Regulation of nitric oxide synthase by nitric oxide

Nitric oxide (NO) is a recently discovered messenger for the activation of soluble guanylate cyclase in a wide variety of cell types. Although enzymes involved in NO synthesis have been discovered, the regulation of their action is not clear. The possibility of NO regulating the activity of a crude NO synthase (EC 1.14.23) preparation from bovine cerebellum was investigated. Authentic NO (50-400 microM) produced a marked attenuation of NO synthase activity, as measured by the stoichiometric conversion of L-[3H]arginine to L-[3H]citrulline. This inhibition was mimicked by the nitrovasodilators S-nitroso-N-acetylpenicillamine, sodium nitroprusside, and glyceryl trinitrate. NO was most potent in inhibiting the enzyme activity, followed by S-nitroso-N-acetylpenicillamine, sodium nitroprusside, and glyceryl trinitrate. The effects of NO and the nitrovasodilators were concentration dependent and reversible. Oxyhemoglobin (50 microM), a scavenger of NO, partially prevented the inhibition of NO synthase activity by NO. Inorganic nitrite (5 mM), the oxidation product of NO, did not produce any effect on the enzyme activity. The Km for L-arginine was not significantly changed by NO (200 microM) (from 6.4 +/- 0.8 microM to 10.6 +/- 1.6 microM), whereas the Vmax of the enzyme was markedly decreased (from 80 +/- 4 to 45 +/- 4 pmol/min/mg of protein). This study suggests that NO production may be regulated by a direct effect of NO on the activity of NO synthase.