Direct and reversible inhibition of endothelial nitric oxide synthase by nitric oxide

The objective of this study was to investigate the regulation of endothelial nitric oxide (NO) synthase by NO. Partially purified endothelial NO synthase was exposed to authentic NO (10-200 μM) and to the nitrovasodilators sodium nitroprusside (SNP; 10-1,000 μM) and S-nitroso-N- acetylpenicillamine (SNAP; 100-1,000 μM), and enzyme activity was assayed by measuring the conversion of L-[³H]arginine to L-[³H]citrulline in the presence of added cofactors. NO, SNP, and SNAP inhibited NO synthase activity in a dose-dependent manner, NO being the most potent inhibitor. The Michaelis constant for L-arginine was not altered (4.87 μM) by NO (50 μM), whereas the maximal velocity of the enzyme decreased from 784 to 633 pmol · mg^-1 · min^-1. Oxyhemoglobin (10 μM) partially prevented the inhibition of NO synthase by NO (50 μM). The data also suggest that NO inhibits endothelial NO synthase activity by directly interacting with the NO synthase and not by an indirect mechanism such as limitation of cofactor or oxygen availability. Dialysis of NO synthase treated with NO (50 μM) partially restored the enzyme activity. This study demonstrates a direct and reversible inhibition of NO synthase by NO, suggesting a feedback mechanism in vivo.