Main Alcohol-Metabolizing Enzymes in the Liver
The liver processes most alcohol (ethanol) through two primary enzymes: alcohol dehydrogenase (ADH) and aldehyde dehydrogenase (ALDH). ADH converts ethanol to acetaldehyde in the cytosol, while ALDH then oxidizes acetaldehyde to acetate, mainly in the mitochondria.[1][2]
ADH exists in multiple isoforms (e.g., ADH1A, ADH1B, ADH1C), with varying activity levels that influence alcohol tolerance across populations.[3] ALDH has key forms like ALDH2, where certain genetic variants (common in East Asians) reduce activity, leading to acetaldehyde buildup and flushing reactions.[4]
Role of Cytochrome P450 Enzymes
The microsomal ethanol oxidizing system (MEOS), led by cytochrome P450 2E1 (CYP2E1), handles excess alcohol when ADH/ALDH are saturated. CYP2E1 induction by chronic drinking increases reactive oxygen species, contributing to liver damage like steatosis.[1][5]
Catalase's Minor Contribution
Catalase, a peroxisomal enzyme, breaks down alcohol using hydrogen peroxide but plays a small role (under 10% of metabolism) compared to ADH and MEOS, mainly in specific conditions like high peroxide levels.[2]
How Genetic Variations Affect Enzyme Activity
Polymorphisms in ADH1B and ALDH2 alter metabolism speed. Fast ADH variants raise acetaldehyde quickly, deterring heavy drinking, while slow ALDH2 causes accumulation and aversion.[3][4] These explain why some people experience rapid intoxication or hangovers.
Why These Enzymes Matter for Liver Disease
Impaired enzyme function leads to toxic acetaldehyde buildup, promoting fatty liver, inflammation, and cirrhosis. Chronic use upregulates CYP2E1, worsening oxidative stress.[5]
Pathways Beyond the Liver
While the liver dominates (90%+), small intestine ADH contributes, and extrahepatic CYP2E1 handles minor metabolism.[1]
[1]: NIAAA: Alcohol Metabolism
[2]: NCBI Bookshelf: Biochemistry of Alcohol
[3]: Nature Reviews Genetics: ADH/ALDH Genetics
[4]: PLoS Genetics: ALDH2 Deficiency
[5]: Hepatology: CYP2E1 in Alcoholic Liver Disease