Human recombinant pro-dipeptidyl peptidase I (cathepsin C) can be activated by cathepsins L and S but not by autocatalytic processing

Human dipeptidyl peptidase I was expressed in the insect cell/baculovirus s ystem and purified in its active (rhDPPI) and precursor (pro-rhDPPI) forms. RhDPPI was very similar to the purified enzyme (hDPPI) with respect to gly cosylation, enzymatic processing, oligomeric structure, CD spectra, and cat alytic activity. The precursor, which was a dimer, could be activated simil ar to 2000-fold with papain. Cathepsin L efficiently activated pro-rhDPPI i n vitro at pH 4.5 (k(app) similar to 2 x 10(3) min(-1) M-1), and two cleava ge pathways were characterized. The initial cleavage was within the pro reg ion between the residual pro part and the activation peptide. Subsequently, the activation peptide was cleaved from the catalytic region, and the latt er was cleaved into the heavy and light chains. Alternatively, the pro regi on was first separated from the catalytic region. Cathepsin S was a less ef ficient activating enzyme. Cathepsin B and rhDPPI did not activate pro-rhDP PI, and the proenzyme was incapable of autoactivation. Incubation of both p ro-rhDPPI and rhDPPI with cathepsin D resulted in degradation. Cystatin C a nd stefins A and B inhibited rhDPPI with K-i values in the nanomolar range (K-i = 0.5-1.1 nM). The results suggest that cathepsin L could be an import ant activator of DPPI in vivo and that cathepsin D and possibly the cystati ns may contribute to DPPI downregulation.