Nitric oxide modulates the activity of the hemoproteins prostaglandin I-2 synthase and thromboxane A(2) synthase

Nitric oxide modulates the activity of the hemoprotein isomerase enzymes that transform prostaglandin H-2 into prostaglandin I-2 and thromboxane A(2). Two nitric oxide donors, 1-hexanamine, 6-(2-hydroxy-1-methyl-2-nitrosohydrazino)-N-methyl-nitroso- hydrazine (MMNN) and 1,1-diethyl-2-hydroxy-2-nitrosohydrazine, modulated prostaglandin I-2 synthase activity in a bidirectional maimer, 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 A(2) synthase activity; however, its effects on thromboxane A(2) synthase differed from its effects on prostaglandin I-2 synthase in three ways: (i) I-2 inhibited thromboxane A(2) synthase in a concentration-dependent manner. The IC50 = 4.2 +/- 0.8 mu M MMNN corresponded to an IC50 congruent to 0.1-0.3 mu M nitric oxide. (ii) It did not increase thromboxane A(2) synthase activity at any concentration tested. (iii) Its irreversible inhibition of thromboxane A(2) synthase contrasted with its reversible inhibition of prostaglandin I-2 synthase. Nitric oxide also inhibited cellular formation of thromboxane A(2) by intact platelets in a concentration-dependent manner. The IC50 = 267 +/- 26 mu M MMNN corresponded to an IC50 congruent to 6-18 mu M 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. (C) 1997 Academic Press.