Alteration of the enzymology of chloral hydrate reduction in the presence of ethanol

Abstract

The enzymology of chloral hydrate reduction to trichloroethanol was studied in rat liver slices and homogenates. Two enzymes capable of reducing chloral hydrate are present in rat liver and by their properties were found to be aldehyde reductase and alcohol dehydrogenase. The alcohol dehydrogenase catalyzed reaction was very sensitive to the NAD/NADH ratio of the incubation medium and was found to be virtually incapable of performing the reduction with a simulated in vivo coenzyme ratio. The aldehyde reductase catalyzed reaction was relatively insensitive to the NADP/NADPH ratio. Hence, only one of the two possible enzyme systems appears to catalyze the reduction in vivo. Incubations performed with liver slices in the presence or absence of inhibitors and alternative substrates for the two enzyme systems indicated that in the absence of ethanol only aldehyde reductase catalyzed the reduction of chloral hydrate. About a 1.5-fold increase in the rate of reduction of chloral hydrate was observed when 40 mM ethanol was added to the liver slice incubation. Further, deuterium was incorporated into trichloroethanol when the incubations were performed with deuteroethanol. The increased rate of reduction and the deuterium incorporation were both prevented by the inclusion of alcohol dehydrogenase inhibitors (pyrazole and isobutyramide). Thus, in the presence of ethanol, both alcohol dehydrogenase and aldehyde reductase contribute to the reduction of chloral hydrate. Alcohol dehydrogenase is capable of reducing chloral hydrate in the presence of an oxidizable alcohol because it is converted into an enzyme—NADH complex which can then reduce the compound.