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

Change in consumption patterns for treatment-seeking patients with alcohol use disorder post-bariatric surgery

OBJECTIVE

The aim of this study is to describe the clinical phenotype of alcohol use disorder (AUD) treatment-seeking patients with Roux-en-Y Gastric Bypass Surgery (RYGB) history; and to compare it to AUD obese non-RYGB controls.

METHODS

Retrospective study of electronic medical records for all patients 30-60years treated at the Mayo Clinic Addiction Treatment Program, between June, 2004 and July, 2012. Comparisons were performed with consumption patterns pre-RYGB and at time of treatment; excluding patients with AUD treatments pre-RYGB.

RESULTS

Forty-one out of 823 patients had a RYGB history (4.9%); 122 controls were selected. Compared to controls, the RYGB group had significantly more females [n=29 (70.7%) vs. n=35 (28.7%) p<0.0001]; and met AUD criteria at a significantly earlier age (19.1±0.4 vs. 25.0±1years old, p=0.002). On average, RYGB patients reported resuming alcohol consumption 1.4±0.2years post-surgery, meeting criteria for AUD at 3.1±0.5years and seeking treatment at 5.4±0.3years postoperatively. Pre-surgical drinks per day were significantly fewer compared to post-surgical consumption [2.5±0.4 vs. 8.1±1.3, p=0.009]. Prior to admission, RYGB patients reported fewer drinking days per week vs. controls (4.7±0.3 vs. 5.5±1.8days, p=0.02). Neither RYGB, gender, age nor BMI was associated with differential drinking patterns.

CONCLUSION

The results of this study suggest that some patients develop progressive AUD several years following RYGB. This observation has important clinical implications, calling for AUD-preventive measures following RYGB. Further large-scale longitudinal studies are needed to clarify the association between RYGB and AUD onset.

Contribution of liver alcohol dehydrogenase to metabolism of alcohols in rats

The kinetics of oxidation of various alcohols by purified rat liver alcohol dehydrogenase (ADH) were compared with the kinetics of elimination of the alcohols in rats in order to investigate the roles of ADH and other factors that contribute to the rates of metabolism of alcohols. Primary alcohols (ethanol, 1-propanol, 1-butanol, 2-methyl-1-propanol, 3-methyl-1-butanol) and diols (1,3-propanediol, 1,3-butanediol, 1,4-butanediol, 1,5-pentanediol) were eliminated in rats with zero-order kinetics at doses of 5-20 mmol/kg. Ethanol was eliminated most rapidly, at 7.9 mmol/kgh. Secondary alcohols (2-propanol-d7, 2-propanol, 2-butanol, 3-pentanol, cyclopentanol, cyclohexanol) were eliminated with first order kinetics at doses of 5-10 mmol/kg, and the corresponding ketones were formed and slowly eliminated with zero or first order kinetics. The rates of elimination of various alcohols were inhibited on average 73% (55% for 2-propanol to 90% for ethanol) by 1 mmol/kg of 4-methylpyrazole, a good inhibitor of ADH, indicating a major role for ADH in the metabolism of the alcohols. The Michaelis kinetic constants from in vitro studies (pH 7.3, 37 °C) with isolated rat liver enzyme were used to calculate the expected relative rates of metabolism in rats. The rates of elimination generally increased with increased activity of ADH, but a maximum rate of 6±1 mmol/kg h was observed for the best substrates, suggesting that ADH activity is not solely rate-limiting. Because secondary alcohols only require one NAD(+) for the conversion to ketones whereas primary alcohols require two equivalents of NAD(+) for oxidation to the carboxylic acids, it appears that the rate of oxidation of NADH to NAD(+) is not a major limiting factor for metabolism of these alcohols, but the rate-limiting factors are yet to be identified.

Acute and chronic ethanol exposure differentially alters alcohol dehydrogenase and aldehyde dehydrogenase activity in the zebrafish liver

Chronic ethanol exposure paradigms have been successfully used in the past to induce behavioral and central nervous system related changes in zebrafish. However, it is currently unknown whether chronic ethanol exposure alters ethanol metabolism in adult zebrafish. In the current study we examine the effect of acute ethanol exposure on adult zebrafish behavioral responses, as well as alcohol dehydrogenase (ADH) and aldehyde dehydrogenase (ALDH) activity in the liver. We then examine how two different chronic ethanol exposure paradigms (continuous and repeated ethanol exposure) alter behavioral responses and liver enzyme activity during a subsequent acute ethanol challenge. Acute ethanol exposure increased locomotor activity in a dose-dependent manner. ADH activity was shown to exhibit an inverted U-shaped curve and ALDH activity was decreased by ethanol exposure at all doses. During the acute ethanol challenge, animals that were continuously housed in ethanol exhibited a significantly reduced locomotor response and increased ADH activity, however, ALDH activity did not change. Zebrafish that were repeatedly exposed to ethanol demonstrated a small but significant attenuation of the locomotor response during the acute ethanol challenge but ADH and ALDH activity was similar to controls. Overall, we identified two different chronic ethanol exposure paradigms that differentially alter behavioral and physiological responses in zebrafish. We speculate that these two paradigms may allow dissociation of central nervous system-related and liver enzyme-dependent ethanol induced changes in zebrafish.

Dose-Dependent Change in Elimination Kinetics of Ethanol due to Shift of Dominant Metabolizing Enzyme from ADH 1 (Class I) to ADH 3 (Class III) in Mouse

ADH 1 and ADH 3 are major two ADH isozymes in the liver, which participate in systemic alcohol metabolism, mainly distributing in parenchymal and in sinusoidal endothelial cells of the liver, respectively. We investigated how these two ADHs contribute to the elimination kinetics of blood ethanol by administering ethanol to mice at various doses, and by measuring liver ADH activity and liver contents of both ADHs. The normalized AUC (AUC/dose) showed a concave increase with an increase in ethanol dose, inversely correlating with β. CL(T) (dose/AUC) linearly correlated with liver ADH activity and also with both the ADH-1 and -3 contents (mg/kg B.W.). When ADH-1 activity was calculated by multiplying ADH-1 content by its V(max⁡)/mg (4.0) and normalized by the ratio of liver ADH activity of each ethanol dose to that of the control, the theoretical ADH-1 activity decreased dose-dependently, correlating with β. On the other hand, the theoretical ADH-3 activity, which was calculated by subtracting ADH-1 activity from liver ADH activity and normalized, increased dose-dependently, correlating with the normalized AUC. These results suggested that the elimination kinetics of blood ethanol in mice was dose-dependently changed, accompanied by a shift of the dominant metabolizing enzyme from ADH 1 to ADH 3.

A new view of alcohol metabolism and alcoholism--role of the high-Km Class III alcohol dehydrogenase (ADH3)

The conventional view is that alcohol metabolism is carried out by ADH1 (Class I) in the liver. However, it has been suggested that another pathway plays an important role in alcohol metabolism, especially when the level of blood ethanol is high or when drinking is chronic. Over the past three decades, vigorous attempts to identify the enzyme responsible for the non-ADH1 pathway have focused on the microsomal ethanol oxidizing system (MEOS) and catalase, but have failed to clarify their roles in systemic alcohol metabolism. Recently, using ADH3-null mutant mice, we demonstrated that ADH3 (Class III), which has a high K(m) and is a ubiquitous enzyme of ancient origin, contributes to systemic alcohol metabolism in a dose-dependent manner, thereby diminishing acute alcohol intoxication. Although the activity of ADH3 toward ethanol is usually low in vitro due to its very high K(m), the catalytic efficiency (k(cat)/K(m)) is markedly enhanced when the solution hydrophobicity of the reaction medium increases. Activation of ADH3 by increasing hydrophobicity should also occur in liver cells; a cytoplasmic solution of mouse liver cells was shown to be much more hydrophobic than a buffer solution when using Nile red as a hydrophobicity probe. When various doses of ethanol are administered to mice, liver ADH3 activity is dynamically regulated through induction or kinetic activation, while ADH1 activity is markedly lower at high doses (3-5 g/kg). These data suggest that ADH3 plays a dynamic role in alcohol metabolism, either collaborating with ADH1 or compensating for the reduced role of ADH1. A complex two-ADH model that ascribes total liver ADH activity to both ADH1 and ADH3 explains the dose-dependent changes in the pharmacokinetic parameters (beta, CL(T), AUC) of blood ethanol very well, suggesting that alcohol metabolism in mice is primarily governed by these two ADHs. In patients with alcoholic liver disease, liver ADH3 activity increases, while ADH1 activity decreases, as alcohol intake increases. Furthermore, ADH3 is induced in damaged cells that have greater hydrophobicity, whereas ADH1 activity is lower when there is severe liver disease. These data suggest that chronic binge drinking and the resulting liver disease shifts the key enzyme in alcohol metabolism from low-K(m) ADH1 to high-K(m) ADH3, thereby reducing the rate of alcohol metabolism. The interdependent increase in the ADH3/ADH1 activity ratio and AUC may be a factor in the development of alcoholic liver disease. However, the adaptive increase in ADH3 sustains alcohol metabolism, even in patients with alcoholic liver cirrhosis, which makes it possible for them to drink themselves to death. Thus, the regulation of ADH3 activity may be important in preventing alcoholism development.

Enzymes of glycerol and glyceraldehyde metabolism in mouse liver: effects of caloric restriction and age on activities

The influence of caloric restriction on hepatic glyceraldehyde- and glycerol-metabolizing enzyme activities of young and old mice were studied. Glycerol kinase and cytoplasmic glycerol-3-phosphate dehydrogenase activities were increased in both young and old CR (calorie-restricted) mice when compared with controls, whereas triokinase increased only in old CR mice. Aldehyde dehydrogenase and aldehyde reductase activities in both young and old CR mice were unchanged by caloric restriction. Mitochondrial glycerol-3-phosphate dehydrogenase showed a trend towards an increased activity in old CR mice, whereas a trend towards a decreased activity in alcohol dehydrogenase was observed in both young and old CR mice. Serum glycerol levels decreased in young and old CR mice. Therefore increases in glycerol kinase and glycerol-3-phosphate dehydrogenase were associated with a decrease in fasting blood glycerol levels in CR animals. A prominent role for triokinase in glyceraldehyde metabolism with CR was also observed. The results indicate that long-term caloric restriction induces sustained increases in the capacity for gluconeogenesis from glycerol.

Enhanced rate of ethanol elimination from blood after intravenous administration of amino acids compared with equicaloric glucose

AIMS

To investigate the effect of an amino acid mixture given intravenously (i.v.) on the rate of ethanol elimination from blood compared with equicaloric glucose and Ringer's acetate as control treatments.

METHODS

In a randomized cross-over study, six healthy men (mean age 23 years) fasted overnight before receiving either Ringer's acetate, glucose or the amino acid mixture (Vamin 18 g N/l) by constant rate i.v. infusion over 4.5 h. Ethanol (0.4 g/kg) was given by an i.v. infusion lasting 60 min during the time each of the treatments was administered. At various times post-infusion, blood samples were taken for determination of ethanol by headspace gas chromatography. Blood glucose and heart rate were monitored at regular intervals. Concentration-time profiles of ethanol were plotted for each subject and the rate of ethanol disappearance from blood as well as other pharmacokinetic parameters were compared by repeated measures analysis of variance.

RESULTS

The rate of ethanol elimination from blood was increased significantly (P < 0.001) after treatment with amino acids (mean +/- SD, 0.174 +/- 0.011 g/l/h) compared with equicaloric glucose (0.121 +/- 0.016 g/l/h) or Ringer's acetate (0.110 +/- 0.013 g/l/h). Heart rate was also slightly higher during infusion of the amino acid mixture (P < 0.05).

CONCLUSIONS

When the rate of ethanol elimination from blood is relatively slow, such as after an overnight fast, it can be increased by approximately 60% after treatment with i.v. amino acids. The efficacy of amino acid treatment was not related to the supply of calories because glucose was no more effective than Ringer's acetate. We suggest that amino acids might increase hepatic oxygen consumption, resulting in a more effective conversion of NADH to NAD+ in mitochondria. An important feature of the experimental design was ensuring hepatic availability of amino acids during much of the time that ethanol was being metabolized.

Acute moderate hypoxia reduces ethanol elimination in the conscious rabbit

Hypoxic states decrease the catalytic activity of certain enzymes of the cytochrome P450, leading to decreased clearance of selected xenobiotics. It is not known whether hypoxia can affect the activity of cytosolic enzymes such as liver alcohol dehydrogenase (LADH). This study was carried out to examine the effect of experimentally induced acute moderate hypoxia on the kinetics of ethanol and on the catalytic activity of LADH in the conscious rabbit. To this purpose, the kinetics of elimination of ethanol (70 mg/kg intravenously) were studied in male rabbits (n = 6) kept in an atmosphere with a fractional concentration of O2 (FiO2) of 0.12 for 24 h, and in rabbits (n = 6) breathing room air with a FiO2 approximately 0.21 (controls). Compared with controls, the clearance of ethanol was reduced by 32% (P < 0.05) in rabbits exposed to acute moderate hypoxia. In rabbits that consumed approximately 6 g/kg/day ethanol in their drinking water for 14 days (n = 6), acute moderate hypoxia reduced the clearance of ethanol to the same extent as observed in rabbits receiving a single dose of ethanol. Hypoxia did not change the in vitro Vmax or Km of LADH. This study demonstrates that acute moderate hypoxia reduces the clearance of ethanol following single or multiple administrations.

Wheat gluten-based diet retarded ethanol metabolism by altering alcohol dehydrogenase and not carnitine status in adult rats

The objective of this study was to determine the effect of a lysine-deficient diet on carnitine status in adult rats and subsequently on ethanol metabolism. Adult male rats were fed either the AIN-76 diet (NS), the AIN-76 diet with wheat gluten (WG) replacing casein, the WG diet plus 0.8% L-lysine (LS), or the LS diet plus 0.5% L-carnitine (CS) for 30 days. On the 31st day the rats were given an oral dose of ethanol and blood-ethanol concentrations (BEC) were monitored for the next 8 hours. One week later the rats were given a second dose of ethanol and urine was collected until killed, 3 hours post-ethanol administration (PEA). Besides growth retardation and hypoproteinemia, BEC were significantly elevated in the WG group compared to the other group at hours 3-8 PEA. There were no significant differences in BEC between the LS and CS groups; however, their BEC were significantly higher than that of the NS group. The BEC were inversely related to liver alcohol dehydrogenase (ADH) activities which were significantly lower in WG, LS and CS groups than in the NS group. Plasma, liver and urine carnitine values were significantly higher in the CS group than in the NS, WG and LS groups, wherein the values were similar. It is concluded that the WG diet reduced ADH activity and attenuated ethanol metabolism without significantly altering blood, liver and urinary carnitines in the adult rat.

The effects of ethanol concentration on glycero-3-phosphate accumulation in the perfused rat liver. A reassessment of ethanol-induced inhibition of glycolysis using 31P-NMR spectroscopy and HPLC

The dose-dependent effect of ethanol on the hepatic metabolism of the perfused rat liver has been investigated by (a) 31P-NMR spectroscopy for the follow-up of intracellular phosphorylated metabolites and (b) HPLC for compounds released in the effluents. Perfusion of livers from fed rats with ethanol induced an increase in the level of sn-glycerol 3-phosphate and net accumulations of 3.30 +/- 0.33 and 0.69 +/- 0.15 mumol x g-1 wet liver were reached after 20 min, for 70 mM and 0.5 mM ethanol, respectively. sn-Glycerol-3-phosphate accumulation was fully detected by 31P NMR as indicated by comparing quantitations based on NMR and biochemical assays. Ethanol administration up to a concentration of 10 mM induced a dose-dependent decrease in the release of lactate + pyruvate by the liver. Lactate release decreased from 1129 +/- 39 to 674 +/- 84 nmol x min-1 x g-1, while pyruvate decreased from 230 +/- 9 to 6.2 +/- 0.4 nmol x min-1 x g-1, after 20 min of perfusion with 10 mM ethanol. Nevertheless, the flux through 6-phosphofructo-1-kinase, as measured by both the accumulation of sn-glycerol 3-phosphate and release of lactate + pyruvate, was not affected in the early phase of ethanol oxidation. Finally, data obtained from oxygen consumption, the release of acetate and the accumulation of sn-glycerol 3-phosphate do not support the involvement of the microsomal ethanol-oxidizing system in the catalysis of ethanol oxidation, even at high doses of alcohol.

The importance of alcohol dehydrogenase in regulation of ethanol metabolism in rat liver cells

We used titration with the inhibitors tetramethylene sulphoxide and isobutyramide to assess quantitatively the importance of alcohol dehydrogenase in regulation of ethanol oxidation in rat hepatocytes. In hepatocytes isolated from starved rats the apparent Flux Control Coefficient (calculated assuming a single-substrate irreversible reaction with non-competitive inhibition) of alcohol dehydrogenase is 0.3-0.5. Adjustment of this coefficient to allow for alcohol dehydrogenase being a two-substrate reversible enzyme increases the value by 1.3-1.4-fold. The final value of the Flux Control Coefficient of 0.5-0.7 indicates that alcohol dehydrogenase is a major rate-determining enzyme, but that other factors also have a regulatory role. In hepatocytes from fed rats the Flux Control Coefficient for alcohol dehydrogenase decreases with increasing acetaldehyde concentration. This suggests that, as acetaldehyde concentrations rise, control of the pathway shifts from alcohol dehydrogenase to other enzymes, particularly aldehyde dehydrogenase. There is not a single rate-determining step for the ethanol metabolism pathway and control is shared among several steps.

A new approach to the determination of ethanol elimination rate in vivo: an extension of Widmark's equation

In order to provide more experimental data about a single ethanol elimination sequence, tracer amounts of 14C-labelled ethanol were given intravenously in fed and fasted rats into a preexisting pool of unlabelled ethanol. The profiles of the elimination curves of 14C-labelled ethanol were distinctly different from those of unlabelled ethanol. This necessitated the elaboration of a mathematical model based on a two-compartment system, which by using the early distribution phase of the 14C-labelled ethanol and the linear part of the elimination curve of unlabelled ethanol enables the determination of the time-rate constants for distribution of ethanol, K12 and K21 and ethanol elimination rate Q. It is shown that the ratio K21'/(K21' + K12') is "r", the distribution volume of ethanol in the sense of Widmark, K12' is K12 divided with Va (initial distribution space of ethanol) and K21' is K21 divided with Vb (the peripheral compartment). The mean value +/- S.E.M. is 0.57 +/- 0.05 for fed rats and 0.49 +/- 0.03 for fasted. The slope of the time-concentration curve of ethanol, Widmark's beta, is shown to be K12'/(K12' x K21') x Q where Q is ethanol elimination rate. The mean elimination rate is 0.303 +/- 0.036 mmol x l-1 x min-1 in fed rats and 0.219 +/- 0.015 mmol x l-1 x min.-1 in fasted (P less than 0.05). It is concluded that we are now able to extend Widmark's equation by an independent determination of the distribution factor "r".

Dose-dependent decrease in rat plasma amino acids after acute administration of ethanol

Male rats were given three different doses of ethanol in i.p. injections (0.66, 1.33 and 2.00 g kg-1). A dose-dependent decrease in the concentrations of most plasma amino acids was observed. For the total amino acid concentration this decrease was 5, 16 and 22%, respectively, compared with a saline-treated control group. It has previously been suggested that the oxidation of ethanol plays an important role in the amino acid decreasing effect of ethanol. In this study the lowest dose used (0.66 g kg-1) was calculated to be high enough to keep the enzyme systems involved in ethanol oxidation saturated during the 60 min course of the experiment. The observation that the ethanol-induced decrease in plasma amino acid levels was more pronounced with higher ethanol doses indicates that not only the oxidation of ethanol but also ethanol itself is important in the effect of ethanol on plasma amino acid concentrations.

Swift increase in alcohol metabolism in humans

One hundred and fifteen human male subjects, 19-30 years of age, received ethanol orally as vodka (0.55, 0.7, or 0.85 g/kg) followed by a second drink (0.3-0.4 g/kg) given 3-4 hr later. After both doses, blood ethanol levels reached approximately 100 mg/dl. Breath samples were taken every 20-30 min and rates of ethanol elimination were determined. In addition to the design described above, 100 subjects received 0.7 g/kg ethanol in two separate visits to the laboratory. In a third experimental design, ethanol was given i.v. to 12 subjects. With the single-day experimental design, the frequency distribution of changes in rates of ethanol elimination between the first compared with the second administration of ethanol was not unimodal. Up to 20% of the subjects demonstrated rates more than 40% greater than basal values in response to ethanol. Based on these findings in humans, a Swift Increase in Alcohol Metabolism (SIAM) was defined as an increase in the rate of ethanol elimination of at least 40% over the basal rate. Under these conditions, the frequency of SIAM was dose dependent (studied with 0.55, 0.7, and 0.85 g/kg); nearly 20% of the subjects demonstrated SIAM with a dose of ethanol of 0.85 g/kg. In the two-day experimental design, a SIAM response was also observed in about 10% of 49 well-fed subjects; however, none of 51 subjects tested exhibited a SIAM response following an overnight fast. In addition, a rapid and transient SIAM reflecting a 60% increase in the rate of ethanol elimination above basal values was observed when ethanol was given continuously for 5 hr i.v.(ABSTRACT TRUNCATED AT 250 WORDS)

Roles of hormonal and nutritional factors in the regulation of rat liver alcohol dehydrogenase activity and ethanol elimination rate in vivo

Fasting reduced the liver alcohol dehydrogenase (ADH) activity by 51% (p less than 0.001). Insulin, within 2 hr, increased the ADH activity found in fasted animals by 28% (p less than 0.02). Insulin administration failed to stimulate the reduced ADH activity in diabetic rats. However, ADH activity in the diabetic-fed rats decreased by 52-54% (p less than 0.001) compared to normal-fed rats regardless of whether they were meal-fed or refed the normal chow. Glucagon blocked by 15% (p less than 0.02) the increase in ADH activity associated with refeeding. Furthermore, insulin caused a marginal stimulation of ethanol elimination rate (EER) when administered to fasted rats. All these results imply that insulin and glucagon may not be the only determining factors in the control of liver ADH activity associated with fasting and refeeding. Meal-feeding or refeeding a high carbohydrate fat-free diet compared to the normal chow-diet caused 29% (p less than 0.001) and 36% (p less than 0.05) decreases in ADH activity, respectively. Concomitant decreases in EER caused by high carbohydrate fat-free diet feeding were also observed under identical conditions. These results raise the possibility that the amount and the type of carbohydrate may be crucial in the regulation of ADH and EER. Alternatively, the presence of fat may be important in maintaining the normal level of ADH and EER.

Ethanol metabolism

Alcohol is metabolized by two pathways in humans: the ADH pathway which accounts for the bulk of the metabolism, and the MEOS pathway which contributes to the increased rate of ethanol elimination at high blood alcohol levels. The increased rate of elimination which results from chronic alcohol consumption is due to an increase in MEOS activity. The activities of these pathways are influenced by environmental factors such as smoking, diet, and endocrine factors. In addition, individuals inherit different types of ADH isoenzymes which have different kinetic properties. Individuals with different phenotypic variants, e.g. the beta 1 vs beta 2 isoenzymes, appear to have different rates of ethanol elimination. The cloning of the ADH genes and the availability of molecular hybridization methods now make it possible to genotype individuals and to correlate the genotype with both alcohol elimination rates and with the risk of developing medical complications of alcoholism or even of developing alcoholism itself.

Increased ketogenesis in hyperthyroid rats metabolizing ethanol

The levels of various metabolites were measured in freeze-clamped samples of liver from triiodothyronine-treated and control rats to which either saline or ethanol (2.5 g/kg body weight) had been administered 2 hours earlier. It was found that ethanol led to a sharp increase in the hepatic acetate concentration in both hyperthyroid and euthyroid rats whereas lactate and pyruvate concentrations were lowered in both groups. The lactate/pyruvate ratio rose significantly in euthyroid animals that had received ethanol but the ratio remained relatively low in hyperthyroid rats. The adenine nucleotide phosphorylation potential, already low in hyperthyroid rats, was further lowered by ethanol. However, the most remarkable difference between the responses of euthyroid and hyperthyroid rats to ethanol was in the hepatic concentrations of ketone bodies, particularly 3-hydroxybutyrate. In control animals, administration of ethanol did not affect either the acetoacetate or 3-hydroxybutyrate concentration but, although the level of ketone bodies in the livers of hyperthyroid rats that had not received ethanol was the same as that of controls, there was a greater than fivefold increase in the 3-hydroxybutyrate level when ethanol was given. While this increase in ethanol-dependent ketogenesis is not explicable at this stage, hyperthyroidism did not increase the activity of cytoplasmic acetyl-CoA synthetase, an enzyme that is probably involved in the formation of ketone bodies from ethanol-derived acetate.

The roles of the hepatocellular redox state and the hepatic acetaldehyde concentration in determining the ethanol elimination rate in fasted rats

Ethanol administration (2 g/kg i.p.) to fasted male Wistar rats caused, on average, a 64% decrease in the cytosolic free NAD+:NADH ratio and a 41% decrease in the mitochondrial free NAD+:NADH ratio measured 90 min after ethanol was injected. Treatment of animals with either Naloxone (2 mg/kg i.p.) 1 hr after ethanol or 3-palmitoyl-(+)-catechin (100 mg/kg p.o. 1 hr before ethanol) prevented these ethanol induced redox state changes, without affecting the ethanol elimination rate or the hepatic acetaldehyde concentration measured at 90 min after ethanol administration. The thiol compounds cysteine and malotilate (diisopropyl-1,3-dithiol-2-ylidene malonic acid) significantly lowered the hepatic acetaldehyde concentrations measured at 0.75, 1.5 and 6.0 hr after ethanol, and caused a 29% and 12% increase respectively in the ethanol elimination rate, without affecting the ethanol induced alterations in the NAD+:NADH ratio. Pretreatment of animals with the aldehyde dehydrogenase inhibitor, cyanamide (1 mg/kg or 15 mg/kg p.o. one hour before ethanol), caused increases of up to 23-fold in the hepatic acetaldehyde level, without influencing the cytosolic NAD+:NADH ratio in ethanol dosed rats, while significantly reducing the ethanol elimination rate by up to 44%, compared with controls. These results suggest that ethanol oxidation by cytosolic alcohol dehydrogenase may be regulated in part by the hepatic acetaldehyde concentration achieved during ethanol metabolism rather than NADH reoxidation, either to supply NAD for the dehydrogenase, or to reduce inhibition of the enzyme by NADH, being a rate-limiting factor in ethanol metabolism in fasted rats.

The effect of acute ethanol treatment on rates of oxygen uptake, ethanol oxidation and gluconeogenesis in isolated rat hepatocytes

In hepatocytes isolated from fed rats, acute ethanol pretreatment (at a dose of 5.0 g/kg body wt.) did not change rates of O2 uptake. In cells from starved animals, acute ethanol pretreatment increased O2 uptake by 17-29%. The increased O2 uptake in hepatocytes from starved rats was not accompanied by increased rates of ethanol oxidation, but was accompanied by increased rates of gluconeogenesis under some conditions. The provision of ethanol (10 mM) as a substrate to cells from fed or starved rats decreased O2 uptake in the absence of other substrates or in the presence of lactate, and increased it in the presence of pyruvate or lactate and pyruvate. The results of this study show that the acute effects of ethanol on liver O2 uptake are dependent on the physiological state of the liver. Previously reported large (2-fold) increases in O2 uptake after acute ethanol pretreatment may have been an artefact owing to low control uptake rates (approximately 1.8 micromol/min per g wet wt. of cells) in the liver preparation used. The ATP contents (2.4-2.6 micromol/g wet wt. of cells) and rates of O2 uptake (2.5-5.0 micromol/min per g wet wt. of cells) of cells used in the present study were the same as values reported under conditions close to those in vivo. Therefore the increase in O2 uptake in cells from starved rats after acute ethanol pretreatment is likely to be of physiological significance.

Incorporation of the 1-pro-R and 1-pro-S hydrogen atoms of ethanol in the reduction of acids in the liver of intact rats and in isolated hepatocytes

Ethanol oxidation causes redox effects. The coupling of this oxidation via NADH to intermediary metabolism was studied in order to reveal the underlying mechanisms. Isolated rat hepatocytes were incubated with [1,1-2H2]-, (1R)-[1-2H]- and (1S)-[1-2H]-ethanol and the 2H incorporation was measured in lactate, beta-hydroxybutyrate, fumarate, malate, succinate, alpha-oxoglutarate and citrate. The results differed in the following ways from results obtained in intact rats. Lactate became labelled to an increasing extent, and in more than one position, indicating labelling of pyruvate. A small and constant fraction of malate and fumarate was formed without access to [2H]coenzyme. Addition of aspartate increased this fraction, which was concluded to be formed in the mitochondria. Citrate was essentially unlabelled. The 2H from (1R)-[1-2H]ethanol contributed to malate to a larger extent and to beta-hydroxybutyrate to a smaller extent, and 2H from (1S)-[1-2H]ethanol contributed to lactate to a smaller extent. These results indicate that the exchange via shuttle system was less efficient in isolated hepatocytes than in intact rats. The 2H incorporation was independent of concentration of [1,1-2H2]ethanol when this was above 5mM. Additions known to increase ethanol elimination, and cyanamide, which decreases it, had no marked effect on the 2H incorporation. This indicates equilibration of the NADH bound to alcohol dehydrogenase with free NADH. Disulfiram and cyanamide caused a decrease in the relative incorporation from (1S)-[1-2H]ethanol into malate in liver of intact rats. Addition of cyanamide to incubations with hepatocytes resulted in a decrease of the contribution of 2H from (1S)-[1-2H]ethanol in lactate, beta-hydroxybutyrate and malate. This indicates that acetaldehyde was only oxidized in the mitochondrial compartment.

Comparison of ethanol metabolism in isolated periportal or perivenous hepatocytes: effects of chronic ethanol treatment

Ethanol metabolism in rat hepatocytes isolated either from the periportal (pp) or the perivenous (pv) area by collagenase gradient perfusion was compared to reveal metabolic factors that could be associated with the development of perivenous alcoholic liver damage. Cells were also isolated from rats given ethanol (E) chronically by addition to the drinking fluid. One group (EM) received in addition the alcohol dehydrogenase inhibitor 4-methylpyrazole, which potentiated the ethanol treatment by causing sustained elevated diurnal blood ethanol levels. Fatty degeneration ensued in only one-third of the E rats but in all of the EM rats. The periportal/perivenous activity distributions of alanine aminotransferase (ALAT) and glutamate dehydrogenase (GLDH) were 2.2 and 0.75, respectively. Both ethanol treatments significantly decreased the ALAT and increased the GLDH activities, but did not change their pp/pv distributions. Ethanol treatment also increased ethanol and acetaldehyde oxidation, but to the same extent in pp and pv cells. The increase was more marked in cells from EM rats despite their more severe liver fatty degeneration. Ethanol incubation also increased the lactate/pyruvate ratio to the same extent in pp and pv cells both from control or ethanol-treated rats. Our results indicate that periportal and perivenous hepatocytes convert ethanol via acetaldehyde to acetate equally well and with similar effects even after chronic ethanol treatment. Consequently, preferential damage of the perivenous area after chronic ethanol intake is not caused by inherent or acquired differences in ethanol metabolism between perivenous and periportal hepatocytes. Rather, sinusoidal gradients only established in the intact liver may exaggerate the metabolic imbalance by ethanol in the perivenous area, thus explaining its greater vulnerability to damage by alcohol abuse.

Effect of triiodothyronine on alcohol dehydrogenase and aldehyde dehydrogenase activities in rat liver. Implications for the control of ethanol metabolism

Treatment of rats with 20 micrograms of 3,3',5-triiodo-L-thyronine (T3) per 100 g body wt for a period of 6 days led to a 45% decrease in total liver alcohol dehydrogenase and a 36% decrease in total liver aldehyde dehydrogenase. Most of the latter decrease was directly attributable to a 57% fall in the level of the physiologically-important low Km mitochondrial isoenzyme. The high Km isoenzyme of the postmitochondrial and soluble fractions was much less affected by T3-treatment. T3, at concentrations up to 0.1 mM, did not inhibit the activity of aldehyde dehydrogenase in vitro. Despite these large losses of the two enzymes most intimately involved in ethanol metabolism, the rate of ethanol elimination in vivo was the same in T3-treated and control animals. Moreover, there was no difference between the two groups in the susceptibility of ethanol elimination to inhibition by 4-methylpyrazole, making it unlikely that an alternative route of ethanol metabolism had been significantly induced by treatment with T3. As it had been suggested that T3 might create a "hypermetabolic state" in which constraints normally imposed on alcohol dehydrogenase and aldehyde dehydrogenase are removed thereby compensating for any loss in total enzymic activity, 2,4-dinitrophenol (0.1 mmoles/kg body wt) was administered to rats in order to raise the general metabolic rate. However, the uncoupler proved to be lethal to T3-treated animals and did not stimulate ethanol elimination in controls. The results do not support the notion that ethanol elimination in vivo is normally governed either by the level of alcohol dehydrogenase or by that of hepatic aldehyde dehydrogenase. However, the mode of control remains unclear.

Kinetic characterization of two classes of dog liver alcohol dehydrogenase isoenzymes

In order to relate the catalytic properties of alcohol dehydrogenase (ADH), the rate-limiting enzyme for alcohol metabolism, with the pharmacokinetics of ethanol elimination in vivo, the multiple molecular forms of dog liver ADH were purified and their steady state kinetics investigated. Two different classes of ADH forms were identified by starch gel electrophoresis: the class I isoenzymes migrate to the cathode and the class II forms migrate to the anode. Three different patterns of the cathodic class I isoenzymes were identified in different liver specimens. Three molecular forms were observed for patterns A and C, and five for B. The two classes of isoenzymes were separated by affinity chromatography and purified by column chromatography. The three predominant class I isoenzymes, A1, B2, and C1, in type A, B, and C livers, respectively, were isolated by high performance cation-exchange chromatography. The steady state kinetic constants of the A1, B2, and C1 isoenzymes are similar, but differ substantially from those of the class II enzyme. The class II enzyme is much less sensitive to pyrazole inhibition, Ki = 2 mM, than the class I forms, Ki = 0.6 microM. Methanol is not a substrate for the class II enzyme, whereas it is oxidized by the class I isoenzymes. The class I isoenzymes exhibit a lower Km and substrate inhibition Ki for ethanol, 0.4 and 160 mM, respectively, than values for the class II enzyme, 10 and 610 mM, respectively. The properties of class I and II dog liver ADH are similar to those of the respective isoenzymes purified from human and monkey liver.(ABSTRACT TRUNCATED AT 250 WORDS)

Inhibition of mouse liver alcohol dehydrogenase by methyl N-butyl ketone

Methyl N-butyl ketone (MNBK) inhibited the activity of mouse liver alcohol dehydrogenase (LADH) in vitro. Ethanol elimination was reduced in MNBK-treated mice as compared to controls. Ethanol-induced induced loss of righting reflex was significantly prolonged in mice pretreated with MNBK.

Application of the reverse dept polarization-transfer pulse sequence to monitor in vitro and in vivo metabolism of 13C-ethanol by 1H-NMR spectroscopy

Using the reverse 13C----1H DEPT polarization-transfer pulse sequence the metabolism of 13C ethanol in vitro and in vivo has been monitored by 1H-NMR spectroscopy. Using yeast alcohol dehydrogenase, acetaldehyde, the hydrated form of acetaldehyde and acetate were identified as metabolites of [2-13C]-ethanol. The ratio of hydrated to free acetaldehyde was dependent upon the protein concentration of the reaction mixture. Binding of acetaldehyde in an irreversible Schiffs base resulted in optimal enzyme activity. Hepatocytes from rats fasted for 20 h, metabolised [1-13C] and [2-13C]ethanol in a linear fashion, but no [13C]acetaldehyde was detected. Metabolic integrity of the hepatocytes was confirmed with [2-13C]acetate. The addition of disulfiram (50 micron) to hepatocyte suspensions which had been incubated with [1-13C]ethanol, resulted in the resynthesis of [13C]ethanol. The amount of [13C]ethanol resynthesized under these conditions represents intracellular acetaldehyde whose concentration was in the range of 400-800 mumol/g wet weight of hepatocytes when 50 mM ethanol had been originally incubated with the hepatocyte suspension. These studies show how NMR-polarization transfer pulse sequences can be used to monitor the metabolism of 13C-ethanol in vivo, and provide a unique tool to measure in vivo concentrations of acetaldehyde. The studies also suggest that cytoplasmic aldehyde dehydrogenase may play a major role in hepatic ethanol metabolism.

In vivo inhibition of liver alcohol dehydrogenase by ethanol administration

The activity of liver alcohol dehydrogenase (LADH) from rats sacrificed two hours after the administration of ethanol 3, 4 or 5 g/kg intraperitoneally was significantly inhibited compared to the activity of LADH from control rats. LADH activity was inversely related to the dose of ethanol administered, to the concentration of ethanol in the liver, and to the concentration of ethanol in the blood. The clearance of blood ethanol in rats was dose-dependent and was inversely related to the dose administered. The half-life of ethanol elimination increased as the dose of ethanol increased. These results suggest that ethanol-induced inhibition of LADH can occur in vivo and that the level of activity of this enzyme determines the rate of oxidation of ethanol.

Immunochemical evidence for a role of cytochrome P-450 in liver microsomal ethanol oxidation

Antibodies to cytochrome P-450 isozyme 3a, the ethanol-inducible isozyme in rabbit liver, were used to determine the role of this enzyme in the microsomal oxidation of alcohols and the p-hydroxylation of aniline. P-450 isozymes, 2, 3b, 3c, 4, and 6 did not crossreact with anti-3a IgG as judged by Ouchterlony double diffusion, and radioimmunoassays indicated a crossreactivity of less than 1%. Greater than 90% of the activity of purified form 3a toward aniline, ethanol, n-butanol, and n-pentanol was inhibited by the antibody in the reconstituted system. The catalytic activity of liver microsomes from control or ethanol-treated rabbits was unaffected by the addition of either desferrioxamine (up to 1.0 mM) or EDTA (0.1 mM), suggesting that reactions involving the production of hydroxyl radicals from H2O2 and any contaminating iron in the system did not make a significant contribution to the microsomal activity. The addition of anti-3a IgG to hepatic microsomes from ethanol-treated rabbits inhibited the metabolism of ethanol, n-butanol, n-pentanol, and aniline by about 75, 70, 80, and 60%, respectively, while the inhibition of the activity of microsomes from control animals was only about one-half as great. The rate of microsomal H2O2 formation was inhibited to a lesser extent than the formation of acetaldehyde, thus suggesting that the antibody was acting to prevent the direct oxidation of ethanol by form 3a. Under conditions where purified NADPH-cytochrome P-450 reductase-catalyzed substrate oxidations was minimal, the P-450 isozymes other than 3a had low but significant activity toward the four substrates examined. The residual activity at maximal concentrations of the antibody most likely represents the sum of the activities of P-450 isozymes other than 3a present in the microsomal preparations. The results thus indicate that the enhanced monooxygenase activity of liver microsomes from ethanol-treated animals represents catalysis by P-450 isozyme 3a.

Effect of age on metabolic tolerance and hepatomegaly following chronic ethanol administration

Chronic consumption of ethanol often results in an increased rate of ethanol metabolism (metabolic tolerance) and in hepatomegaly. However, the extent of these changes is highly variable. We have found that these two phenomena are greatly influenced by age. We studied the effect of age on the development of metabolic tolerance and hepatomegaly and on the increase in hepatic oxygen consumption produced by chronic ethanol administration. The latter has been proposed to contribute to metabolic tolerance to ethanol. Ethanol was administered to female Sprague-Dawley rats with different initial ages (4, 6, 8, 11, and 17 weeks) for a 4-week period in a high-fat liquid diet. Control animals were pair-fed an isocaloric liquid diet in which ethanol was replaced with carbohydrate. Metabolic tolerance and hepatomegaly following chronic ethanol consumption were markedly dependent on the initial age of the animal, with young animals showing the largest increases. Although showing a similar general trend with age, the degree of metabolic tolerance was not linked proportionally with the degree of hepatomegaly. Perfused livers from young rats fed chronically with ethanol showed increases in ethanol metabolism and oxygen consumption, whereas no increase were observed in those from older animals. These findings support the hypothesis that an elevated rate of hepatic oxygen consumption contributes to metabolic tolerance. Total hepatic alcohol dehydrogenase activity was not increased by chronic ethanol consumption in any age group, demonstrating that an increase in the levels of this enzyme is not obligatory for metabolic tolerance.(ABSTRACT TRUNCATED AT 250 WORDS)

A radiochemical method for determination of ethanol oxidation

A radiochemical method for the measurement of ethanol oxidation by tissue preparations is described. Ethanol oxidation is determined from the production of [14C]acetaldehyde, quantified as the semicarbazone derivative, from [1-14C]ethanol. The assay is quantitative, reproducible and highly correlated with the NADH-enzymic-spectrophotometric procedure.

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.

Effect of fasting on the activity and turnover of rat liver alcohol dehydrogenase

Alcohol dehydrogenase activity in rat liver decreased with fasting to about 60% of the fed level, but the specific activities of the enzyme purified from livers of fed and 12- or 48-hr fasted animals were similar, 3.2-3.4 U/mg protein. Therefore, the decrease in enzyme activity with fasting should have resulted from a decrease in the amount of enzyme protein. Accordingly, the turnover of alcohol dehydrogenase was examined in fed and fasted rats. The fractional rate of enzyme synthesis (ks) in fed rats was determined by radioisotopic methods to be 0.13 day-1 and it increased to 0.18 day-1 after a 12- or 48-hr fast. The absolute rate of synthesis (V) and the fractional rate of degradation (kd) were calculated from these ks values and the total enzyme content in livers from animals that were fasted for 8 to 72 hr. After 48-72 hr of fasting, V decreased 16% and kd increased about 20% with respect to the fed values. Together, these changes accounted for the lowered enzyme activity in the fasted state. The rapid decrease in enzyme activity with fasting, t1/2 congruent to 16 hr, was found to be due to a rapid increase in kd from 0.14-0.16 day-1 in fed animals to 0.61 day-1 during the first 8 hr after the initiation of fast. Thereafter, kd decreased steadily to reach 0.18 day-1 after 48-72 hr of fasting.

Rat liver alcohol dehydrogenase. I. Purification and characterization

Alcohol dehydrogenase was purified in 14 h from male Fischer-344 rat livers by differential centrifugation, (NH4)2SO4 precipitation, and chromatography over DEAE-Affi-Gel Blue, Affi-Gel Blue, and AMP-agarose. Following HPLC more than 240-fold purification was obtained. Under denaturing conditions, the enzyme migrated as a single protein band (Mr congruent to 40,000) on 10% sodium dodecyl sulfate-polyacrylamide gels. Under nondenaturing conditions, the protein eluted from an HPLC I-125 column as a symmetrical peak with a constant enzyme specific activity. When examined by analytical isoelectric focusing, two protein and two enzyme activity bands comigrated closely together (broad band) between pH 8.8 and 8.9. The pure enzyme showed pH optima for activity between 8.3 and 8.8 in buffers of 0.5 M Tris-HCl, 50 mM 2-(N-cyclohexylamino)ethanesulfonic acid (CHES), and 50 mM 3-(cyclohexylamino)-1-propanesulfonic acid (CAPS), and above pH 9.0 in 50 mM glycyl-glycine. Kinetic studies with the pure enzyme, in 0.5 M Tris-HCl under varying pH conditions, revealed three characteristic ionization constants for activity: 7.4 (pK1); 8.0-8.1 (pK2), and 9.1 (pK3). The latter two probably represent functional groups in the free enzyme; pK1 may represent a functional group in the enzyme-NAD+ complex. Pure enzyme also was used to determine kinetic constants at 37 degrees C in 0.5 M Tris-HCl buffer, pH 7.4 (I = 0.2). The values obtained were Vmax = 2.21 microM/min/mg enzyme, Km for ethanol = 0.156 mM, Km for NAD+ = 0.176 mM, and a dissociation constant for NAD+ = 0.306 mM. These values were used to extrapolate the forward rate of ethanol oxidation by alcohol dehydrogenase in vivo. At pH 7.4 and 10 mM ethanol, the rate was calculated to be 2.4 microM/min/g liver.

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.

Inhibition of alcohol dehydrogenase by chloral hydrate and trichloroethanol: possible role in the chloral hydrate-ethanol interaction

Both chloral hydrate and trichloroethanol inhibited mouse liver alcohol dehydrogenase (LADH) in vitro. The inhibition of LADH by chloral hydrate appears to be non-competitive in nature with an inhibition constant (Ki) of about 2.7 X 10(-4) M. The inhibition of LADH by trichloroethanol was competitive and the (Ki) was about 2.7 X 10(-5) M. The elimination of ethanol from the blood and brain was significantly reduced in chloral hydrate- or trichloroethanol-pretreated mice. Since reduced elimination of ethanol could result in the prolongation of its central depressant activity, we suggest that this should be considered as a factor in the enhanced pharmacological effects of ethanol-chloral hydrate mixtures.

Steady-state kinetic properties of purified rat liver alcohol dehydrogenase: application to predicting alcohol elimination rates in vivo

The rate of ethanol elimination in fed and fasted rats can be predicted based on the liver content of alcohol dehydrogenase (EC 1.1.1.1), the steady-state rate equation, and the concentrations of substrates and products in liver during ethanol metabolism. The specific activity, kinetic constants, and multiplicity of enzyme forms are similar in fed and fasted rats, although the liver content of alcohol dehydrogenase falls 40% with fasting. The two major forms of the enzyme were separated and found to have very similar kinetic properties. The rat alcohol dehydrogenase is subject to substrate inhibition by ethanol at concentrations above 10 mM and follows a Theorell-Chance mechanism. The steady-state rate equation for this mechanism predicts that the in vivo activity of the enzyme is limited by NADH product inhibition at low ethanol concentrations and by both NADH inhibition and substrate inhibition at high ethanol concentrations. When the steady-state rate equation and the measured concentrations of substrates and products in freeze-clamped liver of fed and fasted rats metabolizing alcohol are employed to calculate alcohol oxidation rates, the values agree very well with the actual rates of ethanol elimination determined in vivo.

New inhibitors of alcohol dehydrogenase: studies in vivo and in vitro in the rat

Two compounds bearing an amide group, p-butoxyphenol acetamide (BPA) and N-(p-butoxybenzyl)formamide (BBF) were studied as inhibitors of alcohol dehydrogenase (ADH) and their action compared with that of 4-methyl-pyrazole (4-MP), a known inhibitor of this enzyme. In vitro studies on pure horse liver ADH showed that BPA and BBF were noncompetitive inhibitors with respect to ethanol and that their Ki values were 22 and 0.14 micrometer, respectively. The apparent Ki values of BPA and BBF for rat liver ADH were found to be 90 and 2.3 micrometers, respectively (noncompetitive inhibition). Several in vivo experiments were carried out in the rat. Administration intraperitoneally of the substance (460 mumol/kg) 1 hr before intraperitoneal injection of alcohol (2 g/kg body weight) led to a significant decrease in ethanol catabolism. Injection of the substances at 460 mumol/kg brought about a decrease in rat liver ADH activity, but the activity of mitochondrial aldehyde dehydrogenase was only decreased in animals treated with BBF.

Properties of alcohol dehydrogenase and ethanol oxidation in vivo and in hepatocytes

Studies in this laboratory have been concerned with testing the properties of alcohol dehydrogenase (ADH) in vitro as predictors of ethanol oxidation in the rat in vivo. With the kinetic constants for the extracted enzyme determined under physiological conditions (pH 7.3, ionic strength = 0.25, 38 degrees C), it was possible to predict rates of ethanol elimination in the rat in vivo within +/- 15%. The results indicate that the level of ADH is the major rate determining factor and that physiological levels of free cytosolic NADH have a minor influence (less than or equal to 20%) on the rate of ethanol oxidation in vivo. Those conclusions are supported by results with isolated hepatocytes which, when incubated without other substrates, oxidize ethanol at 1/3 the rate in vivo. Under that condition, titrations with 4-pentylpyrazole show that ADH is not rate determining, and acceleration of NADH reoxidation stimulates ethanol removal. When hepatocyte incubations are supplemented with substrates, ethanol oxidation proceeds at rates similar to those in vivo, and the rates are, as in vivo, determined largely by the cellular content of ADH.

Relationship between kinetics of liver alcohol dehydrogenase and alcohol metabolism

Since alcohol dehydrogenase (ADH) catalyzes the rate-limiting step for ethanol metabolism, knowledge of the steady-state kinetics of ADH in liver is fundamental to the understanding of the pharmacokinetics of ethanol elimination. Accordingly, we have determined the kinetic properties of purified ADH isoenzymes in rat and human liver. At low ethanol concentrations, rat liver ADH obeys the Theorell-Chance mechanism and the equation predicts that activity in vivo is limited below Vmax mainly by NADH inhibition. At ethanol concentrations above 10 mM, substrate inhibition, consistent with the formation a dead-end ADH-NADH-ethanol complex, also becomes a rate-limiting factor. ADH activity, calculated from this equation and the concentrations of substrates and products present in liver during ethanol oxidation, agrees well with ethanol elimination rates measured in vivo. With human liver ADH, large differences are observed in the kinetic properties of 5 homodimeric isoenzymes: gamma 1 gamma 1 and gamma 2 gamma 2 exhibit negative cooperativity for ethanol saturation, while alpha alpha, beta 1 beta 1 and beta ind beta ind obey Michaelis-Menten kinetics. At pH 7.5, Km values for ethanol and Vmax values range 0.048 mM and 9 min-1 for beta 1 beta 1 to 64 mM and 560 min-1 for beta ind beta ind, respectively. Therefore, individuals with different ADH phenotypes should display different ethanol elimination profiles.

Kinetics of malate dehydrogenase and control of rates of ethanol metabolism in rats

The theory that the rate of ethanol oxidation is governed by rates of NADH reoxidation is based in part on the observation that the ratio of free cytosolic [NADH]/[NAD+] increases during ethanol metabolism. However, it has recently been suggested that the amount of alcohol dehydrogenase governs rates of ethanol metabolism, which then leaves the change in cytosolic redox state unexplained. In this paper the kinetic parameters for rat liver malate dehydrogenase, determined at 37 degrees C and pH 7.4, are used to provide an explanation for the change in cytosolic redox state that is compatible with rate control by alcohol dehydrogenase.

NAD+-dependent ethanol oxidation: redox effects and rate limitation

Effects of ethanol on interconversion of cyclohexanol and cyclohexanone was studied in isolated hepatocytes. Oxidation and reduction catalyzed by alcohol dehydrogenase were markedly inhibited and stimulated, respectively. The changed ratio between the rates indicated that the ratio of NAD+ to NADH bound to alcohol dehydrogenase decreased several hundred times. This is much more than for the NAD+ system used by, e.g., lactate dehydrogenase, and deuterium from [1,1-2H2] ethanol was incorporated in cyclohexanol much more than in, e.g., lactate. These results indicate that the coenzyme bound to alcohol dehydrogenase is not equilibrated with free coenzyme. Thus, the dissociation of NADH might be rate-limiting for ethanol oxidation. Deuterium transfer from chiral [1-2H] ethanols and [2-2H] glycerol in hepatocytes indicated that cytosolic malate dehydrogenase and lactate dehydrogenase were not completely equilibrated, whereas there was no difference in the utilization of NADH formed at alcohol dehydrogenase and at glycerol-3-phosphate dehydrogenase. Fluxes in redox reactions during ethanol oxidation may be too high for equilibration of cytosolic dehydrogenases.

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%.

Comparison of liver alcohol dehydrogenases in Fischer-344 and Sprague-Dawley rats

Livers of Sprague-Dawley rats contain 30-100% more alcohol dehydrogenase activity than livers of Fischer-344 rats. When weight-matched rats from both strains are injected with the same dose of ethanol (1.2 g/kg), Sprague-Dawley rats achieve lower blood alcohol levels than Fischer-344 rats at all the time-points tested. Purified alcohol dehydrogenases from both strains of rats exhibit identical electrophoretic mobilities in SDS-polyacrylamide Section and in isoelectric focusing slab gels, pH optima, peptide maps, Km for ethanol, and capacities to bind monospecific rabbit antibodies. Quantitative differences in alcohol dehydrogenase activity between these strains of rats are due to differences in their liver alcohol dehydrogenase levels.