Rate-limiting factors in ethanol oxidation by isolated rat-liver parenchymal cells. Effect of ethanol concentration, fructose, pyruvate and pyrazole

The fructose-dependent acceleration of ethanol metabolism

The aim of the present study was to elucidate how fructose is able to increase the rate of ethanol metabolism in the liver, an observation previously termed the fructose effect. Previous studies suggest that an increase in ATP consumption driven by glucose synthesis from fructose stimulates the oxidation of NADH in the mitochondrial respiratory chain, allowing faster oxidation of ethanol by alcohol dehydrogenase; however, this idea has been frequently challenged. We tested the effects of fructose, sorbose and tagatose both in vitro and in vivo. Both ethanol and each sugar were either added to isolated hepatocytes or injected intraperitoneally in the rat. In the in vitro experiments, samples were taken from the hepatocyte suspension in a time-dependent manner and deproteinized with perchloric acid. In the in vivo experiments, blood samples were taken every 15 min and the metabolites were determined in the plasma. These metabolites include ethanol, glucose, glycerol, sorbitol, lactate, fructose and sorbose. Ethanol oxidation by rat hepatocytes was increased by more than 50% with the addition of fructose. The stimulation was accompanied by increased glucose, glycerol, lactate and sorbitol production. A similar effect was observed with sorbose, while tagatose had no effect. The same pattern was observed in the in vivo experiments. This effect was abolished by inhibiting alcohol dehydrogenase with 4-methylpyrazole, whereas inhibition of the respiratory chain with cyanide did not affect the fructose effect. In conclusion, present results provide evidence that, by reducing glyceraldehyde and glycerol and fructose to sorbitol, respectively, NADH is consumed, allowing an increase in the elimination of ethanol. Hence, this effect is not linked to a stimulation of mitochondrial re-oxidation of NADH driven by ATP consumption.

Lactate-stimulated ethanol oxidation: Revisiting an old hypothesis

Liver slices from starved rats and incubated without other substrates oxidized ethanol at a rate of 4.1 µmols • h^-1 • g^-1. Addition of 10 mmols • L^-1 lactate increased this rate 2-fold. 4-methylpyrazole (4-MP), an alcohol dehydrogenase (ADH) inhibitor, drastically decreased the rate of ethanol oxidation, but did not inhibit the stimulation due to lactate. In the same context, liver acetaldehyde production, as the main by-product of ethanol oxidation, appeared to be much less inhibited by 4-MP in the presence of lactate. Aminotriazole (a catalase inhibitor), however, completely inhibited the stimulation. Furthermore, 2-hydroxybut-3-ynoate, an alpha-hydroxy acid oxidase inhibitor, completely abolished the stimulated ethanol oxidation promoted by lactate. Moreover, to determine the origin of the H₂O₂ produced, we did liver subcellular fractionation and then analyzed their content in peroxisomes, mitochondria and catalase. We observed that cytoplasm and peroxisomes appears to be the main producers of H₂O₂, and that the acceleration of ethanol oxidation by lactate is completely dependent on catalase. In conclusion, the H₂O₂ necessary to boost the catalase-dependent oxidation of ethanol appears to come from cytoplasm and peroxisomes, and is produced by the enzyme lactate oxidase.

Establishment of steady-state metabolism of ethanol in perfused rat liver: the quantitative analysis using kinetic mechanism-based rate equations of alcohol dehydrogenase

Alcohol dehydrogenase (ADH) catalyzes oxidation of ingested ethanol to acetaldehyde, the first step in hepatic metabolism. The purpose of this study was to establish an ex vivo rat liver perfusion system under defined and verified steady states with respect to the metabolites and the metabolic rates, and to quantitatively correlate the observed rates with simulations based on the kinetic mechanism-based rate equations of rat liver ADH. Class I ADH1 was isolated from male Sprague-Dawley rats and characterized by steady-state kinetics in the Krebs-Ringer perfusion buffer with supplements. Nonrecirculating liver perfusion with constant input of ethanol at near physiological hepatic blood flow rate was performed in situ. Ethanol and the related metabolites acetaldehyde, acetate, lactate, and pyruvate in perfusates were determined. Results of the initial velocity, product, and dead-end inhibition studies showed that rat ADH1 conformed to the Theorell-Chance Ordered Bi Bi mechanism. Steady-state metabolism of ethanol in the perfused liver maintained up to 3h as evidenced by the steady-state levels of ethanol and metabolites in the effluent, and the steady-state ethanol disappearance rates and acetate production rates. The changes of the metabolic rates were qualitatively and in general quantitatively correlated to the results from simulations with the kinetic rate equations of ADH1 under a wide range of ethanol, in the presence of competitive inhibitor 4-methylpyrazole and of uncompetitive inhibitor isobutyramide. Preliminary flux control analysis estimated that apparent C(ADH)(J) in the perfused liver may approximate 0.7 at constant infusion with 1-2 mM ethanol, suggesting that ADH plays a major but not the exclusive role in governing hepatic ethanol metabolism. The reported steady-state rat liver perfusion system may potentially be applicable to other drug or drug-ethanol interaction studies.

Human class IV alcohol dehydrogenase: kinetic mechanism, functional roles and medical relevance

Human alcohol dehydrogenase (ADH) constitutes a complex family. Class IV ADH (ADH4) is characteristic in its epithelial expression in the aerodigestive tract and high V(max) and K(m) for oxidation of ethanol. ADH4 exhibits the highest catalytic efficiency for retinol oxidation in human ADH family. Initial velocity, product inhibition, and dead-end inhibition studies indicate that ADH4, when functioning as ethanol dehydrogenase, conforms to an ordered sequential mechanism with coenzyme binding first and releasing last in catalytic cycle. When functioning as retinol dehydrogenase, the mechanism of ADH4 deduced from steady-state kinetic and equilibrium-binding studies is best described as a rapid equilibrium random mechanism with two dead-end ternary complex for retinol oxidation and a rapid equilibrium ordered mechanism with one dead-end ternary complex for retinal reduction, a unique mechanistic form for zinc-containing ADHs in the medium chain dehydrogenase/reductase superfamily. Kinetic and genetic studies support the proposal that ADH4 may play two important physiological roles, i.e., as a major contributor to first-pass metabolism of ethanol in stomach as well as involvement in the synthesis of retinoic acid, a hormonal ligand controlling a nuclear receptor signaling pathway that regulates growth, development, and epithelial maintenance. Quantitative simulation studies indicate that retinol metabolism through ADH pathway can be inhibited to a significant extent during alcohol consumption. The perturbation of retinoic acid synthesis by ethanol may underlie the pathogenesis of fetal alcohol syndrome and alcohol-related upper digestive tract cancer.

Effect of magnesium ions on fermentative and respirative functions in Pichia stipitis under oxygen-restricted growth

Mg2+ level affected growth, xylitol and ethanol production by P. stipitis grown under microaerophilic conditions. Low Mg2+ level (1 mM) directed the C flux from ethanol to xylitol, with no effect on xylose consumption rate. The addition of pyrazole, an alcohol dehydrogenase (ADH) inhibitor, had the same effect, even in conditions of Mg2+ excess (4 mM), indicating a negative interaction between ADH and Mg2+ ions (p << 0.01). Cells grown either with pyrazole or Mg limitation increased their intracellular NADH concentration about 3 times, but displayed no significant differences in ADH specific activities (1,000 U/mg protein, +/- 10%). In contrast, no interaction was measured between Mg and antimycin A, excluding the possibility that Mg2+ limitation interferes with respiration.

Impairment of the asialoglycoprotein receptor by ethanol oxidation

It is well established that ethanol exposure impairs the process of receptor-mediated endocytosis in hepatic cells, although the molecular mechanism(s) and the physiological consequence(s) of this impairment are unclear. Because addressing these mechanistic questions is difficult in vivo, we have developed a recombinant cell line of hepatic origin capable of metabolizing ethanol. In this study, we have used these recombinant cells, designated HAD cells, to investigate the ethanol-induced impairment to the receptor-mediated endocytosis of the hepatic asialoglycoprotein receptor. Comparing the binding of the ligand asialoorosomucoid in both the parental Hep G2 cells and the recombinant HAD cells, maintained in the presence and absence of ethanol, revealed decreased ligand binding in the HAD cells. This impairment was accentuated by prolonging the ethanol exposure, reaching approximately 40% in both surface and total receptor populations by 7 days. Addition of the alcohol dehydrogenase inhibitor pyrazole to the ethanol-containing medium abolished this impairment, indicating that the decreased binding was a result of the alcohol dehydrogenase-mediated oxidation of ethanol. Furthermore, using antibody specific to the asialoglycoprotein receptor, it was demonstrated that the ethanol-induced impairment in ligand binding was a consequence of decreased ligand binding and not a result of diminished receptor numbers. These results indicated that ethanol oxidation was required for the ethanol-induced impairment in ligand binding, and that the reduced ligand binding was a result of a decrease in the ability of the ligand to bind to the receptor.

Acetylcarnitine-mediated inhibition of ethanol oxidation in hepatocytes

Carnitine-mediated prevention of ethanol-induced hepatic steatosis is related to the attenuation of ethanol metabolism by carnitine in the intact rat. Although carnitine retards ethanol oxidation in the intact animal, the in vitro activities of ethanol-metabolizing enzymes remain unaltered. Therefore, hepatocytes were targeted to understand the mechanism of carnitine effect on ethanol metabolism. Rat hepatocytes were isolated by a collagenase-perfusion technique and incubated in albumin-containing medium with ethanol in the presence or absence of added carnitine or related compounds. Ethanol oxidation was determined by the loss of ethanol as well as by the products formed. The rate of ethanol oxidation in the presence of carnitine was one-half the rate in the absence of carnitine (14 vs. 25 nmol.min-1.million-1 cells). It took 100 times the concentration of carnitine to equal the maximal inhibition produced by acetylcarnitine and the effect of acetylcarnitine was without a lag time. It is concluded that acetylcarnitine is the mediator of carnitine inhibition of ethanol oxidation.

Effect of ethanol on lipoprotein secretion in two human hepatoma cell lines, HepG2 and Hep3B

The two human hepatoma cell lines, HepG2 and Hep3B, have been demonstrated to metabolize ethanol efficiently even in the absence of alcohol dehydrogenase. By using specific metabolic inhibitors, it was found that the microsomal ethanol-oxidizing system (MEOS) plays a significant role in ethanol metabolism in these two cell lines. There is a strong positive correlation between the rates of ethanol metabolism and the total cytochrome P-450 levels in the hepatoma cells. The involvement of the cytochrome P-450 system was further supported by the induction of aniline p-hydroxylase activity after ethanol treatment. However, the 3- to 4-fold elevation in aniline p-hydroxylase activity was not accompanied by an increase in cytochrome P450IIE1 mRNA level. Exposure of HepG2 and Hep3B cells to ethanol resulted in an increase of accumulation of apoA-I (15%-45% over control) in a dose-dependent manner (from 5 to 50 mM) of ethanol over a 24-hr period. All other major apolipoproteins which included apo CII, apo CIII and apoE, with the exception of apoB, were not affected by these treatments. At a concentration of ethanol of 25 mM or greater, accumulation of apoB, VLDL and LDL triglyceride were increased by 20% to 25% over the control level. Elevation of HDL cholesterol (40%-70% over control) was observed when the cells were exposed to an ethanol concentration of > or = 10 mM. Metyrapone, which inhibited the MEOS, was capable of blocking the induction of apoAI caused by ethanol treatment.

Novel 3-hydroxylated leukotriene b4 metabolites from ethanol-treated rat hepatocytes

Coincubations of radiolabeled leukotriene B4 (LTB4) and ethanol with isolated rat hepatocytes led to formation of one dihydroxylated and two novel β-oxidized metabolites of LTB4. The major radioactive peaks from reverse-phase-high performance liquid chromatography (RP-HPLC) eluted with material absorbing UV light maximally at 270 nm, with shoulders at 260 and 280 nm, indicating retention of the conjugated triene structure of the parent molecule in each metabolite structure. Following purification, catalytic reduction, and derivatization, mass spectrometric analysis revealed that all three metabolites were hydroxylated at the C-3 carbon atom based on characteristic ions at m/z 201 and 175 in the electron ionization mass spectra of the metabolites. Negative-ion electron capture mass spectrometry of the metabolites as pentafluorobenzyl (PFB) ester, trimethylsilyl ether derivatives aided structural characterizations while revealing interesting fragmentations. A ketene-containing ion appeared to result from the loss of both PFB groups (one as PFB alcohol), while a lactone alkoxide ion appeared to result following loss of PFB and bis (trimethylsilyl) ether. From these data three novel LTB4 metabolites were suggested to be 3,20-dihydroxy-LTB4 (3,20-diOH-LTB4), 3-hydroxy-18-carboxy-LTB4 (3-OH-18-COOH-LTB4), and 3-hydroxy-16-carboxy-LTB3 (3-OH-16-COOH-LTB3). The significance of the almost exclusive formation of these 3-hydroxylated LTB4 metabolites in the presence of ethanol is currently unknown, but may result from interrupted β-oxidation from the C-1 carboxyl moiety.

In vitro interaction between psychotropic drugs and alcohol dehydrogenase activity

A series of CNS-stimulating and -depressant drugs have been studied for their in vitro interaction with horse liver alcohol dehydrogenase (ADH) activity. The depressant drugs studied included barbital, phenobarbital, thiopental, nitrazepam, chlorpromazine, sulpiride, clomethiazole, Li2CO3, diazepam, phenytoin, ethosuximide, morphine, and codeine. The stimulant drugs were theophylline, caffeine, amphetamine, imipramine, chlorimipramine, amitriptyline, and tranylcypromine. The results were as follows. First, ADH activity was inhibited by the action of chlorpromazine, tranylcypromine, imipramine, chlorimipramine, amitriptyline, sulpiride, amphetamine, codeine, ethosuximide, morphine, clomethiazole, nitrazepam, Li2CO3, theophylline, and phenobarbital, in descending order of inhibitory effect. Second, inhibition followed by activation of ADH activity was observed for imipramine and chlorimipramine. Third, activation of ADH activity was observed for phenytoin. Finally, the following drugs were not seen to exert any effect on ADH activity: barbital, thiopental, diazepam, and caffeine.

Iron mediates production of a neutrophil chemoattractant by rat hepatocytes metabolizing ethanol

Ethanol metabolism in hepatocytes is accompanied by release of a potent lipid chemoattractant for neutrophils. Production of the factor may initiate the inflammation associated with alcoholic hepatitis. In previous studies with a cytosol system from liver, production was blocked by iron chelators as well as by catalase and superoxide dismutase, suggesting the involvement of oxyradicals in formation of the chemoattractant. These studies have examined the role of iron in intact hepatocytes using cells from rats fed an iron-deficient diet, a control diet or a diet containing 3% carbonyl iron. The iron content averaged 1.4 nmol/mg protein in iron-deficient cells, 6.3 in controls and 135.3 in iron-loaded cells. Hepatocytes from all groups were established in primary culture and incubated with ethanol (10 mM); the medium was assayed for chemoattractant activity for human neutrophils. Cultures from chow-fed or iron-loaded animals produced chemoattractant as previously reported. By contrast, chemoattractant production was undetectable in the iron-deficient cultures. Addition of ferric citrate (10 microM) restored chemoattractant production while increasing cellular iron in the deficient cells less than 50% (to 2.3 nmol/mg protein). Addition of desferrioxamine mesylate to cultures of iron-loaded cells ablated chemoattractant production. The data provide evidence for the importance of hepatocellular iron in production of this alcohol-related lipid chemoattractant and suggest that a small intracellular pool of "free" iron plays a critical role.

Deuterium D(V/K) isotope effects on ethanol oxidation in hepatocytes: importance of the reverse ADH-reaction

The kinetic deuterium isotope effect, D(V/K), on ethanol oxidation was measured on hepatocytes from rat and pig by the radiometric competitive method using 14C-labelled ethanol containing deuterium in the (1-R)-position. The corrected D(V/K) values of 2.68 and 2.80 for rat and pig hepatocytes respectively were significantly different, suggesting differences in the amount of non-ADH ethanol oxidizing activity. The apparent isotope effects declined rapidly with time when acetaldehyde was present in the medium as a result of the reduction to ethanol of the [14C]-acetaldehyde formed from the double labelled ethanol by alcohol dehydrogenase (ADH). Fructose and cyanamide caused the acetaldehyde concentration during ethanol oxidation to increase by entirely different mechanisms, and the isotope effect to decrease with time, as did also the addition of acetaldehyde. The apparent first order rate constant for the reverse ADH reaction, assuming the reactants to be acetaldehyde and the ADH-NADH complex, was determined by two methods giving comparable results. In the presence of semicarbazide, which removes acetaldehyde, the isotope effect was nearly constant. This was the case also when the acetaldehyde concentration was very low (less than 1 microM) for other reasons, as in hepatocytes from starved animals. A mathematical formula describing the expected decrease of the apparent isotope effect with time was derived. The different response of pig and rat hepatocytes to addition of fructose (the 'fructose effect') is suggested to be caused by differences in activity of aldehyde dehydrogenases in the two species.

Generation of chemotactic activity for neutrophils by liver cells metabolizing ethanol

The mechanism of polymorphonuclear leukocyte (PMN) infiltration of the liver in acute alcohol-related liver injury is unknown. We have reported that ethanol metabolism by hepatocytes incubated with moderate concentrations of ethanol (2-50 mM) results in the release into the medium of a chemoattractant for human PMN. This response to ethanol is time- and concentration-dependent with peak activity at 10 mM ethanol. Generation of the factor is specific for hepatocytes and is blocked by inhibiting ethanol metabolism with 4-methylpyrazole. It does not appear to be due to cell death. The activity has been partially characterized: it behaves as a polar lipid, possibly an arachidonic acid metabolite distinct from leukotriene B4. Preliminary studies indicate that a cell-free system derived from liver generates a similar activity. In that system scavengers of oxygen-derived free radicals block production of the factor.

Oxidation of pyrazole to 4-hydroxypyrazole by intact rat hepatocytes

4-Hydroxypyrazole has been identified as a major metabolite found in the urine of rats and mice after in vivo administration of pyrazole, a potent inhibitor of alcohol dehydrogenase and of ethanol metabolism. The locus and the enzyme systems responsible for the oxidation of pyrazole have not been identified. In the current report, isolated hepatocytes from fed rats were shown to oxidize pyrazole to 4-hydroxypyrazole. An HPLC procedure employing UV and electrochemical detection was utilized to separate and quantify the 4-hydroxypyrazole. The apparent Km for pyrazole by intact hepatocytes was about 2 mM, whereas the apparent Vmax was about 0.06 nmol 4-hydroxypyrazole per min per mg liver cell protein. The production of 4-hydroxypyrazole was inhibited by carbon monoxide and metyrapone, as well as by competitive drug substrates such as aniline or aminopyrine. These results implicate a role for cytochrome P-450 in the oxidation of pyrazole by the hepatocytes. Ethanol was an effective inhibitor of pyrazole oxidation. Hepatocytes were also isolated from rats treated with acetone and 4-methylpyrazole, to attempt to evaluate whether pyrazole oxidation is induced. The rate of 4-hydroxypyrazole production by hepatocytes after acetone and 4-methylpyrazole treatment was actually lower than that of controls. Kinetic assays suggested the presence of an endogenous inhibitor (perhaps the inducer itself) in the induced hepatocytes. In contrast, hepatocytes isolated from rats fasted for 48 hr showed a 2-fold increase in the oxidation of pyrazole to 4-hydroxypyrazole. The Km for pyrazole was the same in hepatocytes from fasted and fed rats, whereas Vmax was increased after fasting. The locus and enzyme system responsible for the oxidation of pyrazole to 4-hydroxypyrazole, and the site of sensitivity to ethanol, appears to be the cytochrome P-450 system of the hepatocyte.

Ethanol metabolism and hepatotoxicity. Does sex make a difference?

The role of gender as a variable that might affect the metabolism of ethanol and thus the hepatotoxicity of ethanol is evaluated. First, the pharmacodynamics of ethanol are reviewed with particular attention to hormone effects on ethanol absorption and metabolism. Specific differences between males and females relative to ethanol pharmacokinetic parameters are discussed, including gender differences in the volume of distribution and putative hormonal effects on achieved blood alcohol levels. In addition, attention is directed toward the metabolic capacity of alcohol dehydrogenase and the microsomal ethanol-oxidizing system with respect to effects of both sex differences and hormonal manipulations on the activity of these ethanol-metabolizing enzymes. Finally, the studies on the concept of sex-related differences in susceptibility to alcohol hepatotoxicity are examined.

International Commission for Protection against Environmental Mutagens and Carcinogens. ICPEMC Working Paper No. 15/2. Metabolism and metabolic effects of ethanol, including interaction with drugs, carcinogens and nutrition

Different pathways of alcohol metabolism, the alcohol dehydrogenase pathway, the microsomal ethanol-oxidizing system and the catalase pathway are discussed. Alcohol consumption leads to accelerated ethanol metabolism by different mechanisms including an increased microsomal function. Microsomal induction leads to interactions of ethanol with drugs, hepatotoxic agents, steroids, vitamins and to an increased activation of mutagens/carcinogens. A number of ethanol-related complications may be explained by the production of its first metabolite, acetaldehyde, such as alterations of mitochondria, increased lipid peroxidation and microtubular alterations with its adverse effects on various cellular activities, including disturbances of cell division. Nutritional factors in alcoholics such as malnutrition are discussed especially with respect to its possible relation to cancer.

In vivo and in vitro adenosine stimulation of ethanol oxidation by hepatocytes, and the role of the malate-aspartate shuttle

In this study, a pronounced increase of ethanol oxidation was found in hepatocytes obtained from adenosine-treated rats, or after in vitro additional of the nucleoside; this finding was accompanied by a maintenance of the normal cytoplasmic redox state. These results suggest a higher availability of cytoplasmic NAD in these cells. Therefore, the metabolic pathways which carry out the reoxidation of cytosolic reducing equivalents, namely, malate-aspartate and alpha-glycerophosphate shuttles, were examined. Isolated mitochondria from adenosine-treated rats had an increased NADH oxidation by the malate-aspartate shuttle; furthermore, in vivo and in vitro addition of adenosine to the hepatocytes induced changes in the equilibrium of the malate-aspartate shuttle, as evidenced by the subcellular distribution of the intermediates of this pathway. Acetaldehyde removal was also increased by adenosine and this fact was related to an elevated NAD/NADH ratio in the mitochondria. Thus, under these conditions, an increased ethanol uptake was accompanied by enhanced acetaldehyde removal in the animal. In conclusion, adenosine administration stimulates the transport of cytoplasmic reducing equivalents to the mitochondria, mainly through the malate-aspartate shuttle. This action, which may be located at the level of the mitochondrial membrane, is reflected by an enhancement of ethanol and acetaldehyde oxidations.

Body temperature influences on ethanol elimination rate

The effects of varying doses of ethanol injected IP on body temperature and the linear elimination rate were determined in long-sleep (LS) and short-sleep (SS) and C57BL6 mice. As the ethanol dose was increased, decreases in body temperature and rate of ethanol elimination were detected in all three mouse stocks. The correlation coefficients between body temperature and ethanol elimination rate were 0.74, 0.82, and 0.75 for the LS, SS, and C57BL mice, respectively. The SS eliminated ethanol more quickly than did the LS at most ethanol doses but if elimination rates at equal body temperatures were measured the two mouse lines did not differ in elimination rate. The temperatures of C57BL mice were also modified by pretreating them with graded doses of phenobarbital or DFP 1 hr before injection with a 1 g/kg ethanol dose. These treatments resulted in graded decreases in body temperature and an attendant decrease in ethanol elimination rate with correlation coefficients of 0.82 for phenobarbital-treated and 0.85 for DFP-treated animals. Lastly, the hypothermia elicited by a 5 g/kg dose of DFP was prevented by incubating the animals at 37 degrees C. This treatment almost completely prevented the effects of this dose of DFP on ethanol elimination rate. These results demonstrate that body temperature influences the rate of ethanol elimination. Therefore, studies of ethanol elimination rate should take body temperature into account when attempting to measure parameters such as metabolic tolerance.

A differential effect between the acute and chronic administration of ethanol on the endocytotic rate constant, ke, for the internalisation of asialoglycoproteins by hepatocytes

The endocytotic rate constant, ke, originally described for the quantification of epidermal growth factor by fibroblasts (Wiley, H.S. and Cunningham, D.D. (1982) J. Biol. Chem. 257, 4222-4229) has been adapted to measure receptor-mediated endocytosis of asialoglycoproteins by hepatocytes. A ke value of 0.21 min-1 was obtained for the internalisation of beta-D-galactosyl bovine serum albumin by freshly isolated hepatocytes. The addition of ethanol to the incubation medium had a biphasic effect on ke. The value of ke was increased by up to 30% by low concentrations of ethanol, whereas higher concentrations progressively decreased ke and in 500 mM ethanol the ke value was 0.1 min-1. The amount of ligand bound to the cell surface was independent of the extracellular concentration of ethanol and the changes in ke were exclusively due to changes in the amount of internalised ligand. There was a progressive decrease in the value of ke in hepatocytes prepared from rats that were maintained on an ethanol-impregnated liquid diet for up to 20 days. The decrease was already apparent by day 2 when blood alcohol levels were only 50 mg%, indicating that the effect of chronic alcoholism on endocytosis are manifested at an early stage.

Assessment of the role of non-ADH ethanol oxidation in vivo and in hepatocytes from deermice

Deermice genetically lacking alcohol dehydrogenase (ADH-) were used to quantitate the effect of 4-methylpyrazole (4-MP) on non-ADH pathways in hepatocytes and in vivo. Although primarily an inhibitor of ADH, 4-methylpyrazole was also found to inhibit competitively the activity of the microsomal ethanol-oxidizing system (MEOS) in deermouse liver microsomes. The degree of 4-MP inhibition in ADH- deermice then served to correct for the effect of 4-MP on non-ADH pathways in deermice having ADH (ADH+). In ADH+ hepatocytes, the percent contributions of non-ADH pathways were calculated to be 28% at 10 mM and 52% at 50 mM ethanol. When a similar correction was applied to in vivo ethanol clearance rates in ADH+ deermice, non-ADH pathways were found to contribute 42% below 10 mM and 63% at 40-70 mM blood ethanol. The catalase inhibitor 3-amino-1,2,4-triazole, while reducing catalase-mediated peroxidation of ethanol by 83-94%, had only a slight effect on blood ethanol clearance at ethanol concentrations below 10 mM, and no effect at all at 40-70 mM ethanol. These results indicate that non-ADH pathways (primarily MEOS) play a significant role in ethanol oxidation in vivo and in hepatocytes in vitro.

Depression of iron uptake from transferrin by isolated hepatocytes in the presence of ethanol is a pH-dependent consequence of ethanol metabolism

Incubation of freshly isolated rat hepatocytes with highly purified radiolabeled rat transferrin in weakly buffered medium in the presence of 10 mM ethanol resulted in a marked diminution of iron uptake by these cells, associated with a greater pH depression than in ethanol-free control studies. This effect on iron uptake persisted, even when the cells were preincubated for 90 min with ethanol before the addition of transferrin. Increasing the buffering capacity of the system or the addition of a metabolic inhibitor of alcohol dehydrogenase (4-methylpyrazole) returned iron uptake to control values. Acetaldehyde, acetate, lactate (products of ethanol metabolism), and 3-butanol (an alcohol not metabolized by alcohol dehydrogenase) had no influence on iron uptake. Further investigation of iron uptake over the pH range 6-8.5 revealed a marked dependency of iron uptake on the extracellular pH. Leucine incorporation into cell protein was also found to be pH dependent. It is suggested that, in the light of current understanding of transferrin recycling by other cell types, the disturbances of iron homeostasis observed in alcoholics can be partially accounted for by alterations in their acid-base metabolism.

Acute interaction of halothane and enflurane with the metabolism of ethanol in isolated hepatocytes and liver cytosol preparations from the rat

The effects of halothane and enflurane on ethanol (40 mM) oxidation were studied in isolated rat hepatocytes. Anaesthetic (halothane, enflurane and diethyl ether) effect on the activity of alcohol dehydrogenase (ADH) was studied in incubations of cytosol preparations from rat liver. Mean rates of ethanol metabolism ranged from 0.44 to 0.49 mumol ethanol metabolized/mg cell protein/hour in control hepatocytes from fasted and fed animals. These rates were enhanced by 2- and 3-fold in hepatocytes from fed and fasted animals, respectively, when pyruvate (5 mM) was added. Halothane and enflurane both caused dose dependent inhibition of ethanol metabolism (15-40%) in all hepatocytes without exogenous addition of pyruvate. The inhibitory effect was present also after pyruvate stimulation in hepatocytes from fasted animals, but disappeared in hepatocytes from fed animals when pyruvate was added. The rate of ethanol oxidation by cells from fed rats was enhanced by approximately 40% when the concentration of ethanol was increased from 20 mM to 80 mM. The anaesthetic inhibition of ethanol metabolism was about 20% more pronounced at the higher ethanol concentration compared to the lower concentration when no pyruvate was added. In the presence of pyruvate the effect of anaesthetics was again reversed regardless of ethanol concentration. Halothane (2 mM) and enflurane (2 mM) both caused about 25% inhibition of the ADH-activity in cytosol preparations while ether (30 mM) caused more than 50% inhibition. No inhibition of hepatocyte uptake of ethanol was caused by any of the three anaesthetics.(ABSTRACT TRUNCATED AT 250 WORDS)

In vivo roles of alcohol dehydrogenase (ADH), catalase and the microsomal ethanol oxidizing system (MEOS) in deermice

The relative importance of ADH and MEOS for ethanol oxidation in the liver has yet to be elucidated. The discovery of a strain of deermice genetically lacking ADH (ADH-) which can consume ethanol at greater than 50% of the rates seen in deermice having ADH (ADH+) suggested a significant role for non-ADH pathways in vivo. To quantitate contributions of the various pathways, we examined first the ethanol oxidation rates with or without 4-methylpyrazole in isolated deermice hepatocytes. 4-Methylpyrazole significantly reduced the ethanol oxidation in both ADH+ and ADH- hepatocytes. The reduction seen in ADH- cells can be applied to correct for the effect of 4-methylpyrazole on non-ADH pathways of ADH+ deermouse hepatocytes. After correction, non-ADH pathways were found to contribute 28% of ethanol metabolism at 10 mM and 52% at 50 mM. When using a different approach namely measurement of the isotope effect, MEOS was calculated to account for 35% at low and about 70% at high blood ethanol concentrations. Thus, we found that two different complementary approaches yielded similar results, namely that non-ADH pathways play a significant role in ethanol oxidation even in the presence of ADH.

Production of chemotactic activity for polymorphonuclear leukocytes by cultured rat hepatocytes exposed to ethanol

Acute alcoholic hepatitis is characterized by infiltration of the liver parenchyma with polymorphonuclear leukocytes. As a possible explanation for this phenomenon, we have found that ethanol stimulates cultured rat hepatocytes to generate potent chemotactic activity. Hepatocytes (greater than 99% pure), isolated from the livers of Sprague-Dawley rats, responded to incubation with ethanol (2.0-10 mM) by releasing chemotactic activity for human polymorphonuclear leukocytes into culture supernatants in a time- and concentration-dependent fashion. Chemotactic activity was maximal after incubation of hepatocytes with 10 mM ethanol for 6 h. It was undetectable in the absence of ethanol and was reduced in the presence of either the alcohol dehydrogenase inhibitor, 4-methylpyrazole, or the acetaldehyde dehydrogenase inhibitor, cyanamide. Ethanol failed to stimulate generation of chemotactic activity by either rat dermal fibroblasts, hepatic sinusoidal endothelial cells, or Kupffer cells. The chemotactic activity generated by ethanol-treated rat hepatocytes was recovered from culture supernatants in the lipid phase after extraction with chloroform/methanol. Thin-layer chromatography and high performance liquid chromatography of chloroform/methanol extracts demonstrated that the chemotactic factor probably is a polar lipid. This chemotactic lipid may account, in part, for the leukocytic infiltration of the liver parenchyma that is observed during the course of acute alcoholic hepatitis.

Kinetics of inhibition of ethanol metabolism in rats and the rate-limiting role of alcohol dehydrogenase

If liver alcohol dehydrogenase were rate-limiting in ethanol metabolism, inhibitors of the enzyme should inhibit the metabolism with the same type of kinetics and the same kinetic constants in vitro and in vivo. Against varied concentrations of ethanol, 4-methylpyrazole is a competitive inhibitor of purified rat liver alcohol dehydrogenase (Kis = 0.11 microM, in 83 mM potassium phosphate and 40 mM KCl buffer, pH 7.3, 37 degrees C) and is competitive in rats (with Kis = 1.4 mumol/kg). Isobutyramide is essentially an uncompetitive inhibitor of purified enzyme (Kii = 0.33 mM) and of metabolism in vivo (Kii = 1.0 mmol/kg). Low concentrations of both inhibitors decreased the rate of metabolism as a direct function of their concentrations. Qualitatively, therefore, alcohol dehydrogenase activity appears to be a major rate-limiting factor in ethanol metabolism. Quantitatively, however, the constants may not agree because of distribution in the animal or metabolism of the inhibitors. At saturating concentrations of inhibitors, ethanol is eliminated by inhibitor-insensitive pathways, at about 10% of the total rate at a dose of ethanol of 10 mmol/kg. Uncompetitive inhibitors of alcohol dehydrogenase should be especially useful for inhibiting the metabolism of alcohols since they are effective even at saturating levels of alcohol, in contrast to competitive inhibitors, whose action is overcome by saturation with alcohol.

Metabolism and metabolic effects of alcohol

The author provides an excellent overview of the three major pathways for the metabolism of ethanol. Many of the toxic effects of ethanol can be attributed to two specific products, hydrogen and acetaldehyde, and these effects are explored in detail.

Ethanol oxidation in systems containing soluble and mitochondrial fractions of rat liver. Regulation by acetaldehyde

Systems containing soluble fraction of rat liver, with or without mitochondrial fraction, oxidised [1-14C] ethanol to acetaldehyde, 14CO2 and non-volatile 14C-products of which acetate was the principal, and possibly the only, component. Ethanol oxidation was stimulated by pyruvate which served as an electron sink thereby allowing rapid regeneration of NAD. When no mitochondria were present acetaldehyde accumulated, rapidly at first but eventually reaching a plateau. The rate of ethanol oxidation in these systems was much lower than the measured maximum activity of alcohol dehydrogenase (ADH) and it was concluded that ADH was inhibited by the accumulated acetaldehyde. Mitochondria, because of their relatively high aldehyde dehydrogenase (ALDH) activity, prevented the accumulation of acetaldehyde, or quickly removed acetaldehyde already accumulated. This action was accompanied by a sharp increase in the rate of ethanol oxidation, presumably due to the deinhibition of ADH. Cyanamide, an inhibitor of mitochondrial ALDH, blocked the stimulatory effect of mitochondria on ethanol oxidation. It was concluded that, in the reconstituted systems, acetaldehyde played a dominant role in controlling the rate of ethanol oxidation. The possible importance of acetaldehyde in governing ethanol oxidation in vivo is discussed.

Accumulation of triacylglycerol in cultured rat hepatocytes is increased by ethanol and by insulin and dexamethasone

Isolated hepatocytes from female rats were cultured in HI-WO/BA medium for 6 days. To the medium was added oleate, ethanol, dexamethasone and insulin. With oleate To alone, triacylglycerol accumulated; ethanol augmented the accumulation by 90%. To the best of our knowledge, this is the first demonstration that ethanol in vitro increases the content of triacylglycerol in liver cells. Further addition of dexamethasone or insulin did not alter the accumulation of triacylglycerol, indicating that these hormones did not play permissive roles for the effect of ethanol in the present system. Dexamethasone and insulin, in the absence of ethanol, increased the accumulation of triacylglycerol by 30% and 50% respectively. The concentration of glycerol 3-phosphate was increased in the presence of ethanol; however, with time the concentration of glycerol 3-phosphate declined almost to control values, while the accumulation of triacylglycerol continued linearly; this suggests that the effect of ethanol was not mediated via fluctuations in the concentration of glycerol 3-phosphate. These results are discussed in relation to earlier investigations in vivo and in vitro.

Factors influencing rates of ethanol oxidation in isolated rat hepatocytes

The stimulation of ethanol oxidation by fructose which has frequently been observed in isolated hepatocytes was found to occur only in unsupplemented cells. In the presence of other substrates (lactate, pyruvate) which accelerate ethanol oxidation, fructose had no additional effect. Acceleration of ethanol oxidation by fructose was not directly related to the ATP demand created by fructose. The effects of fructose on ethanol oxidation rates were not due to changes in acetaldehyde concentration. In cells from fed animals, acetaldehyde concentrations rose as high as 200 microM in some incubations, and therefore became a significant factor limiting ethanol oxidation rates. In hepatocytes isolated from starved rats incubated with pyruvate, where acetaldehyde concentrations were very low, (1-2 microM) it was possible to assess the effect of changes in [lactate]/[pyruvate] (and hence free cytosolic NADH) on rates of ethanol oxidation. The results showed that the increase in free cytosolic [NADH] usually found during ethanol oxidation in vivo would inhibit rates of ethanol clearance by a maximum of 20%.

Metabolic heterogeneity in the perfused rat liver

New methods have been developed to monitor metabolic events non-invasively within periportal and pericentral regions of perfused rat livers. These techniques utilize two-fiber micro-light guides and miniature oxygen electrodes positioned on identified lobular regions of the perfused liver based on differential pigmentation of periportal and pericentral areas. Two-fiber micro-light guides detect the fluorescence of native and introduced fluors and are used to monitor redox changes of endogenous pyridine nucleotides and the generation of fluorescent products (e.g., 7-hydroxycoumarin) from exogenous substrates. Changes in fluorescence detected with two-fiber micro-light guides are correlated with changes measured with large, multi-fiber light guides and with whole organ rates of metabolism. Subsequently, local rates are estimated. With these techniques, we show that (a) rates of ethanol and acetaldehyde metabolism are similar in periportal and pericentral regions of the liver lobule; (b) mixed-function oxidation predominantes in pericentral regions in livers from phenobarbital-treated rats; (c) rates of sulfation of 7-hydroxycoumarin are greater in periportal than in pericentral hepatocytes; and (d) oxygen uptake is approximately 3-fold greater in periportal than in pericentral areas of the liver lobule.

Microsomal ethanol oxidizing system (MEOS): interaction with ethanol, drugs and carcinogens

Several studies in our unit showed that in men, baboons, rats and deermice, blood ethanol clearance is significantly accelerated at ethanol concentrations higher than the levels needed to effectively saturate the low Km forms of ADH present in animals, thereby incriminating a high Km non-ADH system such as microsomal ethanol oxidizing system (MEOS). Furthermore, kinetics of blood ethanol clearance were consistent with the Km of MEOS. After chronic ethanol consumption, there was an increase in rates of ethanol elimination and in the activity of MEOS. There was an associated rise in microsomal cytochrome P-450, including a form (different from that of a non-ADH pathway of ethanol metabolism and its increase after chronic ethanol consumption was most conclusively shown in ADH-negative deermice. Microsomal induction was also associated with enhanced metabolism of other drugs, resulting in metabolic drug tolerance. In addition, there was increased activation of known hepatotoxic agents (such as CCl4 and acetaminophen) which may explain the enhanced susceptibility of alcoholics to the toxicity of solvents and commonly used drugs. There was enhanced activation of procarcinogens, sometimes at concentrations much lower than those required for other microsomal inducers. Moreover, catabolism of retinoic acid was accelerated possibly contributing to hepatic vitamin A depletion. In conclusion, after chronic ethanol consumption, enhanced MEOS activity and concomitant cytochrome P-450 changes may contribute to accelerated ethanol and drug metabolism and associated activation of hepatotoxic agents and carcinogens.

Rapid oxidation of NADPH via the reconstituted malate-aspartate shuttle in systems containing mitochondrial and soluble fractions of rat liver: implications for ethanol metabolism

In an attempt to assess whether hydrogen shuttle capacity might serve as the rate-limiting factor in the hepatic oxidation of ethanol, the malate-aspartate shuttle was reconstituted in systems containing mitochondrial and soluble fractions of rat liver. Oxidation of NADH was stimulated slightly by the addition of either glutamate or malate but when both substrates were added the stimulation was far stronger. This effect was greatly enhanced by aspartate indicating that, when not added to the system, extramitochondrial aspartate was limiting. It was found that the rate of oxidation of NADH was directly related to the amount of mitochondrial protein present but extramitochondrial reactions became restrictive when the 'soluble protein/mitochondrial protein' ratio fell below 0.8. When calculated on a whole tissue basis the maximum rate of oxidation of NADH by the reconstituted shuttle was substantially higher than reported rates of ethanol oxidation in vivo. The results are discussed in relation to the normal control of ethanol metabolism.

Sex-dependency of hepatic alcohol metabolizing enzymes

In mature female rats the administration of testosterone led to a striking reduction of hepatic alcohol dehydrogenase activity, whereas the hepatic microsomal ethanol oxidizing system as well as catalase were both increased in activity under these experimental conditions. Conversely, estradiol left the activities of all hepatic alcohol metabolizing enzymes virtually unchanged. Ovariectomy also had little if any influence on the activity levels of the enzymes. There was a clear difference between the sexes in the hepatic alcohol metabolizing enzymes with higher enzymic activities of the microsomal ethanol oxidizing system and catalase in male than in female rats, whereas the opposite constellation was found for alcohol dehydrogenase activity. These data therefore indicate the sex-dependent nature of alcohol dehydrogenase, the hepatic microsomal ethanol oxidizing system and catalase activities in rat liver.

The effects of high ethanol doses on rates of ethanol oxidation in rats. A reassessment of factors controlling rates of ethanol oxidation in vivo

1. Ethanol was oxidised more slowly by rats which were given an ethanol dose of 5.1 g/kg than by rats which were given an ethanol dose of 1.4 g/kg. 2. A positive correlation was found between [lactate]/[pyruvate] ratios and rates of ethanol oxidation. 3. Acetaldehyde concentrations varied widely between rats, but in some cases were high enough to influence rates of ethanol oxidation. 4. Liver alcohol dehydrogenase levels were just sufficient to account for ethanol oxidation rates observed in vivo. 5. Pre-administration of a large ethanol dose (6.5 g/kg) did not alter mean [lactate]/[pyruvate] ratios or ethanol oxidation rates during metabolism of test doses of 2.5 g/kg. 6. Injection of pyruvate did not increase rates of ethanol oxidation. 7. The results do not support suggestions that a high-Km ethanol oxidising system plays an important role in vivo, that increased rates of ethanol oxidation can be induced by large, acute ethanol doses or that the rate of NADH reoxidation controls rates of ethanol metabolism. 8. The results support other evidence which has indicated that the level of alcohol dehydrogenase is the major factor limiting rates of ethanol oxidation in vivo.

Ether inhibition of ethanol metabolism in isolated rat liver parenchymal cells

The effect of diethyl ether on ethanol metabolism was studied in isolated rat hepatocytes and ether was found to inhibit ethanol oxidation in a dose-dependent manner. At ethanol concentrations of approximately 30 mM, diethyl ether inhibited ethanol oxidation by approximately 58%, 40%, and 20% at ether concentrations of 30, 20, and 10 mM, respectively. This inhibition was also seen at a low ethanol concentration (5.4 mM) and in pyruvate (5 mM)-stimulated hepatocytes which exhibited increased rates of ethanol metabolism closer to in vivo rates. Accumulation of acetaldehyde from ethanol in cyanamide (400 micron M)-treated hepatocyte suspensions was also reduced by approximately 16% by 30 mM ether. It was concluded that inhibition of ethanol metabolism by diethyl ether might be of practical importance in studies of ethanol metabolism in ether anesthetized animals.