Contribution of liver alcohol dehydrogenase to metabolism of alcohols in rats

Abstract

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.

Keywords: Alcohol dehydrogenase; Alcohol metabolism; Enzyme specificity; Inhibition; Rat metabolism.

Fig. 1

Elimination of primary alcohols. Doses were 20 mmole/kg ethanol (●), 10 mmole/kg 1-propanol (■), 8 mmole/kg 1-butanol (○), 10 mmole/kg 2-methyl-1-propanol (□), and 7 mmole/kg 3-methyl-1-butanol (◇). Data are for one rat.

Fig. 2

Elimination of diols with a primary hydroxyl group. All doses were 10 mmole/kg: 1,3-propanediol (□), 1,3-butanediol (●), 1,4-butanediol (■), 1,5-pentanediol (○).Data are for one rat.

Fig. 3

Elimination of secondary alcohols. In each sub-figure, the concentration of alcohol is represented by filled circle (●), its corresponding ketone by a filled square (■), and the results when another rat was given 1 mmole/kg of 4-methylpyrazole are represented by the open symbols, alcohol (○) and ketone (□). (A) 2-propanol, 10 mmole/kg; (B) 2-propanol-d₇, 15 mmole/kg; (C) 2-butanol (racemic mixture of isomers), 10 mmole/kg; (D) 3-pentanol, 5 mmole/kg; (E) cyclopentanol, 5 mmole/kg; (F) cyclohexanol, 5 mmole.kg. The lines represent the fitted values from simultaneous non-linear least squares fits of data for alcohol and ketone for the uninhibited (or inhibited) state to the differential equations for the first order, sequential reactions.

Fig. 4

Correlation of rates of elimination of alcohols in rats with the rates of oxidation by isolated rat liver alcohol dehydrogenase. Rates are calculated for 5 mM alcohol, as shown in Table 4. The circles represent the data points, and the enclosed number is for the alcohol, which is also labeled.