Red blood cells: a new major modality for acetaldehyde transport from liver to other tissues

Cardioprotection induced by a brief exposure to acetaldehyde: role of aldehyde dehydrogenase 2

Aims

We previously demonstrated that acute ethanol administration protects the heart from ischaemia/reperfusion (I/R) injury thorough activation of aldehyde dehydrogenase 2 (ALDH2). Here, we characterized the role of acetaldehyde, an intermediate product from ethanol metabolism, and its metabolizing enzyme, ALDH2, in an ex vivo model of cardiac I/R injury.

Methods and results

We used a combination of homozygous knock-in mice (ALDH2*2), carrying the human inactivating point mutation ALDH2 (E487K), and a direct activator of ALDH2, Alda-1, to investigate the cardiac effect of acetaldehyde. The ALDH2*2 mice have impaired acetaldehyde clearance, recapitulating the human phenotype. Yet, we found a similar infarct size in wild type (WT) and ALDH2*2 mice. Similar to ethanol-induced preconditioning, pre-treatment with 50 μM acetaldehyde increased ALDH2 activity and reduced cardiac injury in hearts of WT mice without affecting cardiac acetaldehyde levels. However, acetaldehyde pre-treatment of hearts of ALDH2*2 mice resulted in a three-fold increase in cardiac acetaldehyde levels and exacerbated I/R injury. Therefore, exogenous acetaldehyde appears to have a bimodal effect in I/R, depending on the ALDH2 genotype. Further supporting an ALDH2 role in cardiac preconditioning, pharmacological ALDH2 inhibition abolished ethanol-induced cardioprotection in hearts of WT mice, whereas a selective activator, Alda-1, protected ALDH2*2 against ethanol-induced cardiotoxicity. Finally, either genetic or pharmacological inhibition of ALDH2 mitigated ischaemic preconditioning.

Conclusion

Taken together, our findings suggest that low levels of acetaldehyde are cardioprotective whereas high levels are damaging in an ex vivo model of I/R injury and that ALDH2 is a major, but not the only, regulator of cardiac acetaldehyde levels and protection from I/R.

Acetaldehyde adducts in alcoholic liver disease

Chronic alcohol abuse causes liver disease that progresses from simple steatosis through stages of steatohepatitis, fibrosis, cirrhosis, and eventually hepatic failure. In addition, chronic alcoholic liver disease (ALD), with or without cirrhosis, increases risk for hepatocellular carcinoma (HCC). Acetaldehyde, a major toxic metabolite, is one of the principal culprits mediating fibrogenic and mutagenic effects of alcohol in the liver. Mechanistically, acetaldehyde promotes adduct formation, leading to functional impairments of key proteins, including enzymes, as well as DNA damage, which promotes mutagenesis. Why certain individuals who heavily abuse alcohol, develop HCC (7.2-15%) versus cirrhosis (15-20%) is not known, but genetics and co-existing viral infection are considered pathogenic factors. Moreover, adverse effects of acetaldehyde on the cardiovascular system and hematologic systems leading to ischemia, heart failure, and coagulation disorders, can exacerbate hepatic injury and increase risk for liver failure. Herein, we review the role of acetaldehyde adducts in the pathogenesis of chronic ALD and HCC.

Modification of carbonic anhydrase II with acetaldehyde, the first metabolite of ethanol, leads to decreased enzyme activity

BACKGROUND

Acetaldehyde, the first metabolite of ethanol, can generate covalent modifications of proteins and cellular constituents. However, functional consequences of such modification remain poorly defined. In the present study, we examined acetaldehyde reaction with human carbonic anhydrase (CA) isozyme II, which has several features that make it a suitable target protein: It is widely expressed, its enzymatic activity can be monitored, its structural and catalytic properties are known, and it contains 24 lysine residues, which are accessible sites for aldehyde reaction.

RESULTS

Acetaldehyde treatment in the absence and presence of a reducing agent (NaBH3(CN)) caused shifts in the pI values of CA II. SDS-PAGE indicated a shift toward a slightly higher molecular mass. High-resolution mass spectra of CA II, measured with and without NaBH3(CN), indicated the presence of an unmodified protein, as expected. Mass spectra of CA II treated with acetaldehyde revealed a modified protein form (+26 Da), consistent with a "Schiff base" formation between acetaldehyde and one of the primary NH2 groups (e.g., in lysine side chain) in the protein structure. This reaction was highly specific, given the relative abundance of over 90% of the modified protein. In reducing conditions, each CA II molecule had reacted with 9-19 (14 on average) acetaldehyde molecules (+28 Da), consistent with further reduction of the "Schiff bases" to substituted amines (N-ethyllysine residues). The acetaldehyde-modified protein showed decreased CA enzymatic activity.

CONCLUSION

The acetaldehyde-derived modifications in CA II molecule may have physiological consequences in alcoholic patients.

Reduction of ethanol-derived acetaldehyde induced motivational properties by L-cysteine

BACKGROUND

Experimental evidences suggest that acetaldehyde (ACD) contributes to the positive motivational properties of ethanol (EtOH) as assessed by the place conditioning paradigm; indeed, we found that by reducing ACD production and/or by using ACD-sequestrating agents, EtOH is deprived from its motivational properties. Thiol products, such as the amino acid cysteine, are known to be effective ACD-sequestering agents. Cysteine is able to covalently bind ACD thereby forming a stable, nontoxic 2-methyl-thiazolidine-4-carboxylic acid compound. Thus, we treated rats with l-cysteine before intragastric administration of EtOH or ACD.

METHODS

Male Wistar rats were pretreated intraperitoneally with saline or l-cysteine (10, 20, or 30 mg/kg), before intragastric administration of saline, EtOH (1 g/kg), or ACD (20 mg/kg). The specificity of l-cysteine effect was addressed using morphine-induced conditioned place preference (cpp) (2.5 mg/kg, i.p.).

RESULTS

l-cysteine dose-dependently prevented both EtOH and ACD-induced cpp but did not interfere with morphine-induced cpp, suggesting that l-cysteine specifically modulates the motivational properties of EtOH.

CONCLUSION

The present results further underscore the role of EtOH-derived ACD in EtOH-induced motivational properties. l-cysteine, by binding EtOH-derived ACD, would deprive it of its rewarding properties and reduce its abuse liability.

The role of acetaldehyde in pregnancy outcome after prenatal alcohol exposure

It is not known why some heavy-drinking women give birth to children with alcohol-related birth defects (ARBD) whereas others do not. The objective of this study was to determine whether the frequency of elevated maternal blood acetaldehyde levels among alcoholics is in the range of ARBD among alcoholic women. MEDLINE was searched from 1980 to 2000 using the key words acetaldehyde, pharmacokinetics, and alcoholism for controlled trials reporting blood or breath acetaldehyde levels in alcoholics and nonalcoholics. Separately, using the key words fetal alcohol syndrome, epidemiology, prevalence, incidence, and frequency, articles were identified reporting ARBD incidences among the offspring of heavy drinkers. Of 23 articles reporting acetaldehyde levels in alcoholics, four met the inclusion criteria. Forty-three studies reported on the rate of ARBD in heavy drinkers, and 14 were accepted. Thirty-four percent of heavy drinkers had a child with ARBD, and 43% of chronic alcoholics had high acetaldehyde levels. The similar frequencies of high acetaldehyde levels among alcoholics and the rates of ARBD among alcoholic women provide epidemiologic support to the hypothesis that acetaldehyde may play a major role in the cause of ARBD.

The effects of aminotriazole and acetaldehyde on an ethanol drug discrimination with a conditioned taste aversion procedure

The present study was designed to investigate whether acetaldehyde shares stimulus properties with ethanol using the conditioned taste aversion (CTA) baseline of drug discrimination learning. Animals were trained to discriminate ethanol (0.8 g/kg, i.p.) from saline using 11 consecutive cycles consisting of a pairing day and three nonpairing days. On pairing days, all animals were injected with ethanol 30 min prior to a 20-min limited access to a saccharin solution (0.1% w/v) and then immediately injected with either LiCl (0.15 M, 1.8 meq) or distilled water. On the three following nonpairing days, animals were injected with saline and 30 min later presented with the same saccharin solution for 20 min. No injections followed on these nonpairing days. Results showed that animals acquired discriminative stimulus control for ethanol after seven pairings. Pretreatment with the catalase inhibitor did not alter the discriminative control for ethanol. Generalization tests revealed that acetaldehyde substituted for ethanol at a dose of 0.3 g/kg. The results of the present study suggest that catalase inhibition did not reverse or alter the discriminative stimulus effects of ethanol. However, generalization tests showed that acetaldehyde (0.3 g/kg) will substitute for ethanol suggesting that these two drugs share some similar properties.

Ethanol consumption, amino acid and glutathione blood levels in patients with and without chronic liver disease

BACKGROUND

Ethanol abuse and liver cirrhosis cause a reduction of glutathione blood levels; liver cirrhosis induces an alteration of the plasma amino acid pattern. We evaluated whether or not ethanol abuse affects amino acid levels, particularly those that are involved in metabolizing glutathione in the plasma and erythrocytes of chronic alcohol abusers with or without liver cirrhosis.

METHODS

We studied 10 chronic alcohol abusers without liver cirrhosis, 10 with alcoholic cirrhosis, 10 affected by hepatitis C virus-related cirrhosis, and 10 healthy subjects. Glutathione, y-glutamyl-cysteine, and cysteine were determined by fluorescent HPLC, glutamic acid, glycine, and other free amino acids by cation exchange chromatography both in the plasma and erythrocytes of all studied subjects.

RESULTS AND CONCLUSIONS

In both alcoholics and cirrhotics, we found a significant increase of plasma-aromatic amino acid and methionine levels, whereas glutathione was significantly reduced. The erythrocytes of these patients showed a significant increase of cysteine, glutamic acid, and glycine; gamma-glutamylcysteine was normal; and glutathione and other free amino acids were significantly decreased. Data suggest that, independent of liver cirrhosis, ethanol abuse affects the metabolism of amino acids and glutathione in both the plasma and the erythrocytes.

Oxidation of acetaldehyde by isolated aortic rings of UChA and UChB rats

The rate of acetaldehyde metabolism was measured in aortic rings from rat strains genetically bred for high (UChB) and low (UChA) voluntary ethanol consumption. The results show that in aortic rings from naive UChB rats, acetaldehyde oxidation rates were significantly greater than the rates observed in aortic rings from naive UChA rats. These strain differences are explained by different activity of vascular low-Km aldehyde dehydrogenase (ALDH). Chronic feeding of ethanol to UChA rats did not alter their aortic ALDH activity. The results of the present study provides additional evidence that the activity variation for the low-Km ALDH between both rat strains exists in various organs and tissues.

Ethanol metabolism by macrophages: possible role in organ damage

Macrophages and hepatocytes oxidize ethanol to acetate in vitro at comparable rates but by different biochemical pathways. Ethanol metabolism by macrophages is largely ADH-independent and mainly based on cytochrome P450 and on the extracellular release of superoxide anion radicals. There is also evidence that during ethanol metabolism, macrophages release more acetaldehyde extracellularly than hepatocytes; the high concentrations of acetaldehyde around macrophages may damage surrounding tissue cells. Some of this acetaldehyde forms unstable cytotoxic complexes with serum albumin and with erythrocytes. The superoxide anion radicals released by macrophages may not only oxidize ethanol to acetaldehyde but also react with and damage cells in their immediate vicinity. After exposure to ethanol, macrophage-depleted rodents show markedly reduced levels of cytotoxic acetaldehyde-albumin complexes in the blood and reduced levels of hydroxyethyl radicals in the bile compared to control animals, indicating that the generation of such potentially pathogenic molecules is, to a large extent, dependent on macrophage activity. Macrophage-depleted animals also show less early liver damage than control animals. The reduction in ethanol-induced liver damage in macrophage-depleted mice and rats may be due to a reduction or elimination of the generation of various Kupffer-cell-derived hepatotoxic substances, including acetaldehyde and reactive oxygen radicals, in such animals. These data suggest that ethanol metabolism by tissue macrophages may play an important role in mediating ethanol-related tissue damage.

Activated oxazaphosphorines are transported predominantly by erythrocytes

PURPOSE

Oxazaphosphorines are metabolised by a variety of pathways, one of which leads to activation and the formation of alkylating compounds. However, the transport forms conveying activated oxazaphosphorines to the tumour cell have not been fully characterised. There is increasing recognition of the importance of the erythrocyte as a carrier of compounds in the circulation, and we have recently described higher concentrations of 4-hydroxycyclophosphamide within the erythrocyte compartment compared to plasma. We have now determined the concentrations of ifosfamide and seven of its metabolites in the plasma and erythrocytes of patients receiving a six-hour intravenous infusion of ifosfamide.

PATIENTS AND METHODS

Red cells from five patients, receiving a total of eight cycles of ifosfamide, were separated from plasma using the MESED instrument, and analysis of red cells and plasma performed using Gas Chromatography-Mass Spectrometry (GC/MS).

RESULTS

The concentration of all compounds in the erythrocyte compartment was higher than or equal to those in plasma, and isophosphoramide mustard and carboxyifosfamide showed a particular affinity for the erythrocyte. The red cell fraction can contain as much as 77% of the total blood concentration of isophosphoramide mustard.

CONCLUSIONS

Erythrocyte associated isophosphoramide mustard is an important transport form of activated ifosfamide. Red cells may have a role in the delivery of activated oxazaphosphorines to tissues.

Catalase and the production of brain acetaldehyde: a possible mediator of the psychopharmacological effects of ethanol

This review represents an attempt to assess the available data on the role of catalase in the mediation of the behavioral actions of ethanol and the regulation of voluntary ethanol consumption. It is argued that acetaldehyde may be formed in brain through the peroxidatic activity of catalase. Furthermore, acetaldehyde formed centrally through the activity of this enzyme, may be responsible, at least in part, for some of the motivational, behavioral and neurotoxic effects of ethanol.

The formation and stability of imidazolidinone adducts from acetaldehyde and model peptides. A kinetic study with implications for protein modification in alcohol abuse

The kinetics of the reaction of acetaldehyde (AcH) with the alpha-amino group of several di- and tripeptides to form 2-methylimidazolidin-4-one adducts were determined at pH 7, 4, 37 degrees C, using reverse phase HPLC to separate peptides from adducts. The imidazolidin-4-one structure of the adducts was confirmed by 13C NMR spectroscopy. The reaction of val-gly-gly with AcH was shown to follow second-order kinetics over a wide range of concentrations of both reactants, with k2 = 0.734 +/- 0.032 M(-1) min(-1). Under conditions similar to those in the liver of an alcoholic during chronic ethanol oxidation ([Ach]o = 50-910 microm; [free peptide alpha-amino groups]o = 1.5 mM), the reaction proceeded until effectively all of the AcH had been consumed. The side chain of the N-terminal amino acid was shown not to have a marked effect on the rate of imidazolidinone formation. The decomposition of the imidazolidinone adduct of val-gly-gly and AcH was observed at 60-100 degrees C. Extrapolation of an Arrhenius plot to 37 degrees C provided an estimate of K(obs) of 0.002 h-1 (t1/2 approximately 14 days). Based on these kinetic studies, it is concluded that imidazolidinone adducts of AcH with proteins may be present in the liver and, possibly, in the blood of alcoholics.

The influence of brain acetaldehyde on oxidative status, dopamine metabolism and visual discrimination task

The toxic effect of acetaldehyde on brain oxidative capacity and dopamine metabolism has been investigated in rat brains after a single intraperitoneal injection of acetaldehyde (5 mmol/kg) and the results compared with those from chronically ethanol fed rats. Acetaldehyde was present in rat brain 120 hr after a single dose of acetaldehyde, confirming that it is able to cross the blood-brain barrier. Brain catalase increased significantly after acetaldehyde or chronic ethanol administration although there were no other significant changes in the total brain activity of superoxide dismutase, glutathione peroxidase or glutathione reductase. Dopamine turnover was increased in both experimental groups. The acute dose of acetaldehyde reduced the ability of the rats to relearn a computer visual discrimination task.

Acetaldehyde inhibits the anti-elastase activity of alpha 1-antitrypsin

There are genetic and exogenous factors responsible for alpha 1-antitrypsin (alpha 1-AT) deficiency which may lead to cirrhosis of the liver and emphysema. The present study was initiated on a biochemical level in order to determine whether acetaldehyde, the major product of ethanol metabolism, is capable of influencing the physiological effect of alpha 1-AT upon elastase, an enzyme which is capable of inducing emphysema. The effects of acetaldehyde and ethanol upon elastase and alpha 1-AT were tested. Acetaldehyde at 0.3-M and 1.2-M concentrations inhibited the anti-elastase activity of alpha 1-AT. Acetaldehyde at 0.03-M and 0.07-M concentrations did not affect elastase activity and had a slight effect at 0.12-M levels. Equivalent amounts of ethanol were without influence upon elastase activity or alpha 1-AT function. These data provide biochemical support for the possibility that heterozygous males with lower than normal alpha 1-AT levels may be at much higher risk to develop liver disease, emphysema, and alpha 1-AT deficiency as a consequence of chronic exposure to ethanol and concomitant circulating acetaldehyde levels.

Leucine and glucose turnover in chronic alcoholics during early abstinence and after an ethanol load

We studied leucine turnover using a primed infusion of [1-14C]-L-leucine and glucose turnover using a primed infusion of [6-3H]-D-glucose in five alcoholic patients without liver damage and five age-matched controls. Infusions were maintained for 6 hr, and at the end of the 3rd hour, a 0.8 g/kg iv ethanol load was administered in 20 min. Leucine flux, nonoxidative disposal and oxidation rates, and glucose rate of appearance were calculated during the 3rd and 6th hours of infusion. Ethanol disappearance rate and the percentage completely metabolized to CO2 and H2O in 3 hr were also calculated. Compared with controls, alcoholics had significantly higher basal leucine flux (55.6 +/- 12 vs. 37.3 +/- 9.3 microM/m2/min) and nonoxidative disposal (48.7 +/- 8.7 vs. 31.1 +/- 7.5 microM/m2/min). No differences were observed in basal glucose appearance rates in alcoholics and controls (397.6 +/- 115.2 vs. 349.4 +/- 120.6 microM/m2/min). Compared with controls, alcoholics had a higher alcohol disappearance rate (2.72 +/- 0.59 vs. 1.84 +/- 0.43 mM/kg/min) and percentage of ethanol metabolized to CO2 and H2O in 3 hr (40.6 +/- 10.2 vs. 22.9 +/- 6.9%). After the ethanol load, both leucine turnover and glucose rate of appearance decreased significantly only in alcoholics. There was a positive correlation between the change in leucine flux and ethanol disappearance rate and percentage metabolized to CO2 and H2O in alcoholics.

Aetiology and pathogenesis of alcoholic liver disease

Until the 1960s, liver disease of the alcoholic patient was attributed exclusively to dietary deficiencies. Since then, however, our understanding of the impact of alcoholism on nutritional status has undergone a progressive evolution. Alcohol, because of its high energy content, was at first perceived to act exclusively as 'empty calories' displacing other nutrients in the diet, and causing primary malnutrition through decreased intake of essential nutrients. With improvement in the overall nutrition of the population, the role of primary malnutrition waned and secondary malnutrition was emphasized as a result of a better understanding of maldigestion and malabsorption caused by chronic alcohol consumption and various diseases associated with chronic alcoholism. At the same time, the concept of the direct toxicity of alcohol came to the forefront as an explanation for the widespread cellular injury. Some of the hepatotoxicity was found to result from the metabolic disturbances associated with the oxidation of ethanol via the liver alcohol dehydrogenase (ADH) pathway and the redox changes produced by the generated NADH, which in turn affects the metabolism of lipids, carbohydrates, proteins and purines. Exaggeration of the redox change by the relative hypoxia which prevails physiologically in the perivenular zone contributes to the exacerbation of the ethanol-induced lesions in zone 3. In addition to ADH, ethanol can be oxidized by liver microsomes: studies over the last twenty years have culminated in the molecular elucidation of the ethanol-inducible cytochrome P450IIE1 (CYP2E1) which contributes not only to ethanol metabolism and tolerance, but also to the selective hepatic perivenular toxicity of various xenobiotics. Their activation by CYP2E1 now provides an understanding for the increased susceptibility of the heavy drinker to the toxicity of industrial solvents, anaesthetic agents, commonly prescribed drugs, 'over the counter' analgesics, chemical carcinogens and even nutritional factors such as vitamin A. Ethanol causes not only vitamin A depletion but it also enhances its hepatotoxicity. Furthermore, induction of the microsomal pathway contributes to increased acetaldehyde generation, with formation of protein adducts, resulting in antibody production, enzyme inactivation and decreased DNA repair; it is also associated with a striking impairment of the capacity of the liver to utilize oxygen. Moreover, acetaldehyde promotes glutathione depletion, free-radical mediated toxicity and lipid peroxidation. In addition, acetaldehyde affects hepatic collagen synthesis: both in vivo and in vitro (in cultured myofibroblasts and lipocytes), ethanol and its metabolite acetaldehyde were found to increase collagen accumulation and mRNA levels for collagen. This new understanding of the pathogenesis of alcoholic liver disease may eventually improve therapy with drugs and nutrients.

Metabolism of acetaldehyde by rat isolated aortic rings: does endothelial tissue contribute to its extrahepatic metabolism?

Acetaldehyde (AcH) metabolism in isolated aortic rings was studied by assessing in vitro-added AcH disappearing rate by head space gas chromatography. It was found that AcH was metabolized by aortic rings or by homogenates prepared in 0.1 M phosphate buffer containing Triton X-100, by an NAD-dependent enzyme with characteristics similar to those of aldehyde dehydrogenase (AIDH) present in mitochondria from rat liver and brain. This enzyme appears to be present in the vascular endothelium, since the action of aortic rings showed a remarkable decrease by its removal. Extrahepatic metabolism of AcH was assessed by the differences between AcH levels found in samples of blood obtained from the suprahepatic vein, carotid artery, femoral vein, and tail cut of rats. The in vitro activity of aortic rings, as well as the extrahepatic AcH metabolism, were significantly decreased by pretreatment of rats with disulfiram. The wide distribution of vascular endothelium throughout the body suggests that this tissue could contribute to AcH extrahepatic metabolism.

Comparative evaluation of the clinical utility of three markers of ethanol intake: the effect of gender

We evaluated three markers of ethanol intake [whole blood associated acetaldehyde (WBAA), serum beta-hexosaminidase, and gamma-glutamyl transpeptidase (GGT)] in four groups of subjects: teetotalers (n = 104), random insurance applicants or "normals" (n = 1,010), subjects enrolling in an alcohol treatment program or "alcoholics" (n = 31), and subjects attending outpatient drug/alcohol treatment follow-up clinics (n = 128). Significant differences (p < 0.004 for each assay and each comparison) were found in the mean values between teetotalers and normals and normals and alcoholics. Male teetotalers and normals had significantly (p < 0.002) higher levels of WBAA than females of the same group. Male normals had significantly higher levels of GGT than females (p < 0.001). GGT increased with age in the normal population into the fifth decade and decreased thereafter. WBAA was the most sensitive assay with 97% of alcoholics having values above the 99th percentile for the teetotaler population (vs. 66% for serum beta-hexosaminidase and 70% for GGT). None of the alcoholic subjects had values for all three assays below the 99th percentile for teetotalers compared with 21% of those in follow-up and 72% of normals. We conclude that WBAA appears to be the best of the three markers studied and that measurement of multiple markers for ethanol use appears clinically useful and incremental.

Acetaldehyde adducts of proteins: diagnostic and pathogenic implications in diseases caused by excessive alcohol consumption

Alcohol abuse and alcoholism continue to be a major threat to human health. Given their increasing incidence and the detrimental impact on society, it is actually surprising that no objective, specific indicators for the early detection of alcohol-related health problems are available. A diagnostic test for a disease involving excessive alcohol consumption should be extremely specific in order to achieve positive predictive power, and: ideally it should also be very sensitive in order to identify problem drinkers in broad screening programs. The present research indicates that such a test for alcohol abuse may be provided by measurements of covalent chemical addition products (adducts) of acetaldehyde with biologically stable macromolecules. It was recently demonstrated that proteins modified with acetaldehyde are formed in vivo and can induce an antibody response as a result of alcohol consumption. Monoclonal and polyclonal antibodies raised by immunizations against acetaldehyde-modified proteins recognize acetaldehyde adducts irrespective of the nature of the carrier protein. Use of such antibodies in sensitive two-site immunoenzymatic or immunofluorometric assays has indicated that high acetaldehyde adduct concentrations exist in the erythrocytes of alcohol abusers, in healthy volunteers after a bout of drinking, and also in alcohol consuming mothers who subsequently give birth to children with foetal alcohol effects. We have developed the first immunohistochemical techniques for the detection of acetaldehyde adducts in human tissues. The centrilobular region of the liver of alcohol abusers with an early stage of histological tissue damage was found to contain acetaldehyde-modified epitopes, whereas the adducts were more widespread in advanced liver disease. The diagnostic superiority of acetaldehyde adducts as markers of ethanol consumption is due to the fact that they represent true metabolites of ethanol and allow estimations of past alcohol consumption after the ethanol has been eliminated from the body. Investigations into the formation of acetaldehyde adducts in alcohol consumers do not only have diagnostic applications but also help to explain the pathogenesis of alcohol-induced organ damage. Many types of hypersensitivity and immune responses are brought about by acetaldehyde-modified proteins. In addition, such metabolites of ethanol also aggravate liver disease through disturbed protein function and stimulation of fibrogenesis.

The identification and partial characterization of acetaldehyde adducts of hemoglobin occurring in vivo: a possible marker of alcohol consumption

Chromatographic, peptide mapping and mass spectrometric analysis were used to examine hemoglobin (Hb) from heavy drinkers and abstainers for alcohol consumption-related modifications. Heavy drinker and abstainer hemoglobin samples contained similar amounts of glycosylated Hb and significantly different (p < 0.05) amounts of "fast" hemoglobin. The presence of higher amounts of "fast" Hb in heavy drinker relative to abstainer samples suggested the presence of alcohol-consumption related modifications. To further examine Hb for modifications, tryptic peptides of the "fast" hemoglobin HbA1c were isolated and analyzed by plasma desorption mass spectrometry (PDMS). [14C]acetaldehyde (AcH)-Hb was synthesized in vivo for use as a standard. Specific peptides were chosen based on co-migration with radiolabeled peptides from a tryptic digest of the [14C]acetaldehyde-Hb. The masses obtained by PDMS for two heavy drinker peptides were identical to two radiolabeled peptides; the two pairs of peptides co-migrated on HPLC. A comparison of the observed mass for the peptides with the theoretical masses for acetaldehyde-modified Hb peptides suggested that the peptides were AcH-modified alpha and beta chain N-termini of Hb. The modified peptides were found in five of six heavy drinker samples. This is the first description of site-specific AcH-Hb adducts occurring in vivo. The routine detection of such adducts has potential for characterizing usual alcohol intake.

Modification of proteins and other biological molecules by acetaldehyde: adduct structure and functional significance

1. Chronic ethanol consumption is a major cause of liver disease. The modification of hepatic proteins by acetaldehyde (AcH), the primary metabolite of ethanol, has for some time been suggested as one of the major events initiating alcoholic liver disease. 2. These alterations in protein structure are believed to affect liver cell function, and may serve to activate the immune system. 3. This review considers the interaction between AcH and macromolecules and its functional implications.

Enzymatic production of acetaldehyde from ethanol in rat brain tissue

The capacity for the brain to produce acetaldehyde (AcHO) from ethanol was determined in rat brain homogenates. Rat brains were perfused with saline-heparin solution and homogenized in a phosphate buffer. Varying amounts of tissue were incubated with ethanol (0-100 mM) for periods of up to 60 min. The reaction was stopped by the addition of desferrioxamine and ice-cold perchloric acid. Supernatants were treated with dinitrophenylhydrazine reagent, extracted with isooctane in the presence of an internal standard, and the derivatives were separated by HPLC. The addition of 4-methyl pyrazole (an alcohol dehydrogenase inhibitor) or metyrapone (a cytochrome P450 inhibitor) had no effect on the amount of recovered AcHO. On the other hand, treatment with the catalase inhibitors sodium azide, cyanamide, or 3-amino-1,2,4-triazole blocked the production of AcHO while the addition of exogenous peroxide or a peroxide-generating system enhanced the production of AcHO. Overall, these results suggest that AcHO may be produced in the brain during alcohol intoxication, through the action of the enzyme catalase.

Utility and evaluation of biochemical markers of alcohol consumption

Biochemical markers of alcohol consumption have a variety of clinical and research applications. Currently available markers such as the serum gamma-glutamyl transpeptidase (GGT), serum transaminases, and the mean corpuscular volume (MCV) lack sufficient sensitivity and specificity to be used for screening of alcoholism in ambulatory patients. However, these tests can be helpful in corroborating a clinical suspicion of alcoholism. A number of special laboratory markers of alcoholism recently have been developed which may have increased diagnostic accuracy. Promising potential markers include serum carbohydrate-deficient transferrin (CDT), red blood cell acetaldehyde, and acetaldehyde adducts. The application of reliable and practical markers of alcohol consumption could lead to significant improvements in the treatment of alcoholism and in the assessment of clinical trials.

Hepatic, metabolic and toxic effects of ethanol: 1991 update

Until two decades ago, dietary deficiencies were considered to be the only reason for alcoholics to develop liver disease. As the overall nutrition of the population improved, more emphasis was placed on secondary malnutrition and direct hepatotoxic effects of ethanol were established. Ethanol is hepatotoxic through redox changes produced by the NADH generated in its oxidation via the alcohol dehydrogenase pathway, which in turn affects the metabolism of lipids, carbohydrates, proteins, and purines. Ethanol is also oxidized in liver microsomes by an ethanol-inducible cytochrome P-450 (P-450IIE1) that contributes to ethanol metabolism and tolerance, and activates xenobiotics to toxic radicals thereby explaining increased vulnerability of the heavy drinker to industrial solvents, anesthetic agents, commonly prescribed drugs, over-the-counter analgesics, chemical carcinogens, and even nutritional factors such as vitamin A. In addition, ethanol depresses hepatic levels of vitamin A, even when administered with diets containing large amounts of the vitamin, reflecting, in part, accelerated microsomal degradation through newly discovered microsomal pathways of retinol metabolism, inducible by either ethanol or drug administration. The hepatic depletion of vitamin A is strikingly exacerbated when ethanol and other drugs were given together, mimicking a common clinical occurrence. Microsomal induction also results in increased production of acetaldehyde. Acetaldehyde, in turn, causes injury through the formation of protein adducts, resulting in antibody production, enzyme inactivation, decreased DNA repair, and alterations in microtubules, plasma membranes and mitochondria with a striking impairment of oxygen utilization. Acetaldehyde also causes glutathione depletion and lipid peroxidation, and stimulates hepatic collagen production by the vitamin A storing cells (lipocytes) and myofibroblasts.(ABSTRACT TRUNCATED AT 250 WORDS)

Antibodies against acetaldehyde-modified epitopes: an elevated IgA response in alcoholics

Several recent reports have shown that antibodies reactive with acetaldehyde (AcH)-modified epitopes are present in alcoholics. However, similar antibodies have also been found in patients with non-alcoholic liver disease and control subjects. In each of these studies total immunoglobulin binding to the AcH-modified proteins was measured, with no attempt being made to identify the classes of immunoglobulin involved. In the present study we employed an enzyme-linked immunosorbent assay (ELISA) to assess the classes of immunoglobulin involved in this response, using plasma samples from 97 alcoholics with varying degrees of liver disease, 35 patients with non-alcoholic liver disease and 33 control subjects. All three groups exhibited a large IgM response and a negligible IgG response. However, the alcoholics exhibited a significantly higher IgA response than either of the other groups. This suggests that the measurement of the IgA response to AcH-modified epitopes may be a specific marker of ethanol abuse.

A comparative blinded study in miniature swine of whole blood-, hemoglobin-, platelet-, plasma-, and lymphocyte-associated acetaldehyde as markers for ethanol intake

Blood samples were obtained from miniature swine maintained on 0, 2, or 6 g/kg/24 hr ethanol for 8 months (N = 6 in each group). Samples from drinking pigs were taken after 8 hr of ethanol abstinence and all were coded and sent for "blinded" analysis. A fluorigenic high performance liquid chromatographic assay was used to quantify whole blood-associated acetaldehyde, hemoglobin-associated acetaldehyde, plasma-associated acetaldehyde, platelet-associated acetaldehyde, and lymphocyte-associated acetaldehyde. Detectable levels of acetaldehyde were found in each sample in both drinking and nondrinking pigs. Analysis of whole blood-associated acetaldehyde was most discriminatory in distinguishing nondrinking from drinking pigs (mean 21.4 +/- 1.0 microM for nondrinkers vs. 24.6 +/- 1.5 SD for the group consuming 2 g/kg ethanol, p = 0.001). Measurements of hemoglobin-associated acetaldehyde normalized to protein concentration (250 +/- 47 nmoles/g vs. 203 +/- 33 SD, p less than 0.05 drinking vs. nondrinking pigs) and platelet-associated acetaldehyde (0.46 0.34 vs. 0.15 +/- 0.16 nmoles/3 x 10(8) platelets, p = 0.05 drinking vs. nondrinking pigs) were also useful in discriminating drinking from nondrinking animals. Analysis of plasma-associated acetaldehyde and lymphocyte-associated acetaldehyde were not useful as markers of ethanol consumption.(ABSTRACT TRUNCATED AT 250 WORDS)

Studies of whole blood associated acetaldehyde as a marker for alcohol intake in mice

Thirty C57BI mice were randomized into two groups. Group 1 served as controls while Group 2 was given 10% V/V ethanol with the drinking water. Whole blood- associated acetaldehyde (WBAA) was measured on capillary blood samples using a fluorigenic high performance chromatographic assay. WBAA peaked at Day 2. A stable mean plateau of 263 +/- 71 SD with a range of 160-400 nmoles/g hemoglobin WBAA was found in the group consuming ethanol compared with 122 +/- 17 SD and a range of 88-150 nmoles/g hemoglobin for controls (p less than 0.001). When ethanol was discontinued, levels of WBAA declined and became similar to those of controls by 9 days following cessation of ethanol. The quantitative difference between ethanol-consuming and control animals and also the rapid rise of whole blood-associated acetaldehyde and the relatively slow decline following cessation of ethanol intake indicate that such a test might be a useful monitor of drinking behavior.

Characterization of the increased binding of acetaldehyde to red blood cells in alcoholics

Using equilibrium dialysis, we found that acetaldehyde, at the levels commonly occurring after ethanol ingestion, did not bind detectably to plasma proteins, but there was significant binding to red blood cells, more in alcoholics than in nonalcoholics. The binding to red blood cells was inhibited by pyridoxal phosphate and N-ethylmaleimide, suggesting adduction to amino and thiol groups. Binding kinetics were consistent with at least two sites. The one with the highest affinity for acetaldehyde corresponded to hemoglobin. Its affinity and Bmax were not changed in alcoholics, but these binding sites accounted for only 44% of the sites available in the red blood cells of alcoholics and 80% of those in controls. Moreover, this binding was not inhibited by N-ethylmaleimide. There was no detectable binding to red cell ghosts. Nonprotein binding was then assessed by changes in NADH produced by the addition of protein-free fractions of the cells to an alcohol dehydrogenase system in equilibrium; this revealed a second binder of lower affinity, larger capacity and with sensitivity to both inhibitors. This binding (possibly due to thiazolidine formation with cysteine) was enhanced in alcoholics, whose red blood cell cysteine content was doubled. Levels of red blood cell cysteine and acetaldehyde remained high for 2 weeks after withdrawal. Because of the prolonged persistence after withdrawal, these changes may provide new markers of alcoholism.

Impaired oxygen utilization. A new mechanism for the hepatotoxicity of ethanol in sub-human primates

The role of oxygenation in the pathogenesis of alcoholic liver injury was investigated in six baboons fed alcohol chronically and in six pair-fed controls. All animals fed alcohol developed fatty liver with, in addition, fibrosis in three. No evidence for hypoxia was found, both in the basal state and after ethanol at moderate (30 mM) or high (55 mM) levels, as shown by unchanged or even increased hepatic venous partial pressure of O2 and O2 saturation of hemoglobin in the tissue. In controls, ethanol administration resulted in enhanced O2 consumption (offset by a commitant increase in splanchnic blood flow), whereas in alcohol fed animals, there was no increase. At the moderate ethanol dose, the flow-independent O2 extraction, measured by reflectance spectroscopy on the liver surface, tended to increase in control animals only, whereas a significant decrease was observed after the high ethanol dose in the alcohol-treated baboons. This was associated with a marked shift in the mitochondrial redox level in the alcohol-fed (but not in control) baboons, with striking rises in splanchnic output of glutamic dehydrogenase and acetaldehyde, reflecting mitochondrial injury. Increased acetaldehyde, in turn, may aggravate the mitochondrial damage and exacerbate defective O2 utilization. Thus impaired O2 consumption rather than lack of O2 supply characterizes liver injury produced by high ethanol levels in baboons fed alcohol chronically.

Hepatic aldehyde dehydrogenase activity in liver diseases, with particular emphasis on alcoholic liver disease

Hepatic aldehyde dehydrogenase isozyme activity was measured in 51 patients with various types of liver diseases, including 24 patients with alcoholic liver disease, to elucidate the relationship between hepatic aldehyde dehydrogenase activity and liver disease, especially alcoholic liver disease. The levels of low-Km and total aldehyde dehydrogenase activity in the liver decreased both in alcoholic and nonalcoholic liver disease patients, who showed an isoelectric focusing pattern of the usual type. There was no significant difference in the aldehyde dehydrogenase activity between alcoholic and nonalcoholic liver disease. In alcoholic liver disease, the decrease in the activity was significantly correlated with the progression of liver histology. The activity in liver cirrhosis was significantly lower than that in the other types of alcoholic liver disease. In nonalcoholic liver disease patients, the unusual type of hepatic aldehyde dehydrogenase activity observed was not different from the unusual type observed in nonhepatobiliary disease patients. These results indicate that the reduction of hepatic low-Km aldehyde dehydrogenase activity is a change that occurs subsequent to liver damage. Genetic abnormality in aldehyde dehydrogenase may not be important in the pathogenesis of alcoholic liver injury.

Methionine lowers circulating levels of acetaldehyde after ethanol ingestion

Methionine, administered to ethanol treated mice and rats, significantly reduced circulating acetaldehyde levels without altering circulating levels of ethanol. Hepatic levels of acetaldehyde were also lowered by methionine. Methionine was effective when given prior to or after the administration of ethanol, but the time course of the action of methionine suggested the necessity for metabolic transformation of this amino acid in order for the acetaldehyde-lowering effect to be evidenced. Studies with humans, given methionine doses of approximately one-tenth of those used with mice, indicated that methionine can also lower acetaldehyde in humans ingesting ethanol. Given the toxic characteristics of acetaldehyde, methionine may prove effective in reducing the damaging effects of ethanol ingestion.

Effects of acetaldehyde on human red cell metabolism: evidence for the formation of enzyme inhibitors

Since red cells transport and metabolize acetaldehyde in vivo, the effects of acetaldehyde on human red cell enzyme activities were studied. Incubation of intact red cells or undiluted red cell lysates at 37 degrees C for 4 h with 1-10 mmol/l acetaldehyde decreased only GOT, GPT and aldolase activities among the 26 enzymes tested. No inhibition occurred at 4 degrees C or when acetaldehyde was incubated with dilute hemolysates. Incubation of lysates with other reducing substrates or with acetate inhibited aldolase but not GOT or GPT. Preincubation of lysates with cyanate or fluoride markedly decreased acetaldehyde-mediated transaminase inhibition but not aldolase inhibition. Addition of pyridoxal phosphate, the vitamin B6 transaminase coenzyme, to GOT and GPT assay mixes did not reverse acetaldehyde-mediated transaminase inhibition. These findings suggest that acetaldehyde-mediated aldolase inhibition results from oxidation of acetaldehyde while transaminase inhibition results from nonoxidative acetaldehyde metabolism. When 100-200 mumol/l acetaldehyde is added to lysates at 2-h intervals and when lysates are incubated with ethanol, alcohol dehydrogenase and an NAD-regenerating system, enzyme inhibition occurs at acetaldehyde levels approaching those seen in vivo. Thus, the role of acetaldehyde-mediated enzyme inhibition in the toxicity of alcohol abuse warrants further study.

The role of alcoholism and liver disease in the appearance of serum antibodies against acetaldehyde adducts

We recently presented preliminary data indicating the presence of antibodies against acetaldehyde adducts in sera of over 70% of alcoholic patients. To assess the respective roles of liver disease and alcohol consumption as well as the specificity of this immune response, 141 patients in various stages of alcoholic and nonalcoholic liver diseases were tested by a hemagglutination assay. Sixty-three (73%) of 86 alcoholics had antibody titers above control levels (p less than 0.0001). Alcohol consumption of these individuals was significantly higher (p less than 0.001) than that of those alcoholics with normal titers. Twenty-two patients (39%) with nonalcoholic liver diseases also had elevated levels of antibodies against acetaldehyde adducts (p less than 0.0005); of these, 8 had primary biliary cirrhosis (7 in Stages III and IV), 9 had chronic active hepatitis (6 with cirrhosis) and 5 had acute (virus- or drug-induced) hepatitis. Antibody titers did not correlate with levels of transaminase or alkaline phosphatase activity, nor with bilirubin, and albumin. However, in 52 alcoholics and in nonalcoholic patients with biopsy-confirmed liver disease, the highest titers were seen in the more advanced stages of liver damage. Thus, in addition to alcohol consumption, severity of liver disease may play a role in the appearance of circulating antibodies against acetaldehyde adducts.

Effect of abstinence on the blood acetaldehyde response to a test dose of alcohol in alcoholics

Following an acute dose of alcohol (0.15 g/kg intravenously), blood levels of acetaldehyde were significantly higher in nonabstinent alcoholics than in controls. After 2 weeks of abstinence, this blood acetaldehyde response significantly decreased in alcoholics and the acetaldehyde returned towards levels comparable to those observed in nonalcoholics. These results suggest that elevated blood acetaldehyde levels in the alcoholics are secondary to the chronic alcohol consumption rather than reflecting a primary preexisting defect.

The reaction of acetaldehyde with brain microtubular proteins: formation of stable adducts and inhibition of polymerization

Stable adducts were formed by treatment of bovine brain microtubular proteins (MTP) with acetaldehyde, followed by gel filtration to remove excess acetaldehyde. The extent of stable adduct formation was determined using [14C]acetaldehyde and was correlated with acetaldehyde concentration and reaction time. Significant inhibition of MTP polymerization was observed at adduct concentrations of 0.6 mol acetaldehyde/mol tubulin dimer. The data suggest that repeated exposure of MTP to low concentrations of acetaldehyde, as would occur in the brain and other tissues of alcoholics, may inhibit MTP polymerization with neurological consequences.