ALDH rapidly and efficiently converts the acetaldehyde produced by the three previously described pathways to acetate. In humans, there are 19 ALDH isozymes, with different tissue distributions and kinetic properties. However, only ALDH1 and ALDH2 isozymes are involved in acetaldehyde oxidation (120) (Table 2). ALDH1, a cytosolic enzyme with relatively low kinetic activity for acetaldehyde, is found in nearly all tissues, including the brain. In contrast, ALDH2 is a mitochondrial enzyme highly expressed in the liver and gastrointestinal tract, exhibiting high catalytic efficiency for acetaldehyde oxidation (121). As a result, except for individuals with non-functional ALDH2 (i.e., ALDH2*2, discussed in the section on genetic polymorphisms), circulating acetaldehyde levels in the blood remain extremely low —over 1,000 times lower than BAC ) – as exemplified in a study where individuals with a BAC of approximately 7,000 μM (equivalent to 0.3 g/L or 0.03%) had blood acetaldehyde levels below 2.4 μM (122).
