Purification and characterization of two different precursor forms of the cathepsin B-like proteinase from human malignant ascitic fluid.

We have purified two different precursors of a cathepsin B-like proteinase (PCBT) from malignant ascitic fluid. The molecular mass of these proteins were 45-47 kDa and 36 kDa, respectively. This report is the first which shows cathepsin-B precursors as purified proteins. By using sheep immunoglobulins directed against denatured lysosomal cathepsin B, we have found that both precursors, together with the 33-kDa pepsin generated cathepsin B-like proteinase, reacted in immunoblotting: the three components are thus cathepsin B-related. These antibodies allow us the show that during pepsin activation the 45-47 kDa precursor is converted to the 33-kDa cathepsin B-like proteinase together with the 36-kDa PCBT. We have also prepared sheep immunoglobulins directed against the 36-kDa precursor. The generation of the cathepsin B-like proteinase by pepsin digestion of purified precursors followed a time and dose dependent process. This latter result argues for activation through peptide bond cleavages.