Human lymphocyte X-prolyl aminopeptidase (aminopeptidase P)-like protein. A new member of the proline peptidase family?

Peptidases are grouped according to their mechanism of catalysis in serine-type, cysteine-type, aspartic-type and metallo-type peptidases. Amongst these groups, the metallopeptidases represent the most diverse group, comprising 25 different families (1,2). Most of the Zn-binding metallopeptidases, named zincins, have the HEXXH motif for Zn-binding. Some other Zn-binding motifs have been defined, as there are the HXXEH, the HXXE, and the HXH motif (3). X-prolyl aminopeptidase (aminopeptidase P, EC 3.4.11.9), is an aminopeptidase that has been reported to bind zinc, and is activated by manganese ions, but does not contain any of these Zn-binding motifs. Aminopeptidase P from Escherichia coli was classified in the peptidase family M24, a family of metallopeptidases in which the ligands for metal ion binding are predominantly carboxylic acids (2). Apart from E. coli aminopeptidase P, this family comprises E. coli methionyl aminopeptidase (EC 3.4.11.18) and E. coli and human proline dipeptidase (EC 3.4.13.9). We have been studying the soluble form of aminopeptidase P in human lymphocytes and platelets (4,5), and were interested in determining the nucleotide sequence that would provide useful information for the development of potent and specific inhibitors. Aminopeptidase P is a proline-specific metallo-aminopeptidase that catalyses specifically the removal of any unsubstituted N-terminal amino acid that is adjacent to a penultimate proline residue. Due to its specificity towards proline, it has been suggested that aminopeptidase P is important