Horse Liver Alcohol Dehydrogenase

The ethanol‐active isoenzyme of horse liver alcohol dehydrogenase has been carboxymethylated with iodo[ 14 C]acetate and different samples of the labelled protein treated with trypsin (both with and without previous maleylation of the substrate), chymotrypsin, pepsin or cyanogen bromide. In each case most of the resultant peptides have been purified and characterized. Data are given for those peptides originating from an N‐terminal region comprising about 40% of the protein chain and the amino acid sequence of the first 140 residues is deduced. It is established that over this region the two chains of the enzyme have an identical sequence. The N‐terminus is an acetylated serine residue and the “active site” cysteine residue occupies position no. 46. One of the two tryptophans of the chain, but none of the four tyrosines, is in the sequence given. The region around position 100 is rich in cysteine residues. Some serine residues were found to have reacted during maleylation of the protein. The amino acid sequence is compared to the previously known sequences of glyceraldehyde 3‐phosphate dehydrogenase from two species. Similarities strongly suggest a true homology, at least of an N‐terminal region, and support theories of a common ancestral origin for these enzymes. The reactive cysteine residues are, however, at widely different positions in the primary structures.