Proteolytic activation of human procathepsin D.

Procathepsin D is a short-lived inactive precursor of the lysosomal aspartyl protease, cathepsin D. Pulse-chase analysis using radiolabeled amino acids demonstrated the existence of several biosynthetic intermediates during formation of mature cathepsin D (summarized in Figure 1). Procathepsin D is capable of autocatalytic cleavage to pseudocathepsin D. This was demonstrated using small quantities of procathepsin D isolated from cell culture media as well as using a non-glycosylated form of procathepsin D synthesized in a bacterial expression system. Complete conversion to the single-chain cathepsin D appears to require a second enzyme which is inhibited by leupeptin. This conclusion was drawn from the inability to produce single-chain enzyme from either procathepsin D or pseudocathepsin D in vitro as well as observations from addition of protease inhibitors to cell cultures. It appears that the conversion of procathepsin D to active single-chain enzyme falls between the paradigms of pepsinogen autoactivation and prorenin conversion by a separate enzyme.