Procathepsin D is a rapidly processed precursor form of the lysosomal proteinase cathepsin D. The enzymic properties of procathepsin D have been studied by examining the pepstatin-binding characteristics of both the precursor and the mature enzyme. Procathepsin D bound to immobilized pepstatin at 4 degrees C in pH 3.5 buffer but not in pH 5.3 buffer, whereas mature forms of cathepsin D bound to immobilized pepstatin at both pH values. These characteristics of procathepsin D were exploited to isolate the proenzyme from mature forms and to determine whether activation of the proenzyme is an autocatalytic process. After incubation at 37 degrees C in pH 3.5 buffer, the proenzyme underwent pepstatin-inhibitable proteolysis resulting in a dramatically increased affinity of purified procathepsin D for pepstatin at pH 5.3. The low concentration of enzyme used in these studies suggests that procathepsin D cleavage to single-chain cathepsin D may occur via a unimolecular mechanism.