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

Ethanol Metabolism in the Liver, the Induction of Oxidant Stress, and the Antioxidant Defense System

The liver metabolizes ethanol through three enzymatic pathways: alcohol dehydrogenase (ADH), cytochrome p450 (also called MEOS), and catalase. Alcohol dehydrogenase class I (ADH1) is considered the most important enzyme for the metabolism of ethanol, MEOS and catalase (CAT) are considered minor alternative pathways. However, contradicting experiments suggest that the non-ADH1 pathway may have a greater relevance for the metabolism of ethanol than previously thought. In some conditions, ethanol is predominately metabolized to acetaldehyde via cytochrome P450 family 2 (CYP2E1), which is involved in the generation of reactive oxygen species (ROS), mainly through electron leakage to oxygen to form the superoxide (O₂^•−) radical or in catalyzed lipid peroxidation. The CAT activity can also participate in the ethanol metabolism that produces ROS via ethanol directly reacting with the CAT-H₂O₂ complex, producing acetaldehyde and water and depending on the H₂O₂ availability, which is the rate-limiting component in ethanol peroxidation. We have shown that CAT actively participates in lactate-stimulated liver ethanol oxidation, where the addition of lactate generates H₂O₂, which is used by CAT to oxidize ethanol to acetaldehyde. Therefore, besides its known role as a catalytic antioxidant component, the primary role of CAT could be to function in the metabolism of xenobiotics in the liver.

Involvement of cell oxidant status and redox state in the increased non-enzymatic ethanol oxidation by the regenerating rat liver

Ethanol administration is capable of inhibiting or delaying the partial hepatectomy (PH)-induced liver regeneration, probably altering liver metabolism by means of its oxidative metabolism. Since the regenerating liver has increased capacity for oxidizing ethanol, the present study was aimed to address the contribution of the ethanol-oxidizing metabolic pathways in the regenerating liver cells. Isolated hepatocytes were prepared from control livers and from animals subjected to two-thirds PH. In both preparations, ethanol oxidation was largely increased by incubation with glucose and was highly sensitive to inhibitors of ethanol-oxidizing enzymatic pathways (alcohol dehydrogenase, catalase and cytochrome P-4502E1 activities). The latter led to a total blockade of ethanol disposal by control hepatocytes, while liver cells from PH-rats only showed an early 70-75% inhibition of ethanol catabolism with the inhibitors used. In regenerating hepatocytes, the enhanced ethanol oxidation was blocked by scavengers of reactive oxygen species, an effect that correlated with enhanced cytoplasmic lipid peroxidation by-products. Both cell preparations showed similar sensitivity to inhibitors for the malate-aspartate shuttle and mitochondrial electron transport chain; the shift of the cytoplasmic redox state was also quite similar after ethanol oxidation. A more oxidized mitochondrial redox state was found in hepatocytes from PH-rats and more shifted to the reduced state during ethanol oxidation this effect was not abolished by inhibiting alcohol dehydrogenase activity. In conclusion, data clearly show that an important fraction of ethanol is metabolized through a non-enzymatic-mediated oxidative event, which could largely contribute to the deleterious effect of ethanol on the proliferating liver.

Inhibition of canine and feline alcohol dehydrogenase activity by fomepizole

OBJECTIVE

To determine and compare substrate specificity and kinetic rate constants of feline and canine alcohol dehydrogenase (ADH) with ethanol (EtOH) and ethylene glycol (EG) as substrates in vitro, with and without fomepizole.

SAMPLE POPULATION

Livers from 3 dogs and 3 cats.

PROCEDURE

Canine and feline ADH activity, in cytosolic fractions of homogenized liver, was determined by use of various concentrations of nicotinamide adenine dinucleotide (NAD), EtOH, or EG as substrates. Initial reaction velocities were calculated, and kinetic inhibition rate constants (Ki) for fomepizole were determined.

RESULTS

Substrate specificity of canine and feline ADH for EtOH or EG was not significantly different. A 2-fold difference was detected in the maximal velocity of canine, compared with feline, ADH, using either substrate. Fomepizole Ki in feline hepatic homogenates was significantly greater than Ki in canine hepatic homogenates when either EtOH or EG was used as substrate (10- and 30-fold, respectively). A 6-fold increase in the concentration of fomepizole was required to achieve ADH inhibition, with feline homogenates equivalent to those of canine homogenates.

CONCLUSIONS AND CLINICAL RELEVANCE

Feline ADH has lower enzymatic capacity for turnover or is less concentrated in liver than canine ADH with regard to EtOH and EG catalysis. Canine ADH was more effectively inhibited by fomepizole than feline ADH. Results suggest that higher dosages of fomepizole may be more effective to treat cats with EG intoxication than dosages reported to treat dogs.

Ethanol metabolism in patients with liver cirrhosis

Aspects of human metabolism of ethanol are reviewed with the main focus on the rate of ethanol clearance from blood in patients suffering from liver cirrhosis. Studies in humans and experimental animals do not support the notion of a slower rate of ethanol metabolism in patients with liver cirrhosis compared with those with normal liver function. The rate of ethanol disappearance from blood in healthy non-alcoholic subjects falls within the range 9-20 mg/dL/h and there is no compelling evidence to suggest that this should be much different in cirrhotic patients.

Effect of the cytochrome P-450IIE1 genotype on ethanol elimination rate in alcoholics and control subjects

We studied an influence of genetic polymorphisms in the cytochrome P-450IIE1 (CYP2E1) gene on ethanol elimination rate in alcoholic patients and healthy subjects. The CYP2E1 genotype was determined by polymerase chain reaction-restriction fragment length polymorphism method for 124 alcoholics and 54 healthy subjects. There was no significant difference in the gene frequency of CYP2E1 between alcoholics and healthy control subjects. Blood ethanol concentrations in the 65 alcoholics on admission ranged from 0.32 to 4.22 mg/ml. In the patients with the c1/c2 genotype, the elimination rate was significantly correlated with blood ethanol concentration. In each of the three genotypes of CYP2E1, the patients were divided into three groups based on ethanol concentrations. The average of the ethanol elimination rate in the patients with c1/c2 having blood ethanol levels of > or = 2.5 mg/ml was significantly higher than the rates in the two other groups of c1/c2. When blood ethanol levels were > or = 2.5 mg/ml, the elimination rate in the patients with c1/c2 was significantly higher than that in those with c1/c1. Regardless of the CYP2E1 genotype, the elimination rate in the alcoholics was higher than that in the control subjects when blood ethanol levels were < 1.0 mg/ml. These results suggest the possibility that the c2 allele of CYP2E1 Influences the rate of ethanol elimination at high ethanol levels. The rate of ethanol elimination was independent of liver disorder judged by serum total bilirubin values.

NMR investigation of the futile cycling of ethanol in chronic alcoholic rats

Weight gain efficiency differences previously reported between alcohol-fed rats and their controls were investigated. Additionally, the futile cycling of ethanol proposed to explain such differences was studied by NMR spectroscopy. Male Sprague-Dawley rats were fed a nutritionally adequate diet containing 36% of the calories as alcohol, and their paired controls were fed an isocaloric diet for 11 weeks to establish conditions of chronic alcohol feeding. Normalized metabolic efficiencies varied significantly during the initial 2-week period (6.86 +/- 0.51 vs. 2.83 +/- 0.18 g/kcal x 10(-2) for control and alcohol-fed groups, respectively, and to a lesser extent over the entire feeding period (6.41 +/- 0.78 vs. 4.60 +/- 0.27 g/kcal x 10(-2) for control and alcohol-fed groups, respectively. Alcohol-induced weight gain inefficiency in metabolism has previously been studied and explained by a variety of different biochemical and physiological mechanisms. One possible pathway of energy wastage may occur due to ethanol futile cycling from ethanol to acetaldehyde through the microsomal ethanol oxidation system pathway, and simultaneously from acetaldehyde to ethanol via the ADH pathway. This futile cycle represents a net loss of 6 ATP/cycle, corresponding to the loss of two reducing equivalents (NADH and NADPH). 1H NMR spectroscopy was used to test for this cycling in blood extracts after administration of 1,1-2H2 ethanol. No futile cycling was detected either during the initial 2 weeks of feeding or after the entire feeding period.

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.

Respective roles of the microsomal ethanol oxidizing system and catalase in ethanol metabolism by deermice lacking alcohol dehydrogenase

To evaluate the roles of MEOS (microsomal ethanol oxidizing system) and catalase in non-alcohol dehydrogenase (ADH) ethanol metabolism, MEOS and catalase activities in vitro and ethanol oxidation rates in hepatocytes from ADH-negative deermice were measured after treatment with catalase inhibitors and/or a stimulator of H2O2 generation. Inhibition of ethanol peroxidation by 3-amino-1,2,4-triazole (aminotriazole) was found to be greater than 85% up to 3 h and 80% at 6 h in liver homogenates. Urate (1 mM) which stimulates H2O2 production in living systems, increased ethanol oxidation fourfold in control but not in cells from aminotriazole-treated animals, documenting effective inhibition of catalase-mediated ethanol peroxidation by aminotriazole. While aminotriazole slightly depressed (15%) basal ethanol oxidation in hepatocytes, in vitro experiments showed a similar decrease in MEOS activity after aminotriazole pretreatment. Azide (0.1 mM), a potent inhibitor of catalase in vitro, also did not affect ethanol oxidation in control cells. By contrast, 1-butanol, a competitive inhibitor of MEOS, but neither a substrate nor an inhibitor of catalase, decreased ethanol oxidation rates in a dose-dependent manner. These results show that, in deermice lacking ADH, catalase plays little if any role in hepatic ethanol oxidation, and that MEOS mediates non-ADH metabolism.

The microsomal ethanol oxidizing system and its interaction with other drugs, carcinogens, and vitamins

The interaction of ethanol with the oxidative drug-metabolizing enzymes present in liver microsomes results in a number of clinically significant side effects in the alcoholic. Following chronic ethanol consumption, the activity of the microsomal ethanol oxidizing system (MEOS) increases. This enhancement of MEOS activity is primarily due to the induction of a unique microsomal cytochrome P-450 isozyme, which has a high capacity for ethanol oxidation, as shown in reconstituted systems. Normally present in liver microsomes at low levels, this form of cytochrome P-450 increases dramatically after chronic ethanol intake in many species, including baboons. The in-vivo role of cytochrome P-450 in hepatic ethanol oxidation, especially following chronic ethanol consumption, has been conclusively demonstrated in deer-mice lacking liver ADH. Induction of microsomal cytochrome P-450 by ethanol is associated with the enhanced oxidation of other drugs as well, resulting in metabolic tolerance to these agents. There is also increased cytochrome P-450-dependent activation of known hepatotoxins such as carbon tetrachloride and acetaminophen, which may explain the greater susceptibility of alcoholics to the toxicity of industrial solvents and commonplace analgesics. In addition, the ethanol-inducible form of cytochrome P-450 has the highest capacity of all known P-450 isozymes for the activation of dimethylnitrosamine, a potent (and ubiquitous) carcinogen. Moreover, cytochrome P-450-catalyzed oxidation of retinol is accelerated in liver microsomes, which may contribute to the hepatic vitamin A depletion seen in alcoholics. In contrast to chronic ethanol consumption, acute ethanol intake inhibits the metabolism of other drugs via competition for shared microsomal oxidation pathways. Thus, the interplay between ethanol and liver microsomes has a profound impact on the way heavy drinkers respond to drugs, solvents, vitamins, and carcinogens.

Catalase-dependent ethanol metabolism in vivo in deermice lacking alcohol dehydrogenase

Pathways of ethanol elimination in alcohol dehydrogenase (ADH)-positive and -negative deermice were studied using the catalase inhibitor, 3-amino-1,2,4-triazole. To verify that aminotriazole inhibited catalase effectively, the characteristic decrease in catalase-H2O2 which occurs in saline-treated controls when ethanol is peroxidized was monitored at 660-640 nm in perfused deermouse livers. Following 1.5 hr of pretreatment with aminotriazole (1.5 g/kg), the peroxidatic activity of catalase measured in vitro was inhibited by greater than 99%. Under these conditions, ethanol did not decrease catalase-H2O2 in perfused livers, indicating that catalase was inhibited. Ethanol and aniline oxidation by microsomes were also inhibited by about 67-90% after 1.5 hr of pretreatment with aminotriazole. In ADH-positive deermice, pretreatment with aminotriazole for 1.5 hr prior to injection of ethanol (2.0 g/kg) decreased rates of ethanol elimination in vivo from 13.2 +/- 0.8 to 10.2 +/- 0.4 mmoles/kg/hr. In ADH-negative deermice, similar treatment decreased rates of ethanol elimination in vivo from 4.5 +/- 0.4 to 1.1 +/- 0.6 mmoles/kg/hr. Following pretreatment with aminotriazole (1.0 g/kg) for 6 hr, rates of ethanol elimination in ADH-negative deermice returned to near basal values. Under these conditions, the peroxidatic activity of catalase measured in vitro and the ethanol-dependent decrease in catalase-H2O2 in perfused livers also returned to near basal levels; however, the oxidation of ethanol by cytochrome P-450 was inhibited completely. It is concluded, therefore, that time of pretreatment with aminotriazole is an important variable which must be controlled carefully to inhibit catalase completely. Since catalase was active while cytochrome P-450 was not following 6 hr of pretreatment with aminotriazole, it is concluded that ethanol elimination occurs predominantly via catalase-H2O2 in ADH-negative deermice under these conditions.

The deuterium isotope effect on ethanol metabolism in perfused rat liver: effect of reversed perfusion on ethanol and oxygen uptake

Livers from rats fasted for 24 hr were subjected to nonrecirculating perfusion with Krebs-Ringer bicarbonate solution containing 10 mM ethanol. The deuterium isotope effect was measured using (1-R)-[1-14C,1-2H]ethanol. A value of 2.57 +/- 0.09 (SD) was obtained independent of the direction of perfusion. Oxygen uptake and ethanol metabolism in contrast were significantly increased when reverse perfusion (i.e., from vena cava to vena portae) was used. The magnitude of the isotope effect indicates that contribution from microsomal ethanol-oxidizing system if this is the only supplementary system is 9.8% under the experimental conditions. At high ethanol concentrations, the contribution would approach 18%. Equal activities of microsomal ethanol-oxidizing system and catalase under the experimental conditions would mean that both contribute 7.3% of the total ethanol metabolism. At high ethanol concentrations (80 mM), however, catalase will be 6.8% and microsomal ethanol-oxidizing system is calculated to 13.3%. Preliminary experiments with rats pretreated with phenobarbital showed no change in the isotope effect or in the rate of ethanol metabolism, but a 40-50% increase in oxygen consumption. The acetaldehyde concentration in the effluent medium was below 1 microM.

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.