N omega-hydroxy-L-arginine: a novel arginine analog capable of causing vasorelaxation in bovine intrapulmonary artery

Participation of nitric oxide pathway in the relaxation response induced by E-cinnamaldehyde oxime in superior mesenteric artery isolated from rats

For many years, nitric oxide (NO) has been studied as an important mediator in the control of vascular tone. Endothelial deficiencies that diminish NO production can result in the development of several future cardiovascular diseases, such as hypertension and arteriosclerosis. In this context, new drugs with potential ability to donate NO have been studied. In this study, 3 aromatic oximes [benzophenone oxime, 4-Cl-benzophenone oxime, and E-cinnamaldehyde oxime (E-CAOx)] induced vasorelaxation in endothelium-denuded and intact superior mesenteric rings precontracted with phenylephrine. E-CAOx demonstrated the most potent effect, and its mechanism of action was evaluated. Vascular reactivity experiments demonstrated that the effect of E-CAOx was reduced by the presence of 2-phenyl-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide, 1H[1,2,4,]oxadiazolo[4,3-a]quinoxalin-1-one, and (Rp)-8-(para-chlorophenylthio)guanosine-3',5'-cyclic monophosphorothioate, suggesting the participation of NO/sGC/PKG pathway. NO donation seems to be mediated through nicatinamide adenine dinucleotide phosphate-dependent reductases because 7-ethoxyresorufin decreased the effect of E-CAOx on vascular reactivity and reduced NO formation as detected by flow cytometry using the NO indicator diaminofluorescein 4,5-diacetate. Further downstream of NO donation, K+ subtype channels were also shown to be involved in the E-CAOx vasorelaxant effect. The present study showed that E-CAOx acts like an NO donor, activating NO/sGC/PKG pathway and thus K+ channels.

The vascular effects of different arginase inhibitors in rat isolated aorta and mesenteric arteries

BACKGROUND AND PURPOSE

Arginase and nitric oxide (NO) synthase share the common substrate L-arginine, and arginase inhibition is proposed to increase NO production by increasing intracellular levels of L-arginine. Many different inhibitors are used, and here we have examined the effects of these inhibitors on vascular tissue.

EXPERIMENTAL APPROACH

Each arginase inhibitor was assessed by its effects on isolated rings of aorta and mesenteric arteries from rats by: (i) their ability to preserve the tolerance to repeated applications of the endothelium-dependent agonist acetylcholine (ACh); and (ii) their direct vasorelaxant effect.

KEY RESULTS

In both vessel types, tolerance (defined as a reduced response upon second application) to ACh was reversed with addition of L-arginine, (S)-(2-boronethyl)-L-cysteine HCl (BEC) or N(G)-Hydroxy-L-arginine (L-NOHA). On the other hand, N(omega)-hydroxy-nor-L-arginine (nor-NOHA) significantly augmented the response to ACh, an effect that was partially reversed with L-arginine. No effect on tolerance to ACh was observed with L-valine, nor-valine or D,L, alpha-difluoromethylornithine (DFMO). BEC, L-NOHA and nor-NOHA elicited endothelium-independent vasorelaxation in both endothelium intact and denuded aorta while L-valine, DFMO and nor-valine did not.

CONCLUSIONS AND IMPLICATIONS

BEC and L-NOHA, but not nor-NOHA, L-valine, DFMO or nor-valine, significantly reversed tolerance to ACh possibly conserving L-arginine levels and therefore increasing NO bioavailability. However, both BEC and L-NOHA caused endothelium-independent vasorelaxation in rat aorta, suggesting that these inhibitors have a role beyond arginase inhibition alone. Our data thus questions the interpretation of many studies using these antagonists as specific arginase inhibitors in the vasculature, without verification with other methods.

Generation of nitroxyl by heme protein-mediated peroxidation of hydroxylamine but not N-hydroxy-L-arginine

The chemical reactivity, toxicology, and pharmacological responses to nitroxyl (HNO) are often distinctly different from those of nitric oxide (NO). The discovery that HNO donors may have pharmacological utility for treatment of cardiovascular disorders such as heart failure and ischemia reperfusion has led to increased speculation of potential endogenous pathways for HNO biosynthesis. Here, the ability of heme proteins to utilize H2O2 to oxidize hydroxylamine (NH2OH) or N-hydroxy-L-arginine (NOHA) to HNO was examined. Formation of HNO was evaluated with a recently developed selective assay in which the reaction products in the presence of reduced glutathione (GSH) were quantified by HPLC. Release of HNO from the heme pocket was indicated by formation of sulfinamide (GS(O)NH2), while the yields of nitrite and nitrate signified the degree of intramolecular recombination of HNO with the heme. Formation of GS(O)NH2 was observed upon oxidation of NH2OH, whereas NOHA, the primary intermediate in oxidation of L-arginine by NO synthase, was apparently resistant to oxidation by the heme proteins utilized. In the presence of NH2OH, the highest yields of GS(O)NH2 were observed with proteins in which the heme was coordinated to a histidine (horseradish peroxidase, lactoperoxidase, myeloperoxidase, myoglobin, and hemoglobin) in contrast to a tyrosine (catalase) or cysteine (cytochrome P450). That peroxidation of NH2OH by horseradish peroxidase produced free HNO, which was able to affect intracellular targets, was verified by conversion of 4,5-diaminofluorescein to the corresponding fluorophore within intact cells.

The activation of metabolites of nitric oxide synthase by metals is both redox and oxygen dependent: a new feature of nitrogen oxide signaling

Nitrite (NO(2)-), N (G)-hydroxy-L-arginine (NOHA), and hydroxylamine (NH(2)OH) are products of nitric oxide synthase (NOS) activity and can also be formed by secondary reactions of nitric oxide (NO). These compounds are commonly considered to be rather stable and as such to be dosimeters of NO biosynthesis. However, each can be converted via metal-catalyzed reactions into either NO or other reactive nitrogen oxide species (RNOS), such as nitrogen dioxide (NO(2)) and nitroxyl (HNO), which have biologic activities distinct from those of the parent molecules. Consequently, certain aspects of tissue regulation controlled by RNOS may be dictated to a significant extent by metal-dependent reactions, thereby offering unique advantages for cellular and tissue regulation. For instance, because many metal-catalyzed reactions depend on the redox and oxygen status of the cellular environment, such reactions could serve as redox indicators. Formation of RNOS by metal-mediated pathways would confine the chemistry of these species to specific cellular sites. Additionally, such mechanisms would be independent both of NO and NOS, thus increasing the lifetime of RNOS that react with NO. Thus metal-mediated conversion of nitrite, NOHA, and NH(2)OH into biologically active agents may provide a unique signaling mechanism. In this review, we discuss the biochemistry of such reactions in the context of their pharmacologic and biologic implications.

Discriminating formation of HNO from other reactive nitrogen oxide species

Nitroxyl (HNO) exhibits unique pharmacological properties that often oppose those of nitric oxide (NO), in part due to differences in reactivity toward thiols. Prior investigations suggested that the end products arising from the association of HNO with thiols were condition-dependent, but were inconclusive as to product identity. We therefore used HPLC techniques to examine the chemistry of HNO with glutathione (GSH) in detail. Under biological conditions, exposure to HNO donors converted GSH to both the sulfinamide [GSONH2] and the oxidized thiol (GSSG). Higher thiol concentrations generally favored a higher GSSG ratio, suggesting that the products resulted from competitive consumption of a single intermediate (GSNHOH). Formation of GSONH2 was not observed with other nitrogen oxides (NO, N2O3, NO2, or ONOO(-)),indicating that it is a unique product of the reaction of HNO with thiols. The HPLC assay was able to detect submicromolar concentrations of GSONH2. Detection of GSONH2 was then used as a marker for HNO production from several proposed biological pathways, including thiol-mediated decomposition of S-nitrosothiols and peroxidase-driven oxidation of hydroxylamine (an end product of the reaction between GSH and HNO) and NG-hydroxy-l-arginine (an NO synthase intermediate). These data indicate that free HNO can be biosynthesized and thus may function as an endogenous signaling agent that is regulated by GSH content.

Nomega-hydroxy-L-arginine homologues and hydroxylamine as nitric oxide-dependent vasorelaxant agents

Endothelium-independent relaxant activities of N(omega)-hydroxy-L-arginine (L-NOHA) homologues and hydroxylamine, a possible intermediate in nitric oxide (NO) formation, were examined in rat aortic rings. Addition of one -CH(2)- group to the -(CH(2))(x)- chain between the alpha-amino acid and the hydroxyguanidine group (x=4) almost abolished-while deletion of one or two -CH(2)- (x=1 or 2) enhanced-the relaxant activity of L-NOHA homologues. N(omega)-hydroxy-nor-L-arginine- (x=2) and hydroxylamine-induced relaxations were blunted by a NO scavenger and by inhibitors of the guanylyl cyclase pathway, but not by NO synthase or cytochrome P(450) inhibitors (except 7-ethoxyresorufin). However, aortic NO formation was detected (using electron paramagnetic resonance) in the presence of concentrations of these compounds higher than those producing relaxation. These findings support the view that endothelium-independent vasorelaxations induced by both L-NOHA homologues with a required chain length x</=3 and hydroxylamine are mediated by NO-dependent activation of guanylyl cyclase, through a 7-ethoxyresorufin-inhibited mechanism.

Role of the adventitia in the cyclic GMP-mediated relaxant effect of N-hydroxy-L-arginine in rat aorta

N(omega)-hydroxy-L-arginine (L-NOHA), the stable intermediate of the nitric oxide synthase (NOS)-catalyzed reaction, can induce NO/cyclic GMP-dependent relaxation in the rat aorta, in an endothelium- and NOS-independent manner. In this study, the role of the adventitia in the endothelium-independent effect of L-NOHA was investigated. Despite a decrease in norepinephrine (NE)-induced precontraction, adventitia removal in the rat aorta did not markedly alter the relaxant effect of forskolin, S-nitroso-N-acetylpenicillamine or glyceryl trinitrate. In contrast, both inhibition of NE-induced contraction and relaxation of NE-precontracted rings produced by L-NOHA were diminished in the absence of adventitia. Moreover, exposure to L-NOHA significantly enhanced the cyclic GMP level in the media of the aorta with, but not without adventitia. These findings demonstrate the role of the adventitia in the L-NOHA-induced decrease in tone and increase in cyclic GMP in the endothelium-denuded rat aorta. They suggest that NO or an NO-related compound formed from L-NOHA in the adventitia may produce paracrine effects.

Activation and potentiation of the NO/cGMP pathway by NG-hydroxyl-L-arginine in rabbit corpus cavernosum under normoxic and hypoxic conditions and ageing

1 When nitric oxide synthase (NOS) produces NO from N(G)-hydroxy-L-arginine (OH-arginine) instead of L-arginine, the total requirement of molecular oxygen and NADPH to form NO is reduced. The aim of this work was to evaluate the effects of OH-arginine on the contractility of rabbit corpus cavernosum (RCC) and to compare the capacities of L-arginine and OH-arginine to enhance NO-mediated responses under normoxic and hypoxic conditions and in ageing, as models of defective NO production. 2 OH-arginine, but not L-arginine, was able to relax phenylephrine-contracted rabbit trabecular smooth muscle. OH-arginine-induced relaxation was inhibited by the NOS-inhibitor, L-NNA (300 microM), and by the guanylyl cyclase inhibitor, ODQ (20 microM), while it was not affected by the cytochrome P450 oxygenase inhibitor, miconazole (0.1 mM). Administration of OH-arginine, but not L-arginine, produced a significant increment of cGMP accumulation in RCC tissue. 3 Relaxation elicited by OH-arginine (300 microM) was still observed at low oxygen tension. The increase of cGMP levels induced by ACh (30 microM) in RCC was significantly enhanced by addition of OH-arginine (300 microM) in normoxic conditions, as well as under hypoxia, while L-arginine did not alter the effects of ACh on cGMP accumulation. 4 Endothelium-dependent and nitrergic nerve-mediated relaxations were both significantly reduced in RCC from aged animals (>20-months-old) when compared with young adult rabbits (5-months-old). Treatment with OH-arginine (300 microM) significantly potentiated endothelium-dependent and neurogenic relaxation in corpus cavernosum from aged rabbits, while L-arginine (300 microM) did not have significant effects. 5 Results show that OH-arginine promotes NO-mediated relaxation of RCC and potentiates the NO-mediated responses induced by stimulation of endogenous NO generation in hypoxic and aged tissues. We propose that the use of OH-arginine could be of interest in the treatment of erectile dysfunction, at least in those secondary to defective NO production.

Involvement of NO in the endothelium-independent relaxing effects of N(omega)-hydroxy-L-arginine and other compounds bearing a C=NOH function in the rat aorta

The mechanisms of vasorelaxation elicited by N(omega)-hydroxy-L-arginine (L-NOHA) and other compounds bearing a C=NOH function and the structural determinants governing this effect were investigated in rat aorta. L-NOHA, formamidoxime, five aromatic monosubstituted amidoximes, and one aromatic monosubstituted ketoxime elicited relaxation in endothelium-denuded rings. N-Hydroxyguanidine and substituted N-hydroxyguanidines were markedly less active. Relaxations induced by L-NOHA and by the most active studied compound, 4-chlorobenzamidoxime (ClBZA), were unmodified by the presence of endothelium. In endothelium-denuded rings, they were blunted by the NO scavenger 2-phenyl-4,4,5,5-tetramethyl-imidazoline-1-oxyl-3-oxide (300 microM) and by the inhibitor of guanylyl-cyclase activation 1H[1,2,4,]oxadiazolo[4,3-a]quinoxalin-1-one (1 microM). In addition, L-NOHA- and ClBZA both caused cGMP accumulation. L-Arginine, but not D-arginine (1 mM), antagonized the effect of L-NOHA but not ClBZA. Both L-NOHA- and ClBZA-induced relaxations were inhibited by the NAD(P)H-dependent enzymes inhibitor diphenyliodonium (30 microM) and the NAD(P)H-dependent reductases inhibitor 7-ethoxyresorufin (10 microM), but they were unmodified by the cytochrome P450 (P450) inhibitor proadifen (10 microM) and by the NO synthase inhibitor N(omega)-nitro-L-arginine methyl ester (L-NAME, 300 microM). These results show that L-NOHA and other compounds with a C=NOH function can cause endothelium-independent relaxation in the rat aorta. They suggest that activation of guanylyl cyclase and NO formation is implicated in relaxation and that a 7-ethoxyresorufin-sensitive NAD(P)H-dependent pathway is involved. On one hand, L-NOHA and amidoximes may be useful tools for characterizing this pathway in blood vessels and, on the other, may offer a novel approach for treating vascular diseases with impaired endothelial NO activity.

Regulation of striatal dopamine neurotransmission by nitric oxide: effector pathways and signaling mechanisms

An important role for the reactive gas nitric oxide (NO) in regulating striatal dopaminergic neurotransmission was identified shortly after initial observations indicated that this unorthodox neurotransmitter mediates many of the influences of glutamatergic neurotransmission in the cerebellum, cortex, and hippocampus. While the precise actions of NO on striatal presynaptic and postsynaptic elements remain to be fully characterized, the recent application of sophisticated anatomical, neurochemical, and electrophysiological approaches to the study of nitrergic signaling has revealed that NO exerts a powerful influence both on tonic extracellular dopamine (DA) levels and phasic DA neuron spike activity via the modulation of intrinsic striatal mechanisms and striatonigral feedback loops. Although the nature of the NO-mediated modulatory influence on DA neurotransmission was initially clouded by seemingly conflicting neurochemical observations, a growing body of literature and understanding of the diverse signaling mechanisms and effector pathways utilized by NO indicates that NO exerts a primary facilitatory influence over tonic and phasic dopaminergic neurotransmission under physiological conditions. A review of neurochemical and electrophysiological studies examining the influence of endogenous and exogenous NO on DA neurotransmission indicates that NO signaling exerts multiple effects on local striatal circuits and projection neurons involved in regulating basal ganglia output and nigrostriatal DA neuron activity. In addition to summarizing these influences, the current review focuses on the mechanisms utilized by striatal NO signaling pathways involved in modulating DA transmission at the level of the terminal and cell body and attempts to integrate these observations into a functional model of NO-dependent regulation of basal ganglia systems.

Decreased plasma concentrations of L-hydroxy-arginine as a marker of reduced NO formation in patients with combined cardiovascular risk factors

Patients with metabolic syndrome represent a group with extensive cardiovascular risk factors for the development of atherosclerosis, which may be preceded by an impairment of endothelial function. Endothelial dysfunction is characterized by a reduced availability of bioactive nitric oxide, the principal mediator of endothelium-dependent vasodilation. In the present study we assessed NO synthesis in vivo by measuring the NO-related amino acids L-arginine and L-citrulline and in particular the stable intermediate compound N(omega)-hydroxy-L-arginine (L-NHA) in patients with metabolic syndrome by using high-performance liquid chromatography (HPLC) analysis. As a prerequisite to our study, we measured the amino acid concentrations in 31 healthy volunteers to investigate gender and age differences. To prove whether blood drawn from peripheral veins reflects plasma concentrations of the whole vessel system, several blood samples from different regions were obtained from patients undergoing elective left and right heart catheterization. In the latter group, no significant differences were noted among the plasma concentrations between the different sample sites. In healthy volunteers, there were no significant differences in plasma concentrations of any one specific amino acid between males and females or age groups. The main finding of the study is that the intermediate product of NO synthesis, L-NHA, is significantly reduced in the plasma samples of patients with a metabolic syndrome as compared with samples from healthy control subjects. The plasma concentrations of the NO precursor L-arginine and the end product of NO synthesis, L-citrulline, were unchanged. In conclusion, our results suggest that plasma levels of L-NHA are independent of age and gender and are not different at various locations within the vascular system. In a group of patients at high risk for the development of atherosclerosis, we found reduced plasma concentrations of L-NHA, either caused by a decreased endothelial NO synthase activity or caused by an increased breakdown of L-NHA by pathways independent of NO synthase, resulting in a reduced availability of L-NHA for NO synthesis.

Striatal nitric oxide signaling regulates the neuronal activity of midbrain dopamine neurons in vivo

A major component of the cortical regulation of the nigrostriatal dopamine (DA) system is known to occur via activation of striatal efferent systems projecting to the substantia nigra. The potential intermediary role of striatal nitric oxide synthase (NOS)-containing interneurons in modulating the efferent regulation of DA neuron activity was examined using single-unit recordings of DA neurons performed concurrently with striatal microdialysis in anesthetized rats. The response of DA neurons recorded in the substantia nigra to intrastriatal artificial cerebrospinal fluid (ACSF) or drug infusion was examined in terms of mean firing rate, percent of spikes fired in bursts, cells/track, and response to electrical stimulation of the orbital prefrontal cortex (oPFC) and striatum. Intrastriatal infusion of NOS substrate concurrently with intermittent periods of striatal and cortical stimulation increased the mean DA cell population firing rate as compared with ACSF controls. This effect was reproduced via intrastriatal infusion of a NO generator. Infusion of either a NOS inhibitor or NO chelator via reverse microdialysis did not affect basal firing rate but increased the percentage of DA neurons responding to striatal stimulation with an initial inhibition followed by a rebound excitation (IE response) from 40 to 74%. NO scavenger infusion also markedly decreased the stimulation intensity required to elicit an IE response to electrical stimulation of the striatum. In single neurons in which the effects of electrical stimulation were observed before and after drug delivery, NO antagonist infusion was observed to decrease the onset latency and extend the duration of the initial inhibitory phase induced by either oPFC or striatal stimulation. This is the first report showing that striatal NO tone regulates the basal activity and responsiveness of DA neurons to cortical and striatal inputs. These studies also indicate that striatal NO signaling may play an important role in the integration of information transmitted to basal ganglia output centers via corticostriatal and striatal efferent pathways.

NG-hydroxy-L-arginine and nitric oxide inhibit Caco-2 tumor cell proliferation by distinct mechanisms

The objective of this study was to elucidate the role and mechanism of nitric oxide (NO) synthase (NOS) in modulating the growth of the Caco-2 human colon carcinoma cell line. The two novel observations reported here are, first, that NG-hydroxy-L-arginine (NOHA) inhibits Caco-2 tumor cell proliferation, likely by inhibiting arginase activity, and, second, that NO causes cytostasis by mechanisms that might involve inhibition of ornithine decarboxylase (ODC) activity. Both arginase and ODC are enzymes involved in the conversion of arginine to polyamines required for cell proliferation. Cell growth was monitored by cell count, cell protein analysis, and DNA synthesis. NOHA (1-30 microM) and NO in the form of DETA/NO (1-30 microM) inhibited cell proliferation by 30-85%. The cytostatic effect of NOHA was prevented by addition of excess ornithine, putrescine, spermidine, or spermine to cell cultures, whereas the cytostatic effect of NO (DETA/NO) and alpha-difluoromethylornithine (ODC inhibitor) was unaffected by ornithine but was prevented by putrescine, spermidine, or spermine. The cytostatic effect of NOHA appeared to be independent of its conversion to NO, and the effect of NO appeared to be independent of cGMP. NOHA inhibited urea production by Caco-2 cells and inhibited arginase catalytic activity (85% at 3 microM), whereas NO (DEA/NO and SNAP) inhibited ODC activity (>/=60% at 30 microM) without affecting arginase activity. Coculture of Caco-2 cells with lipopolysaccharide/cytokine-activated rat aortic endothelial cells markedly slowed Caco-2 cell proliferation, and this was blocked by NOS inhibitors. These observations that NOHA and NO may inhibit sequential steps in the arginine-polyamine pathway suggest a novel biological role for NOS in the inhibition of cell proliferation of certain tumor cells and possibly other cell types.

Endogenous nitric oxide facilitates striatal dopamine and glutamate efflux in vivo: role of ionotropic glutamate receptor-dependent mechanisms

We have investigated the influence of the nitric oxide synthase (NOS) substrate, NG-hydroxy-L-arginine (H-ARG) on dopamine (DA) and glutamate (GLU) efflux in vivo using concentric microdialysis probes implanted in the anterior-medial striatum of chloral hydrate-anesthetized rats. Intrastriatal infusion of H-ARG (100 microM, 200 microM, or 1 mM for 120 min) increased DA efflux in a dose-dependent fashion. The facilitatory effect of H-ARG (1 mM) on DA efflux was abolished following pretreatment (80 min) with the constitutive NOS inhibitor 7-nitroindazole (7-NI, 10 microM) but unaffected by L-NG(1-iminoethyl) lysine (100 microM) infusion. As both H-ARG (1 mM) and the NO-generator (+/-)-S-nitroso-N-acetylpenicillamine (1 mM) were observed to increase GLU efflux concurrently with the effect on DA efflux, we evaluated the potential intermediary role of GLU in NO-facilitated DA efflux using ionotropic GLU receptor antagonists. Local infusion of dizocilpine maleate (10 microM) or (+/-)-2-amino-3-[3-(carboxymethoxy)-5-methyl-isoxazol-4-yl] propionic acid (100 microM), attenuated the H-ARG (1 mM)-induced elevation of extracellular DA levels. Conversely, similar treatment with the kainate receptor antagonist d-gamma-glutamyl-aminomethanesulfonic acid did not alter H-ARG-induced DA efflux. To evaluate the regulatory influence of striatal NO on NMDA receptor activation, NMDA (100 microM) was co-perfused with either H-ARG (2 mM) or 7-NI (10 microM). While co-perfusion with 7-NI potentiated NMDA-induced DA efflux, similar treatment with H-ARG (2 mM) abolished the effect. These results demonstrate that endogenous NO production, stimulated via H-ARG-dependent activation of type 1 NOS, enhances striatal DA efflux via an increase in glutamatergic tone on ionotropic GLU-receptors. At higher levels of NOS activation (following H-ARG (2 mM) or NMDA infusion), NO may block glutamatergic neurotransmission via inhibition of NMDA receptor function.

Nitric oxide (NO), the only nitrogen monoxide redox form capable of activating soluble guanylyl cyclase

In the present study, we determined that of the redox forms of nitrogen monoxide, NO-, NO and NO+, only NO significantly activates soluble guanylyl cyclase (GTP pyrophosphate-lyase cyclizing, EC 4.6.1.2). Neither of the NO-donors tested, Angeli's salt (Na2N2O3) or Piloty's acid (C6H5SO2NHOH), caused a change in the guanylyl cyclase activity relative to the basal activity level. Interference by other reaction products was eliminated as a possible explanation for the lack of activation. To the extent that NO+ could be stabilized in aqueous solution, by dissolution of the nitrosonium salt NOPF6 in dry organic solvent prior to addition to the enzyme in buffer, NO+ had no effect on the activity of soluble guanylyl cyclase. The counter-ion, PF6-, had a minimal effect on the enzyme activity and, therefore was, not responsible for the lack of activation by NO+. These observations suggest that NO- is the natural activator of soluble guanylyl cyclase and is reasonably identical with endothelium-derived relaxing factor, the physiological regulator of soluble guanylyl cyclase activity.

Mechanisms mediating the vasodilatory effects of N-hydroxy-L-arginine in coronary arteries

We assessed the mechanisms of action of NG-hydroxy-L-arginine in isolated porcine large coronary arterial rings. Increasing (1, 10 and 100 microM) concentrations of NG-hydroxy-L-arginine evoked endothelium-dependent dilation which was eliminated by 100 microM of NG-nitro-L-arginine methyl ester, but not affected by a cytochrome P450 inhibitors (miconazole or 7-ethoxyresorufin). At a given concentration, the dilatory response to NG-hydroxy-L-arginine was stronger than that elicited by L-arginine. NG-Hydroxy-L-arginine (100 microM), but not NG-hydroxy-D-arginine, potentiated the endothelium-dependent dilation of calcium ionophore A23187 but had no effect on endothelium-independent dilation evoked by an NO donor. NO release by endothelium-intact porcine coronary arterial rings was measured with a chemiluminescence analyser. A23187 (10 microM), NG-Hydroxy-L-arginine (100 microM), and to a lesser extent NG-hydroxy-D-arginine (100 microM), significantly increased NO concentration over 15 min observation period. When A23187 and NG-hydroxy-L-arginine were combined, NO concentration increased in an additive fashion. Enhanced NO release by either A23187, NG-hydroxy-L-arginine or NG-hydroxy-D-arginine was attenuated by NG-nitro-L-arginine methyl ester. We conclude that NG-hydroxy-L-arginine exerts its effects on the contractility of coronary arteries by acting as a substrate for the endothelial nitric oxide synthase leading to enhanced NO production. Cytochrome P450 were not involved the dilatory response to NG-hydroxy-L-arginine. In this respect, porcine coronary arteries differ significantly from cultured smooth muscle cells in metabolising NG-hydroxy-L-arginine.

Oxidation of N-hydroxyguanidine by nitric oxide and the possible generation of vasoactive species

It has been reported previously that the N-hydroxyguanidine function of N-hydroxy-L-arginine can react with nitric oxide (NO) to generate other species that can act as potent vasodilators with different biological lifetimes than NO. The identities of these species have yet to be determined. Therefore, we have studied the reaction between NO and N-hydroxyguanidine and determined that N-hydroxyguanidine is capable of reducing NO to yield nitrous oxide (N2O) and possibly other nitroso species. It is likely that at least some of the N2O formation in these reactions is due to the initial generation of nitroxyl (HNO). Since HNO has been shown to be a potent vasorelaxant, it is possible that some of the non-NO-mediated biological activity alluded to in previous studies was due to HNO and that other nitroso-species generated in the reaction may also contribute to the overall pharmacological activity by release of either NO or HNO.

Purification and characterization of nitric oxide synthase (NOSNoc) from a Nocardia species

We previously reported on the occurrence, partial purification, and preliminary characterization of the first reported bacterial nitric oxide synthase. The soluble Nocardia enzyme, designated NOSNoc, has now been purified 1,353-fold by a combination of 2',5'-ADP-agarose affinity chromatography and hydroxylapatite chromatography. NOSNoc runs as a band of M(r) 51,900 on sodium dodecyl sulfate-polyacrylamide gel electrophoresis. The molecular mass was estimated to be 110.6 +/- 0.5 kDa by gel filtration, indicating that the native enzyme exists as a homodimer in solution. An N-terminal 15-amino-acid sequence was determined for NOSNoc, showing it to be different from known mammalian NOSs. NG-Hydroxy-L-arginine was confirmed to be an intermediate in the enzymatic reaction by stoichiometric determinations of oxygen uptake, NADPH oxidation, NO formation as measured by nitrite determinations, citrulline formation, and kinetic studies. NOSNoc was competitively inhibited by NG-methyl- and NG-nitro-L-arginine with either L-arginine or NG-hydroxyl-L-arginine as the substrate. Furthermore, the stability and pH and temperature optima of NOSNoc have been established.

Synthesis of 15N omega-hydroxy-L-arginine and ESR and 15N-NMR studies for the elucidation of the molecular mechanism of enzymic nitric oxide formation from L-arginine

N omega-Hydroxy-L-arginine (2) was prepared by a multi-stage synthesis; the key step was the addition of hydroxylamine to the protected cyanamide 8. The presence of N-hydroxyguanidines was confirmed, above all, by 15N-NMR investigations. 15N omega-Hydroxy-L-arginine (2) was converted quantitatively to 15NO by NO synthases from macrophages. 15NO was identified by ESR-spectroscopy. These experiments confirm that 15N omega-hydroxy-L-arginine (2) is an intermediate in the biosynthesis of NO from arginine (1) and that the N-hydroxylated N-atom is present in the NO formed.

Cytochrome P450-dependent N-hydroxylation of an aminoguanidine (amidinohydrazone) and microsomal retroreduction of the N-hydroxylated product

1. The first example of a P450-dependent N-hydroxylation of an aminoguanidine (amidinohydrazone) is reported for 2-amino-5-chlorobenzophenone amidinohydrazone 1 (G 256) as substrate. 2. The N-hydroxylated metabolite 2 (2-amino-5-chlorobenzophenone N-hydroxyamidinohydrazone NOH-G256) and a further metabolite of 1, the phenol 3, were identified by tlc and ms analysis. 3. The microsomal reduction of an N-hydroxyaminoguanidine (N-hydroxy-amidino-hydrazone) was also demonstrated for the transformation of 2 to 1. 4. Both the N-hydroxylation of the aminoguanidine and the retroreduction of the N-hydroxyaminoguanidine were characterized by quantitative hplc analysis. 5. The conversion of the aminoguanidine 1 to N-hydroxyaminoguanidine 2 may be considered as an analogue of the physiological N-hydroxylation of arginine to N-hydroxyarginine by NO synthases.

Exogenous NG-hydroxyl-L-arginine causes nitrite production in vascular smooth muscle cells in the absence of nitric oxide synthase activity

Nitric oxide (NO) production from exogenous NG-hydroxy-L-arginine (OH-L-Arg) was investigated in rat aortic smooth muscle cells in culture by measuring nitrite accumulation in the culture medium. As well, the interaction between OH-L-Arg and L-arginine uptake via the y+ cationic amino acid transporter was studied. In cells without NO-synthase activity, OH-L-Arg (1-1000 microM) induced a dose-dependent nitrite production with a half-maximal effective concentration (EC50) of 18.0 +/- 1.5 microM (n = 4-7). This nitrite accumulation was not inhibited by the NO-synthase inhibitor NG-nitro-L-arginine methyl ester, L-NAME (300 microM). In contrast, it was abolished by miconazole (100 microM), an inhibitor of cytochrome P450. Incubation of vascular smooth muscle cells with LPS (10 micrograms/ml) induced an L-NAME inhibited nitrite accumulation, but did not enhance the OH-L-Arg induced nitrite production. OH-L-Arg and other cationic amino acids, L-lysine and L-ornithine, competitively inhibited [3H]-L-arginine uptake in rat aortic smooth muscle cells, with inhibition constants of 195 +/- 23 microM (n = 12), 260 +/- 40 microM (n = 5) and 330 +/- 10 microM (n = 5), respectively. These results show that OH-L-Arg is recognized by the cationic L-amino acid carrier present in vascular smooth muscle cells can be oxidized to NO and nitrite in these cells in the absence of NO-synthase, probably by cytochrome P450 or by a reaction involving a cytochrome P450 by-product.

Cytochrome P450 dependent N-hydroxylation of a guanidine (debrisoquine), microsomal catalysed reduction and further oxidation of the N-hydroxy-guanidine metabolite to the urea derivative. Similarity with the oxidation of arginine to citrulline and nitric oxide

The microsomal N-hydroxylation of the strongly basic guanidinium group (debrisoquine) to N-hydroxyguanidine (N-hydroxydebrisoquine) and the retroreduction of the N-hydroxyguanidine are demonstrated for the first time. The reduction of the N-hydroxyguanidine by liver homogenates and hepatocytes is catalysed by a microsomal NADH-dependent system that is strongly inhibited by hydroxylamine or N-methylhydroxylamine. In the presence of these alternate substrates for the reductase the microsomal catalysed N-hydroxylation of debrisoquine is readily characterized. The oxidation was inhibited by antibodies against NADPH cytochrome P450 reductase and the role of the P450 monooxygenase was further verified by studies with partially purified and purified P450 2C3 reconstituted systems. The transformation of N-hydroxydebrisoquine to the corresponding urea derivative was also detected in in vitro experiments with microsomal fractions and enriched P450 fractions as well as with flavin-containing monooxygenase (FMO). Experiments with catalase, superoxide dismutase and H2O2 have shown that the H2O2 or O2-, respectively, formed from the respective enzyme and the substrate, apparently participated in the reaction. Whereas the N-hydroxylation of the guanidine involves the usual monooxygenase activity of cytochrome P450 the resultant N-hydroxyguanidine decouples monooxygenases (cytochrome P450, FMO) and the H2O2 and, above all, O2- thus formed transform the N-hydroxyguanidine further to the corresponding urea derivative. The possibility for the N-hydroxylation of non-physiological guanidines to N-hydroxyguanidines and subsequent oxidative conversion to the respective urea is comparable to the physiological transformation of arginine to citrulline via N-hydroxyarginine with the liberation of nitric oxide (endothelial derived relaxing factor) and could, therefore, contribute to the efficacy of drugs containing guanidine and similar functional groups.

NG-hydroxy-L-arginine prevents the haemodynamic effects of nitric oxide synthesis inhibition in the anaesthetized rat

1. We have investigated the effects of L-hydroxy-L-arginine (L-HOArg), an intermediate in the biosynthesis of nitric oxide (NO) from L-arginine (L-Arg), on the haemodynamic effects (systemic blood pressure and renal blood flow) of the NO synthesis inhibitor NG-nitro-L-arginine methyl ester (L-NAME) in the anaesthetized rat. 2. L-Arg or L-HOArg (3 mg kg-1 min-1), but not D-arginine (D-Arg) or NG-hydroxy-D-arginine (D-HOArg), elicited a slight but significant increase in total renal blood flow (RBF) of 11 +/- 2% and 11 +/- 1%. Since mean arterial blood pressure (MAP) did not change this dose of L-Arg or L-HOArg resulted in a reduced renal vascular resistance (RVR) of the same magnitude. 3. Bolus injections of L-NAME, at 0.3 or 1 mg kg-1 i.v., produced a significant fall in RBF of 11 +/- 2% and 32 +/- 5% and an increase in MAP of 7 +/- 3 mmHg and 22 +/- 5 mmHg, respectively. Consequently, RVR was elevated by 21 +/- 5% and 52 +/- 10%. 4. L-Arg or L-HOArg (3 mg kg-1 min-1) reduced the L-NAME-induced (0.3 or 1 mg kg-1) falls in RBF and increases in RVR by more than 65%. Neither D-Arg nor D-HOArg (3 mg kg-1 min-1) had any significant effect on the changes in RBF or RVR induced by L-NAME. 5. L-Arg or L-HOArg (3 mg kg-' min-') attenuated the pressor effect of L-NAME (3 mg kg-') by 73% and 64%, respectively, while neither the D-isomer of arginine nor hydroxyarginine had any effect.6. These results demonstrate that L-HOArg antagonizes the haemodynamic effects of NO-biosynthesis inhibition in vivo, thus supporting the hypothesis that L-HOArg is an intermediate in the formation of NO from L-Arg.

Chemical oxidation of N-hydroxyguanidine compounds. Release of nitric oxide, nitroxyl and possible relationship to the mechanism of biological nitric oxide generation

N omega-Hydroxy-L-arginine was found to cause vasodilation in arginine-depleted rabbit aorta. It is, therefore, likely to be a biosynthetic intermediate in the conversion of arginine to nitric oxide in this tissue. N-Hydroxyalkylguanidine compounds, including N omega-hydroxy-L-arginine were oxidized with various oxidizing agents and examined for their ability to release nitric oxide. All oxidizing agents tested were capable of oxidizing the N-hydroxyguanidine function but only lead tetra-acetate (Pb(OAc)4) and potassium ferricyanide/hydrogen peroxide (K3FeCN6/H2O2) were capable of generating significant amounts of nitric oxide. Oxidation with K3FeCN6, lead oxide (PbO2) and silver carbonate (Ag2CO3) resulted instead in the release of nitrous oxide (N2O) presumably through the initial release of nitroxyl (HNO).

Nitric oxide and another potent vasodilator are formed from NG-hydroxy-L-arginine by cultured endothelial cells

The hypothesis was investigated that NG-hydroxy-L-arginine (L-HOArg) is an intermediate in the biosynthesis of nitric oxide (.NO) from L-arginine (L-Arg) by the constitutive .NO synthase (NOS) present in endothelial cells (ECs). When infused through a column of bovine aortic ECs on beads, either L-HOArg or D-HOArg (1-10 microM) substantially potentiated relaxations of the bioassay tissues to .NO released from the cells by ADP or bradykinin, and this effect was abolished by coinfusions of NG-nitro-L-arginine (L-NO2Arg) methyl ester (10 microM) or NG-monomethyl-L-arginine (L-MeArg; 30 microM). Both L-HOArg and D-HOArg, irrespective of the presence of ECs, also potentiated relaxations induced by authentic .NO, but not glyceryl trinitrate. This was due to a rapid chemical reaction of either isomer with .NO, resulting in the formation of a potent and more stable vasodilator. When infusions of L-HOArg (3 microM) were consequently made in the presence of D-HOArg (10 microM), the L-isomer no longer had any effect on relaxations induced by authentic .NO, but significantly increased the stimulated release of .NO from the column of ECs. The conclusion that L-HOArg is a substrate for the constitutive NOS in cultured ECs was strongly supported by the L-NO2Arg-sensitive conversion of L-HOArg, but not D-HOArg, to .NO by NOS preparations from these cells. Interestingly, cultured ECs produced from L-HOArg (greater than or equal to 3 microM), but not D-HOArg, a stable vasodilator, the effects of which were inhibited by oxyhemoglobin (0.3-3 microM). However, the formation of this substance was not prevented by L-NO2Arg methyl ester (10 microM) or L-MeArg (10-100 microM), suggesting an enzymatic pathway different from NOS.