Characterization of cathepsin B-like proteinase from ascitic fluid of patients with primary liver cancer.

Ascitic fluid of patients with primary liver cancer was shown to contain a latent thiol proteinase which can be activated by pepsin treatment or by autolysis at acidic environment. This enzyme resembles cathepsin B (EC 3.4.22.1) in many physical-chemical properties including substrate specificity, requirement for thiol activators and inhibition both by thiol blocking reagents and by peptidyl diazomethyl ketones, but has a higher molecular size even after activation. Pepsin treatment and autolysis reduce its Mr from 41 800 to 33 400 and 27 700, respectively, but all these forms are larger than human liver cathepsin B. The latent enzyme may be, therefore, an enzyme--inhibitor complex or an inactive precursor of cathepsin B due to an altered processing in Golgi endoplasmatic reticulum-lysosome compartment.