Int. J. Environ. Res. Public Health 2010, 7 1082 8. A Complex Two-ADH Model to Explain Pharmacokinetics of Blood Alcohol In mice [24,45], the alcohol clearance (CLT: dose/AUC) decreased in a dose-dependent manner up to a dose of 5 g/kg, and exhibited a significant correlation with average total liver ADH activity during alcohol metabolism. The elimination rate for blood alcohol (β) reached a maximum at an ethanol dose of 2 g/kg, which corresponds to the Vmax of Michaelis-Menten (M-M) elimination kinetics. However, above a dose of 2 g/kg,  decreased as the dose increased, as CLT decreased. The decrease in β above a medium dose has also been observed in rats [46,47]. On the other hand, the normalized AUC (AUC/dose), which exhibited a nonlinear, dose-dependent increase, had an inverse correlation with β. To explain how alcohol-metabolizing enzymes regulate the pharmacokinetics of blood alcohol in mice, we have devised a complex two-ADH model in which the total liver ADH activity involves not only ADH1 but also ADH3 [24,45]. According to the model, liver ADH activity is a function of four variables: ADH1 activity, ADH1 content, ADH3 activity, and ADH-3 content. Since liver ADH3 activity is determined not only by the change in content but also by the change in kinetics, as mentioned in the previous section, we make the following definitions: liver ADH3 activity = total liver ADH activity – liver ADH1 activity, and liver ADH1 activity = liver ADH1 content  Vmax/mg for purified ADH1. When the activities of both ADHs are normalized by the ratio of the liver ADH activity of mice to which ethanol was administered mice to that of control mice, the ADH3 activity increased dose-dependently, whereas that of ADH1 decreased. The ADH3/ADH1 activity ratio correlates directly with AUC/dose, and inversely with CLT and β. So, the dose-dependent increase in AUC is explained by the dose-dependent increase in the ADH3/ADH1 activity ratio, which causes the body clearance (CLT) and elimination rate (β) of alcohol metabolism to decrease. Thus, the complex two-ADH model explains the changes in various pharmacokinetic parameters of blood alcohol in mice, suggesting that, as the ethanol dose increases, the elimination kinetics of blood alcohol shift from M-M kinetics dominated by ADH1 to first-order kinetics dominated by ADH3 [45]. The first-order elimination kinetics of blood ethanol has been observed in humans at blood ethanol levels above 100 mM [29,30]. As the ethanol dose increases, the shift in enzyme from a low-Km ADH to a high-Km ADH causes a nonlinear increase in AUC due to the lowered metabolic rate while the ability of alcohol to be metabolized is maintained. These metabolic changes may qualitatively enhance the toxicity of alcohol. 9. Regulation of ADH3 Activity in Chronic Ethanol Feeding and its Role in Alcohol Metabolism Animal experiments on chronic ethanol feeding have usually been performed on rats using a liquid diet (Lieber diet) containing 5% ethanol, which corresponds to 36% of the total caloric intake. In these experiments, liver MEOS shows increased activity, whereas liver ADH shows reduced or unchanged activity after four weeks of feeding, which accelerates alcohol metabolism and causes a severe case of fatty liver [7]. Based on these data, it has been thought that the acceleration of alcohol metabolism in chronic drinking is due to the induction of MEOS. However, it has recently been shown that there is no difference in the elimination rate for blood ethanol between CYP2E1 (a major component of MEOS)-