We recently observed that specific inhibitors of post-proline cleaving aminodipeptidases cause apoptosis in quiescent lymphocytes in a process independent of CD26/dipeptidyl peptidase IV. These results led to the isolation and cloning of a new protease that we have termed quiescent cell proline dipeptidase (QPP). QPP activity was purified from CD26(-) Jurkat T cells. The protein was identified by labeling with [(3)H]diisopropylfluorophosphate and subjected to tryptic digestion and partial amino acid sequencing. The peptide sequences were used to identify expressed sequence tag clones. The cDNA of QPP contains an open reading frame of 1476 base pairs, coding for a protein of 492 amino acids. The amino acid sequence of QPP reveals similarity with prolylcarboxypeptidase. The putative active site residues serine, aspartic acid, and histidine of QPP show an ordering of the catalytic triad similar to that seen in the post-proline cleaving exopeptidases prolylcarboxypeptidase and CD26/dipeptidyl peptidase IV. The post-proline cleaving activity of QPP has an unusually broad pH range in that it is able to cleave substrate molecules at acidic pH as well as at neutral pH. QPP has also been detected in nonlymphocytic cell lines, indicating that this enzyme activity may play an important role in other tissues as well.
A dipeptidyl-peptidase IV was purified from the culture medium of the human-pathogenic fungus Aspergillus fumigatus. The enzyme has an apparent molecular mass of 95 kDa and contained approximately 10 kDa of N-linked carbohydrate. This glycoprotein is antigenic and has all characteristics of the class IV dipeptidyl-peptidases: removal of Xaa-Pro and to a lesser extent Xaa-Ala dipeptides from the N termini of peptides, including bioactive peptides such as neuropeptide Y, [des-Arg1] bradykinin, and glucagon-like peptide 1, activity at neutral pH, and presence in the amino acid sequence of the Gly-X-Ser-X-Gly consensus motif of the serine-hydrolases and the putative catalytic triad (Ser613, Asp690, His725) of the dipeptidyl-peptidases. Moreover, the last 200 amino acids displayed 60 to 65% similarity with the other dipeptidyl-peptidases IV from rat, mouse, human, and yeast. However, unlike the other dipeptidyl-peptidases, the dipeptidyl-peptidase IV of A. fumigatus is a secreted enzyme with a cleavable signal peptide. Expression of a recombinant dipeptidyl-peptidase IV of A. fumigatus has been attained in the yeast Pichia pastoris.
