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 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. Cathepsin V 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. Inhibitors Five cyclic peptides show inhibitory activity towards human cathepsins L, B, H, and K. Several inhibitors have reached clinical trials, targeting cathepsins K and S as promising therapeutics for osteoporosis, osteoarthritis, and chronic pain. Cathepsin K inhibitors, Relacatib, Balicatib, and Odanacatib, were terminated during clinical trials at phases I, II, and III, respectively, owing to adverse side effects. SAR114137, a Cathepsin S inhibitor, did not progress past phase I for chronic pain. In 2022, STI-1558, a Cathepsin L inhibitor, received FDA clearance to begin phase I studies to treat COVID-19. Cathepsin zymography Zymography is a type of gel electrophoresis that uses a polyacrylamide gel co-polymerized with a substrate in order to detect enzyme activity. Cathepsin zymography separates different cathepsins based on their migration through a polyacrylamide gel co-polymerized with a gelatin substrate. The electrophoresis takes place in non-reducing conditions, and the enzymes are protected from denaturation using leupeptin. After protein concentration is determined, equal amounts of tissue protein are loaded into a gel. The protein is then allowed to migrate through the gel. After