On the homology of the active-site peptides of liver carboxylesterases

1. 1.Highly purified preparations of liver carboxylesterases (carboxylic ester hydrolases, EC 3.1.1.1) from pig, sheep, ox and chicken were stoichiometrically labelled with [P]DFP, and then subjected to peptic digestion. 2. 2.Radioactive peptides were isolated from the peptic digests by chromatography on Sephadex G-25, paper chromatography and high voltage electrophoresis. For each species, an octapeptide was isolated as the major radioactive peptide. 3. 3.Amino acid analyses of pig and sheep octapeptides were identical with the previously published analysis of the corresponding octapeptide from horse liver carboxylesterase. Analyses of ox and chicken octapeptides both indicated single amino acid substitutions when compared with the horse octapeptide. 4. 4.The two amino acid substitutions were located by conventional sequencing procedures. In the ox octapeptide, alanine replaces the glycine three residues from the labelled serine towards the C-terminal. In the chicken peptide, isoleucine replaces the glutamic acid residue four removed from the serine towards the C-terminal. The possible significance of the amino acid substitutions is discussed in terms of other properties of the enzymes