Aldehyde dehydrogenase. Covalent intermediate in aldehyde dehydrogenation and ester hydrolysis.

4-trans-(NN-Dimethylamino)cinnamaldehyde (an aldehyde, DACA) and 4-trans-(NN-dimethylamino)cinnamoylimidazole (an amide, DACI) have been shown to be substrates for human aldehyde dehydrogenase (EC 1.2.1.3) which form chromophoric covalent intermediates. The spectra of covalent intermediates from both the cytoplasmic (E1) and mitochondrial (E2) isoenzymes derived from DACA and DACI were compared. The spectra were similar when either substrate was used, and also when the two isoenzymes were compared, and resembled that obtained for 4-trnas-(NN-dimethylamino)cinnamoyl-N-acetylcysteine, but differed from the spectrum of 4-trans-(NN-dimethylamino)cinnamoyl ethyl ester. After extensive digestion of the covalent intermediates from both 3H-labelled DACA and DACI with Pronase and purification, the labelled amino acid was identified as cysteine. Covalent intermediates from both DACA and DACI were also digested with trypsin, and labelled peptides were purified by ion-exchange and reverse-phase chromatography. Amino acid sequence analysis showed that the peptide comprising residues 273-307 was labelled by both DACA and DACI. The radioactive label at cysteine residues 301-303 of the primary structure could be unequivocally identified by employing the DACA derivative. Assignment of label to cysteine-302 was achieved by employing iodoacetamide-labelled E1 isoenzyme (iodoacetamide specifically labels cysteine-302), in which case there was no formation of the covalent intermediate from either DACA or DACI. In addition, cysteine-302 is the only cysteine residue conserved in all aldehyde dehydrogenases sequenced. Thus cysteine-302 is the amino acid residue that forms a covalent intermediate with both aldehyde and ester substrates.