Autoinhibition of neuronal nitric oxide synthase:: distinct effects of reactive nitrogen and oxygen species on enzyme activity

Nitric oxide (NO) synthases (NOSs), which catalyse the oxidation of L-arginine to L-citrulline and an oxide of nitrogen, possibly NO or nitroxyl (NO(-)), are subject to autoinhibition by a mechanism that has yet to be fully elucidated. In the present study we investigated the actions of NO and other NOS-derived products as possible autoregulators of enzyme activity. With the use of purified NOS-I, L-arginine turnover was found to operate initially at V(max) (0-15 min, phase I) although, despite the presence of excess substrate and cofactors, prolonged catalysis (15-90 min, phase II) was associated with a rapid decline in L-arginine turnover. Taken together, these observations suggested that one or more NOS products inactivate NOS. Indeed, exogenously applied reactive nitrogen oxide species (RNSs) decreased V(max) during phase I, although with different potencies (NO- > NO > ONOO(-)); and efficacies (NO > NO(-) = ONOO(-)). The NO scavengers oxyhaemoglobin (HbO(2); 100 mu M) and 1H-imidazol-1-yloxy-2-(4-carboxyphenyl)-4,5-dihydro-4,4,5,5-tetramethyl-3-oxide (CPTIO; 10 mu M) and the ONOO(-) scavenger GSH (7 mM) had no effect on NOS activity during phase I, except for an endogenous autoinhibitory influence of NO and ONOO(-). However, superoxide dismutase (SOD; 300 units/ml), which is thought either to increase the half-life of NO or to convert NO(-) to NO, lowered V(max) in an NO-dependent manner because this effect was selectively antagonized by HbO(2) (100 mu M). This latter observation demonstrated the requirement of SOD to reveal endogenous NO-mediated autoinhibition. Importantly, during phase II of catalysis, NOS became uncoupled and began to form H(2)O(2) because catalase, which metabolizes H(2)O(2), increased enzyme activity. Consistent with this, exogenous H(2)O(2) also inhibited NOS activity during phase I. Thus during catalysis NOS is subject to complex autoinhibition by both enzyme-derived RNS and H(2)O(2), differentially affecting enzyme activity.