How the liver breaks down alcohol

(i) Alcohol dehydrogenase catalyses the oxidation of alcohol to acetaldehyde.
(ii) Acetaldehyde dehydrogenase oxidises the acetaldehyde to acetyl-CoA. In these reactions,
coenzyme NAD+ is reduced to NADH; therefore, when alcohol is metabolised,
NAD+ concentrations are reduced while NADH increases.
During alcohol metabolism, NAD+ becomes unavailable to the many other vital body
processes for which it is needed, including glycolysis, the TCA cycle and the mitochondrial
respiratory chain. Without NAD+, the energy pathway is blocked and alternative routes are
taken, with serious physical consequences:
The accumulation of hydrogen ions shifts the body’s balance towards acid.
The depletion of NAD+ (and the change to the NADH to NAD+ ratio) slows the TCA
cycle, resulting in a build-up of pyruvate and acetyl-CoA. Excess acetyl-CoA increases
fatty acid synthesis and fat deposits in the liver (fatty liver). An accumulation of fat in the
liver can be observed after just a single night of heavy drinking.


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