What enzyme breaks down alcohol in the liver?

Alcohol dehydrogenase breaks down alcohol in the liver.

In humans, this enzyme oxidizes ethanol into acetaldehyde, a reactive and toxic compound. The reaction uses NAD+ and produces NADH. Acetaldehyde is then processed by aldehyde dehydrogenase into acetate, which can eventually be converted into carbon dioxide and water.

Alcohol dehydrogenase is the main pathway for ordinary alcohol metabolism, although catalase and the CYP2E1 enzyme system can also contribute, especially at higher or repeated alcohol exposures. That does not make them the best answer to this question: alcohol dehydrogenase is the standard canonical enzyme named in human liver metabolism.

The enzyme is also found in the stomach and other tissues, and genetic variants can alter how quickly ethanol becomes acetaldehyde. A famous mix-up is confusing alcohol dehydrogenase with aldehyde dehydrogenase: the former handles ethanol first, while the latter handles acetaldehyde next.