Enzymatic Control of Alcohol Metabolism in the Body—The Roles of Class I, II, III, and IV Alcohol Dehydrogenases/NADH Reoxidation System, Microsomal Ethanol Oxidizing System, Catalase/H2O2 System, and Aldehyde Dehydrogenase 2

Alcohol metabolism in the body is a key theme in medical research on alcohol. It is primarily regulated by the alcohol dehydrogenase (ADH) and mitochondrial NADH reoxidation in the liver. Class I ADH1 is a well-known ADH isozyme and a key enzyme in alcohol metabolism, with the lowest Kms for ethanol and the highest sensitivity to pyrazole (Pz) among the ADH isozymes. However, a Pz-insensitive metabolic pathway also plays a role in systemic alcohol metabolism, with increasing metabolic contributions at higher blood alcohol concentrations (BACs) and under chronic alcohol consumption (CAC). The Pz-insensitive pathway is referred to as the non-ADH pathway—specifically, it is a non-ADH1 pathway—and is assumed to involve the microsomal ethanol oxidizing system (MEOS) or catalase, as both enzymes are insensitive to Pz and exhibit higher Kms than ADH1. The MEOS is a favored candidate for this pathway, as its activity markedly increases with the rate of alcohol metabolism under CAC. However, the role of the MEOS in alcohol metabolism has not been proven in vivo (even under CAC conditions), nor has that of catalase. Here, we report Class III ADH3 as a new candidate in the non-ADH1 pathway, as it also has a lower sensitivity to Pz and a higher Km. It is markedly activated by lowering Km following the addition of amphiphilic substances, which increases the solution’s hydrophobicity in the reaction medium; additionally, Nile red staining demonstrates a higher solution hydrophobicity in the cytoplasm of mouse liver cells. The rate of alcohol metabolism in ADH1 knockout (Adh1−/−) mice—which depends solely on the non-ADH1 pathway—increased by more than twice under CAC and was significantly correlated with the amount of liver ADH3 protein, but not with CYP2E1 protein (a main component of the MEOS). The rate of alcohol metabolism in Adh3−/− mice lacking ADH3 decreased in a dose-dependent manner compared with wild mice. The liver ADH3 protein in wild-type mice increased in line with the ADH1 protein under CAC. These data suggest that ADH3 contributes to alcohol metabolism in vivo as a non-ADH1 pathway and to the enhancement of alcohol metabolism under CAC through activation of the ADH1/ADH3/NADH reoxidation system. In alcoholic liver diseases, ADH1 activity decreased with the progression of liver disease, while ADH3 activity increased or was maintained even in alcoholic liver cirrhosis. Therefore, the role of ADH3 in alcohol metabolism may be increased in the context of alcoholic liver diseases, complementing the reduced role of ADH1. It has also been suggested that Class II ADH2, Class IV ADH4, and aldehyde dehydrogenase (ALDH) 2 play roles in alcohol metabolism in vivo under certain limited conditions. However, ADH2 and 4 may not contribute to the enhancement of alcohol metabolism through CAC.