The cytoplasmic isoenzyme of horse liver aldehyde dehydrogenase. Relationship to the corresponding human isoenzyme.

The structural divergence between the cytoplasmic isoenzymes of aldehyde dehydrogenase from different species was investigated by analysis of peptides from the horse protein, and correlation of the results with the complete primary structure of the human isoenzyme. The amino acid sequences of these two proteins show a high degree of homology (91% of residues compared are identical). The differences observed are spread over the entire polypeptide chains, with only one cluster, which is close to a reactive cysteine residue and also adjacent to the most conserved region (covering 68 residues) in the primary structures of the whole enzymes. The secondary structure predicted for the human isoenzyme is mainly unaffected by the residue differences in the horse isoenzyme, although limited conformational changes might be compatible with an unexpected overrepresentation of differences involving isoleucine (12 of 43 exchanges represent a loss of Ile in the horse protein). Two cysteine residues that correlate with catalytic activity are identically positioned in the enzyme from the two species.