title: Exocytosis of active cathepsin B enzyme activity at pH 7.0, inhibition and molecular mass.
abstract: Lysosomal cathepsin B has been implicated in parasitic, inflammatory and neoplastic diseases. Most of these pathologies suggest a role for cathepsin B outside the cells, although the origin of extracellular active enzyme is not well defined. The activity of extracellular cathepsin B is difficult to assess because of the presence of inhibitors and inactivation of the enzyme by oxidizing agents. Therefore, we have developed a continuous assay for measurement of cathepsin B activity produced pericellularly by living cells. The kinetic rate of Z-Arg-Arg-NHMec conversion was monitored and the assay optimized for enzyme stability, cell viability and sensitivity. To validate the assay, we determined that human liver cathepsin B was stable and active under the conditions of the assay and its activity could be inhibited by the selective epoxide derivative CA-074. Via this assay, we were able to demonstrate that active cathepsin B was secreted pericellularly by viable cells. Both preneoplastic and malignant cells secreted active cathepsin B. Pretreatment of cells with the membrane-permeant proinhibitor CA-074Me completely abolished pericellular and total cathepsin B activity whereas pretreatment with the active drug CA-074 had no effect. Immunoprecipitation and immunoblotting experiments suggested that the active enzyme species was 31-kDa single-chain cathepsin B. Exocytosis of cathepsin B was not related to secretion of proenzyme or secretion from mature lysosomes. Our results suggest an alternative pathway for exocytosis of active cathepsin B.
authors.affiliation: Department of Pharmacology, Wayne State University School of Medicine, Detroit, MI 48201, USA.
authors.forename: B E
authors.initials: BE
authors.lastname: Linebaugh
authors.forename: M
authors.initials: M
authors.lastname: Sameni
authors.forename: N A
authors.initials: NA
authors.lastname: Day
authors.forename: B F
authors.initials: BF
authors.lastname: Sloane
authors.forename: D
authors.initials: D
authors.lastname: Keppler
mesh_terms: D002401:Cathepsin B / Q000037:antagonists & inhibitors / Q000737:chemistry / Q000378:metabolism*; D002461:Cell Line, Transformed; D005089:Exocytosis; D005455:Fluorescent Antibody Technique; D006801:Humans; D006863:Hydrogen-Ion Concentration; D007700:Kinetics; D008970:Molecular Weight; D015203:Reproducibility of Results; D012680:Sensitivity and Specificity; D014407:Tumor Cells, Cultured
chemical_list: D002401:Cathepsin B