Lhe elimination of alcohol in the liver takes place in two stages, thanks to two enzymes: ADH (alcohol dehydrogenase) and ALDH (acetaldehyde dehydrogenase). 

1) aDH converts ethanol into acetaldehyde ;
2) aLDH converts acetaldehyde into acetate, which in turn is converted into acetyl coenzyme A.

The most toxic substance, responsible for most of the damage caused by alcohol in the body, is acetaldehyde.

In some individuals, this alcohol-degradation system fails: either ADH breaks down alcohol too quickly into acetaldehyde, or ALDH breaks down acetaldehyde too slowly into acetate. In either case, the result is a particularly high concentration of acetaldehyde in the liver, leading to more severe and rapid reactions, and making alcohol consumption much more toxic for these individuals.
This genetically determined characteristic is fairly rare in Caucasians, but affects almost half of Asians