Aldehyde dehydrogenase content and composition of human liver

Human liver contains only four proteins which catalyze dehydrogenation of acetaldehyde; two of these are tetrameric with MW of 220,000 and are structurally related. These enzymes were purified previously to homogeneity and are now known as the cytoplasmic E1 and mitochondrial E2. The other two proteins do not appear to be structurally related to E1 and E2. The recently isolated E4 enzyme is a dimer of MW of ca. 175,000; the E3 may be a polymorphic enzyme. The Enzyme Commission classification for E1 and E2 is EC 1.2.1.3, that for E4 is at present uncertain since its Michaelis constants for short chain aldehydes are high, making it unlikely that these would be its natural substrates. The relationship between E3 and E4 is also uncertain. Employing a suitably designed assay, E1 and E2 are assayed as "low Km" enzymes while E3 and E4 are assayed as "high Km" enzymes. Therefore by employing such an assay, combined with electrofocusing procedure, an assessment of enzyme content and composition of aldehyde dehydrogenase in human liver can be made.