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 L, a lysosomal endopeptidase, is a member of the papain-like family of cysteine proteinases. It is involved in the promotion of tumor cell invasion, antigen processing and turnover of intracellular and secreted proteins. The SensoLyte®520 Cathepsin L Activity Assay Kit uses a QXL^® 520/HiLyte™ Fluor 488 labeled FRET peptide substrate for measuring enzyme activity. The long wavelength FRET substrate is designed based on a sequence surrounding the cleavage site of Cathepsin L. The QXL^® 520/HiLyte™ Fluor 488 pair is used for optimal quenching of the intact substrate. When active Cathepsin L cleaves the FRET substrate, it results in an increase of HiLyte™ Fluor 488 fluorescence and is monitored at excitation/emission = 490 nm/520 nm. The fluorescent signal from HiLyte™ Fluor 488 is stable at low pH, the optimal pH for Cathepsin L activity.Kit size: 100 assays (96-well plate)