Figure 1.

Pathways involved in alcohol metabolism. In the major metabolic pathway (A) ethanol is oxidized into acetaldehyde. This oxidative process is facilitated by alcohol dehydrogenase (ADH), which is present in high concentration in the cytosol of hepatocytes. In this second step, acetaldehyde enters the mitochondria where it is oxidized into acetate and water. This process is facilitated by mitochondrial aldehyde dehydrogenase (ALDH). For both steps, nicotinamide adenine dinucleotide (NAD⁺) is essential to provide the necessary energy for the conversion, which becomes available when NAD⁺ is converted into NADH + H⁺. A second major pathway for alcohol breakdown, especially active in subjects who chronically drink alcohol, is the microsomal ethanol oxidizing system (MEOS, see (B)). The reaction is catalyzed by CYP2E1 and requires nicotinamide adenine dinucleotide phosphate (NADP⁺) instead of NAD⁺ to convert ethanol into acetaldehyde.