COMPARISON OF HYDROPEROXIDE INITIATOR REQUIREMENTS FOR THE CYCLOOXYGENASE ACTIVITIES OF PROSTAGLANDIN-H SYNTHASE-1 AND SYNTHASE-2

Two isoforms of prostaglandin H synthase have been described: isoform- 1 (PGHS-1), which is ascribed a role in basal or housekeeping prostagl andin synthesis; and isoform-2 (PGHS-2), which has been found to be st rongly inducible in many tissues and has been associated with inflamma tory processes. Recent observations have indicated that cyclooxygenase catalysis by the two isoforms can be differentially regulated when bo th are present simultaneously (Reddy, S. T., and Herschman, H. R. (199 4) J. Biol. Chem. 269, 15473-15480). The requirement of the cyclooxyge nase for hydroperoxide initiator has been proposed as an important lim it on cellular prostaglandin synthesis (Marshall, P. J., Kulmacz, R. J ., and Lands, W. E. M. (1987) J. Biol. Chem. 262, 3510-5517). To compa re the levels of hydroperoxide required for cyclooxygenase initiation in the two PGHS isoforms, we have examined the ability of a hydroperox ide scavenger, glutathione peroxidase, to suppress the cyclooxygenase activity of purified preparations of human PGHS-2, ovine PGHS-2, and o vine PGHS-1. Half-maximal prostaglandin synthetic activity was found t o require a much lower hydroperoxide level with human PGHS-2 (2.3 nM) and ovine PGHS-2 (2.2 nM) than with ovine PGHS-1 (21 nM). Similar resu lts were obtained when cyclooxygenase activity was monitored by chroma tographic analyses of radiolabeled arachidonate metabolites or with ox ygen electrode measurements. Mixing four parts of ovine PGHS-1 with on e part of human PGHS-2 did not markedly change the sensitivity of the overall cyclooxygenase activity to inhibition by glutathione peroxidas e, indicating that the PGHS-1 activity was not easily initiated by PGH S-2 activity in the same vessel. Effective catalysis by PGHS-2 can thu s proceed at hydroperoxide levels too low to sustain appreciable catal ysis by PGHS-1. This difference in catalytic characteristics provides a biochemical mechanism for differential control of prostaglandin synt hesis by the two PGHS isoforms, even when both are present in the same intracellular compartment.