Pgg₂11r-Hpete and 15r/S-Hpete are Formed From Different Conformers of Arachidonic Acid in the Prostaglandin Endoperoxide H Synthase-1 Cyclooxygenase Site

Abstract

Prostaglandin endoperoxide H synthases-1 and -2 (PGHS-1 and -2) catalyze the committed step in the formation of prostanoids (prostaglandins, thromboxane A2 (l-6). PGHSs catalyze two separate reactions: a cyclooxygenase reaction in which arachidonate is converted to prostaglandin G2 (PGG₂) and a peroxidase reaction in which PGG₂undergoes a two-electron reduction to PGH₂. The cyclooxygenase reaction begins with a rate-limiting abstraction of the 13-proS hydrogen from arachidonate to yield an arachidonyl radical (7,8). This is followed by sequential oxygen additions at C-11 and C-15 to yield the prostaglandin endoperoxide PGG₂. PGHSs exhibit some lipoxygenase activity producing small amounts of 11-hydroperoxy-5Z,8Z,12E,14Z-eicosatetraenoic acid (11-HPETE) and 15-hydroperoxy-5Z,8Z,11Z,13E-eicosatetraenoic acid (15-HPETE) from arachidonic acid (9, 10). Aspirin-acetylated PGHS-2, which has no cyclooxygenase activity, synthesizes 15R-HPETE (10,11). Studies comparing native and aspirinacetylated PGHS-2 have raised the possibility that arachidonate can bind in distinct orientations in the PGHS-2 active site to produce either PGG₂, 1 l R-HPETE or 15RHPETE (10). Here we develop the concept that arachidonate can be bound in the cyclooxygenase active site of ovine (o)PGHS-1 in at least three different, catalytically competent arrangements that lead to PGG₂, 11R-HPETE, and 15R/S-HPETE, respectively, and that these three arrangements of arachidonate occur subsequent to its entry into the cyclooxygenase active site.