Increased hepatic efflux of glutathione after chronic ethanol feeding

Glutathione depletion and recovery after acute ethanol administration in the aging mouse

Glutathione (GSH) plays an important role in the detoxification of ethanol (EtOH) and acute EtOH administration leads to GSH depletion in the liver and other tissues. Aging is also associated with a progressive decline in GSH levels and impairment in GSH biosynthesis in many tissues. Thus, the present study was designed to examine the effects of aging on EtOH-induced depletion and recovery of GSH in different tissues of the C57Bl/6NNIA mouse. EtOH (2-5 g/kg) or saline was administered i.p. to mice of ages 6 months (young), 12 months (mature), and 24 months (old); and GSH and cyst(e)ine concentrations were measured 0-24h thereafter. EtOH administration (5 g/kg) depleted hepatic GSH levels >50% by 6h in all animals. By 24h, levels remained low in both young and old mice, but recovered to baseline levels in mature mice. At 6h, the decrease in hepatic GSH was dose-dependent up to 3g/kg EtOH, but not at higher doses. The extent of depletion at the 3g/kg dose was dependent upon age, with old mice demonstrating significantly lower GSH levels than mature mice (P<0.001). Altogether these results indicate that aging was associated with a greater degree of EtOH and fasting-induced GSH depletion and subsequent impaired recovery in liver. An impaired ability to recover was also observed in young animals. Further studies are required to determine if an inability to recover from GSH depletion by EtOH is associated with enhanced toxicity.

Should a lower treatment line be used when treating paracetamol poisoning in patients with chronic alcoholism?: a case for

A lower threshold for treatment of paracetamol (acetaminophen) poisoning has been advocated in chronic heavy users of alcohol, based originally on animal studies indicating that chronic alcohol ingestion increased hepatotoxicity. This was attributed to increased production of the toxic metabolite, N-acetyl-p-benzoquinoneimine, by cytochrome P450 (CYP)2E1 induction. The clinical evidence for increased risk is limited to four retrospective studies with potential for referral and reporting bias and conflicting results. No study has specifically addressed the issue of the treatment threshold for acute paracetamol overdose in chronic alcohol users. However, animal studies in multiple species have consistently shown a lower dose of paracetamol is required to produce hepatotoxicity after chronic alcohol use. The knowledge of potential mechanisms has expanded to include effects of other alcohols, such as isopentanol, induction of CYP enzymes other than CYP2E1 and glutathione depletion. There are no convincing reasons or data to suggest these findings do not apply to humans. However, further human toxicokinetic and clinical research is required to quantify the extent of the interaction. Arguments about treating overdoses should not be confused with those about whether there is an alcohol-paracetamol interaction at therapeutic doses. Halving the threshold dose/concentration for treatment is a conservative educated guess that has been widely adopted. In overdose, the potential benefits of treatment at this lower threshold clearly outweigh the minimal risks of acetylcysteine.

Gamma glutamyl transferase

Serum gamma-glutamyl transferase (GGT) has been widely used as an index of liver dysfunction and marker of alcohol intake. The last few years have seen improvements in these areas and advances in understanding of its physiological role in counteracting oxidative stress by breaking down extracellular glutathione and making its component amino acids available to the cells. Conditions that increase serum GGT, such as obstructive liver disease, high alcohol consumption, and use of enzyme-inducing drugs, lead to increased free radical production and the threat of glutathione depletion. However, the products of the GGT reaction may themselves lead to increased free radical production, particularly in the presence of iron. There have also been important advances in the definition of the associations between serum GGT and risk of coronary heart disease, Type 2 diabetes, and stroke. People with high serum GGT have higher mortality, partly because of the association between GGT and other risk factors and partly because GGT is an independent predictor of risk. This review aims to summarize the knowledge about GGT's clinical applications, to present information on its physiological roles, consider the results of epidemiological studies, and assess how far these separate areas can be combined into an integrated view.

Interaction of 1-hydroxyethyl radical with antioxidant enzymes

There is considerable interest in the role of the 1-hydroxyethyl radical (HER) in the toxic effects of ethanol. The goal of this study was to evaluate the effects of HER on classical antioxidant enzymes. The interaction of acetaldehyde with hydroxylamine-o-sulfonic acid has been shown to produce 1, 1'-dihydroxyazoethane (DHAE); this compound appears to be highly unstable, and its decomposition leads to the generation of HER. Addition of DHAE into a solution of PBN led to the appearance of the typical EPR spectra of PBN/HER adduct. No PBN/HER spin adduct was detected when DHAE was incubated with 0.1 M PBN in the presence of GSH. In the absence of PBN, DHAE oxidized ascorbic acid to semidehydroascorbyl radical, presumably via an ascorbate-dependent one-electron reduction of HER back to ethanol. Catalase was progressively inactivated by exposure to DHAE-generated HER in a time and HER concentration-dependent manner. Ascorbic acid and PBN gave full protection to catalase against HER-dependent inactivation. The antioxidants 2-tert-butyl-4-methylphenol, propylgallate, and alpha-tocopherol-protected catalase against inactivation by 84, 88, and 39%, respectively. Other antioxidant enzymes were also sensitive to exposure to HER. Glutathione reductase, glutathione peroxidase, and superoxide dismutase were inactivated by 46, 36, and 39%, respectively, by HER. The results reported here plus previous results showing HER interacts with GSH, ascorbate, and alpha-tocopherol suggest that prolonged generation of HER in cells from animals chronically exposed to ethanol may lower the antioxidant defense status, thereby contributing to mechanisms by which ethanol produces a state of oxidative stress and produces toxicity.

Regulation of hepatic glutathione metabolism and its role in hepatotoxicity

The regulation of hepatic glutathione (GSH) metabolism have been reviewed. Key steps in the regulation of hepatic GSH are GSH biosynthesis, the GSH-redox-cycle, the cystathionine pathway, and the carrier-mediated export processes. Possible influences of xenobiotics on these different pathways are discussed. GSH fulfills several essential functions; detoxification of oxygen-derived free radicals, thioldisulfide exchange and storage and transfer of cysteine. GSH is present in all mammalian cells, but may be especially important for organs with intensive exposure to exogenous toxins such as the liver. Within the cell mitochondrial GSH is the main defense against physiological oxidative stress generated by cellular respiration and may be a critical target for toxins. Glutathione homeostasis of the organism is a highly complex process, which is predominantly regulated by the liver but also by skeletal muscle, lung and kidney.

Prooxidant and antioxidant hepatic factors in rats chronically fed an ethanol regimen and treated with an acute dose of lindane

While acute lindane treatment and chronic ethanol feeding to rats have been associated with hepatic oxidative stress, the possible roles of these stresses in the pathogenesis of hepatic lesions reported in acute lindane intoxication and in those observed in some models of chronic alcoholism have not been established. Our previous studies in rats chronically fed ethanol regimens and then treated with a single intraperitoneal (i.p.) dose of lindane (20 mg/kg) showed that while lindane per se was invariably associated with hepatic oxidative stress, chronic ethanol feeding only produced this stress when the dietary level of vitamin E was relatively low. Chronic ethanol pretreatment did not significantly affect the lindane-associated oxidative stress, and neither chronic ethanol feeding nor acute lindane, single or in combination, produced any histologic and biochemical evidence of liver damage. In the present experiment, the acute dose of lindane was increased to 40 mg/kg, and we have studied a larger number of prooxidant and antioxidant hepatic factors. Male Wistar rats (115.5 +/- 5.4 g) were fed ad lib for 11 weeks a calorically well-balanced and nutritionally adequate basal diet, or the same basal diet plus a 32% ethanol/25% sucrose solution, also ad lib, and were then injected i.p. with a single dose of lindane or with equivalent amounts of corn oil. The results indicated that acute lindane treatment to naive rats increased practically all the prooxidant hepatic factors examined (cytochromes P450 and b5, NADPH cytochrome c reductase, NADPH oxidase), as well as the generation of microsomal superoxide radical and thiobarbituric acid reactive substances of liver homogenates, but did not modify any of the antioxidant hepatic factors studied. Conversely, the chronic administration of ethanol alone did not significantly affect the prooxidant hepatic factors but reduced some of the antioxidants (i.e., the activities of GSH-Px and the contents of alpha-tocopherol and ubiquinols 9 and 10). Although chronic ethanol pretreatment further increased the superoxide generation induced by lindane per se, it did not increase but generally reduced the effects of lindane per se on the other prooxidant factors studied. Furthermore, although acute lindane administration to ethanol-pretreated rats was associated with decreases in GSH and catalase (not affected by ethanol or lindane treatment alone), it did not substantially modify the reducing effects of ethanol feeding per se on GSH-Px, alpha-tocopherol, and ubiquinols. Once again, neither chronic ethanol feeding nor lindane treatment, single or in combination, was associated with any evidence of liver damage.

In vitro effect of S-adenosyl methionine on ethanol embryopathy in the rat

S-adenosyl methionine (SAM) is a universal methyl donor for biological systems. Chronic consumption of ethanol results in depletion of available SAM and reduces its biosynthesis in the transmethylation pathway. Administration of excess SAM may reduce the embryopathic effects of ethanol. The in-vitro effects of SAM on ethanol embryopathy was investigated by culturing 9.5 day old whole rat embryos for 48 hours in ethanol alone (Group II), 0.05 mM SAM (Group III), ethanol + 0.05 mM SAM (Group IV), ethanol + 0.1 mM SAM (Group V), ethanol + 1 mM SAM (Group VI), and in ethanol + 3 mM SAM (Group VII). In Group VII embryos, cardiovascular, nervous, auditory, visual, craniofacial and musculoskeletal systems were retarded in development; crown-rump length, yolk-sac diameter, as well as morphological scores, were reduced compared to those in embryos treated with ethanol alone (Group II). There were, however, significant differences between Group II and Group IV embryos with respect to crown-rump length, yolk sac diameter and somite number. The mean crown-rump length, yolk sac diameter and somite number in Group II were 2.3 +/- 0.2, 2.8 +/- 0.3 and 22.4 +/- 3.5 respectively, compared to 2.6 +/- 0.2, 3.1 +/- 0.2 and 25.3 +/- 3.1 in Group IV. These results suggest that simultaneous administration of S-adenosyl methionine and ethanol may protect against the embryopathic effects of ethanol.

A physiologically based pharmacokinetic model for butadiene and its metabolite butadiene monoxide in rat and mouse and its significance for risk extrapolation

The gas 1,3-butadiene (BU) is an important industrial chemical and an environmental air pollutant. BU has been shown to be a weak carcinogen in the rat but a potent carcinogen in the B6C3F1 mouse. This species difference makes risk extrapolation to humans difficult and the underlying mechanism should be clarified before meaningful risk extrapolation to humans can be made. One possible explanation for the species differences in cancer response is that there are quantitative species differences in the formation of genotoxic epoxides. To investigate this possibility a physiologically based pharmacokinetic (pbpk) model for BU together with its first reactive metabolite 1,2-epoxybutene-3 (butadiene monoxide, BMO) was developed. Previously reported values on hepatic glutathione (GSH) turnover, depletion of hepatic GSH in rodents exposed to BU, and in vitro metabolic data of BU and BMO were included in the model, which incorporates intrahepatic first-pass hydrolysis of BMO and the ordered sequential, ping-pong mechanism to describe the enzyme kinetics of BMO-GSH conjugation. In vitro studies were carried out to obtain tissue: air partition coefficients of BU and BMO in rat tissue homogenates. The simulated pharmacokinetics of BU, BMO, and GSH agreed with previously published experimental observations in rat and mouse obtained in closed and open chamber experiments. According to the model, the internal dose of BMO (expressed either as the concentration in mixed venous blood or as the area under the concentration-time curve) is approximately 1.6 times higher in the mouse than in the rat for exposure to BU below 1000 ppm. At higher exposure levels, GSH depletion occurs in the mouse, but not in the rat, after about 6-9 h. This GSH depletion results in up to 2-3 times higher internal doses in the mouse than in the rat. The clear but relatively small species differences in body burdens of BMO indicated from our model can only partly explain the marked species difference in cancer response between mice and rats exposed to BU.

Glutathione homeostasis in rats chronically treated with ethanol. Evidence for an increased hepatic GSH export in vivo

The influence of chronic ethanol feeding to rats on the hepatic glutathione (GSH and GSSG) system (synthesis, catabolism, export) and on the GSH and GSSG concentrations in extrahepatic tissues was investigated. Histological examination of livers from ethanol pretreated rats revealed a minor dilatation of the hepatic sinusoids. After ethanol administration the distribution pattern of gamma-glutamyltranspeptidase (enzymehistochemistry) was nearly unchanged, but the hepatic activity of this enzyme was increased. The ethanol pretreatment led to a decrease in hepatic GSH content. The hepatic activity of the GSSG-reductase were increased after ethanol treatment whereas the activities of the GSH synthesizing enzymes (gamma-glutamyl-cysteinyl-synthetase and GSH-synthetase) were not affected. A strong increase in sinusoidal GSH export was found in the ethanol-pretreated rats. The GSH- and GSSG concentrations of brain, lung, kidney and skeletal muscle were unchanged. It can be concluded that the ethanol-induced alteration of the hepatic GSH metabolism is caused mainly by changes of the sinusoidal membrane of the hepatocytes (direct effect of ethanol on the sinusoidal GSH carrier) leading to an increased GSH export into plasma. This effect should not due to an increased extrahepatic requirement for GSH.

Cloning and nucleotide sequence of human liver cDNA encoding for cystathionine gamma-lyase

We have cloned and sequenced a full-length cDNA (1083 bp) encoding the human liver cystathionine-gamma-lyase enzyme (cystathionase). The human cystathionase sequence presented a substantial deletion of 132 bases (44 amino acids) compared to that reported for rat cystathionase, and of 135 bases (45 amino acids) compared to that reported for yeast cystathionase. After re-alignment for the missing nucleotides, the human cDNA sequence shows significant amino acid homology to that for the rat enzyme (85%) and the yeast enzyme (50%). A search for an undeleted cDNA, by the polymerase chain reaction, yielded a second clone which contained the missing 132 bases. Flanking nucleotides in the latter clone were identical to those in the cDNA clone containing the deletion. The two forms of human cystathionase deduced from the two cDNA clones may be derived from two different genes or may be splice variants.

Implication of free radical mechanisms in ethanol-induced cellular injury

Numerous experimental data reviewed in the present article indicate that free radical mechanisms contribute to ethanol-induced liver injury. Increased generation of oxygen- and ethanol-derived free radicals has been observed at the microsomal level, especially through the intervention of the ethanol-inducible cytochrome P450 isoform (CYP2E1). Furthermore, an ethanol-linked enhancement in free radical generation can occur through the cytosolic xanthine and/or aldehyde oxidases, as well as through the mitochondrial respiratory chain. Ethanol administration also elicits hepatic disturbances in the availability of non-safely-sequestered iron derivatives and in the antioxidant defense. The resulting oxidative stress leads, in some experimental conditions, to enhanced lipid peroxidation and can also affect other important cellular components, such as proteins or DNA. The reported production of a chemoattractant for human neutrophils may be of special importance in the pathogenesis of alcoholic hepatitis. Free radical mechanisms also appear to be implicated in the toxicity of ethanol on various extrahepatic tissues. Most of the experimental data available concern the gastric mucosa, the central nervous system, the heart, and the testes. Clinical studies have not yet demonstrated the role of free radical mechanisms in the pathogenesis of ethanol-induced cellular injury in alcoholics. However, many data support the involvement of such mechanisms and suggest that dietary and/or pharmacological agents able to prevent an ethanol-induced oxidative stress may reduce the incidence of ethanol toxicity in humans.

Glutathione metabolism and its role in hepatotoxicity

Glutathione (GSH) fulfills several essential functions: Detoxification of free radicals and toxic oxygen radicals, thiol-disulfide exchange and storage and transfer of cysteine. GSH is present in all mammalian cells, but may be especially important for organs with intense exposure to exogenous toxins such as the liver, kidney, lung and intestine. Within the cell mitochondrial GSH is the main defense against physiological oxidant stress generated by cellular respiration and may be a critical target for toxic oxygen and electrophilic metabolites. Glutathione homeostasis is a highly complex process, which is predominantly regulated by the liver, lung and kidney.

Can methionine and zinc prevent the embryopathic effects of alcohol?

Alcoholism is a major and world-wide health problem. A characteristic pattern of congenital defects, known as the fetal alcohol syndrome, has been detected in the offspring of mothers who are chronic alcoholics. The mechanism underlying the teratogenic effects of ethanol is not known. An important hypothesis is that ethanol becomes teratogenic by inducing maternal/fetal nutritional deficiencies. Nutrients affected by ethanol consumption include methionine and zinc. Epidemiological and experimental studies have shown that zinc deficiency is associated with both fetal growth retardation and developmental defects. Furthermore, the ethanol metabolizing enzyme, alcohol dehydrogenase, is a zinc metalloenzyme. Zinc deficiency decreases alcohol dehydrogenase activity and thus slows down elimination of ethanol. Methionine, an essential amino acid, is a lipotrope, as well as a universal methyl donor. It is involved in hepatic detoxification. Studies on the effects of methionine and zinc might lead to appropriate therapeutic intervention in pregnancies of alcoholic mothers who are at a risk of giving birth to an impaired offspring. Furthermore, the results of such investigations will provide a better understanding of the mechanism of action of ethanol on the embryo.

Impaired uptake of glutathione by hepatic mitochondria from chronic ethanol-fed rats. Tracer kinetic studies in vitro and in vivo and susceptibility to oxidant stress

Isolated hepatocytes incubated with [35S]-methionine were examined for the time-dependent accumulation of [35S]-glutathione (GSH) in cytosol and mitochondria, the latter confirmed by density gradient purification. In GSH-depleted and -repleted hepatocytes, the increase of specific activity of mitochondrial GSH lagged behind cytosol, reaching nearly the same specific activity by 1-2 h. However, in hepatocytes from ethanol-fed rats, the rate of increase of total GSH specific radioactivity in mitochondria was markedly suppressed. In in vivo steady-state experiments, the mass transport of GSH from cytosol to mitochondria and vice versa was 18 nmol/min per g liver, indicating that the half-life of mitochondrial GSH was approximately 18 min in controls. The fractional transport rate of GSH from cytosol to mitochondria, but not mitochondria to cytosol, was significantly reduced in the livers of ethanol-fed rats. Thus, ethanol-fed rats exhibit a decreased mitochondrial GSH pool size due to an impaired entry of cytosol GSH into mitochondria. Hepatocytes from ethanol-fed rats exhibited a greater susceptibility to the oxidant stress-induced cell death from tert-butylhydroperoxide. Incubation with glutathione monoethyl ester normalized the mitochondrial GSH and protected against the increased susceptibility to t-butylhydroperoxide, which was directly related to the lowered mitochondrial GSH pool size in ethanol-fed cells.

Glutathione and glutathione conjugate efflux from cultured liver cells

Efflux of glutathione (GSH) and GSH-conjugates from cultured rat liver epithelial cell lines; the non-tumorigenic ARL-15C1 and the gamma-glutamyl transpeptidase containing, tumorigenic ARL-16T2, has been assessed under basal condition and during chronic treatment with 75 and 150 microM ethacrynic acid (EA). The intracellular level of GSH increased in proportion to EA concentration during chronic exposure. The rates of GSH and GSH-EA conjugate efflux increased with intracellular GSH in both ARL cell lines. Glutathione-S-transferase activity measured with EA as substrate increased over the experimental time course after treatment with 150, but not 75 microM EA. When intracellular GSH content was increased by treatment with the cysteine pro-drug, 2-L-oxothiazolidine 4-carboxylic acid, the rate of GSH efflux was increased, but not the rate of GS-EA conjugate export. Inhibition of gamma-glutamyl transpeptidase by acivicin (AT-125) increased the GSH and GS-EA conjugate efflux rate in ARL-16T2 cells by factors of approximately 2 and 15, respectively. Acivicin treatment of ARL-16T2 cells chronically treated with EA elevated GSH efflux rate by 10-fold and GS-EA efflux by 40-fold versus control samples. These studies show that GSH and GSH conjugate efflux are accomplished as independently regulated processes. Efflux of GSH is enhanced by increased intracellular GSH, but increase in the conjugate transport rate requires the presence of the GSH conjugate. The response of the efflux process to treatment with a chronic GSH depleting agent was identical in two cell lines in which the metabolic fate of glutathione is known to differ fundamentally.

Antioxidant protection systems of rat lung after chronic ethanol inhalation

The effect of chronic ethanol administration on pulmonary antioxidant protection systems was investigated in male Sprague-Dawley rats exposed to room air or room air containing ethanol vapors for 5 weeks. Blood ethanol concentrations in ethanol-exposed rats were usually between 200 and 300 mg/dl. Glutathione, vitamin E, and malondialdehyde concentrations were measured in lung homogenates, and antioxidant enzyme activities (catalase, glutathione peroxidase, Cu/Zn-superoxide dismutase, glutathione reductase) were determined in the supernatant fractions. For comparison, the measurements were also made using liver fractions. Ethanol treatment increased the activities of catalase (117%) and Cu/Zn-superoxide dismutase (25%) in lung but not in liver. Although chronic ethanol inhalation lowered hepatic glutathione (19%) and hepatic vitamin E (33%), there was no increase in malondialdehyde content in either liver or lung of ethanol-exposed rats. The elevation of pulmonary antioxidant enzyme activities could be interpreted to mean that lung is a target for ethanol-induced oxidative stress. However, as there was no loss of pulmonary GSH or vitamin E and no increase in malondialdehyde formation, it appears that long-term ethanol exposure did not produce a significant degree of oxidative stress in rat lung.

Increased plasma levels of glutathione and malondialdehyde after acute ethanol ingestion in humans

The effect of acute ethanol consumption on plasma glutathione (GSH) and malondialdehyde (MDA) concentrations was studied in two groups of healthy male subjects. The first group (n = 15) received an acute dose of ethanol (1.5 g/kg p.o. over a period of 3 h); in the control group (n = 15), ethanol was replaced isocalorically with carbohydrates. Blood samples were taken at 0 time (ethanol/carbohydrates ingestion) and every 60 min for 6 h. A significant increase in plasma MDA concentration as well as in plasma GSH values were observed in subjects receiving ethanol compared to controls. The enhancement of plasma GSH was accompanied by a concomitant increase of oxidized glutathione (GSSG). These data support the hypothesis of an increase of lipid peroxidation as a possible mechanism of acute ethanol toxicity. The enhancement of plasma GSH and GSSG may reflect an increased utilization and loss of the tripeptide from the liver induced by ethanol.

Effects of ethanol on microsomal drug metabolism in aging female rats. I. Induction

The purpose of this study was to determine how aging affects the induction by ethanol or acetone of the hepatic microsomal monooxygenase system of female Fischer 344 rats. Young-adult, middle-aged and old rats (4, 14 and 25 months) were fed an ethanol-containing or control liquid diet for 15 days. Cytochrome P-450, cytochrome c reductase, aniline hydroxylase, nitrophenol hydroxylase, nitroanisole O-demethylase and benzphetamine N-demethylase activities were measured in hepatic microsomes. All of the drug metabolism activities except benzphetamine N-demethylase were 20-35% lower in old than in young-adult rats fed the control diet. In addition, the increase in drug metabolism produced by feeding the regular ethanol diet (36% of calories as ethanol) was 50-60% lower in the old rats. However, there was no difference in the magnitude of ethanol induction when ethanol intakes were matched. The effects of chronic acetone consumption (1.2g/day per kg body weight for 15 days) paralleled those of ethanol consumption, except that the extent of induction was greater with acetone. Acetone-induced levels of hepatic microsomal cytochrome P-450, nitrophenol hydroxylase, nitroanisole O-demethylase and aniline hydroxylase were similar in all three age groups. The results of this study indicate that induction of hepatic microsomal drug metabolism by ethanol or acetone is unaffected by the aging process.

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 chronic ethanol feeding on rat hepatocytic glutathione. Relationship of cytosolic glutathione to efflux and mitochondrial sequestration

Chronic ethanol feeding to rats increases the sinusoidal component of hepatic glutathione (GSH) efflux, despite a lower steady-state GSH pool size. In the present studies, no increase of biliary GSH efflux in vivo was found in chronic ethanol-fed cells. Studies were performed on ethanol-fed and pair-fed cells to identify the kinetic parameters of cellular GSH concentration-dependent efflux. The relationship between cytosolic GSH and the rate of efflux was modeled by the Hill equation, revealing a similar Vmax, 0.22 +/- 0.013 vs. 0.20 +/- 0.014 nmol/min per 10(6) cells for ethanol-fed and pair-fed cells, respectively, whereas the Km was significantly decreased (25.3 +/- 2.3 vs. 33.5 +/- 1.4 nmol/10(6) cells) in ethanol-fed cells. The difference in Km was larger when the data were corrected for the increased water content in ethanol-fed cells. We found a direct correlation between mitochondria and cytosolic GSH, revealing that mitochondria from ethanol-fed cells have less GSH at all cytosolic GSH values. The rate of resynthesis in depleted ethanol-fed cells in the presence of methionine and serine was similar to control cells and gamma-glutamylcysteine synthetase remained unaffected by chronic ethanol. However, the reaccumulation of mitochondrial GSH as the cytosolic pool increased was impaired in the ethanol cells. The earliest time change in GSH regulation was a 50% decrease in the mitochondrial GSH at 2 wk.

Biochemical basis for alcohol-induced liver injury

Chronic ethanol ingestion leads to hepatocellular injury and alcoholic liver disease (ALD) only if multiple factors combine to favor centrilobular hepatocellular hypoxia. It is hypothesized that these factors include a shift in the redox state, the induction of the microsomal ethanol oxidizing system (MEOS), a high blood alcohol level (BAL), a high polyunsaturated fat diet and episodic decreased O2 supply to the liver. The shift in the redox state favors a low cellular pH, decreased fatty acid oxidation and increased triglyceride formation. The increased MEOS activity increases O2 consumption and portal-central O2 gradient as well as favors acetaldehyde toxic effects including retention of hepatic lipids and export proteins causing cell swelling. The resultant increase in the concentration of acetaldehyde and lactate may stimulate fibrosis as they stimulate collagen synthesis in vitro. The resultant fatty liver narrows the sinusoids slowing sinusoid blood flow. The combination of events reduces available O2 leading to decreased levels of ATP and cellular pH making the liver vulnerable to episodes of systemic hypoxia. The role of membrane changes are reviewed, i.e., 1) membrane fluidity as related to changes in the species of phospholipids, 2) mitochondrial function as related to the changes in the lipid environment of the electron transport chain, and 3) linoleic acid-prostaglandin metabolism. Acute ethanol in vitro has been shown to affect liver cell metabolism regulation by triggering and increasing protein phosphorylation through the Ca2+-phospholipase C pathway. A high fat diet enhances the liver injury caused by chronic ethanol ingestion.

Ethanol decreases plasma sulphydryls in man: effect of disulfiram

Based on animal experiments, interactions of ethanol and its metabolites with sulphydryls have been implicated in the toxicity of ethanol, but acute effects of ethanol on sulphydryls have not been documented in man. Plasma free glutathione and cysteine were therefore measured following the administration of 0.2 g kg-1 ethanol to normal healthy volunteers and chronic alcoholics on disulfiram, where the effects of high concentrations of acetaldehyde can be observed. In both groups, plasma glutathione decreased shortly following ethanol, and a sustained decreased in glutathione was seen in the subjects on disulfiram. In patients on disulfiram, but not the healthy controls, plasma cysteine decreased significantly. The decrease in plasma cysteine was correlated to the rise in acetaldehyde, suggesting that cysteine, but not glutathione, forms an adduct with acetaldehyde in man. We conclude that even moderate doses of ethanol may disturb the sulphydryl homeostasis and could interfere with biologically important processes that depend on sulphydryl groups.

Effect of chronic ethanol feeding on rat hepatocytic glutathione. Compartmentation, efflux, and response to incubation with ethanol

Hepatocytes from rats that were fed ethanol chronically for 6-8 wk were found to have a modest decrease in cytosolic GSH (24%) and a marked decrease in mitochondrial GSH (65%) as compared with pair-fed controls. Incubation of hepatocytes from ethanol-fed rats for 4 h in modified Fisher's medium revealed a greater absolute and fractional GSH efflux rate than controls with maintenance of constant cellular GSH, indicating increased net GSH synthesis. Inhibition of gamma-glutamyltransferase had no effect on these results, which indicates that no degradation of GSH had occurred during these studies. Enhanced fractional efflux was also noted in the perfused livers from ethanol-fed rats. Incubation of hepatocytes in medium containing up to 50 mM ethanol had no effect on cellular GSH, accumulation of GSH in the medium, or cell viability. Thus, chronic ethanol feeding causes a modest fall in cytosolic and a marked fall in mitochondrial GSH. Fractional GSH efflux and therefore synthesis are increased under basal conditions by chronic ethanol feeding, whereas the cellular concentration of GSH drops to a lower steady state level. Incubation of hepatocytes with ethanol indicates that it has no direct, acute effect on hepatic GSH homeostasis.

Effects of ethanol feeding and withdrawal on plasma glutathione elimination in the rat

Chronic ethanol feeding increases hepatic turnover and sinusoidal efflux of glutathione in rats. The present study was performed to determine whether the observed increase in glutathione efflux was due to increased extrahepatic requirements for glutathione. The concentration and disposition of plasma glutathione were determined in rats fed liquid diets containing 36% of calories as ethanol or pair-fed an isocaloric mixture with carbohydrate replacing ethanol calories for 5 to 8 weeks. The half-life and plasma clearance of [35S]glutathione were found to be similar in ethanol-fed and control rats and in rats withdrawn 24 hr from ethanol. Uptakes of the sulfur moiety of [35S]glutathione by kidney, jejunal mucosa, liver, lung, spleen, muscle and heart were also unchanged by ethanol feeding. The plasma glutathione concentration was significantly higher in ethanol-withdrawn rats 22.30 +/- 3.06 nmoles per ml (p less than 0.05) compared to pair-fed controls (13.51 +/- 2.04), while rats continuing to drink ethanol had intermediate levels (16.96 +/- 2.22). Plasma cysteine levels were slightly, but not significantly, higher in ethanol-fed rats. These findings suggest that increased sinusoidal efflux of glutathione in ethanol-fed rats is due to a direct effect of ethanol on hepatic glutathione transport and not due to an alteration in extrahepatic disposition of glutathione. In order to characterize further the effects of ethanol feeding on glutathione-dependent detoxification, activities of glutathione S-transferase, glutathione reductase and gamma-glutamyltransferase were determined.(ABSTRACT TRUNCATED AT 250 WORDS)