Rat mammary gland in culture secretes a stable high molecular weight form of cathepsin L.

Culture medium from rat mammary gland explants was analyzed for the presence of cysteine proteinases. In addition to a putative precursor of the lysosomal enzyme cathepsin B, a cysteine proteinase with enzymatic properties similar to those reported for cathepsin L was found. Further evidence of the cathepsin L-like nature of this activity was provided by its high sensitivity towards the diazomethane inhibitors Z-Phe-Phe-CHN2 and Z-Phe-Ala-CHN2 and towards leupeptin. The secreted form of cathepsin L is distinguished from the lysosomal form by its increased stability at alkaline pH and by its larger molecular size. It may thus represent an incompletely processed precursor form of the lysosomal enzyme.