Factors that Modify the Metabolism of Ethanol in Rat Liver and Adaptive Changes Produced by its Chronic Administration
BY L. .VIDELA* AD Y. ISRAEL* Laboratory of Biochemistry, University of Chile, Casilla 233, Santiago, Chile (Received 31 December 1969)
1. 2,4-Dinitrophenol (0.1mm) increases by 100-160% therateofethanolmetabolism by rat liver slices incubated in a medium saturated with a gas mixture containing 02 +C02 +N2 (18: 5: 77). Similar effects are produced by relatively low concentrations of arsenate (10mM). At higher concentrations (37.5 and 50mM) arsenate inhibits the rate of ethanol metabolism. 2. When liver slices are incubated under an atmosphere containing 02 + C02 (95: 5) the metabolism ofethanolincreases by about 100% over that obtained with 02+C02 +N2 (18:5:77). However, under these conditions the activating effect of dinitrophenol is no longer observed. 3. Chronic administration of ethanol to rats for 3-4 weeks, in doses from 3 to 8g/kg per day, increases by 70-90% the ability of the liver to metabolize ethanol. In the liver slices of these rats, although an 02 +C02 +N2 (18: 5: 77) mixturewas used, dinitrophenol does not further increase the metabolism of ethanol. If the chronic administration of ethanol is discontinued for two weeks, the rate of ethanol metabolism is lowered to control values and the activating effect of dinitrophenol is recovered. 4. No change in alcohol dehydrogenase activity was found in the liver of the rats in which the metabolism of ethanolhad been increasedas a result of the chronic ethanol treatment; a 40 % increase in the activity of succinate dehydrogenase was observed.
Ethanol is known to be oxidized to acetaldehyde by alcohol dehydrogenase (alcohol-NAD oxidoreductase, EC 1.1.1. 1). In the liver cell, the metabolism of ethanol leads to an increase in the NADH/ NAD ratio (Smith & Newman, 1959; Reboucas & Isselbacher, 1961; Raiha & Oura, 1962;Cherrick & Leevy, 1965). Ethanol has also been found to increase the lactate/pyruvate ratio in the liver (Forsander, 1966; Kreisberg, 1967; Krebs, Freedland, Hems & Stubbs, 1969). This redox pair is in equilibrium with the NADH/NAD+ pair in the cell extramitochondrial compartment (Schimassek, 1963). The NADH formed by the action of alcohol dehydrogenase, although in the cytosol, is known to increasethe cell ,B-hydroxybutyrate/acetoacetate ratio (Forsander, Maenpiia & Salaspuro, 1965; Rawat, 1968; Lindros & Aro, 1969), a redox couple in equilibrium with the intramitochondrial NADH/ NAD+ pair (Krebs, 1967). At least three limiting steps in the metabolism of ethanol in the liver cell are conceivable: (a) the amount (maximum activity) of alcohol dehydrogenase; (b) the transfer of reducingequivalents into the mitochondria; (c) the rate of NADH reoxidation by the mitochondria.
* Present address: Department of Pharmacology, University of Toronto, Toronto, Ont., Canada.
Theorell & Chance (1951) have shown, by using a purified liver alcohol dehydrogenase in an isolated system, that the limiting step for the oxidation of ethanol is the dissociation of the enzyme-NADH complex. Thisinformation has been interpreted by most investigators as an indication that the limiting factor in the metabolism of ethanol in the liver cell is the amount of alcohol dehydrogenase and there have been several studies in which the amount of enzyme (maximum activity of alcohol dehydrogenase assayed with saturating concentrations of NAD+) has been determined as an indication of the ability of the liver tometabolize ethanol (see Hawkins, Kalant & Khanna, 1966). Smith & Newman (1959) have shown that the rate of ethanol oxidation is slower in starved than fed animals, despite the fact that ethanol increases the cell NADH/NAD ratio to a greater extent in the starved animals. These results indicate that the oxidation of NADH, rather than the amount of enzyme, could be an important factor in...