Nitroxyl analogs as inhibitors of aldehyde dehydrogenase. C-nitroso compounds

Direct and nitroxyl (HNO)-mediated reactions of acyloxy nitroso compounds with the thiol-containing proteins glyceraldehyde 3-phosphate dehydrogenase and alkyl hydroperoxide reductase subunit C

Nitroxyl (HNO) reacts with thiols, and this reactivity requires the use of donors with 1-nitrosocyclohexyl acetate, pivalate, and trifluoroacetate, forming a new group. These acyloxy nitroso compounds inhibit glyceraldehyde 3-phosphate dehydrogenase (GAPDH) by forming a reduction reversible active site disulfide and a reduction irreversible sulfinic acid or sulfinamide modification at Cys244. Addition of these acyloxy nitroso compounds to AhpC C165S yields a sulfinic acid and sulfinamide modification. A potential mechanism for these transformations includes nucleophilic addition of the protein thiol to a nitroso compound to yield an N-hydroxysulfenamide, which reacts with thiol to give disulfide or rearranges to sulfinamides. Known HNO donors produce the unsubstituted protein sulfinamide as the major product, while the acetate and pivalate give substituted sulfinamides that hydrolyze to sulfinic acids. These results suggest that nitroso compounds form a general class of thiol-modifying compounds, allowing their further exploration.

Water soluble acyloxy nitroso compounds: HNO release and reactions with heme and thiol containing proteins

Nitroxyl (HNO) has gained interest as a potential treatment of congestive heart failure through the ability of the HNO donor, Angeli's salt (AS), to evoke positive inotropic effects in canine cardiac muscle. The release of nitrite during decomposition limits the use of AS requiring other HNO sources. Acyloxy nitroso compounds liberate HNO and small amounts of nitrite upon hydrolysis and the synthesis of the water-soluble 4-nitrosotetrahydro-2H-pyran-4-yl acetate and pivalate allows for pig liver esterase (PLE)-catalysis increasing the rate of decomposition and HNO release. The pivalate derivative does not release HNO, but the addition of PLE catalyzes hydrolysis (t(1/2)=39 min) and HNO formation (65% after 30 min). In the presence of PLE, this compound converts metmyoglobin (MetMb) to iron nitrosyl Mb and oxyMb to metMb indicating that these compounds only react with heme proteins as HNO donors. The pivalate in the presence and the absence of PLE inhibits aldehyde dehydrogenase (ALDH) with IC(50) values of 3.5 and 3.3 μM, respectively, in a time-dependent manner. Reversibility assays reveal reversible inhibition of ALDH in the absence of PLE and partially irreversible inhibition with PLE. Liquid chromatography-mass spectrometry (LC-MS) reveals formation of a disulfide upon incubation of an ALDH peptide without PLE and a mixture of disulfide and sulfinamide in the presence of PLE. A dehydroalanine residue forms upon incubation of this peptide with excess AS. These results identify acyloxy nitroso compounds as unique HNO donors capable of thiol modification through direct electrophilic reaction or HNO release.

Cyanamide-mediated Inhibition of N-acetyltransferase 1

Cyanamide has been used for decades for medical intentions in the treatment of alcoholism and for agricultural purposes as a plant growth regulator and bud-breaking agent. Its therapeutic effect is mediated by reversible inhibition of aldehyde dehydrogenase and it was reported to be metabolized in vivo mainly via coenzyme A dependent N-acetylation by N-acetyltransferases. Although described to be a substrate for N-acetyltransferases (NATs), cyanamide has a different molecular structure to arylamines and hydrazines, the preferred substrates for N-acetyltransferases. Therefore, a more detailed investigation of its interrelations with N-acetyltransferases was performed. We analyzed the impact of cyanamide on NAT1 activities of human monocytes (monocytic THP-1 cells) using the classical substrate p-aminobenzoic acid. We found that a 24h treatment with physiologically relevant concentrations of cyanamide decreased the NAT1 activity significantly. Based on this observation we performed additional experiments using recombinant human NAT1 and NAT2 to achieve further insights. In detail a significant dose- and time-dependent inhibition of NAT1 activity was observed for 100 and 1000μM cyanamide using recombinant human NAT1*4. However, cyanamide did not inhibit recombinant NAT2*4. Experiments testing cyanamide as substrate did not provide evidence that cyanamide is metabolized via coenzyme A dependent N-acetylation in vitro by human NAT1 or NAT2, THP-1 or human liver cytosol. Therefore we can conclude that the observed enzyme inhibition (around 50% and 25% after treatment with 0.5 and 0.25mM CA, respectively) is not based on substrate-dependent down-regulation of NAT1. Further mechanistic and kinetic studies indicated that cyanamide reacts with the active site cysteine residue of NAT1, leading to its rapid inhibition (significant inhibition after 30min and 2h for 1000 and 100μM CA, respectively). Addition of the reduction agent dithiothreitol (DTT) did not modify the effect, indicating that oxidative processes that can be reversed by 5mM DTT are not likely involved in the inhibition. Taken together our results show that cyanamide is able to inhibit NAT1 most likely via interaction with the active site cysteine residue. Thereby cyanamide might modulate NAT1 dependent detoxification and activation of arylamines.

The chemistry of nitroxyl-releasing compounds

Nitroxyl (HNO) demonstrates a diverse and unique biological profile compared to nitric oxide, a redox-related compound. Although numerous studies support the use of HNO as a therapeutic agent, the inherent chemical reactivity of HNO requires the use of donor molecules. Two general chemical strategies currently exist for HNO generation from nitrogen-containing molecules: (i) the disproportionation of hydroxylamine derivatives containing good leaving groups attached to the nitrogen atom and (ii) the decomposition of nitroso compounds (X-N=O, where X represents a good leaving group). This review summarizes the synthesis and structure, the HNO-releasing mechanisms, kinetics and by-product formation, and alternative reactions of six major groups of HNO donors: Angeli's salt, Piloty's acid and its derivatives, cyanamide, diazenium diolate-derived compounds, acyl nitroso compounds, and acyloxy nitroso compounds. A large body of work exists defining these six groups of HNO donors and the overall chemistry of each donor requires consideration in light of its ability to produce HNO. The increasing interest in HNO biology and the potential of HNO-based therapeutics presents exciting opportunities to further develop HNO donors as both research tools and potential treatments.

Reactions of HNO with heme proteins: new routes to HNO-heme complexes and insight into physiological effects

The formation and interconversion of nitrogen oxides has been of interest in numerous contexts for decades. Early studies focused on gas-phase reactions, particularly with regard to industrial and atmospheric environments, and on nitrogen fixation. Additionally, investigation of the coordination chemistry of nitric oxide (NO) with hemoglobin dates back nearly a century. With the discovery in the early 1980s that NO is biosynthesized as a molecular signaling agent, the literature has been focused on the biological effects of nitrogen oxides, but the original concerns remain relevant. For instance, hemoglobin has long been known to react with nitrite, but this reductase activity has recently been considered to be important to produce NO under hypoxic conditions. The association of nitrosyl hydride (HNO; also commonly referred to as nitroxyl) with heme proteins can also produce NO by reductive nitrosylation. Furthermore, HNO is considered to be an intermediate in bacterial denitrification, but conclusive identification has been elusive. The authors of this article have approached the bioinorganic chemistry of HNO from different perspectives, which have converged because heme proteins are important biological targets of HNO.

Dual mechanisms of HNO generation by a nitroxyl prodrug of the diazeniumdiolate (NONOate) class

Here we describe a novel caged form of the highly reactive bioeffector molecule, nitroxyl (HNO). Reacting the labile nitric oxide (NO)- and HNO-generating salt of structure iPrHN−N(O)=NO⁻Na⁺ (1, IPA/NO) with BrCH₂OAc produced a stable derivative of structure iPrHN-N(O)=NO−CH₂OAc (2, AcOM-IPA/NO), which hydrolyzed an order of magnitude more slowly than 1 at pH 7.4 and 37 °C. Hydrolysis of 2 to generate HNO proceeded by at least two mechanisms. In the presence of esterase, straightforward dissociation to acetate, formaldehyde, and 1 was the dominant path. In the absence of enzyme, free 1 was not observed as an intermediate and the ratio of NO to HNO among the products approached zero. To account for this surprising result, we propose a mechanism in which base-induced removal of the N−H proton of 2 leads to acetyl group migration from oxygen to the neighboring nitrogen, followed by cleavage of the resulting rearrangement product to isopropanediazoate ion and the known HNO precursor, CH₃−C(O)−NO. The trappable yield of HNO from 2 was significantly enhanced over 1 at physiological pH, in part because the slower rate of hydrolysis for 2 generated a correspondingly lower steady-state concentration of HNO, thus, minimizing self-consumption and enhancing trapping by biological targets such as metmyoglobin and glutathione. Consistent with the chemical trapping efficiency data, micromolar concentrations of prodrug 2 displayed significantly more potent sarcomere shortening effects relative to 1 on ventricular myocytes isolated from wild-type mouse hearts, suggesting that 2 may be a promising lead compound for the development of heart failure therapies.

The reaction of nitroxyl (HNO) with nitrosobenzene gives cupferron (N-nitrosophenylhydroxylamine)

Nitroxyl (HNO), a penultimate product in the NOS-catalyzed conversion of L-arginine to L-citrulline, generated from Angeli's salt (AS) was determined by trapping it with nitrosobenzene (NB) to produce cupferron. The cupferron thus produced was characterized by complexation with Fe3+, Al3+, Cu2+, or Sn2+. UV/VIS spectra of the solubilized (in CHCl3) precipitates formed from NB and nitroxyl generated from AS in the presence of the iron, aluminum, copper, or tin salts were identical to those of their corresponding cupferron complexes. The identities of the Fe3+ and Cu2+ complexes formed from NB and HNO were further confirmed by their identical retention times on HPLC when compared to authentic Fe3+ and Cu2+ cupferron complexes. It was possible to detect 5 x 10(-6) M of the cupferron Fe3+ complex spectrophotometrically and to measure its production from the nitroxyl generators AS and methanesulfohydroxamic acid (MSHA) in the presence of 10(-4) M NB. The yield of cupferron was 51 and 62% of the amount of nitroxyl possible from AS or MSHA, respectively, after taking into account the relative rates of nitroxyl generation from these donors.

Prodrugs of nitroxyl and nitrosobenzene as cascade latentiated inhibitors of aldehyde dehydrogenase

The prototypic aromatic C-nitroso compound, nitrosobenzene (NB), was shown previously to mimic the effect of nitroxyl (HN=O), the putative active metabolite of cyanamide, in inhibiting aldehyde dehydrogenase (AlDH). To minimize the toxicity of NB in vivo, pro-prodrug forms of NB, which were designed to be bioactivated either by an esterase intrinsic to AlDH or the mixed function oxidase enzymes of liver microsomes, were prepared. Accordingly, the prodrug N-benzenesulfonyl-N-phenylhydroxylamine (3) was further latentiated by conversion to its O-acetyl (1a), O-methoxycarbonyl (1b), O-ethoxycarbonyl (1c), and O-methyl (2) derivatives. Similarly, pro-prodrug forms of nitroxyl were prepared by derivatization of the hydroxylamino moiety of methanesulfohydroxamic acid with N, O-bis-acetyl (7a), N,O-bis-methoxycarbonyl (7b), N, O-bis-ethoxycarbonyl (7c), and N-methoxycarbonyl-O-methyl (7d) groups. It was expected that the bioactivation of these prodrugs would initiate a cascade of nonenzymatic reactions leading to the ultimate liberation of NB or nitroxyl, thereby inhibiting AlDH. Indeed, the ester pro-prodrugs of both series were highly active in inhibiting yeast AlDH in vitro with IC50 values ranging from 21 to 64 microM. However, only 7d significantly raised ethanol-derived blood acetaldehyde levels when administered to rats, a reflection of the inhibition of hepatic mitochondrial AlDH-2.

Mechanisms of inhibition of aldehyde dehydrogenase by nitroxyl, the active metabolite of the alcohol deterrent agent cyanamide

Nitroxyl, produced in the bioactivation of the alcohol deterrent agent cyanamide, is a potent inhibitor of aldehyde dehydrogenase (AIDH); however, the mechanism of inhibition of AlDH by nitroxyl has not been described previously. Nitroxyl is also generated from Angeli's salt (Na2N2O3) at physiological pH, and, indeed, Angeli's salt inhibited yeast AlDH in a time- and concentration-dependent manner, with IC50 values under anaerobic conditions with and without NAD+ of 1.3 and 1.8 microM, respectively. Benzaldehyde, a substrate for AlDH, competitively blocked the inhibition of this enzyme by nitroxyl in the presence of NAD+, but not in its absence, in accord with the ordered mechanism of this reaction. The sulfhydryl reagents dithiothreitol (5 mM) and reduced glutathione (10 mM) completely blocked the inhibition of AlDH by Angeli's salt. These thiols were also able to partially restore activity to the nitroxyl-inhibited enzyme, the extent of reactivation being dependent on the pH at which the inactivation occurred. This pH dependency indicates the formation of two inhibited forms of the enzyme, with an irreversible form predominant at pH 7.5 and below, and a reversible form predominant at pH 8.5 and above. The reversible form of the inhibited enzyme is postulated to be an intra-subunit disulfide, while the irreversible form is postulated to be a sulfinamide. Both forms of the inhibited enzyme are derived via a common N-hydroxysulfenamide intermediate produced by the addition of nitroxyl to active site cysteine thiol(s).

Mechanism for the inhibition of aldehyde dehydrogenase by nitric oxide

The inhibition of Saccharomyces cerevisiae aldehyde dehydrogenase (AlDH) by gaseous nitric oxide (NO) in solution and by NO generated from diethylamine nonoate was time and concentration dependent. The presence of oxygen significantly reduced the extent of inhibition by NO, indicating that NO itself rather than an oxidation product of NO such as N2O3 is the inhibitory species under physiological conditions. A cysteine residue at the active site of the enzyme was implicated in this inhibition based on the following observations: a) NAD+ and NADP+, but not reduced cofactors, significantly enhanced inhibition of AlDH by NO; b) the aldehyde substrate, benzaldehyde, blocked inhibition; and c) inhibition was accompanied by loss of free sulfhydryl groups on the enzyme. Activity of the NO-inactivated enzyme was readily restored by treatment with dithiothreitol (DTT), but not with GSH. This difference was attributed, in part, to a redox process leading to the formation of a cyclic DTT disulfide. Based on the chemistry deduced from model systems, the reaction of NO with AlDH sulfhydryls was shown to produce intramolecular disulfides and N2O. These disulfides were shown to be intrasubunit disulfides by nonreducing SDS-PAGE analysis of the NO- inhibited enzyme. Following complete inhibition of AlDH by NO, four of the eight titratable (Ellman's reagent) sulfhydryl groups of AlDH were found to be oxidized to disulfides. These results suggest that a) the sulfhydryl group of active site Cys-302 and a proximal cysteine are oxidized to form an intrasubunit disulfide by NO; b) only two of the four subunits of AlDH are catalytically active; and c) NO preferentially oxidizes sulfhydryl groups of the catalytically active subunits. A detailed mechanism for the inhibition of AlDH by NO is presented.

Changes in serotonin metabolism during treatment with the aldehyde dehydrogenase inhibitors disulfiram and cyanamide

The effect of the aldehyde dehydrogenase inhibitors disulfiram (Antabuse) and cyanamide (calcium carbimide, Dipsan) on the metabolism of serotonin measured as relative amounts of the metabolites 5-hydroxyindole-3-acetic acid and 5-hydroxytryptophol in urine were studied in alcoholic patients. Sixteen out of 23 patients receiving drug therapy showed elevated excretion of 5-hydroxytryptophol. However, there was a marked, 15-fold, variability in 5-hydroxytryptophol excretion rate between patients. A high degree of variability was also seen in another group of patients studied before and after introduction of drug therapy. When patients were followed during the dose interval, a time-dependent response after each single dose could be observed. The disulfiram response lasted over the course of several days whereas the response to cyanamide lasted for less than 12 hr. It is concluded that treatment with disulfiram and cyanamide affects serotonin metabolism leading to increased production of 5-hydroxytryptophol, but there is a marked inter-individual variability in degree of response.

Identification of 4-(N,N-dipropylamino)benzaldehyde as a potent, reversible inhibitor of mouse and human class I aldehyde dehydrogenase

As the physiologic roles for the different classes of aldehyde dehydrogenase (ALDH) enzymes are elucidated, the identification of specific, reversible inhibitors becomes of great pharmacologic interest. Previous structure-function studies identified dialkylamino substituted benzaldehyde compounds as a novel class of reversible inhibitors of class I ALDH. To examine further structural requirements for inhibition, we tested a series of 4-(N,N-dialkylamino)benzaldehyde analogs as inhibitors of propanal oxidation by mouse liver and human erythrocyte class I ALDH. 4-(N,N-dipropylamino)benzaldehyde (DPAB) was identified as the most potent, reversible inhibitor of propanal oxidation by class I ALDH in spectrophotometric enzyme assays. In kinetic studies, DPAB showed mixed-type inhibition with respect to the aldehyde substrates propanal, phenylacetaldehyde, benzaldehyde, and aldophosphamide. DPAB exhibited uncompetitive inhibition with respect to the cofactor NAD. Inhibition constants (Ki) for DPAB, estimated from Dixon plots, were 10 nM (propanal) and 77 nM (phenylacetaldehyde) for mouse ALDH and 3 nM (propanal) and 70 nM (phenylacetaldehyde) for human ALDH. These Ki values are 100-fold lower than those reported for class I specific inhibitors. At low (< 1 microM) DPAB concentrations, inhibition of propanal and aldophosphamide oxidation was > 75%, whereas inhibition of benzaldehyde (32%) and phenylacetaldehyde (19%) oxidation was reduced markedly. These results indicate that DPAB exhibits potent, reversible inhibition of mouse and human class I ALDH. The degree of inhibition was highly dependent on the structure of the aldehyde substrate.

Metabolic activation of n-butyraldoxime by rat liver microsomal cytochrome P450. A requirement for the inhibition of aldehyde dehydrogenase

n-Butyraldoxime (n-BO) is known to cause a disulfiram/ethanol-like reaction in humans, a manifestation of the inhibition of hepatic aldehyde dehydrogenase (AIDH). As with a number of other in vivo inhibitors of AIDH, n-BO does not inhibit purified AIDH in vitro, suggesting that a metabolite of n-BO is the actual inhibitor of this enzyme. In re-examination of the effect of n-BO on blood acetaldehyde levels following ethanol in the Sprague-Dawley rat, we found that pretreatment with substrates and/or inhibitors of cytochrome P450 blocked the n-BO-induced rise in blood acetaldehyde in the following order of decreasing potency: 1-benzylimidazole (0.1 mmol/kg) > 3-amino-1,2,4-triazole (1.0 g/kg) > ethanol (3.0 g/kg) > phenobarbital (0.1% in the drinking water, 7 days) > SKF-525A (40 mg/kg). Rat liver microsomes were shown to catalyze the conversion of n-BO to an active metabolite that inhibited yeast AIDH. This reaction was dependent on NADPH and molecular oxygen and was inhibited by CO and 1-benzylimidazole. Hydroxylamine, postulated by others to be a metabolite of n-BO, inhibited AIDH via a catalase-mediated reaction and not through an NADPH-supported microsome-catalyzed reaction. Using GLC-mass spectrometry, 1-nitrobutane (an N-oxidation product) and butyronitrile (a dehydration product) were identified as metabolites from microsomal incubations of n-BO. However, neither of these metabolic products inhibited AIDH directly or in the presence of liver microsomes and NADPH. We conclude that another NADPH-dependent, cytochrome P450-catalyzed metabolic product of n-BO is responsible for the inhibition of AIDH by n-BO.