Int. J. Environ. Res. Public Health 2010, 7 1080 5. Role of ADH3 in Systemic Alcohol Metabolism and in the Action of Alcohol in the Body— in vivo Evidence from ADH-knockout Mice In order to clarify the roles of ADH1 and ADH3 in systemic alcohol metabolism and in the action of alcohol in the body at various ethanol doses, we investigated the rate of alcohol metabolism (EDR: mg/kg/h) in wild-type mice (Wild), ADH1-null mutant mice (Adh1-/-) and ADH3-null mutant mice (Adh3-/-) by the administration of ethanol at doses ranging from 1.0 to 4.5 g/kg [25]. Compared to the EDR of Wild, that of Adh3-/- decreased markedly as ethanol dose increased and was about 66% of that of Wild at a dose of 4.5 g/kg. These data demonstrate that ADH3 contributes to systemic alcohol metabolism in a dose-dependent manner in vivo, with the contribution being over 30% at large doses. These data also indicate that the contribution of ADH1 to alcohol metabolism decreases dose-dependently because ADH1 is the only major alcohol-metabolizing enzyme in Adh3-/-. In addition, the EDR of Adh1-/- was 21-33% that of Wild, which indicates that the contribution of ADH1 to systemic alcohol metabolism is more than 60% but less than 80%, and decreases dose-dependently. Thus, we conclude that the contribution of ADH3 to alcohol metabolism increases as the dose increases, thereby compensating for the reduced role of ADH1. Moreover, when the ethanol dose is large, ADH1 and ADH3 together account for almost all the alcohol metabolism of Wild. The duration of the loss of the righting reflex (LORR) in mice, which is the length of time that a mouse sleeps after the administration of enough ethanol to produce acute intoxication, was longer in the order Adh1-/-, Adh3-/-, and Wild. The frequency of mortality at large doses also increased in the order Adh1-/-, Adh3-/-, and Wild. These data demonstrate that, in vivo, ADH3 diminishes acute alcohol intoxication together with ADH1, by participating in alcohol metabolism in a dose-dependent manner. 6. The Role of ADH3 in First-Pass Metabolism After drinking alcohol, it is partially metabolized in the digestive organs including the liver, before entering the systemic circulation of blood. The concentration of alcohol reaches a molar level in these organs. It is possible for ADH3 to play an important role in this first-pass metabolism (FPM) because it has a very high Km for ethanol and is present not only in the liver but also in the stomach and small intestine. Indeed, the ADH-3 activity in the stomach and the FPM are smaller for women than for men [37]. Furthermore, we found that, in mouse liver, ADH3 highly distributes in sinusoidal endothelial cells, in contrast to ADH1, which is present only in parenchymal cells [19]. So, it seems that, when a high concentration of alcohol is carried from a portal vein to the liver, it is first metabolized by high-Km ADH3 in sinusoidal endothelial cells; and then, after ADH3 has lowered the concentration, the remained ethanol is metabolized by low-Km ADH1 in parenchymal cells. Thus, the FPM of alcohol in the liver may involve a two-step process employing both ADHs, which have different Kms for ethanol and different tissue distributions. ADH3 may play the role of a gatekeeper that protects the parenchymal cells from the high toxicity of high concentrations of alcohol.