Oxidation of Ethanol Coupled to Reduction of Retinals Catalyzed by Alcohol Dehydrogenase 4
Bryce V. Plapp, Jennifer L. Mitchell, and Kristine B. Berst
Department of Biochemistry, The University of Iowa, Iowa City, IA 52242-1109
Liver alcohol dehydrogenases detoxify ethanol and other ingested alcohols, but additional
physiological functions of the various enzymes may be proposed because of their broad
specificity for a wide range of substrates. In particular, ADH4, the Class IV (sigma or mu)
enzymes found in stomach mucosa and other tissues have higher catalytic efficiency with longer
chain substrates and retinoids than with ethanol and acetaldehyde. Because metabolism of
retinoids is critical for development, differentiation and metabolic signalling, the contribution of
ADH4 to retinoid metabolism and the effects of ethanol must be assessed. Ethanol is a
competitive inhibitor of retinol oxidation by ADH4, but it is also important to recognize that
during ethanol metabolism, the steady-state levels of NADH and the alcohol dehydrogenase-
NADH complex will increase and lead to increased rates of reduction of retinals. This coupled
reaction can be very efficient since it by-passes the steps for dissociation of coenzyme, which are
often slower than the chemical steps. We find that the reduction of all-trans- or 9-cis-retinals by
NAD+ and saturating concentrations of ethanol catalyzed by the purified mouse enzyme under
physiological conditions is as efficient as the reduction with NADH. However, the Michaelis
constant for ethanol is about 90 mM, which indicates that at physiologically relevant
concentrations (20 mM or lower) the rate of reduction would be less than maximal. In contrast,
the coupled reaction of ethanol and all-trans-retinal catalyzed by human ADH4 exhibits an
apparent Km for ethanol of about 3 mM. It appears that reduction of retinals catalyzed by human
ADH4, together with the inhibition of oxidation of retinols, could be significant physiological
consequences of ethanol metabolism, leading to decreased steady-state levels of retinals and
retinoic acids. The reduction of other aldehydes and ketones can also be expected to be
facilitated by alcohol dehydrogenases during ethanol metabolism. Simulations of these reactions
with rate constants estimated for the steps in the mechanism can illustrate the steady-state
progress curves. These studies contribute to the goal of describing quantitatively the roles of the
many enzymes involved in the metabolism of alcohols and carbonyl compounds.