Nitric oxide modulates the activity of the hemoproteins prostaglandin I2 synthase and thromboxane A2 synthase

Nitric oxide modulates the activity of the hemoprotein isomerase enzymes that transform prostaglandin H2 into prostaglandin I2 and 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 I2 synthase 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 A2 synthase activity; however, its effects on thromboxane A2 synthase differed from its effects on prostaglandin I2 synthase in three ways: (i) It inhibited thromboxane A2 synthase in a concentration-dependent manner. The IC50 = 4.2 +/- 0.8 microM MMNN corresponded to an IC50 congruent with 0.1-0.3 microM nitric oxide. (ii) It did not increase thromboxane A2 synthase activity at any concentration tested. (iii) Its irreversible inhibition of thromboxane A2 synthase contrasted with its reversible inhibition of prostaglandin I2 synthase. Nitric oxide also inhibited cellular formation of thromboxane A2 by intact platelets in a concentration-dependent manner. The IC50 = 267 +/- 26 microM MMNN corresponded to an IC50 congruent with 6-18 microM 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.