Regulation of prostaglandin H-2 synthase activity by nitrogen oxides

Nitric oxide and its derivatives have been shown to both activate and inhib it prostaglandin H-2 synthase 1 (PGHS-1), We set out to determine the mecha nisms by which different nitrogen oxide derivatives modulate PGHS-1 activit y. To this end, we show that 3-morpholinosydnonimine hydrochloride (SIN-1), a compound capable of generating peroxynitrite, activates purified PGHS-1 and also stimulates PGE(2) production in arterial smooth muscle cells in th e presence of exogenous arachidonic acid. The effect of SIN-1 in smooth mus cle cells was abrogated by superoxide and peroxynitrite inhibitors, which s upports the hypothesis that peroxynitrite is an activating species of PGHS- 1. Indeed, authentic peroxynitrite also induced PGE(2) production in arachi donic acid-stimulated cells. In contrast, when cells were exposed to the ni tric oxide-releasing compound 1-hydroxy-2-oxo-3-[(methylamino)propyl]-3-met hyl-1-triazene (NOC-7), PGHS-1 enzyme activity was inhibited in the presenc e of exogenous arachidonic acid. Finally, in lipid-loaded smooth muscle cel ls, we demonstrate that SIN-1 stimulates arachidonic acid-induced PGE(2) pr oduction; albeit, the extent of activation is reduced compared to that unde r normal conditions. These results indicate that formation of peroxynitrite is a key intermediary step in PGHS-1 activation. However, other forms of N Ox inhibit PGHS-1. These results may have implications in the regulation of vascular function and tone in normal and atherosclerotic arteries.