Human and Rat Class IV Alcohol Dehydrogenases. In: Weiner H, Holmes RS, Wermuth B, editors. Enzymology and Molecular Biology of Carbonyl Metabolism 5. Boston: Springer US; 1995. pp. 331-9. [DOI: 10.1007/978-1-4615-1965-2_40]

A new view of alcohol metabolism and alcoholism--role of the high-Km Class III alcohol dehydrogenase (ADH3)

The conventional view is that alcohol metabolism is carried out by ADH1 (Class I) in the liver. However, it has been suggested that another pathway plays an important role in alcohol metabolism, especially when the level of blood ethanol is high or when drinking is chronic. Over the past three decades, vigorous attempts to identify the enzyme responsible for the non-ADH1 pathway have focused on the microsomal ethanol oxidizing system (MEOS) and catalase, but have failed to clarify their roles in systemic alcohol metabolism. Recently, using ADH3-null mutant mice, we demonstrated that ADH3 (Class III), which has a high K(m) and is a ubiquitous enzyme of ancient origin, contributes to systemic alcohol metabolism in a dose-dependent manner, thereby diminishing acute alcohol intoxication. Although the activity of ADH3 toward ethanol is usually low in vitro due to its very high K(m), the catalytic efficiency (k(cat)/K(m)) is markedly enhanced when the solution hydrophobicity of the reaction medium increases. Activation of ADH3 by increasing hydrophobicity should also occur in liver cells; a cytoplasmic solution of mouse liver cells was shown to be much more hydrophobic than a buffer solution when using Nile red as a hydrophobicity probe. When various doses of ethanol are administered to mice, liver ADH3 activity is dynamically regulated through induction or kinetic activation, while ADH1 activity is markedly lower at high doses (3-5 g/kg). These data suggest that ADH3 plays a dynamic role in alcohol metabolism, either collaborating with ADH1 or compensating for the reduced role of ADH1. A complex two-ADH model that ascribes total liver ADH activity to both ADH1 and ADH3 explains the dose-dependent changes in the pharmacokinetic parameters (beta, CL(T), AUC) of blood ethanol very well, suggesting that alcohol metabolism in mice is primarily governed by these two ADHs. In patients with alcoholic liver disease, liver ADH3 activity increases, while ADH1 activity decreases, as alcohol intake increases. Furthermore, ADH3 is induced in damaged cells that have greater hydrophobicity, whereas ADH1 activity is lower when there is severe liver disease. These data suggest that chronic binge drinking and the resulting liver disease shifts the key enzyme in alcohol metabolism from low-K(m) ADH1 to high-K(m) ADH3, thereby reducing the rate of alcohol metabolism. The interdependent increase in the ADH3/ADH1 activity ratio and AUC may be a factor in the development of alcoholic liver disease. However, the adaptive increase in ADH3 sustains alcohol metabolism, even in patients with alcoholic liver cirrhosis, which makes it possible for them to drink themselves to death. Thus, the regulation of ADH3 activity may be important in preventing alcoholism development.

A review of alcohol clearance in humans

The level of blood or brain alcohol is considered to influence alcohol ingestion by causing subjective perceptions or neural activations that are reinforcing or rewarding. Alcohol-dependent people may try to maintain some desired tissue level, drinking to replace the millimolar levels that were cleared from the blood by metabolism. The biomedical literature describes many approaches to understanding the role of blood alcohol levels in human physiology and behavior, and this review examines some of the published results. They include the general kinetics of intake and removal of beverage alcohol as well as the characteristics of many different catalysts that can interact with alcohol. Because ingested alcohol creates blood levels that are a 1000-fold greater than those normally experienced during abstinence, ethanol may impose itself as an alternate substrate for the many oxidoreductases that act physiologically on other endogenous alcohols. Many enzymes that can act on millimolar ethanol have been isolated, and their structural genes are sequenced. Unfortunately, the genetic sequence does not indicate the physiological material upon which the translated gene product may act. In a sense, the set of enzymes with catalytic sites occupied by millimolar ethanol during alcohol drinking might constructively be regarded as "orphan gene products" whose physiological role remains to be clarified. This review is designed to indicate some of what is known, what is not known, and what needs to be known to improve the interpretations regarding adaptations to beverage alcohol and the ability of millimolar levels of alcohol to diminish dysphoria. The dysphoria may be influenced by ethanol, by ethanol metabolites, or by altered metabolism of currently unspecified endogenous substrates. A major challenge is to evaluate the multiple alternative variables within a context that stimulates curiosity and encourages quantitative tests of the relative contribution of each variable to the overall physiology of an individual.