Human thimet oligopeptidase.

We have purified human thimet oligopeptidase to homogeneity from erythrocytes, and compared it with the enzyme from rat testis and chicken liver. An antiserum raised against rat thimet oligopeptidase also recognized the human and chicken enzymes, suggesting that the structure of the enzyme has been strongly conserved in evolution. Consistent with this, the properties of the human enzyme were very similar to those for the other species. Thus human thimet oligopeptidase also is a thiol-dependent metallo-oligopeptidase with M(r) about 75,000. Specificity for cleavage of a number of peptides was indistinguishable from that of the rat enzyme, but Ki values for the four potent reversible inhibitors tested were lower. In discussing the results, we consider the determinants of the complex substrate specificity of thimet oligopeptidase. We question whether substrates containing more than 17 amino acid residues are cleaved, as has been suggested. We also point out that the favourable location of a proline residue and a free C-terminus in the substrate may be as important as the hydrophobic residues in the P2, P1 and P3' positions that have been emphasized in the past.