Characterization of a cysteine proteinase secreted by mouse mammary gland

The cysteine proteinase activity secreted by cultured mammary gland was characterized to determine its relationship to a similar enzyme secreted by explants of mouse mammary tumors. Enzymic characterization showed that the secreted enzyme was similar to the lysosomal cysteine proteinase cathepsin B, and physical characterization showed properties identical to a stable cysteine proteinase secreted from mammary tumors reported previously. The secreted enzyme cross-reacted with mouse cathepsin B isolated from liver in a radioimmunoassay. The secreted enzymes are stable at alkaline pH, but irreversible conformational changes can be induced in vitro which render them unstable and thus similar to lysosomal cathepsin B, which is also unstable at alkaline pH. Tissue homogenates from fresh and cultured mammary gland contain mainly pH-unstable cathepsin B; however, the molecular size for the tissue cathepsin B, while smaller than that of the secreted enzymes, was found to be larger than that reported for mouse liver cathepsin B. Isoelectric focusing profiles were also slightly different as compared to those of mouse liver. These data suggest that there might be differences in the processing of cathepsin B between different tissues and organs, and the high degree of similarity between the forms of cathepsin B secreted from mouse mammary tumors, mouse mammary gland, and human malignant breast tumors suggests a similar mechanism for their extracellular release in these tissues.