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 B is a member of the cathepsin family consisting of 12 cysteine proteases with broad exo- and endopeptidase activity. Elevated levels of cathepsin B were detected in metastases and neurological disorders including Alzheimer"s disease (AD). The SensoLyte® 520 Cathepsin B Activity Assay Kit provides a QXL^® 520/HiLyte™ Fluor 488 labeled FRET peptide substrate for 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 nm/520 nm. With superior fluorescence quantum yield and longer emission wavelength, the QXL^® 520/HiLyte™ Fluor 488 based FRET peptide has less interference from the autofluorescence of test compounds and cellular components and provides better assay sensitivity. The kit can be used to detect the activity of Cathepsin B 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• Human liver Cathepsin B • Inhibitor • Fluorescence reference standard for calibration • A detailed protocolKit size: 100 assays (96-well plate)