Oxygen reduction by nitric-oxide synthases.

Nitric-oxide synthases (NOS, EC 1.14.13.39) are hemeproteins that catalyze oxidation of L-arginine to NOz and L-citrulline. Three main isozymes exist in mammals that are regulated by distinct genes (1–6): a constitutive neuronal NOS (nNOS or NOS I), (7, 8), an endotoxinand cytokine-inducible NOS (iNOS or NOS II) (9, 10), and a constitutive endothelial NOS (eNOS or NOS III) (11). Although NOS have some unique features that distinguish them from other hemeprotein monooxygenases such as cytochrome P-450s, there are, nevertheless, a number of similarities that allow comparisons to be made (12). One such finding is that both enzymes inadvertently secrete O2 . (13, 14). Indeed, NOSs contain four redox active prosthetic groups (FAD, FMN, iron protoporphyrin IX (heme), and (6R)-tetrahydrobiopterin (H4B)) that could conceivably pass electrons to O2. Understanding the extent to which this occurs independent of NOz synthesis is important from a mechanistic standpoint. In particular, how does the enzyme control O2 activation? From a biologic perspective, NOz and O2 . initiate different cell signaling pathways (15, 16). This is further complicated by the fact that NOzand O2 . combine to form peroxynitrite (17), which has physiological activities that differ greatly from those of the parent free radicals (18). In this review we examine the electron transport chain of NOS with special emphasis on O2 . production and interpret these findings with a view toward NOS structure-function and the kinetics of the electron transfer reactions.