Erythrocyte aldehyde dehydrogenase and disulfiram-like side effects of hypoglycemics and antianginals

Site-directed mutagenesis of aldehyde dehydrogenase-2 suggests three distinct pathways of nitroglycerin biotransformation

To elucidate the mechanism underlying reduction of nitroglycerin (GTN) to nitric oxide (NO) by mitochondrial aldehyde dehydrogenase (ALDH2), we generated mutants of the enzyme lacking the cysteines adjacent to reactive Cys302 (C301S and C303S), the glutamate that participates as a general base in aldehyde oxidation (E268Q) or combinations of these residues. The mutants were characterized regarding acetaldehyde dehydrogenation, GTN-triggered enzyme inactivation, GTN denitration, NO formation, and soluble guanylate cyclase activation. Lack of the cysteines did not affect dehydrogenase activity but impeded GTN denitration, aggravated GTN-induced enzyme inactivation, and increased NO formation. A triple mutant lacking the cysteines and Glu268 catalyzed sustained formation of superstoichiometric amounts of NO and exhibited slower rates of inactivation. These results suggest three alternative pathways for the reaction of ALDH2 with GTN, all involving formation of a thionitrate/sulfenyl nitrite intermediate at Cys302 as the initial step. In the first pathway, which predominates in the wild-type enzyme and reflects clearance-based GTN denitration, the thionitrate apparently reacts with one of the adjacent cysteine residues to yield nitrite and a protein disulfide. The predominant reaction catalyzed by the single and double cysteine mutants requires Glu268 and results in irreversible enzyme inactivation. Finally, combined lack of the cysteines and Glu268 shifts the reaction toward formation of the free NO radical, presumably through homolytic cleavage of the sulfenyl nitrite intermediate. Although the latter reaction accounts for less than 10% of total turnover of GTN metabolism catalyzed by wild-type ALDH2, it is most likely essential for vascular GTN bioactivation.

Aldehyde dehydrogenase 2 in cardiac protection: a new therapeutic target?

Mitochondrial aldehyde dehydrogenase 2 (ALDH2) is emerging as a key enzyme involved in cytoprotection in the heart. ALDH2 mediates both the detoxification of reactive aldehydes such as acetaldehyde and 4-hydroxy-2-nonenal and the bioactivation of nitroglycerin to nitric oxide. In addition, chronic nitrate treatment results in ALDH2 inhibition and contributes to nitrate tolerance. Our laboratory recently identified ALDH2 to be a key mediator of endogenous cytoprotection. We reported that ALDH2 is phosphorylated and activated by the survival kinase protein kinase C epsilon and found a strong inverse correlation between ALDH2 activity and infarct size. We also identified a small molecule ALDH2 activator which reduces myocardial infarct size induced by ischemia/reperfusion in vivo. In this review, we discuss evidence that ALDH2 is a key mediator of endogenous survival signaling in the heart, suggest possible cardioprotective mechanisms mediated by ALDH2 and discuss potential clinical implications of these findings.

Organic nitrates and nitrate tolerance--state of the art and future developments

The hemodynamic and antiischemic effects of nitroglycerin (GTN) are lost upon chronic administration due to the rapid development of nitrate tolerance. The mechanism of this phenomenon has puzzled several generations of scientists, but recent findings have led to novel hypotheses. The formation of reactive oxygen and nitrogen species in the mitochondria and the subsequent inhibition of the nitrate-bioactivating enzyme mitochondrial aldehyde dehydrogenase (ALDH-2) appear to play a central role, at least for GTN, that is, bioactivated by ALDH-2. Importantly, these findings provide the opportunity to reconcile the two "traditional" hypotheses of nitrate tolerance, that is, the one postulating a decreased bioactivation and the concurrent one suggesting a role of oxidative stress. Furthermore, recent animal and human experimental studies suggest that the organic nitrates are not a homogeneous group but demonstrate a broad diversity with regard to induction of vascular dysfunction, oxidative stress, and other side effects. In the past, attempts to avoid nitrate-induced side effects have focused on administration schedules that would allow a "nitrate-free interval"; in the future, the role of co-therapies with antioxidant compounds and of activation of endogeneous protective pathways such as the heme oxygenase 1 (HO-1) will need to be explored. However, the development of new nitrates, for example, tolerance-free aminoalkyl nitrates or combination of nitrate groups with established cardiovascular drugs like ACE inhibitors or AT(1)-receptor blockers (hybrid molecules) may be of great clinical interest.

Partially irreversible inactivation of mitochondrial aldehyde dehydrogenase by nitroglycerin

Mitochondrial aldehyde dehydrogenase (ALDH2) may be involved in the biotransformation of glyceryl trinitrate (GTN), and the inactivation of ALDH2 by GTN may contribute to the phenomenon of nitrate tolerance. We studied the GTN-induced inactivation of ALDH2 by UV/visible absorption spectroscopy. Dehydrogenation of acetaldehyde and hydrolysis of p-nitrophenylacetate (p-NPA) were both inhibited by GTN. The rate of inhibition increased with the GTN concentration and decreased with the substrate concentration, indicative of competition between GTN and the substrates. Inactivation of p-NPA hydrolysis was greatly enhanced in the presence of NAD⁺, and, to a lesser extent, in the presence of NADH. In the presence of dithiothreitol (DTT) inactivation of ALDH2 was much slower. Dihydrolipoic acid (LPA-H₂) was less effective than DTT, whereas glutathione, cysteine, and ascorbate did not protect against inactivation. When DTT was added after complete inactivation, dehydrogenase reactivation was quite modest (≤16%). The restored dehydrogenase activity correlated inversely with the GTN concentration but was hardly affected by the concentrations of acetaldehyde or DTT. Partial reactivation of dehydrogenation was also accomplished by LPA-H₂ but not by GSH. We conclude that, in addition to the previously documented reversible inhibition by GTN that can be ascribed to the oxidation of the active site thiol, there is an irreversible component to ALDH inactivation. Importantly, ALDH2-catalyzed GTN reduction was partly inactivated by preincubation with GTN, suggesting that the inactivation of GTN reduction is also partly irreversible. These observations are consistent with a significant role for irreversible inactivation of ALDH2 in the development of nitrate tolerance.

The enigma of nitroglycerin bioactivation and nitrate tolerance: news, views and troubles

Nitroglycerin (glyceryl trinitrate; GTN) is the most prominent representative of the organic nitrates or nitrovasodilators, a class of compounds that have been used clinically since the late nineteenth century for the treatment of coronary artery disease (angina pectoris), congestive heart failure and myocardial infarction. Medline lists more than 15 000 publications on GTN and other organic nitrates, but the mode of action of these drugs is still largely a mystery. In the first part of this article, we give an overview on the molecular mechanisms of GTN biotransformation resulting in vascular cyclic GMP accumulation and vasodilation with focus on the role of mitochondrial aldehyde dehydrogenase (ALDH2) and the link between the ALDH2 reaction and activation of vascular soluble guanylate cyclase (sGC). In particular, we address the identity of the bioactive species that activates sGC and the potential involvement of nitrite as an intermediate, describe our recent findings suggesting that ALDH2 catalyses direct 3-electron reduction of GTN to NO and discuss possible reaction mechanisms. In the second part, we discuss contingent processes leading to markedly reduced sensitivity of blood vessels to GTN, referred to as vascular nitrate tolerance. Again, we focus on ALDH2 and describe the current controversy on the role of ALDH2 inactivation in tolerance development. Finally, we emphasize some of the most intriguing, in our opinion, unresolved puzzles of GTN pharmacology that urgently need to be addressed in future studies.

Bioactivation of nitroglycerin by purified mitochondrial and cytosolic aldehyde dehydrogenases

Metabolism of nitroglycerin (GTN) to 1,2-glycerol dinitrate (GDN) and nitrite by mitochondrial aldehyde dehydrogenase (ALDH2) is essentially involved in GTN bioactivation resulting in cyclic GMP-mediated vascular relaxation. The link between nitrite formation and activation of soluble guanylate cyclase (sGC) is still unclear. To test the hypothesis that the ALDH2 reaction is sufficient for GTN bioactivation, we measured GTN-induced formation of cGMP by purified sGC in the presence of purified ALDH2 and used a Clark-type electrode to probe for nitric oxide (NO) formation. In addition, we studied whether GTN bioactivation is a specific feature of ALDH2 or is also catalyzed by the cytosolic isoform (ALDH1). Purified ALDH1 and ALDH2 metabolized GTN to 1,2- and 1,3-GDN with predominant formation of the 1,2-isomer that was inhibited by chloral hydrate (ALDH1 and ALDH2) and daidzin (ALDH2). GTN had no effect on sGC activity in the presence of bovine serum albumin but caused pronounced cGMP accumulation in the presence of ALDH1 or ALDH2. The effects of the ALDH isoforms were dependent on the amount of added protein and, like 1,2-GDN formation, were sensitive to ALDH inhibitors. GTN caused biphasic sGC activation with apparent EC₅₀ values of 42 ± 2.9 and 3.1 ± 0.4 μm in the presence of ALDH1 and ALDH2, respectively. Incubation of ALDH1 or ALDH2 with GTN resulted in sustained, chloral hydrate-sensitive formation of NO. These data may explain the coupling of ALDH2-catalyzed GTN metabolism to sGC activation in vascular smooth muscle.

Risks of combined alcohol/medication use in older adults

BACKGROUND

Many older adults (ie, those aged >65 years) drink alcohol and use medications that may be harmful when consumed together.

OBJECTIVE

This article reviews the literature on alcohol and medication interactions, with a focus on older adults.

METHODS

Relevant articles were identified through a search of MEDLINE and International Pharmaceutical Abstracts (1966-August 2006) for English-language articles. The following medical subject headings and key words were used: alcohol medication interactions, diseases worsened by alcohol use, and alcohol metabolism, absorption, and distribution. Additional articles were identified by a manual search of the reference lists of the identified articles, review articles, textbooks, and personal reference sources.

RESULTS

Many older adults drink alcohol and take medications that may interact negatively with alcohol. Some of these interactions are due to age-related changes in the absorption, distribution, and metabolism of alcohol an medications. Others are due to disulfiram-like reactions observed with some medications, exacerbation of therapeutic effects and adverse effects of medications when combined with alcohol, and alcohol's interference with the effectiveness of some medications.

CONCLUSIONS

Older adults who drink alcohol and who take medications are at risk for a variety of adverse consequences depending on the amount of alcohol and the type of medications consumed. It is important for clinicians to know how much alcohol their older patients are drinking to be able to effectively assess their risks and to counsel them about the safe use of alcohol and medications. Similarly, it is important for older adults to understand the potential risks of their combined alcohol and medication use to avoid the myriad of problems possible with unsafe use of these substances..

Bioactivation of nitroglycerin by the mitochondrial aldehyde dehydrogenase

The mitochondrial aldehyde dehydrogenase (ALDH2, mtALDH) was recently found to catalyze the reduction of nitroglycerin (glyceryl trinitrate [GTN]) to generate nitrite and 1,2-glyceryl dinitrate. The nitrite generated within the mitochondria is metabolized further to generate nitric oxide (NO)-based bioactivity, by reduction to NO and/or by conversion to S-nitrosothiol, as revealed by a series of biochemical, pharmacologic, and genetic studies. These studies also demonstrated that mechanism-based inactivation of mtALDH is involved in the development of GTN tolerance. In mice in which the mtALDH gene was selectively deleted (mtALDH(-/-)), vascular responsiveness to low but not to high GTN concentrations was eliminated, indicating the existence of an additional mechanism of GTN biotransformation ("high K(m)" pathway). In addition, bioactivation of isosorbide dinitrate/mononitrate vasodilators is independent of mtALDH. Induction of GTN tolerance in vitro in aortae from normal mice selectively affected responsiveness to low doses of GTN, and the remaining responsiveness to high doses of GTN in mtALDH(-/-) vasculature did not exhibit tolerance. These findings suggest strongly that the high K(m) pathway is not involved in the development of GTN tolerance that is mechanism-based. Notably, recent studies indicate that individuals of East Asian origin with the common E487K mutation of mtALDH, which results in decreased mtALDH activity, are significantly less responsive to GTN. These observations in toto provide strong support for the conclusion that mtALDH provides the necessary and sufficient enzymatic mechanism for biotransformation of clinically relevant concentrations of GTN to NO-based vasoactivity and indicate in addition that inactivation of mtALDH plays a significant role in the development of mechanism-based GTN tolerance.

Photochemical reactions of thiols with organic nitrates — Oxygen atom transfer via a thionitrate

Nitroglycerin is an organic nitrate that has been used in the clinical treatment of angina for 130 years, yet important details of its mechanism of action remain unanswered. The biological activity of nitrates suggests that they are bioactivated to NO via a three-electron reduction. The involvement of free or bound protein thiols in this reduction has often been proposed. To examine the involvement of thiyl radicals in such a process, the photochemical generation of benzenethiyl radical from thiol and disulfide precursors was studied in the presence of isopropyl nitrate. Analysis of reaction products and kinetics led to the conclusion that photolysis of the nitrate to NO2dominated the observed photochemistry. Formation of sulfonothioate and NO as products, and trapping of NO2by 4-chlorophenol, indicated a mechanism involving oxygen atom transfer from N to S via a thionitrate intermediate. The results of the study did not indicate a rapid reaction between thiyl radical and organic nitrate. Despite weak nitrate absorption of light >300 nm and a relatively high BDE for homolysis to give NO2, the photochemistry under thiyl-generating conditions was driven by nitrate photolysis to NO2. A novel nitrate, containing a phenyl disulfanyl group linked to nitrate groups, did not undergo photolysis to NO2or generate sulfonothioate, but did yield NO. These observations suggest that reaction between thiyl radicals and nitrates leading to NO release is a viable pathway, but it is subservient to other competing reactions, such as photolysis, in the case of IPN, and reaction with thiolate, in the case of the novel nitrate.Key words: nitrate, photolysis, thiyl radical, nitrogen dioxide, nitric oxide.

Explaining the phenomenon of nitrate tolerance

During the last century, nitroglycerin has been the most commonly used antiischemic and antianginal agent. Unfortunately, after continuous application, its therapeutic efficacy rapidly vanishes. Neurohormonal activation of vasoconstrictor signals and intravascular volume expansion constitute early counter-regulatory responses (pseudotolerance), whereas long-term treatment induces intrinsic vascular changes, eg, a loss of nitrovasodilator-responsiveness (vascular tolerance). This is caused by increased vascular superoxide production and a supersensitivity to vasoconstrictors secondary to a tonic activation of protein kinase C. NADPH oxidase(s) and uncoupled endothelial nitric oxide synthase have been proposed as superoxide sources. Superoxide and vascular NO rapidly form peroxynitrite, which aggravates tolerance by promoting NO synthase uncoupling and inhibition of soluble guanylyl cyclase and prostacyclin synthase. This oxidative stress concept may explain why radical scavengers and substances, which reduce oxidative stress indirectly, are able to relieve tolerance and endothelial dysfunction. Recent work has defined a new tolerance mechanism, ie, an inhibition of mitochondrial aldehyde dehydrogenase, the enzyme that accomplishes bioactivation of nitroglycerin, and has identified mitochondria as an additional source of reactive oxygen species. Nitroglycerin-induced reactive oxygen species inhibit the bioactivation of nitroglycerin by thiol oxidation of aldehyde dehydrogenase. Both mechanisms, increased oxidative stress and impaired bioactivation of nitroglycerin, can be joined to provide a new concept for nitroglycerin tolerance and cross-tolerance. The consequences of these processes for the nitroglycerin downstream targets soluble guanylyl cyclase, cGMP-dependent protein kinase, cGMP-degrading phosphodiesterases, and toxic side effects contributing to endothelial dysfunction, such as inhibition of prostacyclin synthase, are discussed in this review.

Nitrates and NO release: contemporary aspects in biological and medicinal chemistry

Nitroglycerine has been used clinically in the treatment of angina for 130 years, yet important details on the mechanism of action, biotransformation, and the associated phenomenon of nitrate tolerance remain unanswered. The biological activity of organic nitrates can be said to be nitric oxide mimetic, leading to recent, exciting progress in realizing the therapeutic potential of nitrates. Unequivocally, nitroglycerine and most other organic nitrates, including NO-NSAIDs, do not behave as NO donors in the most fundamental action: in vitro activation of sGC to produce cGMP. The question as to whether the biological activity of nitrates results primarily or exclusively from NO donation will not be satisfactorily answered until the location, the apparatus, and the mechanism of reduction of nitrates to NO are defined. Similarly, the therapeutic potential of nitrates will not be unlocked until this knowledge is attained. Aspects of the therapeutic and biological activity of nitrates are reviewed in the context of the chemistry of nitrates and the elusive efficient 3e- reduction required to generate NO.

Role of mitochondrial aldehyde dehydrogenase in nitrate tolerance

Glyceryl trinitrate (GTN) is used in the treatment of angina pectoris and cardiac failure, but the rapid onset of GTN tolerance limits its clinical utility. Research suggests that a principal cause of tolerance is inhibition of an enzyme responsible for the production of physiologically active concentrations of NO from GTN. This enzyme has not conclusively been identified. However, the mitochondrial aldehyde dehydrogenase (ALDH2) is inhibited in GTN-tolerant tissues and produces NO2- from GTN, which is proposed to be converted to NO within mitochondria. To investigate the role of this enzyme in GTN tolerance, cumulative GTN concentration-response curves were obtained for both GTN-tolerant and -nontolerant rat aortic rings treated with the ALDH inhibitor cyanamide or the ALDH substrate propionaldehyde. Tolerance to GTN was induced using both in vivo and in vitro protocols. The in vivo protocol resulted in almost complete inhibition of ALDH2 activity and GTN biotransformation in hepatic mitochondria, indicating that long-term GTN exposure results in inactivation of the enzyme. Treatment with cyanamide or propionaldehyde caused a dose-dependent increase in the EC50 value for GTN-induced relaxation of similar magnitude in both tolerant and nontolerant aorta, suggesting that although cyanamide and propionaldehyde inhibit GTN-induced vasodilation, these inhibitors do not affect the enzyme or system involved in tolerance development to GTN. Treatment with cyanamide or propionaldehyde did not significantly inhibit 1,1-diethyl-2-hydroxy-2-nitrosohydrazine-mediated vasodilation in tolerant or nontolerant aorta, indicating that these ALDH inhibitors do not affect the downstream effectors of NO-induced vasodilation. Immunoblot analysis indicated that the majority of vascular ALDH2 is present in the cytoplasm, suggesting that mitochondrial biotransformation of GTN by ALDH2 plays a minor role in the overall vascular biotransformation of GTN by this enzyme.

Mechanisms of nitrate tolerance: potential roles of folate

More than 100 years since their introduction in cardiovascular therapy, nitrates continue to be widely used in ischaemic heart disease despite incomplete knowledge of their intimate mechanism of action. Particularly, the development of a progressive attenuation of their efficacy over prolonged use (tolerance) continues to be the subject of current investigation. Newer findings point to the role of increased intracellular oxidative stress as a mechanism for tolerance and to folic acid derivatives as pharmacologic means to attenuate its development. This paper reviews nitrate mechanism of action, the history of nitrate tolerance and newer findings related to the use of folate to prevent this phenomenon.

Identification of the enzymatic mechanism of nitroglycerin bioactivation

Nitroglycerin (glyceryl trinitrate, GTN), originally manufactured by Alfred Nobel, has been used to treat angina and heart failure for over 130 years. However, the molecular mechanism of GTN biotransformation has remained a mystery and it is not well understood why "tolerance" (i.e., loss of clinical efficacy) manifests over time. Here we purify a nitrate reductase that specifically catalyzes the formation of 1,2-glyceryl dinitrate and nitrite from GTN, leading to production of cGMP and relaxation of vascular smooth muscle both in vitro and in vivo, and we identify it as mitochondrial aldehyde dehydrogenase (mtALDH). We also show that mtALDH is inhibited in blood vessels made tolerant by GTN. These results demonstrate that the biotransformation of GTN occurs predominantly in mitochondria through a novel reductase action of mtALDH and suggest that nitrite is an obligate intermediate in generation of NO bioactivity. The data also indicate that attenuated biotransformation of GTN by mtALDH underlies the induction of nitrate tolerance. More generally, our studies provide new insights into subcellular processing of NO metabolites and suggest new approaches to generating NO bioactivity and overcoming nitrate tolerance.

Moderate alcohol consumption and adverse drug reactions among older adults

PURPOSE

To assess the effect of moderate alcohol consumption on Adverse Drug Reactions (ADRs) among older adults admitted to acute care hospitals and to examine the consistency of this effect across gender and age groups.

METHODS

We used the GIFA (Italian Group of Pharmacoepidemiology in the Elderly) database, which includes information on patients admitted to 81 medical centers in Italy. For this study we examined exclusively the ADRs detected at hospital admission that were classified as definite or probable based on the Naranjo algorithm.

RESULTS

Among 22,778 participants, 894 were found to have one or more ADRs (3.9%). Gastrointestinal complications (n = 210; 0.9% of the population) were the most frequent ADRs, followed by metabolic/endocrine (n = 156; 0.7%), dermatological/allergic (n = 102; 0.4%) and arrhythmic (n = 78; 0.3%) complications. Diuretics were the most frequent culprit drugs, followed by NSAIDs and digoxin. An ADR was recorded in 383/10,427 (3.7%) non-drinkers and in 511/12,351 (4.1%) moderate drinkers. After adjusting for potential confounders, moderate alcohol consumption was associated with a 24% increased risk of ADRs (OR 1.24; 95%CI: 1.08-1.43). This effect seemed more evident among women (OR 1.30; 95%CI: 1.09-1.55), than men (OR 1.14; 95%CI: 0.90-1.43), while it was similar across different age groups (< 65 years OR 1.28; 95%CI: 0.99-1.66; 65-79 years OR 1.22; 95%CI: 0.98-1.52; > or = 80 years OR 1.20; 95%CI: 0.93-1.56). Considering the most common ADRs, moderate alcohol users presented a significantly higher risk of drug-related headache (OR 3.89; 95%CI: 1.43-10.61) and metabolic/endocrine complications (OR 1.67; 95%CI: 1.19-2.33).

CONCLUSIONS

Moderate alcohol intake is associated with an increased risk of ADRs; this effect seems more evident among women than men, and it does not differ across age groups.

Pharmacotherapy of type 2 diabetes mellitus

OBJECTIVE

To review the drug treatments and some of the popular, nontraditional remedies now available for type 2 diabetes mellitus, as well as selected investigational agents; to describe each medication's place in the overall approach to treatment.

DATA SOURCES

English-language journals, abstracts, review articles, and newspaper accounts.

DATA SYNTHESIS

In the past five years, there has been tremendous progress in the pharmacotherapy of diabetes, particularly type 2 diabetes. Several new agents have entered the clinical arena, and many more are in the late stages of investigation leading to approval. Sulfonylureas stimulate the production and release of insulin; these drugs must be used in patients with an intact pancreas. The meglitinides are nonsulfonylurea agents that are also insulin secretagogues. Unlike the sulfonylureas, repaglinide appears to require the presence of glucose to close the adenosine triphosphate-sensitive potassium channels and induce calcium influx. Metformin reduces hepatic glucose production in some patients and increases peripheral glucose utilization, but its use is hampered by a high percentage of adverse reactions. Disaccharidase inhibitors effectively compensate for the defective early-phase insulin release by slowing the production of sugars from carbohydrates. Thiazolidinediones appear to activate peroxisome proliferator-activated receptor gamma, which is involved in the metabolism of lipids. Short-acting insulin and the role of weight-loss agents are also discussed.

CONCLUSIONS

The availability of new options for diabetes therapy provides a chance for successful therapy in a larger number of patients. However, it is important to consider how much true benefit these new forms of treatment will have on the diabetic community. The best choice for a patient remains controversial.

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.

Diazepam- and chlordiazepoxide-mediated increases in erythrocyte aldehyde dehydrogenase activity and its possible implications

Erythrocyte ALDH activity was assayed in alcoholic (n = 70) and nonalcoholic (n = 40) subjects. In general, alcoholics without any prior medications (n = 57) were found to have a decreased ALDH activity (mean +/- SD: 3.38 +/- 1.7 mU; p less than 0.001) as compared to control group (5.10 +/- 1.57 mU). However, a group of alcoholics who were detoxified with benzodiazepines (n = 13) prior to blood collection for enzyme assay were found to have higher ALDH activity (4.92 +/- 2.46 mU; p less than 0.05) as compared to alcoholics who were not detoxified. In vitro experiments demonstrated that both diazepam (DZM) and chlordiazepoxide (CDP) could activate the ALDH. The magnitude of enzyme activation by DZM and CDP appear to correlate with their relative potency of tranquilizing effect. Further, the observed ability of DZM to reverse the inhibition of ALDH mediated by disulfiram may explain the biochemical basis of the reported ability of benzodiazepines (BDZ) to reduce the intensity of disulfiram ethanol reaction (DER).

Atherosclerosis and acetaldehyde metabolism in blood

Acetaldehyde elimination in blood homogenates and erythrocyte aldehyde dehydrogenase (ALDH) activity were studied in 64 patients operated before the age of 60 years because of symptomatic stenosis of aorta, iliac, or carotid arteries and in 38 healthy controls. The disappearance of acetaldehyde in blood homogenates was biphasic. Patients showed an enhanced elimination of acetaldehyde during the second phase (30-60 min), as compared to controls (T1/2 of acetaldehyde was 103 +/- 47 and 198 +/- 93 min, respectively, P less than 0.001). No correlation was found between ALDH activity and acetaldehyde elimination rate. Acetaldehyde elimination in blood homogenates and [14C]acetaldehyde binding to plasma proteins, hemoglobin, and erythrocyte membranes were studied in 10 patients with atherosclerotic disease and in 12 healthy controls. There was a significant correlation between unstable binding of [14C]acetaldehyde to plasma proteins and the half-life of acetaldehyde in the elimination test (p = 0.74, P less than 0.005). Fractionation of plasma proteins after incubation with [14C]acetaldehyde revealed no difference between patients and controls in the distribution of radioactivity. The binding of [14C]acetaldehyde to hemoglobin or erythrocyte membranes did not differ between patients and controls. These results indicate that patients with angiopathy and an enhanced acetaldehyde elimination in blood have reduced binding of acetaldehyde to plasma proteins. As unstable binding of acetaldehyde to proteins is known to involve free amino groups of amino acid residues, modification of these residues in atherosclerotic disease is conceivable.

Increased frequency of acetaldehyde-induced sister-chromatid exchanges in human lymphocytes treated with an aldehyde dehydrogenase inhibitor

Acetaldehyde, the first metabolite of ethanol oxidation, in concentrations ranging from 100 microM to 400 microM caused a dose-dependent linear increase in the frequency of sister-chromatid exchanges (SCE) in cultured human peripheral lymphocytes. The SCE frequency was on an average 2-fold higher when the cells were exposed to the acetaldehyde after 24 h incubation instead of at the time of mitogen stimulation (0 h). When acetaldehyde was added together with the potent aldehyde dehydrogenase inhibitor 1-aminocyclopropanol (0.1 mM), the SCE response was significantly (p less than 0.05) increased. The present results indicate that acetaldehyde is metabolized within human lymphocytes, and, moreover, that alcohol consumption during treatment with drugs that inactivate aldehyde dehydrogenase may cause a further increased incidence of acetaldehyde-induced SCE and concomitant lesions.

Reactions of biogenic aldehydes with hemoglobin

The aldehyde derivatives of dopamine and serotonin, 3,4-dihydroxyphenylacetaldehyde (DOPAL) and 5-hydroxyindole-3-acetaldehyde (5-HIAL), respectively, were incubated with human hemoglobin under physiological conditions. Both DOPAL and 5-HIAL, as well as dopamine, showed a time-dependent disappearance during the incubations, whereas this was not observed with serotonin. The amounts of free aldehydes recovered after incubation with hemoglobin, as analysed by high-performance liquid chromatography with electrochemical detection, corresponded to the amounts of acid metabolites formed in enzymatic assays, when the samples instead were incubated with aldehyde dehydrogenase. Incubations with DOPAL, 5-HIAL, or dopamine, and hemoglobin also resulted in distinct increases in the absorption spectra between 250-350 nm, whereas no similar increase was observed with serotonin. Addition of sodium borohydride to the incubates, which is used to stabilize Schiff base adducts between aldehydes and proteins, resulted in reduction of DOPAL and 5-HIAL to their corresponding alcohol metabolites. However, a rapid initial disappearance of the aldehydes, as analysed from the recoveries of the alcohol metabolites, was observed, followed by a more slow disappearance rate throughout the incubation period.

The clinical use of disulfiram (Antabuse): a review

Disulfiram is a potent alcohol-sensitizing drug, the effectiveness of which remains unproven in the treatment of alcoholism after 40 years of use. Its clinical utility is more closely associated with nonspecific, nonpharmacological factors (such as social class, patient compliance, patient personality characteristics, and treatment structure) than with its aversive biochemistry. Disulfiram is not effective as a sole alternative to a structured treatment program. Disulfiram retains a place in standard alcoholism treatment programs because clinicians have found this agent useful for selected alcoholic patients. Clinical studies and clinical lore describe these patients as older, relapse-prone, socially stable, cognitively intact, not depressed, compulsive, capable of following rules, and tolerant of dependence. Another distinctly responsive (but evasive) group is court-probated patients. These characteristics also describe patients who are well-known to have good outcomes without disulfiram, thus they do not help clinicians to select suitable patients for this medication. Consequently, this article proposes the following selection criteria: (1) patients who can tolerate a treatment relationship; (2) patients who are relapse-prone (but in treatment); (3) patients who have failed with less structured approaches; (4) patients in early abstinence who are in crisis or under severe stress; (5) patients in established recovery for whom individual or group psychotherapy is a relapse risk; and (6) patients who specifically request it. With or without disulfiram, a treatment program needs to be highly structured and predictable in order to be useful to newly recovering patients. Recovery is a process with discernible phases of development, and the provision of structure is the core of early treatment, where behavior change is more important than insight. A well-structured program will have phases through which a patient may progress. Generally speaking, disulfiram is most useful early to establish sobriety and to allow time for other support structures, such as AA, therapist-patient relationships, and new personal relationships, to take hold. Disulfiram is best given to patients with prior treatment failures, early in treatment, briefly during crises in established sobriety, or to support unusual stresses, such as psychotherapy. Prescriptions should be short-term and not allow automatic refills. It should be necessary to attend a treatment program in order to obtain them. Supervision and monitoring dramatically increase compliance.(ABSTRACT TRUNCATED AT 400 WORDS)

Hydrogen peroxide-induced glutathione depletion and aldehyde dehydrogenase inhibition in erythrocytes

To study relationships between lipid peroxidation and aldehyde dehydrogenase (ALDH) inhibition, the Stocks and Dormandy model of H2O2-induced lipid peroxidation in erythrocytes was employed. Hydrogen peroxide treatment of erythrocytes and erythrocyte lysates caused a dose-dependent inhibition and depletion of ALDH and reduced glutathione (GSH) respectively. Complete ALDH inhibition and glutathione depletion occurred before significant lipid peroxidation was detected by HPLC analysis of malondialdehyde-thiobarbituric acid adducts. Hydroxyl radical scavengers did not antagonize the hydrogen peroxide-induced enzyme inhibition. Studies with the iron chelator desferrioxamine suggested that the hydrogen peroxide-induced ALDH inhibition was mediated by iron in erythrocyte lysates but not in semi-purified (and Chelex-treated) ALDH preparations. Glutathione peroxidase reduction of H2O2 exhibited an anomalous GSH dependence which was not in agreement with the accepted reaction mechanism. Reduced glutathione also antagonized the hydrogen peroxide-induced ALDH inhibition by possible complex formation with the enzyme. A hypothetical model is presented which accounts for the observed responses to hydrogen peroxide.

Assay of human blood aldehyde dehydrogenase activity by high-performance liquid chromatography

A simple and sensitive method for routine analysis of aldehyde dehydrogenase (ALDH, EC 1.2.1.3) activity in human blood has been developed. The assays were performed by incubating diluted whole blood samples in sodium pyrophosphate buffer in the presence of NAD. The aldehyde derived from dopamine, or alternatively the aldehyde from serotonin, was used as the substrate and the acid formed was measured using high-performance liquid chromatography with electrochemical detection. The present method can be performed with a small sample (10-25 microliters) of whole blood and no time-consuming pretreatments of the samples are needed. Six to seven samples can be assayed per hour, and the precision of the method was 2%. For comparison, assays were also performed with two fluorimetric methods, one measuring the formation of indole-3-acetic acid from indole-3-acetaldehyde and the other measuring the rate of acetaldehyde disappearance.