Maturation of human procathepsin B. Proenzyme activation and proteolytic processing of the precursor to the mature proteinase, in vitro, are primarily unimolecular processes.

Recombinant latent human procathepsin B produced in yeast was purified to near homogeneity. The purified recombinant proenzyme is activated in vitro under acidic conditions resulting in rapid conversion into the mature form of the proteinase. Activation as well as proteolytic maturation of the recombinant cathepsin B precursor were shown to be primarily concentration-independent processes indicating a unimolecular (i.e. intramolecular) mechanism. Only one cleavage site was identified, yielding a mature polypeptide with the same amino-terminal sequence as that found in recombinant active human cathepsin B obtained from yeast culture media. The same peptide bond is cleaved during processing of a nonactivatable mutant of procathepsin B by the purified mature enzyme (i.e. intermolecular processing). Thus, the complete proregion is liberated during procathepsin B processing. This peptide may then act as a reversible inhibitor and stabilizer of the mature proteinase, and it appears likely that cathepsin B-propeptide complexes occur transiently during proteolytic maturation.