Simple spectrophotometric assay for calcium-activated neutral proteases (calpains)

A simple spectrophotometric assay for calcium-activated neutral proteases (calpains) is a simple modification of other popular spectrophotometric assays using casein substrate and precipitation by trichloroacetic acid (TCA). The assay presented here, however, uses azocasein as substrate, and activity is measured by the amount of azo chromophore that remains in solution after TCA precipitation. Because the chromophore is specific to the substrate and the azo substrate absorbs strongly, this assay provides improved spectrophotometric performance in a practical and sensitive procedure.