Nitric oxide synthase inhibitors: amino acids.
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Amino Acids↗
Biomedical subjects
O W Griffith
Publications and source records attributed to O W Griffith.
Characterization by electron paramagnetic resonance of the interactions of L-arginine and L-thiocitrulline with the heme cofactor region of nitric oxide synthase.
Nitric oxide synthase (NOS) catalyzes sequential NADPH- and O2-dependent mono-oxygenase reactions converting L-arginine to N omega-hydroxy-L-arginine and N omega-hydroxy-L-arginine to citrulline and nitric oxide. The homodimeric enzyme contains one heme/monomer, and that cofactor is thought to mediate both partial reactions. Here we show by electron paramagnetic resonance spectroscopy that binding of substrate L-arginine to neuronal NOS perturbs the heme cofactor binding pocket without directly interacting as a sixth axial heme ligand; heme iron is exclusively high spin. In contrast, binding of L-thiocitrulline, a NOS inhibitor, produces both high and low spin iron spectra; L-thiocitrulline sulfur is a sixth axial heme ligand in one, but not all, of the low spin forms. The high spin forms of the L-thiocitrulline NOS complex display a distortion in the opposite direction to that caused by L-arginine binding. The findings elucidate the binding interactions of L-arginine and L-thiocitrulline to neuronal NOS and demonstrate that each causes a unique perturbation to the heme cofactor pocket of NOS.
Animals↗
S-alkyl-L-thiocitrullines. Potent stereoselective inhibitors of nitric oxide synthase with strong pressor activity in vivo.
Nitric oxide synthase catalyzes the oxidation of a guanidino nitrogen of L-arginine to nitric oxide with concomitant formation of citrulline. Enzyme activity is inhibited by a variety of N omega-monosubstituted L-arginine analogs including N omega-alkyl-, N omega-amino-, and N omega-nitro-L-arginine derivatives. We report here that both constitutive and inducible isoforms of nitric oxide synthase are strongly inhibited by S-alkyl-L-thiocitrullines (N delta-(S-alkyl)isothioureido-L-ornithines) with n-alkyl groups of one to three carbons. These compounds represent a novel class of inhibitors and are the most potent nitric oxide synthase-inhibiting amino acids described to date. Inhibition is reversible, stereoselective, and competitive with L-arginine. Spectral studies show no direct interaction of inhibitor sulfur with heme iron, a result in contrast to that seen previously with the parent compound, L-thiocitrulline. The S-alkyl-L-thiocitrullines have strong pressor activity in normotensive control rats; S-methyl-L-thiocitrulline reverses hypotension in a rat model of septic peritonitis and in dogs administered endotoxin. These latter findings suggest that the inhibitors may have therapeutic utility in treating hypotension due to the overproduction of nitric oxide.
Amino Acid Oxidoreductases↗
The effect of L-amino acid oxidase on activity of melphalan against an intracranial xenograft.
We have previously shown that diet restriction-induced depletion of large neutral amino acids (LNAAs) in murine plasma to 46% of control significantly enhances intracranial delivery of melphalan without enhancing delivery to other organs. Studies have now been conducted to determine whether more substantial LNAA depletion could further enhance intracranial delivery of melphalan. Treatment with L-amino acid oxidase (LOX) significantly depleted murine plasma LNAAs: phenylalanine, leucine, and tyrosine (> 95%); methionine (83%); isoleucine (70%); and valine (46%). Experiments evaluating the intracellular uptake of melphalan and high-pressure liquid chromatography quantitation of melphalan metabolites revealed, however, that melphalan is rapidly degraded in the presence of LOX, and that the timing of the administration of melphalan following the use of LOX to deplete LNAAs is crucial. Conditions were found under which LOX-mediated degradation of melphalan was minimized and LNAA depletion was maximized, resulting in a potentiation of the antitumor effect of melphalan on human glioma xenografts in nude mice. Such potentiation could not be obtained using diet restriction alone.
Amino Acid Oxidoreductases↗
NG-methyl-L-arginine, an inhibitor of nitric oxide synthase, reverses interleukin-2-induced hypotension.
OBJECTIVE: To evaluate the role of NG-methyl-L-arginine as a modulator of the hyperdynamic shock induced by the administration of interleukin-2 (IL-2). DESIGN: A prospective, pilot clinical study. SETTING: Intensive care unit of a tertiary care center. PATIENTS: Three sequential patients with metastatic renal cell carcinoma who developed hypotension during their first course of treatment with high-dose IL-2 (18 x 10(6) IU/m2/day by continuous infusion for 5 days). INTERVENTIONS: Upon developing hypotension during their subsequent therapy with IL-2, patients were administered 12 mg/kg of NG-methyl-L-arginine. Thereafter, a dose of 4 mg/kg was given every 4 hrs, as needed, to maintain the systolic blood pressure above 100 mm Hg. MEASUREMENTS AND MAIN RESULTS: Invasive hemodynamic monitoring was instituted before the initiation of treatment with IL-2. Differences noted before, and 15 mins after, the administration of NG-methyl-L-arginine were analyzed using the paired t-test. NG-methyl-L-arginine (12 mg/kg) induced a significant antihypotensive effect (mean blood pressure increased from 87 +/- 4 to 121 +/- 7 mm Hg), accompanied by an increase of the systemic vascular resistance (549 +/- 51 to 860 +/- 167 dyne.sec/cm5) and pulmonary vascular resistance (81 +/- 16 to 117 +/- 29 dyne.sec/cm5). A decrease in the cardiac index was also documented (4.5 +/- 0.5 to 3.6 +/- 0.3 L/min/m2). No significant changes in pulmonary artery occlusion and central venous pressures were observed. Maintenance doses of 4 mg/kg of NG-methyl-L-arginine induced similar hemodynamic results, although the duration of the antihypotensive effect of NG-methyl-L-arginine decreased with sequential doses. CONCLUSIONS: The hemodynamic effects induced by IL-2 administration are reversed by NG-methyl-L-arginine, a nitric oxide synthesis inhibitor. These results provide evidence for the biological activity of NG-methyl-L-arginine when administered alone to hypotensive patients. While no adverse effects were observed in this preliminary study, issues of toxicity and effectiveness need to be defined further in formal clinical trials. NG-methyl-L-arginine may play a therapeutic role in the modulation of the extreme vasodilation induced by cytokine administration or in septic shock.
Adult↗
Characterization of the effects of two new arginine/citrulline analogues on constitutive and inducible nitric oxide synthases in rat aorta.
1. New potent inhibitors of nitric oxide synthase (NOS) may be useful in the treatment of septic shock, a disorder characterized by a vascular hyporeactivity to catecholamines caused by an overproduction of nitric oxide (NO-). We examined the effects of L-thiocitrulline (L-TC) and S-methyl-L-thiocitrulline (L-SMTC), novel NOS inhibitors, on the constitutive and inducible NOS in rat aorta and compared those effects with inhibition due to NG-methyl-L-arginine (L-NMA). 2. Phenylephrine evoked similar concentration-contraction curves in the control rings and in the rings treated with these different NOS inhibitors (10 microM), whereas 100 microM of L-NMA, L-TC or L-SMTC increased significantly, and to a similar extent, contractions evoked by phenylephrine in aortic rings with endothelium without significantly affecting the maximal responses. 3. Relaxations evoked by acetylcholine, adenosine triphosphate, or calcium ionophore were significantly inhibited in a dose-dependent manner by L-NMA, L-SMTC, or L-TC (10-100 microM). The potencies of these inhibitors in reducing the relaxations of these vasodilators were not significantly different. 4. In endotoxin-treated preparations with endothelium, the three L-arginine analogues (10 microM) significantly potentiated contractile responses to phenylephrine (pEC50: 6.73 +/- 0.12 and 7.3 +/- 0.12, 7.34 +/- 0.13, or 7.22 +/- 0.14; in the absence and the presence of L-NMA, L-TC, or L-SMTC respectively) and increased maximal contractions from 1.53 +/- 0.15 g to 1.95 +/- 0.13 g, 2.08 +/- 0.12 g, and 2.03 +/- 0.13 g with L-NMA, L-TC, and L-SMTC respectively. A higher concentration of these NOS inhibitors (100 microM)further increased contractions evoked by this alpha1-agonist without further enhancing the maximal contractions; however, contractions evoked by 10 nM phenylephrine were significantly greater in the presence of L-SMTC or L-TC than in the presence of L-NMA (100 microM) (L-NMA: 0.4 +/- 0.11 g; L-TC:0.78 +/- 0.14 g and L-SMTC: 0.82+/-0.17 g). The effects of these inhibitors on NO- synthesis induced by endotoxin were significantly reversed by addition of L-arginine (1 mM) but not by L-citrulline (1 mM). InLPS-treated rings with endothelium, all three NOS inhibitors (100 microM) shifted the concentration contraction curves evoked by phenylephrine significantly to the left (pEC5o: 7.19 +/- 0.03 and 7.79 +/- 0.08,8.01 +/- 0.07, or 8.02 +_ 0.07, in the absence and the presence of L-NMA, L-TC, or L-SMTC, respectively)and increased significantly maximal contractions from 2.05 +/- 0.05 g to 2.38 +/- 0.14 g, 2.5 +/- 0.12 g, and 2.4 +_ 0.21 g with L-NMA, L-TC, and L-SMTC, respectively. L-TC and L-SMTC were significantly more potent than L-NMA in potentiating contractions evoked by 10 nM and 30 nM phenylephrine.5. L-TC and L-SMTC produced dose-dependent increases in tone in LPS-treated aortic rings with and without endothelium. In LPS-treated rings with endothelium, L-NMA induced contractions but in preparations without endothelium low concentrations of L-NMA induced small contractions while high concentrations of this inhibitor evoked relaxations. In both preparations L-TC and L-SMTC were significantly more potent than L-NMA in increasing vascular tone.6. These results suggest that L-SMTC, L-TC and L-NMA were equipotent on basal and agonist stimulated NO- synthesis produced by the constitutive isoform of NOS, whereas the two new L-arginine analogues were more potent than L-NMA in inhibiting the production of NO- induced by endotoxin in rat aorta.
Acetylcholine↗
Nitric oxide synthases: properties and catalytic mechanism.
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Amino Acid Oxidoreductases↗
Pulmonary oxygen toxicity in mice is characterized by alterations in ascorbate redox status.
Pulmonary oxygen toxicity results from disruption of the usual antioxidant defenses of the body. We therefore investigated whether mice that suffer from oxygen toxicity show significant alterations in the redox status of ascorbate, an important antioxidant, as reflected by changes in the relative amounts of its oxidized and reduced forms. Mice were exposed to air or hyperoxia (> 97% O2, 760 mmHg). After 5 days, plasma and saline-perfused lungs were removed and levels of ascorbate (AA), oxidized ascorbate [dehydroascorbate (DHAA)], and total ascorbate species ([AA+DHAA]) were determined by a sensitive and specific high-performance liquid chromatography assay; lungs were also assayed for total glutathione and glutathione disulfide (GSSG), an established marker of oxidative stress. We found that with hyperoxic exposure plasma AA increased by 32%, plasma DHAA increased substantially from previously undetectable levels, and the DHAA-to-[AA+DHAA] ratio increased. In contrast, in lung, [AA+DHAA] decreased by 41%. Plasma AA, DHAA, and [AA+DHAA] each correlated inversely with lung [AA+DHAA] and directly with lung GSSC. We conclude that alterations in plasma ascorbate redox status reflect pulmonary oxygen toxicity in mice. Our results suggest that further investigations are warranted to determine whether similar findings occur in humans and have clinical utility.
Animals↗
L-thiocitrulline. A stereospecific, heme-binding inhibitor of nitric-oxide synthases.
Nitric-oxide synthase (NOS) catalyzes the oxidation of L-arginine to citrulline and nitric oxide (NO). The enzyme is inhibited by a variety of N omega-monosubstituted L-arginine analogs, and some of these compounds are useful in reversing pathologies associated with the overproduction of NO (e.g. the hypotension of septic shock). We report here that L-thiocitrulline (gamma-thioureido-L-norvaline) is a potent, stereospecific inhibitor of the constitutive brain and endothelial isoforms of NOS as well as the isoform induced in vascular smooth muscle cells by lipopolysaccharide and interferon-gamma. Steady state kinetic studies show L-thiocitrulline inhibition is competitive with L-arginine (Ki approximately 4-20% of KArgm), indicating that initial binding is as a substrate/product analog. In contrast to L-arginine and N omega-methyl-L-arginine, the prototypic NOS inhibitor, L-thiocitrulline binding elicits a "Type II" difference spectrum, indicating a high spin to low spin transition of the iron in the heme cofactor. This finding suggests that L-thiocitrulline is contributing the sixth ligand to heme iron, probably through the thioureido sulfur. Such interaction with heme iron neither stimulates nor inhibits the direct flavin-mediated cytochrome c reduction activity of the enzyme, but it does inhibit heme-dependent superoxide formation. In vivo, L-thiocitrulline is a potent pressor agent in both normal and endotoxemic rats, the latter finding suggesting utility in treating the hypotension of septic shock.
Amino Acid Oxidoreductases↗
Inhibition of nitric oxide production is associated with enhanced weight loss, decreased survival, and impaired alloengraftment in mice undergoing graft-versus-host disease after bone marrow transplantation.
The pathophysiologic role of nitric oxide (NO) in graft-versus-host disease (GVHD) was investigated in a murine bone marrow (BM) transplantation model where donor and recipient were H-2-matched but differed at multiple minor histocompatibility antigens. Host AKR/J (H-2K) mice received lethal total body irradiation as pretransplant conditioning followed by transplantation of donor B10.BR (H-2K) BM cells with or without spleen cells as a source of GVH-reactive T cells. NO production, as assessed by serum nitrate and nitrite levels, was increased for up to 3 weeks posttransplant in animals undergoing both moderate and severe GVHD. Administration of NG-methyl-L-arginine (L-NMA), an inhibitor of nitric oxide synthase, to animals undergoing GVHD resulted in effective suppression of NO production when compared with saline-treated GVHD control animals. Suppression of NO production by L-NMA in GVHD animals was associated with enhanced weight loss early posttransplant and decreased overall survival. Histologic analysis of tissues from L-NMA-treated and saline-treated GVHD animals showed that early weight loss was not because of an exacerbation of GVHD, indicating that NO did not appear to play an immunosuppressive role in this experimental model. L-NMA-treated animals with enhanced weight loss were observed to have splenic atrophy, decreased extramedullary hematopoiesis, and a reduction in BM cellularity when compared with GVHD control mice that were weight-matched before transplant. Analysis of T-cell chimerism in the spleen showed that L-NMA treatment impaired donor T-cell repopulation. In vitro colony-forming unit (CFU) assays were performed to further assess the role of NO on BM progenitor cell growth. L-NMA added directly into culture had no effect on CFU-granulocyte/macrophage (CFU-GM) formation in normal murine BM. In contrast, total CFU-GM from L-NMA-treated animals were significantly reduced when compared with GVHD controls or BM control animals who did not develop GVHD. Collectively, these data indicate that inhibition of NO impairs hematopoietic reconstitution and support the premise that NO appears to play a novel role in the facilitation of alloengraftment posttransplant.
Amino Acid Oxidoreductases↗
Elevated DNA polymerase alpha, DNA polymerase beta, and DNA topoisomerase II in a melphalan-resistant rhabdomyosarcoma xenograft that is cross-resistant to nitrosoureas and topotecan.
Previous investigations have revealed that the human TE-671 MR human rhabdomyosarcoma xenograft selected in vivo for melphalan resistance (M. C. Rosenberg, et al., Cancer Res., 49: 6917-6922, 1989) is cross-resistant to a wide variety of alkylating agents and to bleomycin, but is collaterally sensitive to etoposide. Although glutathione levels were noted to be elevated in TE-671 MR compared to the melphalan-sensitive parental TE-671 xenograft, treatment with buthionine sulfoximine to deplete glutathione levels did not fully restore melphalan sensitivity in the TE-671 MR xenograft. The present studies were undertaken to search for additional mechanisms of resistance in the TE-671 MR xenograft. Drug sensitivity testing performed at the dose of agents that was lethal to 10% of the animals revealed that the TE-671 MR xenograft maintained resistance to the bifunctional cross-linking agent 1,3-bis(2-chloroethyl)-1-nitrosourea and was cross-resistant to the topoisomerase I poison topotecan. Treatment with buthionine sulfoximine did not sensitize the TE-671 MR xenograft to 1,3-bis(2-chloroethyl)-1-nitrosourea. Further, even though O6-alkylguanine-DNA alkyltransferase levels were high in both the TE-671 and TE-671 MR xenografts, depletion of O6-alkylguanine-DNA alkyltransferase activity by treatment with O6-benzylguanine substantially sensitized the TE-671 xenografts but not the TE-671 MR xenografts, suggesting an additional mechanism of resistance. Measurement of additional enzyme activities that might be involved in DNA repair revealed significant elevations in DNA polymerase alpha (46 +/- 8 (SD) units/mg protein in TE-671, 69 +/- 6 units/mg protein in TE-671 MR, P < 0.05) and DNA polymerase beta (0.43 +/- 0.01 units/mg protein in TE-671, 0.78 +/- 0.12 units/mg protein in TE-671 MR, P < 0.05) but not DNA polymerase delta or total DNA ligase. Examination of topoisomerases by activity assays and Western blotting revealed a 2-fold increase in topoisomerase II and a 2-fold decrease in topoisomerase I in the TE-671 MR xenograft compared to the parental xenograft, apparently explaining the collateral sensitivity to etoposide and cross-resistance to topotecan. These results suggest that TE-671 MR xenografts contain multiple changes in activities of DNA repair-related proteins and other nuclear proteins that could contribute to alkylating agent resistance.
Alkyl and Aryl Transferases↗
Pharmacodynamics of prolonged treatment with L,S-buthionine sulfoximine.
PURPOSE: To develop dosing criteria for the use of L-buthionine-S-sulfoximine (active diastereoisomer) as a glutathione depletor in the clinic, using a pharmacodynamic and pharmacokinetic in vitro-in vivo approach. METHODS AND MATERIALS: In vitro: L-buthionine-S-sulfoximine uptake was determined in human glioblastoma cells (T98G) and NIH-3T3 cells using 35S-labeled drug. Dose response relationships were derived for inhibition of glutathione synthesis in CHO cells, and for depletion of glutathione in exponentially growing T98G and CHO cells, as a function of extracellular L-buthionine-S-sulfoximine concentration. Steady-state glutathione levels for CHO and NIH-3T3 cells were measured using an enzymatic assay, while glutathione synthesis rates in CHO cells were determined using a flow cytometric assay. In vivo: L-buthionine-S-sulfoximine biodistribution was determined in male nude mice carrying human glioblastomas (T98G) intracranially, using 35S-labeled drug infused subcutaneously by osmotic pump. Tissue glutathione levels were measured using an enzymatic assay. RESULTS AND CONCLUSION: The observed cellular uptake t1/2 of approximately 55 min, coupled with a previously reported, rapid in vivo clearance of buthionine sulfoximine, suggest that continuous infusion would be preferable to bolus dosing. Effective concentrations of L-buthionine-S-sulfoximine (24 h exposure), required to lower cellular glutathione content to 50% of control (EC50), were under 1 mM for both cell lines. The amount of L-buthionine-S-sulfoximine in tissues (estimated from 35S drug disposition) reached steady state within 8 h and was proportional to the rate of infusion. Brain tumors were depleted to approximately 50% of control glutathione by a infusion rate of 0.25 mumoles/h (25 g mice). At lower infusion rates an increase in glutathione content was noted in certain nude mouse tissues including brain tumor xenografts.
3T3 Cells↗
Synthesis of L-thiocitrulline, L-homothiocitrulline, and S-methyl-L-thiocitrulline: a new class of potent nitric oxide synthase inhibitors.
Nitric oxide synthase catalyzes the NADPH- and O2-dependent conversion of L-arginine to L-citrulline and nitric oxide. L-Thiocitrulline, L-homothiocitrulline, and S-methyl-L-thiocitrulline, novel citrulline analogs, have been synthesized and are shown to be potent inhibitors of both the constitutive brain and the inducible smooth muscle isoforms of nitric oxide synthase. Although many N omega-monosubstituted arginine derivatives inhibit nitric oxide synthase, inhibitory citrulline derivatives have not previously been reported. S-Methyl-L-thiocitrulline is significantly more potent than N omega-methyl-L-arginine, the prototypic nitric oxide synthase inhibitor.
Amino Acid Oxidoreductases↗
NG-methyl-L-arginine, an inhibitor of nitric oxide formation, reverses IL-2-mediated hypotension in dogs.
The effects of NG-methyl-L-arginine (L-NMA), an inhibitor of nitric oxide formation, were studied in dogs treated with interleukin-2 (IL-2). The administration of IL-2 to dogs resulted in hypotension within 3 days of treatment. The development of hypotension correlated with accumulation in the serum of nitrate, which is a stable breakdown product of nitric oxide. Administration of L-NMA decreased serum nitrate levels and increased the mean arterial pressure. The antihypotensive effect was dose dependent with a maximum effect observed at a dose of 20 mg/kg. Administration of a continuous infusion of L-NMA (5 mg.kg-1.h-1) maintained the mean arterial pressure for 48 h with concurrent administration of IL-2. Evaluation of IL-2-induced lymphokine-activated killer cell proliferation and tumoricidal activity toward a canine glioblastoma target cell line was unaffected by L-NMA. These studies imply that L-NMA may effectively ameliorate the dose-limiting hypotension associated with administration of IL-2 without adversely affecting the antitumor effects.
Animals↗
NG-methyl-L-arginine, an inhibitor of nitric oxide formation, acts synergistically with dobutamine to improve cardiovascular performance in endotoxemic dogs.
OBJECTIVE: To evaluate the hemodynamic effects of the nitric oxide inhibitor, NG-methyl-L-arginine, and dobutamine during experimental endotoxemia. DESIGN: Prospective, randomized, controlled animal study. SETTING: University research laboratory. SUBJECTS: Adult, male mongrel dogs. INTERVENTIONS: After catheterization with a flow-directed, thermal-dilution pulmonary artery flotation catheter and arterial catheter, awake dogs received either NG-methyl-L-arginine or dobutamine alone or in combination (controls; n = 5). Other animals were administered endotoxin (50 micrograms/kg), then received either NG-methyl-L-arginine alone or in combination with dobutamine after the onset of hypotension (endotoxin-treated; n = 5). MEASUREMENTS AND MAIN RESULTS: Both dobutamine and NG-methyl-L-arginine alone had a small, but significant vasopressor effect on control animals. In contrast, administration of the combination of NG-methyl-L-arginine and dobutamine resulted in a 48.6% increase in mean arterial pressure, an effect which was dose-dependent with respect to NG-methyl-L-arginine. In dogs treated with 50 micrograms/kg of endotoxin, hypotension could be only partially reversed by NG-methyl-L-arginine, mainly due to a decline in cardiac output. Co-infusion of dobutamine reversed this depression of cardiac output and resulted in a complete restoration of blood pressure. CONCLUSIONS: Later stages of septic shock are characterized by hypotension and decreased myocardial performance. A major mediator of hypotension is nitric oxide, a vasodilatory agent derived from L-arginine. Administration of the arginine derivative, NG-methyl-L-arginine, improved systemic vascular resistance but not myocardial performance. The addition of an inotropic agent to NG-methyl-L-arginine, a nitric oxide synthase inhibitor, resulted in an enhancement of the antihypotensive action of NG-methyl-L-arginine through the restoration of cardiac output. The synergistic action between dobutamine and NG-methyl-L-arginine may be of therapeutic value in the treatment of septic shock.
Animals↗
Pathogenetic mechanisms of septic shock.
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Amino Acid Oxidoreductases↗
Oxy radicals and antioxidative responses in cancer: 12th Sapporo Cancer Seminar.
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Antioxidants↗
Irreversible inactivation of macrophage and brain nitric oxide synthase by L-NG-methylarginine requires NADPH-dependent hydroxylation.
L-NG-Methylarginine (NMA) is an established mechanism-based inactivator of murine macrophage nitric oxide synthase (mNOS). In this report, NMA is shown to irreversibly inhibit both mNOS (k(inact) = 0.08 min-1) and the recombinant constitutive brain NOS (bNOS). For both NOS isoforms, metabolism of NMA parallels that of the natural substrate L-arginine (ARG), in that it undergoes a regiospecific, NADPH-dependent hydroxylation to form L-NG-hydroxy-NG-methylarginine (NOHNMA). This intermediate then undergoes further NADPH-dependent oxidation to form L-citrulline (CIT). Authentic NOHNMA, synthesized from L-ornithine, irreversibly inhibited both mNOS (k(inact) = 0.10 min-1) and bNOS in an NADPH-dependent reaction. The conversion of either NMA or NOHNMA to CIT correlated with irreversible enzyme inactivation. Thus, the data suggest that enzyme inhibition occurs as a consequence of oxidative metabolism of the intermediate, NOHNMA. A unified mechanism is proposed that accounts for NO biosynthesis from ARG, for the inactivation of NOS by NMA and for the intermediacy of hydroxylated ARG or NMA derivatives in these processes.
Amino Acid Oxidoreductases↗