Catalytic consumption of nitric oxide by prostaglandin H synthase-1 regulates platelet function

Nitric oxide ((NO)-N-.) plays a central role in vascular homeostasis via regulation of smooth muscle relaxation and platelet aggregation. Although mechanisms for (NO)-N-. formation are well known, removal pathways are less well characterized, particularly in cells that respond to (NO)-N-. through activation of soluble guanylate cyclase. Herein, we report that (NO)-N-. is catalytically consumed by prostaglandin H synthase-1 (PGHS-1) through acting as a reducing peroxidase substrate. With purified ovine PGHS-1, (NO)-N-. consumption requires peroxide (LOOH or H2O2), with a K-m ((app)) for 15(S)hydroperoxyeicosatetraenoic acid (HPETE) of 3.27 +/- 0.35 muM. During this, 2 mol (NO)-N-. are consumed per mol HPETE, and loss of HPETE hydroperoxy group occurs with retention of the conjugated diene spectrum. Hydroperoxide-stimulated (NO)-N-. consumption requires heme incorporation, is not inhibited by indomethacin, and is further stimulated by the reducing peroxidase substrate, phenol. PGHS-1-dependent (NO)-N-. consumption also occurs during arachidonate, thrombin, or A23187 activation of platelets (1-2 muM.min(-1) for typical plasma platelet concentrations) and prevents (NO)-N-. stimulation of platelet soluble guanylate cyclase. Platelet sensitivity to (NO)-N-. as an inhibitor of aggregation is greater using a platelet-activating stimulus (U46619) that does not cause (NO)-N-. consumption, indicating that this mechanism overcomes the anti-aggregatory effects of (NO)-N-., Catalytic consumption of (NO)-N-. during eicosanoid synthesis thus represents both a novel pro-aggregatory function for PGHS-1 and a regulated mechanism for vascular (NO)-N-. removal.