Nitric oxide formation during microsomal hepatic denitration of glyceryl trinitrate: involvement of cytochrome P-450

Mechanisms of action of nitrates

Glyceryl trinitrate, isosorbide dinitrate, and isosorbide-5-mononitrate are organic nitrate esters commonly used in the treatment of angina pectoris, myocardial infarction, and congestive heart failure. Organic nitrate esters have a direct relaxant effect on vascular smooth muscles, and the dilation of coronary vessels improves oxygen supply to the myocardium. The dilation of peripheral veins, and in higher doses peripheral arteries, reduces preload and afterload, and thereby lowers myocardial oxygen consumption. Inhibition of platelet aggregation is another effect that is probably of therapeutic value. Effects on the central nervous system and the myocardium have been shown but not scrutinized for therapeutic importance. Both the relaxing effect on vascular smooth muscle and the effect on platelets are considered to be due to a stimulation of soluble guanylate cyclase by nitric oxide derived from the organic nitrate ester molecule through metabolization catalyzed by enzymes such as glutathione S-transferase, cytochrome P-450, and possibly esterases. The cyclic GMP produced by the guanylate cyclase acts via cGMP-dependent protein kinase. Ultimately, through various processes, the protein kinase lowers intracellular calcium; an increased uptake to and a decreased release from intracellular stores seem to be particularly important.

Biotransformation of organic nitrates and vascular smooth muscle cell function

The organic nitrates are interesting examples of drugs that undergo biotransformation at their site of action to generate the active form of the drug. Furthermore, tolerance to the vasodilator effects of organic nitrates is associated with impairment of this metabolic activation process. Despite considerable research effort, the intracellular processes and the chemical reaction pathways by which organic nitrates are converted to their active form are still unresolved. This review by Brian Bennett and colleagues summarizes the characteristics of organic-nitrate biotransformation in vascular smooth muscle, the difficulties encountered when assessing this biotransformation, and the evidence for the role of two identified vascular biotransformation systems (glutathione-S-transferases and the cytochrome P450 system) in the metabolic activation of organic nitrates.

Nitroblue tetrazolium inhibits oxidation of glyceryl trinitrate to nitric oxide in bovine aortic smooth muscle cells

The effects of nitroblue tetrazolium (NBT), a well-known scavenger of superoxide anions and an inhibitor of nicotinamide adenine dinucleotide (NADPH)-dependent oxidations, were assessed on the metabolism of glyceryl trinitrate (GTN) to nitric oxide (NO) by bovine aortic smooth muscle cells (SMC). The extent of this metabolism was determined by measuring NO formed, using the inhibition of thrombin-induced platelet aggregation and relaxation of rabbit aortic strips as bioassay systems. In addition, NO produced from GTN by SMC was measured as nitrite (NO2-), one of its breakdown products. The antiplatelet effect of GTN (44 microM) was potentiated by SMC (0.12-0.46 x 10(5) cells) treated with indomethacin (10 microM) and this was inhibited in a concentration-dependent manner when the cells were pretreated with NBT (100 microM). NBT (3-100 microM) also reduced the formation of NO2- from GTN (600 microM) by SMC (3 x 10(5) cells). Furthermore, relaxations of endothelium-denuded strips of the rabbit aorta by GTN (10(-9)-10(-6) M) were attenuated when the strips were pretreated with NBT (100 or 500 microM). The formation of NO from L-arginine (L-Arg) by SMC was not affected by NBT. The hypotensive responses to GTN (0.25-1 mg/kg, i.v.) in anaesthetized rats were inhibited by pretreatment with NBT (1.25 mg/kg, i.v.) but NBT did not alter the hypotensive responses induced by SIN-1. Thus, NBT inhibited the bioconversion of GTN to NO both in vitro and in vivo. NBT may be a useful pharmacological tool to investigate the enzymic pathway(s) involved in the conversion of GTN to NO by smooth muscle cells or other cells.

Smooth muscle cell responsiveness to nitrovasodilators in hypertensive and normotensive rats

Endothelium-derived relaxing factor and exogenous nitrovasodilators are thought to produce smooth muscle relaxation by activation of soluble guanylate cyclase. To investigate whether diminished cyclic GMP (cGMP) accumulation underlies the differences in vascular reactivity to nitrovasodilators between Wistar-Kyoto (WKY) rats and spontaneously hypertensive rats (SHR), we determined cGMP formation in aortic smooth muscle cells from the two strains. Both cultured cells and aortic rings from 12- to 14-week-old SHR accumulated greater amounts of cGMP on stimulation with exogenous nitrovasodilators (ie, sodium nitroprusside) than those from WKY rats, whereas there was no difference observed in cells from prehypertensive animals (5- to 6-week old) between the two strains. Responsiveness of smooth muscle cells to endothelium-derived relaxing factor was investigated in cocultures of bovine aortic endothelial cells (BAE) and smooth muscle cells from SHR and WKY rats. cGMP accumulation elicited by endothelium-derived relaxing factor released either basally or in response to bradykinin and the calcium ionophore A23187 was greater in smooth muscle from 12- to 14-week-old SHR than from age-matched WKY rats (80 +/- 17 versus 11 +/- 2 for basal; 152 +/- 12 versus 80 +/- 26 for A23187; 163 +/- 21 versus 40 +/- 12 pmol/mg protein per 15 minutes for bradykinin) in SHR/BAE and WKY/BAE cocultures, respectively. Northern blot analysis of steady-state messenger RNA levels for the beta 1 subunit of soluble guanylate cyclase revealed higher levels of the message in SHR.(ABSTRACT TRUNCATED AT 250 WORDS)

Vascular and anti-platelet actions of 1,2- and 1,3-glyceryl dinitrate

1. The aim of this study was to investigate whether two metabolites of glyceryl trinitrate (GTN), 1,2 and 1,3-glyceryl dinitrate (1,2-GDN and 1,3-GDN) could account for the pharmacological effects of GTN. To this end the formation of nitric oxide (NO) from 1,2- and 1,3-GDN in the presence of bovine aortic smooth muscle cells (SMC) or endothelial cells (EC) was studied. The effects of various thiols on NO formation from these dinitrates was also evaluated. 2. 1,2-GDN or 1,3-GDN (10(-10)-10(-5) M) caused a dose-dependent relaxation of rabbit aortic strips denuded of endothelium and precontracted with phenylephrine. The dinitrates were less than one tenth as potent as GTN. 3. Incubation of 1,2-GDN or 1,3-GDN (75-2400 microM) with SMC for 30 min led to a concentration-dependent increase in nitrite (NO2-) formation but this increase was less than that produced from GTN. Likewise incubation of 1,2-GDN or 1,3-GDN with N-acetylcysteine (NAC), glutathione (GSH) or thiosalicylic acid (TSA) (all at 1 mM) for 30 min at 37 degrees C produced a concentration-dependent increase in NO2- formation. 4. Platelet aggregation induced by thrombin (40 mu ml-1) was not modified by high concentrations of 1,2-GDN or 1,3-GDN (175-700 microM). However, aggregation was inhibited when platelets were exposed to 1,2-GDN or 1,3-GDN (700 microM) in the presence of SMC (0.24-1.92 x 10(5) cells) or EC (0.8-3.2 x 10(5) cells). These effects were abrogated by co-incubation with oxyhaemoglobin (OxyHb, 10 microM) indicating that they were due to NO release. The concentrations of the dinitrates required to inhibit platelet aggregation by 50% were about 15 times higher than for GTN in the presence of the same numbers of SMC or EC.5. When NAC or TSA (both at 0.5 mM) were co-incubated with platelets for 3 min in the presence of1,2-GDN or 1,3-GDN, a concentration-dependent inhibition of platelet aggregation was observed. These anti-platelet effects were abolished by co-incubation with OxyHb (10 microM). Glutathione had no potentiating effects.6. Thus the dinitrate metabolites of GTN are metabolized to NO by SMC or EC and are acted upon by thiols to form NO at concentrations about 10 times higher than those of GTN. In vivo, after oral or intravenous GTN, GDN levels are reached which are more than 10 times higher than those of GTN.These data support the notion that part of the effects of GTN are due to the generation of NO from 1,2-GDN and 1,3-GDN by the cells of the vascular wall.

Biochemical characterization of a membrane-bound enzyme responsible for generating nitric oxide from nitroglycerin in vascular smooth muscle cells

A membrane-bound enzyme responsible for generating nitric oxide (NO) from nitroglycerin (NTG) in vascular smooth muscle cells has been characterized. The enzyme could be solubilized from vascular microsomes by several detergents, the most effective of which was 3-[(3-cholamidopropyl)-dimethylamino]-1-propanesulfonate (CHAPS). A partially purified enzyme preparation was obtained with CHAPS-solubilized vascular microsomes that were processed sequentially through an ion exchange column and a gel filtration column. The activity of this partially purified enzyme showed a dependence on substrate concentration, protein concentration and the duration of incubation. Enzyme activity was enhanced 2.7- to 4.2-fold by several thiols such as cysteine, N-acetylcysteine, reduced glutathione, and dithiothreitol. On the other hand, N-ethylmaleimide, iodoacetic acid, p-chloromercuric benzoic acid and 1-chloro-2,4-dinitrobenzene, reagents known to bind with the free sulfhydryl groups, inactivated the NO-generating activity from NTG. The enzyme activity could be reversibly bound to an organomercurial column. These results suggested the presence of a free thiol group in the enzyme and that this thiol group was required for enzyme activity. The partially purified enzyme was active in the presence of 0.1% sodium dodecyl sulfate (SDS). The enzyme was purified to near homogeneity using several sequential chromatographic steps including DEAE-Sephacel, Biogel A 1.5 m, hydroxylapatite and organomercurial columns, resulting in an increase in enzyme activity of about 94-fold. The subunit of this enzyme, as identified on an SDS-treated electrophoresis gel, had an apparent molecular size of 58 kDa.

Clinical pharmacology of organic nitrates

Although organic nitrates have been used in cardiovascular therapy for many years, various aspects of their pharmacology remain poorly understood. It is now known that organic nitrates produce nitric oxide (NO) in vascular smooth muscle cells, catalyzed by a membrane-bound enzyme that is not glutathione-S-transferase. Other nitrovasodilators, such as organic nitrites, sodium nitroprusside, and S-nitrosothiols, do not utilize the same enzyme for NO generation. The short-term hemodynamic action of various organic nitrates has been shown to be related to their pharmacokinetics, but their long-term therapeutic effects are limited by the development of pharmacologic tolerance. Nitrate sensitivity in patients can be restored daily after a nitrate-free period of 8-12 hours. Coadministration of nitrates with other vasodilators, such as captopril and hydralazine, may avoid the development of nitrate tolerance in patients with congestive heart failure.

Effect of ethacrynic acid, a glutathione-S-transferase inhibitor, on nitroglycerin-mediated cGMP elevation and vasorelaxation of rabbit aortic strips

The effects of ethacrynic acid (ECA), an inhibitor of glutathione-S-transferase, on both the pharmacologic and biochemical responses of aortic tissue to nitroglycerin (GTN) were evaluated. Using the rabbit aortic strip model, relaxation responses to 0.6 microM GTN were measured with and without ECA (0.2 mM) pretreatment. These same strips were frozen, and the concentrations of cGMP in the strips were measured using a 3H-labeled radioimmunoassay. Both the relaxation response and the increase in cGMP upon GTN treatment were reduced significantly by pretreatment of the strips with ECA. A correlation was observed between the decreases in the pharmacodynamic and biochemical responses upon ECA pretreatment. cGMP levels in strips treated with sodium nitroprusside, which generates nitric oxide by mechanisms distinct from that for organic nitrates, were not decreased by ECA pretreatment. These observations suggest that the mechanism of GTN action involves a glutathione-S-transferase-mediated metabolic step for GTN and that the isozyme(s) involved in this activation process may be inhibited by ECA.

Nitroglycerin metabolism in vascular tissue: role of glutathione S-transferases and relationship between NO. and NO2- formation

Nitroglycerin is a commonly employed pharmacological agent which produces vasodilatation by release of nitric oxide (NO.). The mechanism by which nitroglycerin releases NO. remains undefined. Recently, glutathione S-transferases have been implicated as important contributors to this process. They are known to release NO2- from nitroglycerin, but have not been shown to release NO.. The present studies were designed to examine the role of endogenous glutathione S-transferases in this metabolic process. Homogenates of dog carotid artery were incubated anaerobically with nitroglycerin, and NO. and NO2- production was determined by chemiluminescence. The role of glutathione S-transferases was studied by incubating homogenates with nitroglycerin in the presence of 1 mM GSH or 1 mM S-hexyl-glutathione, a potent inhibitor of glutathione S-transferases. Homogenates released 163 pmol of NO./h per mg of protein from nitroglycerin, and 2370 pmol of NO2-/h per mg. Adding GSH decreased NO. production by 82% and increased NO2- production by 98%. S-Hexylglutathione inhibited glutathione S-transferase activity by 96% and decreased NO2- production by 78%, but had no effect on NO. release. A linear relationship between glutathione S-transferase activity and NO2- production was observed, whereas glutathione S-transferase activity and NO. release were unrelated. Western-blot analysis demonstrated that dog carotid vascular smooth muscle contained Pi and Mu forms of glutathione S-transferases, with a predominance of the former. Purified preparations of human Pi and rat Mu isoforms metabolized nitroglycerin only to NO2- and not to NO.. On the basis of these findings, we conclude that (1) glutathione S-transferases do not contribute to the bioconversion of nitroglycerin to NO., but instead act as a degradative pathway for nitroglycerin, and (2) the release of NO. from nitroglycerin is not dependent on the formation of NO2-.

Release of nitric oxide from glyceryl trinitrate by captopril but not enalaprilat: in vitro and in vivo studies

1. The hypotensive effects of glyceryl trinitrate (GTN, 0.5 mg kg-1) but not of 3-morpholino-sydnonimine (SIN-1, 0.125 mg kg-1) in anaesthetized rats were attenuated following a seven day (using a q.i.d. dosing schedule) oral treatment with isosorbide-5-mononitrate (IS-5-MN; 5 mg kg-1) indicative of the induction of tolerance to GTN but not to SIN-1. The hypotensive effects of GTN did not decline when the sulphydryl (SH) containing angiotensin converting enzyme inhibitor (ACE-1), captopril (CPT, 5 mg kg-1) or the structurally unrelated SH-containing, N-acetylcysteine (NAC, 10 mg kg-1) but not the non-SH-containing ACE-I, enalaprilat (ENA, 5 mg kg-1) were given together with IS-5-MN for the seven days treatment. 2. The attenuated hypotensive effects of GTN (0.5 mg kg-1) in rats treated with IS-5-MN were also restored when CPT (1 mg kg-1) or NAC (2.5 mg kg-1) but not ENA (1 mg kg-1) was administered intraperitoneally (i.p.) 30 min before GTN. Furthermore, in control rats, CPT or NAC but not ENA given i.p. 30 min before GTN, potentiated its haemodynamic effects. These effects were blocked by methylene blue (10 mg kg-1). At the same doses, CPT or NAC did not affect the hypotensive effects of SIN-1. 3. The reduced ability of cultured tolerant smooth muscle cells (SMC, 24 x 103 cells) or endothelial cells(EC, 40 x 103 cells) to potentiate the anti-platelet effects of GTN (44 microM) was restored by CPT or NAC but not by ENA or glutathione (all at 0.5 mM). Potentiation of the anti-platelet effects of tolerant SMC or EC by CPT or NAC was abolished by co-incubation with oxyhaemoglobin (Oxy-Hb, 10 microM)indicative of nitric oxide (NO) formation.4. When GTN (150-2400 microM) was incubated with CPT, NAC or glutathione but not ENA (all at 0.1 mM) for 30 min in Krebs buffer at 37 degrees C a concentration-dependent increase in nitrite (NO2-)formation was observed. 5. The antiplatelet effects of GTN (5.5-352 microM) were potentiated by co-incubation with CPT or NAC but not with ENA or glutathione (all at 0.5 mM). The concentration of GTN required to inhibit platelet aggregation by 50% (IC50) was 110 +/- 2 microM for GTN alone, 14 +/- 2 microM for GTN in the presence of NAC and 30 +/- 2 microM for GTN in the presence of CPT. The potentiation of the effects of GTN by CPT or NAC was inhibited by co-incubation with Oxy-Hb (10 microM). By themselves, CPT or NAC did not inhibit platelet aggregation.6. The ability of CPT to restore (a) the haemodynamic effects of GTN in tolerant rats and (b) the reduced capacity of tolerant SMC or EC to potentiate the anti-platelet effects of GTN is not related to its ACE inhibitory activity.7. CPT also potentiated the hypotensive effects of GTN in non-tolerant rats, and in vitro CPT released NO from GTN in the absence of a GTN to NO converting cell, so that it is unlikely that reversal of tolerance by CPT is due to the replenishment of intracellular thiols. Rather it can be explained by the ability of CPT to release NO from GTN in the extracellular space. This extracellular formation of NO from GTN by CPT would then compensate for the impaired enzymic biotransformation of GTN to NO that develops during tolerance as was originally proposed for NAC.

Particular ability of cytochrome P-450 CYP3A to reduce glyceryl trinitrate in rat liver microsomes: subsequent formation of nitric oxide

Glyceryl trinitrate was denitrated in rat hepatic subcellular fractions, with formation of glyceryl dinitrates and glyceryl mononitrates. Among differently treated-rat liver microsomes, the highest microsomal activity was obtained under anaerobic conditions with microsomal preparations from dexamethasone-treated rats and NADPH. The reaction was inhibited by O2, CO, miconazole, dihydroergotamine and troleandomycin showing that it was catalyzed by cytochrome P-450 CYP3A isoforms. The formation of a transient cytochrome P-450 Fe(II)-NO complex during this reaction was shown by visible spectroscopy. The cytosolic activity was shown to be dependent on glutathione and glutathione transferase and was not inhibited by dioxygen. In the hepatic 9000 x g supernatant containing both NADPH and cytochrome P-450 and glutathione and glutathione transferase, the cytochrome P-450-dependent reaction accounts for 30-40% of the total denitration activity observed under anaerobic conditions, using 100 microM GTN.

Inhibition of nitric oxide synthesis by methylene blue

Methylene blue appears to inhibit nitric oxide-stimulated soluble guanylyl cyclase and has been widely used for inhibition of cGMP-mediated processes. We report here that endothelium-dependent relaxation of isolated blood vessels and NO synthase-dependent cGMP formation in cultured endothelial cells were both markedly more sensitive to inhibition by methylene blue than effects induced by direct activation of soluble guanylyl cyclase. These discrepancies were also observed when superoxide dismutase (SOD) was present to protect NO from inactivation by superoxide anion. Subsequent experiments showed that formation of L-citrulline by purified NO synthase was completely inhibited by 30 microM methylene blue (IC50 = 5.3 and 9.2 microM in the absence and presence of SOD, respectively), whereas guanylyl cyclase stimulated by S-nitrosoglutathione was far less sensitive to the drug (50% inhibition at approximately 60 microM, and maximal inhibition of 72% at 1 mM methylene blue). Experimental evidence indicated that oxidation of NADPH, tetrahydrobiopterin or reduced flavins does not account for the inhibitory effects of methylene blue. Our data suggest that methylene blue acts as a direct inhibitor of NO synthase and is a much less specific and potent inhibitor of guanylyl cyclase than hitherto assumed.

Biotransformation of glyceryl trinitrate by rat aortic cytochrome P450

Denitration of glyceryl trinitrate (GTN) by the microsomal fraction of rat aorta was found to be NADPH dependent and followed apparent first-order kinetics (T1/2 70.1 min). Biotransformation of GTN was regioselective for glyceryl-1,2-dinitrate formation, and was inhibited by carbon monoxide, SKF-525A, and oxygen. In aortic microsomes prepared from phenobarbital-pretreated rats, biotransformation was increased 7-fold, and was regioselective for glyceryl-1,3-dinitrate formation. These data strongly suggest the involvement of aortic cytochrome P450 in the biotransformation of GTN.

Conversion of glyceryl trinitrate to nitric oxide in tolerant and non-tolerant smooth muscle and endothelial cells

1. Exposure of smooth muscle cells (SMC) to glyceryl trinitrate (GTN, 75-600 microM) for 30 min led to a concentration-dependent increase in nitrite (NO2-), one of the breakdown products of nitric oxide (NO). This was not affected by 30 min pretreatment of the cells with 0.5 mM of sulphobromophthalein (SBP) an inhibitor of glutathione-S-transferase (GST), by metyrapone or SKF-525A inhibitors of cytochrome P450. These experiments were confirmed by organ bath studies using rabbit aortic strips denuded of endothelium and contracted with phenylephrine. Thus, a 30 min incubation of the strips with 0.5 mM SPB, metyrapone or SKF-525A did not affect the relaxations in response to GTN (10(-10)-10(-6) M). 2. Potentiation of the anti-platelet effect of GTN (44 microM) by endothelial cells (EC, 40 x 10(3) cells) was not affected by prior incubation of EC with SBP, metyrapone or SKF-525A (all at 0.5 mM). 3. Potentiation of the antiplatelet activity of GTN (11-352 microM) by small numbers of SMC (24 x 10(3) cells) or EC (40 x 10(3) cells) treated with indomethacin (10 microM) was attenuated when the SMC or EC were treated in culture with a high concentration of GTN (600 microM) for 18 h beforehand (referred to as 'tolerant' cells). In addition, tolerant SMC produced far less NO2- than non-tolerant SMC. 4. Exposure of non-tolerant SMC or EC (10(5) cells) to GTN (200 microM) for 3 min increased (3-4 fold) the levels of guanosine 3':5'-cyclic monophosphate (cyclic GMP). This increase was much less (< I fold) in the tolerant SMC or EC (105 cells). The basal levels of cyclic GMP were similar in normal or tolerant SMC or EC. Sodium nitroprusside (80 JAM) or atrial natriuretic factor (ANF, I0- M) increased the levels of cyclic GMP in normal or tolerant SMC or EC to the same extent.5 The anti-platelet effects of GTN (44 JM) were potentiated by the sulphydryl donor N-acetylcysteine(NAC, 0.5mM). Incubation of GTN (150-1200fJM) for 30min with NAC (0.1-1mM) led to aconcentration-dependent increase in N02- formation. The reduced ability of tolerant SMC or EC to potentiate the anti-platelet activity of GTN was restored by NAC (0.5 mM). These anti-aggregatory effects were abolished by concurrent co-incubation with oxyhaemoglobin (10 JM) indicating that they were due to NO release.6 Thus, in SMC or EC, metabolism of GTN to NO does not depend on glutathione-S-transferase or the cytochrome P450 system. Furthermore, when compared to normal cells, tolerant SMC or EC metabolize GTN to NO less effectively as assessed by the reduced capacity to potentiate the antiplatelet effects of GTN, to release NO2- and to increase the level of cyclic GMP. This decrease in NO formation shows that tolerance to GTN is mainly due to impaired biotransformation of GTN to NO. NAC, by directly forming NO from GTN, compensates for this failing mechanism.

Mutagenicity of glyceryl trinitrate (nitroglycerin) in Salmonella typhimurium

The recent finding that the clinical nitrovasodilator, glyceryl trinitrate (GTN), is mutagenic in Salmonella typhimurium strain TA1535 has been examined in closer detail, with emphasis on its mechanism of action. GTN increased the number of His+ revertants to a maximum of 4 times over background at a GTN dose of 5 mumol/plate. Hamster liver S9 depressed the toxicity of high GTN doses and increased the maximum number of revertants to 5 times over background at 10 mumol/plate. GTN did not cause significant reversion in any of the six other S. typhimurium strains tested (TA1975, TA102, TA1538, TA100, TA100NR, YG1026), although signs of toxicity were observed. Therefore, the mutagenicity of GTN was manifest only in the repair-deficient (uvrB and lacking in pKM101) strain which is responsive to single base changes. Oligonucleotide probe hybridization of TA1535 revertants showed that virtually all of the GTN-induced mutants contained C-->T transitions in either the first or second base of the hisG46 (CCC) target codon, with a preference for the latter. A similar mutational spectrum was seen previously with a complex of spermine and nitric oxide (NO) which releases nitric oxide. This suggests that NO, which can be derived from GTN via metabolic reduction, may be responsible for GTN's mutagenic action. The known NO scavenger oxymyoglobin did not substantially alter the dose response of GTN, indicating that extracellular NO was not mediating reversion. The data are consistent with the hypothesis that intracellular nitric oxide is responsible for the observed mutations.

Nitrates in different vascular beds, nitrate tolerance, and interactions with endothelial function

The favorable anti-ischemic effect of nitrates is based on the unique distribution pattern of vascular relaxation that they evoke in different vascular sections. Nitrovasodilators reduce cardiac preload and wall tension, and thus myocardial oxygen consumption. They increase precollateral coronary perfusion pressure, thereby augmenting oxygen delivery to ischemic sections, especially to the subendocardial layers. These vasodilator actions are caused by the nitric oxide (NO)-induced activation of soluble guanylyl cyclase, which augments vascular cyclic guanosine monophosphate (cGMP) levels to suppress intracellular Ca2+ concentrations. After some metabolic steps NO is finally cleaved from all nitrovasodilators and is probably identical with, or very closely related to, endothelium-derived relaxing factor (EDRF). A dinitrosyl-iron complex may serve under biologic conditions to stabilize the NO- radical, which has an extremely short half-life. NO derived from nitrovasodilators is used therapeutically to substitute for a deficient endothelium-mediated vascular control and autacoid production.

Cytochrome P450 catalyzes the oxidation of N omega-hydroxy-L-arginine by NADPH and O2 to nitric oxide and citrulline

Rat liver microsomes catalyze the oxidative denitration of N omega-hydroxy-L-arginine (NOHA) by NADPH and O2 with formation of citrulline and nitrogen oxides like NO and NO2-. Besides NO2- and citrulline, whose simultaneous formation is linear for at least 20 min, the formation of NO could be detected under the form of its P450 and P420-Fe(II) complexes by UV-visible and EPR spectroscopy. Classical inhibitors of NO-synthases, like N omega-methyl-and N omega-nitro-arginine, fail to inhibit the microsomal oxidation of NOHA to citrulline and NO2-. On the contrary classical inhibitors of hepatic cytochromes P450 like CO, miconazole, dihydroergotamine and troleandomycin, strongly inhibit this monooxygenase reaction. These results show that the oxygenation of NOHA by NADPH and O2 with formation of citrulline and NO can be efficiently catalyzed by cytochromes P450 (with rates up to 1.5 turnovers per min for the cytochromes of the 3A subfamily).

The metabolism of glyceryl trinitrate to nitric oxide in the macrophage cell line J774 and its induction by Escherichia coli lipopolysaccharide

The metabolism of glyceryl trinitrate (GTN) to nitric oxide (NO) was studied in the mouse macrophage cell line J774 and in the human monocytic cell line U937 in the absence or presence of Escherichia coli lipopolysaccharide (LPS). Two bioassay systems were used: inhibition of platelet aggregation and measurement of cGMP after stimulation by NO of guanylate cyclase in J774 cells. In addition, NO produced from GTN by cells or by cellular fractions was measured as nitrite (NO2-) one of its breakdown products. J774 cells (1.25 x 10(5) cells) treated with indomethacin (10 microM) enhanced the platelet inhibitory activity of GTN (22-352 microM) but not that of sodium nitroprusside (4 microM). This effect was abrogated by co-incubation with oxyhaemoglobin (oxyHb, 10 microM) indicating release of NO from GTN. U937 cells (up to 60 x 10(5)) did not metabolize GTN to NO. LPS (0.5 micrograms/mL for 18 hr) enhanced at least 2-fold the capacity of J774 cells but not that of U937 cells to form NO from GTN and this enhancement was attenuated when cycloheximide (10 micrograms/mL) was incubated together with LPS. In the absence of LPS stimulation, cycloheximide had no effect. Furthermore, when incubated with GTN (200 microM), J774 cells treated with LPS released more NO from GTN as indicated by a 3-fold greater increase in their level of cGMP which was prevented by oxyHb (10 microM). Incubation of J774 cells with GTN (75-600 microM) for 30 min led to a concentration-dependent increase in NO2- which was substantially reduced when the cells were boiled. The microsomal fraction was more potent than the cytosol in producing NO2- from GTN (1.2-2.4 mM). Release of NO2- from GTN by J774 cells was not affected by treating the cells with the NO synthase inhibitor, NG-monomethyl-L-arginine (MeArg, 300 microM). In J774 cells made tolerant to GTN, potentiation of the anti-platelet effects of GTN (11-352 microM) and release of NO2- from GTN was reduced. Thus, J774 cells but not U937 cells convert GTN to NO. This enzymic pathway (present mainly in the microsomal fraction of the J774 cells) is induced by LPS and is not regulated by endogenous NO released from L-Arg by the enzyme NO synthase. Furthermore, when compared to normal cells, tolerant J774 cells metabolize GTN to NO less effectively as assessed by a reduced capacity to potentiate the anti-platelet effect of GTN and to release NO2-.(ABSTRACT TRUNCATED AT 400 WORDS)

Carbon monoxide does not inhibit glyceryl trinitrate biotransformation by or relaxation of aorta

Carbon monoxide (CO) was employed to assess the role of ferrous hemoproteins in the vasodilation and biotransformation of glyceryl trinitrate (GTN) in rabbit aortic strips. These tissues were contracted submaximally with phenylephrine, exposed to CO for 5 min, and then incubated with 0.5 microM [1,3(14) C]GTN in the presence of CO for 2 min or 30 s. The [14C]GTN-induced relaxation of the strips was recorded, and [14C]GTN biotransformation to [14C]glyceryl dinitrate metabolites by the tissues was determined by thin-layer chromatographic-liquid scintillation spectrometric analysis. CO treatment inhibited neither GTN-induced relaxation nor GTN biotransformation to glycerol dinitrate metabolites. These results indicate that ferrous hemoproteins are probably not involved in the biotransformation of GTN that is associated with relaxation of the rabbit aortic strips.

Clinical relevance of endothelium-derived relaxing factor (EDRF)

1. In addition to metabolic and neurohumoral factors endothelium-derived autacoids like the nitric oxide radical NO and prostacyclin are effective regulators of vascular tone and thus tissue perfusion. NO is produced in endothelial cells from L-arginine by a Ca2+/calmodulin-dependent enzyme NO synthase. In addition, the NO radical is ultimately cleaved from all nitrovasodilators and resembles their vasoactive and antiaggregatory principle, which is used under pathological conditions as substitution therapy for impaired endothelial function and autacoid production. Impaired endothelium-dependent vasomotor control has been documented in hypercholesterolaemia, atheromatosis, diabetes, hypertension, and in reperfusion damage. L-arginine supplementation is effective in a few instances.

Nitroglycerin metabolism by Phanerochaete chrysosporium: evidence for nitric oxide and nitrite formation

We have demonstrated that a filamentous fungus Phanerochaete chrysosporium converts glyceryl trinitrate (GTN) into its di- and mononitrate derivatives concurrently with the formation of nitric oxide detected by electron paramagnetic resonance (EPR), and the formation of nitrite. The metabolisms of nitrite and nitrate by the fungus are evaluated and taken into account when considering GTN degradation. Lack of evidence for nitrate formation from GTN suggests that an esterase-type activity is not involved. Furthermore, the kinetics of appearance of the hemoprotein-NO and non-heme protein-NO (FeS-NO) complexes indicate that an enzymatic process producing NO directly from GTN may be involved concurrently with a glutathione transferase-like system.

Mechanisms responsible for the heterogeneous coronary microvascular response to nitroglycerin

Nitroglycerin dilates large (greater than or equal to 100 microns) but not small coronary arterial microvessels, and a putative metabolite of nitroglycerin, S-nitroso-L-cysteine, has been shown in vitro to dilate both large and small coronary microvessels. Based on this evidence, we tested the hypothesis that the lack of response of small coronary microvessels was due to an inability of small coronary microvessels to convert nitroglycerin into its vasoactive metabolite and examined possible explanations for this phenomenon. We studied left ventricular epicardial microvessels in vivo using video microscopy and stroboscopic epi-illumination in anesthetized, open-chest dogs. Diameters were determined while the epicardium was suffused with nitroglycerin, S-nitroso-L-cysteine, or S-nitroso-D-cysteine (all 10 microM) and nitroglycerin in the presence of L- or D-cysteine (100 microM). None of the agents affected systemic hemodynamics. Nitroglycerin dilated large arterioles (20 +/- 2%) but not small arterioles (1 +/- 1%). Both S-nitroso-L-cysteine and S-nitroso-D-cysteine were potent dilators of all size classes of microvessels. Concomitant application of L-cysteine and nitroglycerin evoked dilation in small microvessels (22 +/- 4%, p less than 0.5 versus nitroglycerin alone) and larger microvessels (27 +/- 6%, p = NS versus nitroglycerin alone). D-Cysteine did not alter the microvascular response to nitroglycerin in either small (7 +/- 4%, p = NS versus nitroglycerin alone) or large (18 +/- 3%, p = NS versus nitroglycerin alone) microvessels. Neither L-cysteine nor D-cysteine had a direct effect on microvascular diameter. These findings suggest that 1) sulfhydryl groups are required for the conversion of nitroglycerin to its vasoactive metabolite; 2) the interaction between nitroglycerin and sulfhydryl residues is a stereospecific process, indicating either an intracellular mechanism or a membrane-associated enzymatic reaction; and 3) a lack of available sulfhydryl groups may be responsible for the lack of response of small coronary arterioles to nitroglycerin.

Molecular mechanisms of nitrovasodilator bioactivation

All nitrovasodilators act intracellularly by a common molecular mechanism. This is characterized by the release of nitric oxide (NO). They are, thus, prodrugs or carriers of the active principle NO, responsible for endothelial controlled vasodilation. The rate of NO-formation strongly correlates with the activation of the soluble guanylate cyclase in vitro, resulting in a stimulation of cGMP synthesis. Nitrovasodilators thus are therapeutic substitutes for endogenous EDRF/NO. The pathways of bioactivation, nevertheless, differ substantially, depending on the individual chemistry of the nitrovasodilator. Besides NO, numerous other reaction products such as nitrite and nitrate anions are formed. The guanylate cyclase is only activated if NO is liberated. In the case of organic nitrates such as GTN, NO is only formed if certain thiol compounds are present as an essential cofactor. The rate of NO-formation correlates with the number of nitrate ester groups and proceeds with a simultaneous nitrite formation (with a ratio of 1:14 in the presence of cysteine). Nitrosamines such as molsidomine do not need thiol compounds for bioactivation. They directly liberate NO from the ring-open A-forms. This process basically depends on the presence of oxygen as electron acceptor from the sydnonimine molecule. Therefore, besides NO also superoxide radicals are formed, which may react with the generated NO under formation of nitrate ions. Organic nitrites (such as amyl nitrite) require the preceding interaction with a mercapto group to form a S-nitrosothiol intermediate, from which finally NO radicals are liberated. Nitrosothiols (like S-nitroso-acetyl-penicillamine) and sodium nitroprusside spontaneously release NO. The molecules themselves do not possess a direct enzyme activating potency. In the presence of thiol compounds organic nitrites (e.g., amyl nitrite) and nitrosothiols may act as intermediary products of NO generation.