The concept of a hypermetabolic state to explain metabolic tolerance to
ethanol grew from the recognition that the rate of alcohol metabolism is, in general, limited by the rate at which mitochondria can reoxidize reducing equivalents and thus by the rate at which
oxygen can be consumed by the liver. This relationship appears to be most important in conditions in which the
alcohol dehydrogenase (ADH)/QO2 ratio is high and is not in conflict with observations suggesting that ADH can, under certain conditions, constitute a rate-determining step for
ethanol metabolism in rodents. Liver preparations from animals fed alcohol chronically, in which an increase in
ethanol metabolism is shown, consume
oxygen at higher rates. This effect, concerning which there is discrepancy among investigators, depends on the type of preparation.
Thyroid hormones play a permissive role in the development of the hypermetabolic state, while increased circulating levels of these
hormones are not required.
Antithyroid drugs inhibit both metabolic tolerance in vivo and the hypermetabolic state. While the hypermetabolic state requires an increased
ATP utilization in the form of an
adenosine triphosphatase, or an inhibition of
ATP synthesis, the different mechanisms proposed for such an effect do not quantitatively account for the increases in oxygen consumption. In humans and animals chronically exposed to
ethanol, but withdrawn,
oxygen tensions in blood leaving the liver are significantly reduced. In some situations, low
oxygen tensions in zone 3 of the hepatic acinus can reach critical hypoxic levels and may lead to cell
necrosis. Studies in which the effectiveness of
propylthiouracil is tested in human
alcoholic hepatitis are discussed.