX-1. Increased supply of arachidonic acid by explosive activation of cytosolic phospholipase A2allows COX-1 to be coupled with mPGES.