Nitric Oxide Modulates the Activity of the Hemoproteins Prostaglandin I2Synthase and Thromboxane A2Synthase

Nitric oxide modulates the activity of the hemoprotein isomerase enzymes that transform prostaglandin H2into prostaglandin I2and thromboxane A2. Two nitric oxide donors, 1-hexanamine, 6-(2hydroxy-1-methyl-2-nitrosohydrazino)-N-methyl-nitroso-hydrazine (MMNN) and 1,1-diethyl-2-hydroxy-2-nitrosohydrazine, modulated prostaglandin I2synthase activity in a bidirectional manner. At moderate concentrations, they increased enzyme activity irreversibly and at higher concentrations they inhibited enzyme activity reversibly. We confirmed that these effects originated from nitric oxide. First, we showed that hemoglobin, a substance that sequesters nitric oxide, prevented both the activation and the inhibition of catalysis, stoichiometrically. Second, we showed that solutions depleted of nitric oxide had no effect on catalysis. Nitric oxide also modulated thromboxane A2synthase activity; however, its effects on thromboxane A2synthase differed from its effects on prostaglandin I2synthase in three ways: (i) It inhibited thromboxane A2synthase in a concentration-dependent manner. The IC50= 4.2 ± 0.8 μM MMNN corresponded to an IC50≅ 0.1–0.3 μM nitric oxide. (ii) It did not increase thromboxane A2synthase activity at any concentration tested. (iii) Its irreversible inhibition of thromboxane A2synthase contrasted with its reversible inhibition of prostaglandin I2synthase. Nitric oxide also inhibited cellular formation of thromboxane A2by intact platelets in a concentration-dependent manner. The IC50= 267 ± 26 μM MMNN corresponded to an IC50≅ 6–18 μM nitric oxide. We conclude that nitric oxide can modulate certain hemoprotein enzymes in the biosynthetic cascade that governs the formation of eicosanoid mediators of thrombosis and hemostasis.