Cathepsins are a class of globular lysosomal proteases playing a vital role in mammalian cellular turnover. They degrade polypeptides and are distinguished by their substrate specificities. Cathepsin K is the lysosomal cysteine protease involved in bone remodeling and resorption. It has potential as a drug target in autoimmune diseases and osteoporosis. The SensoLyte® 520 Cathepsin K Activity Assay Kit uses a QXL^® 520/HiLyte™ Fluor 488 FRET peptide substrate for the measurement of enzyme activity. In the intact FRET peptide, the fluorescence of HiLyte™ Fluor 488 is quenched by QXL^® 520. Upon cleavage of the FRET peptide by the active enzyme, the increase of fluorescence can be continuously monitored at excitation/emission = 490/520 nm. The fluorescent signal from HiLyte™ Fluor 488 is stable at low pH, which is optimal for cathepsin activity. The kit can be used to detect the activity of Cathepsin K enzyme in biological samples and purified enzyme preparations. The kit contains: • QXL^® 520/HiLyte™ Fluor 488-based FRET peptide substrate (Ex/Em=490/520 nm upon cleavage) • Assay buffer • Procathepsin K • Activation buffer • Inhibitor • Fluorescence reference standard for calibration • A detailed protocol Kit size: 100 assays (96-well plate)