Cathepsin , lysosomal, properties

Several lines of evidence indicate that neoplastic cells per se are the main source of extracellular thiol proteinase activity. Recent studies (39) have shown that malignant human breast tumors maintained in organ culture secrete high levels of a cathepsin B-like enzyme into the culture medium. Moreover high levels of cathepsin B-like enzyme are present in the serum of patients with a wide variety of cancers, and these levels decrease when the cancer tissue is removed or treated with therapeutic agents (64, 65). Cathepsin B-like enzyme from cultured cells of malignant tumors (39,66) possesses enzymic properties similar to those of cathepsin B with respect to specificity, affinity, and pH optima for synthetic substrates. It hydrolyzes Bz-Arg-Arg-2-naphthylamide and is inhibited by leupeptin. However, the tumor enzyme is much more stable than cathepsin B to inactivation above pH 7. It has a molecular weight of about 33,000-35,000. The distribution of cathepsin B-like activity was determined in fractions of control and neoplastic epithelial cells from human ectocervix (66). The activity is present mainly in the mitochondrial and lysosomal fractions of normal cells but mainly in the plasma membranes and nuclei of neoplastic cells. 

Mammalian liver and muscle frucose bisphosphate aldolases are also very susceptible to limited proteolysis (5,53-55). Cathepsin B, cathepsin L, and papain catalyze the limited proteolysis of rabbit muscle and rat liver aldolases 50,51). In fact, decrease of aldolase activity in liver is observed during starvation 109) and after administration of lysosome-tropic agents 103). Leupeptin caused an increase in osmotic sensitivity of lysosomes and an increase in the activities of free lysosomal proteinases, such as cathepsin A and cathepsin D, and a moderate increase of cathepsin B and L, and resulted in a decrease in aldolase activity. The molecular properties of aldolase isolated from the livers of control rats and leupeptin-treated rats indicated that the decrease of aldolase activity is attributable to hydrolysis of a peptide linkage(s) near the carboxyterminal of the enzyme. However, care is necessary in determining whether proteolytic modification of enzymes... 

During the course of studies on lysosomes in rat thoracic duct lymphocytes (TDL), it was found that cathepsin D, a typical lysosomal enzyme in most cells and tissues, did not show the same distribution as the other lysosomal acid hydrolases after fractionation. In this paper, we report some of our findings concerning the unique properties of this rat TDL enzyme. 

Inhibition by antiserum. An intracellular localization for cathepsin D different from that of the other lysosomal acid hydrolases in rat TDL led us to explore some of the biochemical properties of this enzyme. As illustrated in Figure 3, an antiserum prepared in rabbits against rat liver soluble lysosomal enzymes effectively inhibited rat liver cathepsin D, although it did not inhibit the cathepsin D of rat TDL. In this case the incubations were carried out at pH 5 instead of pH 3.6 to avoid dissociation of the antigen-antibody complex. Both rat liver and rat TDL cathepsin D, however, have identical pH activity curves toward denatured bovine hemoglobin as substrate. 

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