INTERACTION BETWEEN NITRIC-OXIDE AND PROSTAGLANDIN-H SYNTHASE

Prostaglandin H synthase (PGHS) is a hemeprotein, and thus its catalyt ic activity potentially could be modulated by direct interaction with nitric oxide (NO). We have monitored spectroscopic and activity change s in pure ovine PGHS isoform-1 to investigate its interaction with NO in more detail. The binding kinetics for NO and the ferric heme in res ting PGHS were analyzed by stopped-dow spectrophotometry at 21 degrees C. The rate constants for association and dissociation were estimated to be 6.5 X 10(4) M(-1) s(-1) and 60 s(-1), respectively, leading to an equilibrium dissociation constant (K-d) of 0.92 mM. NO thus has a r elatively weak affinity for heme in ferric PGHS, the resting oxidation state of this hemeprotein. NO did react strongly and completely with ferrous PGHS under anaerobic conditions, displacing the proximal histi dine ligand to the prosthetic group. Dissolved NO at up to 2 mM produc ed only slight decreases in the cyclooxygenase activity of microsomal, detergent-extracted, or homogeneous preparations of ovine PGHS. The N O donors sodium nitroprusside and glyceryl trinitrate at levels of up to 1 mM also had little effect on the activity of the PGHS preparation s. Thus, there was no evidence for significant direct interaction of P GHS with NO at concentrations likely to be encountered in vivo. (C) 19 94 Academic Press, Inc.