Cathepsin B is a normal lysosomal proteinase that is expressed in all cells. It is a cysteine endoproteinase structurally and functionally related to the papain family of proteases. Cathepsin B is synthesized as an inactive pro enzyme which, by removal of a 62 amino acid propeptide, is activated to the single chain form of 31 kDa or the two chain form (25 and 5 kDa subunits). Matured Cathepsin B is normally localized in the lysosomes where it functions in protein turnover. Cathepsin B is also shown capable of degrading extracellular matrix proteins at acidic and neutral pH. Endogenous inhibitors of cysteine proteases, the cystatins and stefins, may play a major role in regulating Cathepsin B activity. In tumor cells and in cells exposed to mitogens, cathepsin B displays altered cellular trafficking resulting in the secretion of the 43 kDa precursor. Elevated levels of Cathepsin B correlate with malignancy suggesting that this enzyme may be a useful prognostic marker for several types of human cancers. Cathepsin B has also been implicated in the pathogenesis of rhematoid arthritis, muscular dystrophy and tumour metastasis. It is also known as amyloid precursor protein secretase and is involved in the proteolytic processing of amyloid precursor protein (APP).

Lysosome. Melanosome. Note=Identified by mass spectrometry in melanosome fractions from stage I to stage IV.