Aldehyde dehydrogenases (EC 1.2.1.3 and EC 1.2.1.5) are enzymes which oxidize a wide variety of aliphatic and aromatic aldehydes. In mammals at least four different forms of the enzyme are known [1]: class-1 (or Ald C) a tetrameric cytosolic enzyme, class-2 (or Ald M) a tetrameric mitochondrial enzyme, class-3 (or Ald D) a dimeric cytosolic enzyme, and class IV a microsomal enzyme. Aldehyde dehydrogenases have also been sequenced from fungal and bacterial species. A number of enzymes are known to be evolutionary related to aldehyde dehydrogenases; these enzymes are listed below.

• Plants and bacterial βine-aldehyde dehydrogenase (EC 1.2.1.8) [2], an enzyme that catalyzes the last step in the biosynthesis of βine.
• Plants and bacterial NADP-dependent glyceraldehyde-3-phosphate dehydrogenase (EC 1.2.1.9).
• Escherichia coli succinate-semialdehyde dehydrogenase (NADP+) (EC 1.2.1.16) (gene gabD) [3], which reduces succinate semialdehyde into succinate.
• Escherichia coli lactaldehyde dehydrogenase (EC 1.2.1.22) (gene ald) [4].
• Mammalian succinate semialdehyde dehydrogenase (NAD+) (EC 1.2.1.24).
• Escherichia coli phenylacetaldehyde dehydrogenase (EC 1.2.1.39).
• Escherichia coli 5-carboxymethyl-2-hydroxymuconate semialdehyde dehydrogenase (gene hpcC).
• Pseudomonas putida 2-hydroxymuconic semialdehyde dehydrogenase [5] (genes dmpC and xylG), an enzyme in the meta-cleavage pathway for the degradation of phenols, cresols and catechol.
• Bacterial and mammalian methylmalonate-semialdehyde dehydrogenase (MMSDH) (EC 1.2.1.27) [6], an enzyme involved in the distal pathway of valine catabolism.
• Yeast delta-1-pyrroline-5-carboxylate dehydrogenase (EC 1.5.1.12) [7] (gene PUT2), which converts proline to glutamate.
• Bacterial multifunctional putA protein, which contains a delta-1-pyrroline- 5-carboxylate dehydrogenase domain.
• 26G, a garden pea protein of unknown function which is induced by dehydration of shoots [8].
• Mammalian formyltetrahydrofolate dehydrogenase (EC 1.5.1.6) [9]. This is a cytosolic enzyme responsible for the NADP-dependent decarboxylative reduction of 10-formyltetrahydrofolate into tetrahydrofolate. It is an protein of about 900 amino acids which consist of three domains; the C- terminal domain (480 residues) is structurally and functionally related to aldehyde dehydrogenases.
• Yeast hypothetical protein YBR006w.
• Yeast hypothetical protein YER073w.
• Yeast hypothetical protein YHR039c.
• Caenorhabditis elegans hypothetical protein F01F1.6.

A glutamic acid and a cysteine residue have been implicated in the catalytic activity of mammalian aldehyde dehydrogenase. These residues are conserved in all the enzymes of this family. We have derived two patterns for this family, one for each of the active site residues.