FIGURE 1.

Ethanol metabolism. A: ethanol metabolism occurs in two steps: ethanol is metabolized quickly by alcohol dehydrogenase (ADH) to generate acetaldehyde. Acetaldehyde is then metabolized by the mitochondrial aldehyde dehydrogenase 2 (ALDH2) to acetate. The first step of metabolism, catalyzed by ADH, is a reversible reaction. The second step, catalyzed by ALDH2, is the rate-limiting step in ethanol metabolism, and it takes ∼1 h to metabolize the amount of ethanol that is found in a single alcoholic beverage. B: ethanol metabolism occurs mainly in the liver. In normal ALDH2 individuals, acetaldehyde is quickly oxidized to nontoxic acetate. In ALDH2 enzyme-deficient (ALDH2*2) individuals, a significant amount of acetaldehyde is rapidly accumulated even after a moderate amount of alcohol ingestion. Acetaldehyde, which is very diffusible and crosses biological membranes, can be circulated in the blood and metabolized in all tissues, as ALDH2 is present in all mitochondria.