The sequences of nitric-oxide synthase flavin domains closely resemble that of NADPH-cytochrome P450 reductase (CPR). However, all nitric-oxide synthase (NOS) isoforms are 20 -40 residues longer in the C terminus, forming a "tail" that is absent in CPR. To investigate its function, we removed the 33 and 42 residue C termini from neuronal NOS (nNOS) and endothelial NOS (eNOS), respectively. Both truncated enzymes exhibited cytochrome c reductase activities without calmodulin that were 7-21-fold higher than the nontruncated forms. With calmodulin, the truncated and wild-type enzymes reduced cytochrome c at approximately equal rates. Therefore, calmodulin functioned as a nonessential activator of the wild-type enzymes and a partial noncompetitive inhibitor of the truncated mutants. Truncated nNOS and eNOS plus calmodulin catalyzed NO formation at rates that were 45 and 33%, respectively, those of their intact forms. Without calmodulin, truncated nNOS and eNOS synthesized NO at rates 14 and 20%, respectively, those with calmodulin. By using stopped-flow spectrophotometry, we demonstrated that electron transfer into and between the two flavins is faster in the absence of the C terminus. Although both CPR and intact NOS can exist in a stable, one-electron-reduced semiquinone form, neither of the truncated enzymes do so. We propose negative modulation of FAD-FMN interaction by the C termini of both constitutive NOSs.Nitric-oxide synthases (NOSs) 1 are bidomain, dimeric enzymes that synthesize NO from L-arginine through a series of monooxygenation reactions (for review see Ref. 1). The three isoforms, neuronal NOS (nNOS), inducible NOS (iNOS), and endothelial NOS (eNOS), produce NO by the same mechanism but play very different physiological roles due to the type of cell where they are located. nNOS, located in neurons in the brain and at neuromuscular junctions, is involved in neurotransmission (2, 3); iNOS, located in macrophages, is involved in the immune response (4, 5); and eNOS, located in endothelial cells, is involved in hemodynamic regulation (6, 7). The NO produced by nNOS and eNOS exerts its effects through the stimulation of guanylate cyclase, whereas the NO produced by iNOS exerts its effects directly or by combining with superoxide anion radical to form peroxynitrite anion, both potent oxidants deleterious to proteins and DNA.The NOSs consist of two domains, a heme and H 4 B-containing oxygenase (or heme) domain, which binds the substrate L-arginine, and a flavin-containing reductase (or flavoprotein) domain, which binds the prosthetic group flavins FAD and FMN and the cofactor NADPH. Electrons are transferred into NOS at the FAD moiety and are subsequently passed through the FMN to the heme domain. A calmodulin-binding region bisects the two domains. Calmodulin (CaM) is required for NO production and mediates the transfer of electrons from the FMN of NOS to the heme domain (8). CaM binds NOS in a 1:1 stoichiometry with very high affinity. CaM binds to sequences within the nNOS, eNOS, and iNOS CaM-binding...

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The sequences of nitric-oxide synthase (NOS) flavin domains closely resemble that of NADPH-cytochrome P450 reductase (CPR), with the exception of a few regions. One such region is the C terminus; all NOS isoforms are 20 -40 amino acids longer than CPR, forming a "tail" that is absent in CPR. To investigate its function, we removed the 21-amino acid C-terminal tail from murine macrophage inducible NOS (iNOS) holoenzyme and from a flavin domain construct. Both the truncated holoenzyme and reductase domain exhibited cytochrome c reductase activities that were 7-10-fold higher than the nontruncated forms. The truncated holoenzyme catalyzed NO formation approximately 20% faster than the intact form. Using stopped-flow spectrophotometry, we demonstrated that electron transfer into and between the two flavins and from the flavin to the heme domain is 2-5-fold faster in the absence of the C-terminal tail. The heme-nitrosyl complex, formed in all NOS isoforms during NO catalysis, is 5-fold less stable in truncated iNOS. Although both CPR and intact NOS can exist in a stable, one electron-reduced semiquinone form, neither the truncated holoenzyme nor the truncated flavin domain demonstrate such a form. We propose that this C-terminal tail curls back to interact with the flavin domain in such a way as to modulate the interaction between the two flavin moieties. Nitric oxide synthases (NOSs)1 produce NO from L-arginine through a series of monooxygenation reactions (for review, see Refs. 1-3). The three isoforms, nNOS, iNOS, and eNOS, produce NO by the same mechanism but play very different physiological roles due to the type of cell in which they are located. nNOS, located in neurons in the brain and at neuromuscular junctions, is involved in neurotransmission (4, 5); iNOS, located in macrophages, is involved in the immune response (6, 7); and eNOS, located in endothelial cells, is involved in hemodynamic regulation (8, 9). The NO produced by nNOS and eNOS exerts its effects through the stimulation of guanylate cyclase, whereas the NO produced by iNOS exerts its effects directly or by combining with superoxide to form peroxynitrite, both of which are free radicals that harm proteins and DNA.The NOSs consist of two domains, a heme and (6R)-5,6,7,8-tetrahydrobiopterin (H 4 B)-containing oxygenase (or heme) domain, which binds the substrate L-arginine and a flavin-containing reductase (or flavoprotein) domain, which binds the flavins FAD and FMN as prosthetic groups and the cofactor NADPH. A calmodulin binding region bisects the two domains. Calmodulin is required for NO production, and its binding is dependent on cellular calcium levels. Two of the isoforms (nNOS and eNOS) are constitutive; induction of NO synthesis activity requires an influx of calcium to promote calmodulin binding (5, 10). The iNOS enzyme is induced at the transcriptional level, and calmodulin is bound even at basal calcium concentrations (11).At the time Bredt and Snyder (5) reported the cloning of the first nitric-oxide synthase isoform (nNOS), they note...

5-(Glutathion-S-yl)-alpha-methyldopamine [5-(GSyl)-alpha-MeDA] is a putative metabolite of the serotonergic neurotoxicants 3,4-(+/-)-(methylenedioxy)amphetamine and 3,4-(+/-)-(methylenedioxy)methamphetamine. Glutathione (GSH) conjugates of several polyphenols are biologically (re)active. Therefore, as part of our studies on the role of 5-(GSyl)-alpha-MeDA in MDA-mediated neurotoxicity, we determined the regional brain metabolism of 5-(GSyl)-alpha-MeDA (720 nmol) following intracerebroventricular administration to male Sprague-Dawley rats. 5-(GSyl)-alpha-MeDA was rapidly cleared from all brain regions examined, and regional differences in the distribution of gamma-glutamyl transpeptidase (gamma-GT) correlated with the formation of 5-(cystein-S-yl)-alpha-methyldopamine (5-[CYS]-alpha-MeDA). We also observed the formation of 5-(N-acetyl-L-cystein-S-yl)-alpha-MeDA (5-[NAC]-alpha-MeDA) in all brain regions, indicating that the brain has the ability to synthesize mercapturic acids. Peak concentrations of 5-(NAC)-alpha-MeDA were found in the order: hypothalamus > midbrain/diencephalon/telencephalon > pons/medulla > hippocampus > cortex > striatum. In contrast to 5-(GSyl)-alpha-MeDA and 5-(CYS)-alpha-MeDA, 5-(NAC)-alpha-MeDA was eliminated relatively slowly from the brain. Differences were also found in cystein conjugate N-acetyltransferase activity in microsomes prepared from the various brain regions, but little difference was observed in brain cytosolic N-acetyl-L-cysteine conjugate N-deacetylase activity.(ABSTRACT TRUNCATED AT 250 WORDS)

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It has been proposed that Cys99 of human endothelial nitric oxide synthase (eNOS) is responsible for tetrahydrobiopterin (BH 4 ) binding. To examine this possibility rigorously, we expressed rat neuronal NOS (nNOS) in Escherichia coli, with the homologous Cys 331 to Ala mutation, and characterized structural and functional attributes of the purified, mutated enzyme. C331A-nNOS, as isolated, was catalytically incompetent. Upon prolonged incubation with L-arginine (L-Arg), not only BH 4 binding but also catalytic activity could be restored. In contrast to wild-type nNOS (WT-nNOS), which exhibits an absorbance maximum at 407 nm that shifts immediately upon L-arginine addition to a high spin form, the C331A-nNOS mutant, as isolated, exhibited an absorbance maximum at 420 nm. C331A-nNOS, as isolated, did not bind detectable levels of either

Neuronal nitric oxide synthase (nNOS) is a modular enzyme which consists of a flavin-containing reductase domain and a heme-containing oxygenase domain, linked by a stretch of amino acids which contains a calmodulin (CaM) binding site. CaM binding to nNOS facilitates the transfer of NADPH-derived electrons from the reductase domain to the oxygenase domain, resulting in the conversion of L-arginine to L-citrulline with the concomitant formation of a guanylate cyclase activating factor, putatively nitric oxide. Numerous studies have established that peroxynitrite-derived nitrogen oxides are present following nNOS turnover. Since peroxynitrite is formed by the diffusion-limited reaction between the two radical species, nitric oxide and O2.-, we employed the adrenochrome assay to examine whether nNOS was capable of producing O2.- during catalytic turnover in the presence of L-arginine. To differentiate between the role played by the reductase domain and that of the oxygenase domain in O2.- production, we compared its production by nNOS against that of a nNOS mutant (CYS-331), which was unable to transfer NADPH-derived electrons efficiently to the heme iron under special conditions, and against that of a flavoprotein module construct of nNOS. We report that O2.- production by nNOS and the CYS-331 mutant is CaM-dependent and that O2.- production can be modulated by substrates and inhibitors of nNOS. O2.- was also produced by the reductase domain of nNOS; however, it did not display the same CaM dependency. We conclude that both the reductase and oxygenase domains of nNOS produce O2.-, but that the reductase domain is both necessary and sufficient for O2.- production.

alpha-Methyldopamine (alpha-MeDA) is a metabolite of the serotonergic neurotoxicants 3,4-(+/-)-(methylenedioxy)amphetamine (MDA) and 3,4-(+/-)-(methylenedioxy)methamphetamine (MDMA). alpha-MeDA readily oxidizes, and in the presence of glutathione (GSH) it forms 5-(glutathion-S-yl)-alpha-methyldopamine [5-(glutathion-S-yl)-alpha-MeDA]. Since GSH conjugates of many polyphenols are biologically (re)active, we investigated the role of 5-(glutathion-S-yl)-alpha-MeDA in the acute and long-term neurochemical changes observed after administration of MDA. Intracerebroventricular (icv) administration of 5-(glutathion-S-yl)-alpha-MeDA (720 nmol) to male Sprague-Dawley rats produced behavioral changes similar to those reported after subcutaneous administration of MDA. Thus, animals became hyperactive and aggressive and displayed forepaw treading and Straub tails, behaviors usually seen after administration of serotonin (5-HT) releasers, and consistent with a role for 5-(glutathion-S-yl)-alpha-MeDA in some of the behavioral alterations seen after administration of MDA and MDMA. In addition to the behavioral changes, 5-(glutathion-S-yl)-alpha-MeDA also caused short-term alterations in the dopaminergic, serotonergic, and noradrenergic systems. An increase in dopamine synthesis appears to be a prerequisite for the long-term depletion of brain 5-HT following MDMA administration. However, although 5-(glutathion-S-yl)-alpha-MeDA reproduced some of the effects of MDA on the dopaminergic system and was capable of causing acute increases in 5-HT turnover, a single icv injection of 5-(glutathion-S-yl)-alpha-MeDA did not result in long-term serotonergic toxicity. Thus, although acute stimulation of dopamine turnover may be necessary for long-term serotonergic toxicity, such changes are not sufficient to produce these effects. The effects of a multiple dosing schedule of 5-(glutathion-S-yl)-alpha-MeDA will therefore require investigation before we can define a role for this metabolite in MDA and MDMA mediated neurotoxicity. MDA also produces a pressor response that is related to its ability to release neuronal norepinephrine stores, and 5-(glutathion-S-yl)-alpha-MeDA caused comparable depletions of brain norepinephrine concentrations, indicating that both compounds produce similar effects on the noradrenergic system.