Cathepsins (Ancient Greek kata- “down” and hepsein “boil”; abbreviated CTS) are proteases (enzymes that degrade proteins) found in all animals as well as other organisms. There are approximately a dozen members of this family, which are distinguished by their structure, catalytic mechanism, and which proteins they cleave. Most of the members become activated at the low pH found in lysosomes. Thus, the activity of this family lies almost entirely within those organelles. There are, however, exceptions such as cathepsin K, which works extracellularly after secretion by osteoclasts in bone resorption.
The earliest record of “cathepsin” found in the MEDLINE database (e.g.. via PubMed) is from the Journal of Biological Chemistry in 1949. However, references within this article indicate that cathepsins were first identified and named around the turn of the 20th century. Much of this earlier work was done in the laboratory of Max Bergmann, who spent the first several decades of the century defining these proteases.  Probably word cathepsin have long been determined muscle enzymes active in the acidic environment. Already in 1937 the hemoglobin was using to determined the overall activity of cathepsins (Anson, 1937). First detected cathepsin was in the muscle of fish (Siebert, 1958), and later identified as cathepsin D (Mekinodan and Ikeda, 1969).
Cathepsins have a vital role in mammalian cellular turnover, e.g. bone resorption. They degrade polypeptides and are distinguished by their substrate specificities.
Deficiencies in this protein are linked to multiple forms of galactosialidosis. The cathepsin A activity in lysates of metastatic lesions of malignant melanoma is significantly higher than in primary focus lysates. Cathepsin A increased in muscles moderately affected by muscular dystrophy and denervating diseases.
Cathepsin B seems to actually break down the proteins that cause amyloid plaque, the root of Alzheimer’s symptoms, and may even be a pivotal part of the natural defense against this disease used by people who do not get it. Overexpression of the encoded protein, which is a member of the peptidase C1 family, has been associated with esophageal adenocarcinoma and other tumors. Cathepsin B has also been implicated in the progression of various human tumors including ovarian cancer. Cathepsin B is also involved in apoptosis as well as degradation of myofibrillar proteins in myocardial infarction.
Cathepsin D (an aspartyl protease) appears to cleave a variety of substrates such as fibronectin and laminin. Unlike some of the other cathepsins, cathepsin D has some protease activity at neutral pH. High levels of this enzyme in tumor cells seems to be associated with greater invasiveness. Cathepsin K is the most potent mammalian collagenase. Cathepsin K is involved in osteoporosis, a disease in which a decrease in bone density causes an increased risk for fracture. Osteoclasts are the bone resorbing cells of the body, and they secrete cathepsin K in order to break down collagen, the major component of the non-mineral protein matrix of the bone. Cathepsin K, among other cathepsins, plays a role in cancer metastasis through the degradation of the extracellular matrix. The genetic knockout for cathepsin S and K in mice with atherosclerosis was shown to reduce the size of atherosclerotic lesions. The expression of cathepsin K in cultured endothelial cells is regulated by shear stress. Cathepsin K has also been shown to play a role in arthritis.
Mouse cathepsin L is homologous to human cathepsin V. Mouse cathepsin L has been shown to play a role in adipogenesis and glucose intolerance in mice. Cathepsin L degrades fibronectin, insulin receptor (IR), and insulin-like growth factor 1 receptor (IGF-1R). Cathepsin L-deficient mice were shown to have less adipose tissue, lower serum glucose and insulin levels, more insulin receptor subunits, more glucose transporter (GLUT4) and more fibronectin than wild type controls.
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Reference  Nomura T, Katunuma N (February 2005). “Involvement of cathepsins in the invasion, metastasis and proliferation of cancer cells”. J. Med. Invest. 52 (1–2): 1–9.  Yamashima T (2013). “Reconsider Alzheimer’s disease by the ‘calpain-cathepsin hypothesis’–a perspective review”. PROGRESS IN NEUROLOGY 105: 1–23.  Salminen-Mankonen HJ, Morko J, Vuorio E (February 2007). “Role of cathepsin K in normal joints and in the development of arthritis”. Curr Drug Targets 8 (2): 315–23.