Regulation of Biosynthesis of Nitric Oxide.

Nitric oxide synthases (NOSs)' (L-arginine, NADPH:oxygen oxidoreductases (nitric oxide forming); EC 1.14.13.39) are unique among eukaryotic enzymes in being dimeric, calmodulin-dependent or calmodulin-containing cytochrome P450-like hemoproteins that combine reductase and oxygenase catalytic domains in one monomer, bear both FAD and FMN, and carry out a 5-electron oxidation of a non-aromatic amino acid (L-arginine) with the aid of tetrahydrobiopterin (1).Eukaryotes as phylogenetically ancient and diverse as Limulus polyphemus (2) and Rhodnius prolixus (3) make NO, and species as distantly related as Drosophila' and man (&lo) have yielded NOS cDNAs.Biosynthesis of nitric oxide (NO) appears to be within the repertoire of most mammalian cell types ( l l ) , provided they are appropriately stimulated.NO has been known for the past 6 years to mediate aspects of macrophage cytotoxicity, regulate blood pressure, and participate in neurotransmission (11,12), yet a steady stream of reports continues to enlarge NO'S sphere of action.For example, endogenous NO can contribute to destruction of the host's joints (13, 14) and kidneys ( 14) or to its survival from viral infection (15).Accordingly, regulation of NO biosynthesis has attracted intensive study and is the subject of this review.Other recent surveys deal with the enzymology of NOSs (1, 16) and their physiologic roles (11,12). RecapResearch through 1992 described two constitutive, calmodulindependent NOSs (cNOSs).One is apparently restricted to endothelium.The other is found in central and peripheral neurons, neuroblastomas (7), human but not rat skeletal muscle (6), p cells of pancreatic islets, and epithelial cells of bronchioli, uterus, and stomach (17).The cNOSs lie dormant until and so long as a rise in intracellular Ca2+ sustains the binding of calmodulin, leading to NO release over several minutes (18).An inducible NOS is expressed in many cell types after challenge with immunologic or inflammatory stimuli (11) and thereupon generates large amounts of NO over periods as long as 5 days (19).Tonic activation of this isoform may reflect its ability to bind calmodulin even at the trace levels of Ca2+ found in resting cells (20).This isoform was termed iNOS to signify its independence of elevated Ca2+ and exogenous calmodulin (21).These elements of the picture remain in place.Now, however, the canvas is crowded with new details. How Many NOSs?The human endothelial cNOS gene, residing at 7q35-7q36, comprises 26 exons that span 21 kb (22,23).The 28-exon neural cNOS gene on human chromsome 12q24.2extends over 100 kb (7).3 Hu-