7] Formation of electronically excited states during the oxidation of arachidonic acid by prostaglandin endoperoxide synthase

Abstract

Prostaglandin endoperoxide H synthase (PGHS) is a bifunctional enzyme that catalyzes the oxidation of arachidonic acid to prostaglandin H 2 (PGH 2 ), the committed step in prostanoid biosynthesis. The enzyme contains in a single protein, a cyclooxygenase activity, which catalyzes the incorporation of O 2 into arachidonic acid to form prostaglandin G 2 (PGG 2 ), and a peroxidase activity that catalyzes the reduction of PGG 2 to PGH 2 . A feature of prostaglandin endoperoxide synthase activity is suicide inactivation, probably due to the formation of a potent oxidant produced during the peroxidase-catalyzed conversion of PGG 2 to PGH 2 . Early reports attempted to identify this oxidant as an oxygen-centered radical or as singlet molecular oxygen. Electron paramagnetic resonance (EPR) studies suggested the formation of hydroxyl radical, although this view is difficult to sustain because of the lack of a discernible hyperfine structure of the EPR spectrum. Likewise, studies with arachidonic acid-supplemented sheep vesicular gland microsomes (containing a high PGHS activity) fail to provide unequivocal evidence for the generation of singlet oxygen ( 1 O 2 ) in terms of photoemission or of formation of the 5α-hydroperoxide adduct of cholesterol, a specific probe for singlet oxygen. This chapter addresses the formation of electronically excited states whose relaxation yields low-level chemiluminescence during the oxidation of arachidonic acid by PGHS. This is preceded by a brief description of the photon-counting apparatus used to measure low-level or ultraweak chemiluminescence.