[The biochemistry of alcohol metabolism]

The metabolism of ethanol to acetaldehyde in the liver proceeds via alcohol dehydrogenase (ADH) and the microsomal ethanol-oxidizing system (MOS), whereas catalase plays no significant role. ADH is an enzyme of the cytosol, requires NAD+ as cofactor and exhibits a pH optimum in the alkaline range. The Km of ADH is about 2 mM for ethanol (equivalent to 0.1%). Thus, the enzyme is already saturated at low ethanol concentrations. Conversely, MEOS resides in the endoplasmic reticulum, requires NADPH and O2, is inhibited by CO and exhibits a km of about 10 mM corresponding to 0.5% ethanol. This enzyme system is therefore primarily the pathway of ethanol metabolism at intermediate to high ethanol concentrations. MEOS has many properties in common with other drug metabolizing enzymes and is characterized by inducibility following chronic ethanol consumption, which suggests the involvement of the microsomal system in the adaptive enhancement of ethanol clearance commonly observed in alcoholics. The product of ethanol oxidation by ADH, MEOS and catalase is acetaldehyde. Acetaldehyde is oxidized in the liver to acetate by NAD dependent aldehyde dehydrogenase. Four isozymes have been identified. Lack of isozyme I is responsible for the "flush-syndrome" commonly observed in asian people following alcohol intake. Ethanol metabolism is affected by the aging process and is decreased with advancing age.