IMPROVED CALPAIN ASSAY USING FLUORESCEIN ISOTHIOCYANATE-LABELED CASEIN

A sensitive assay has been developed using a fluorescein isothiocyanat e (FITC)-labeled casein substrate. Fluorescence of TCA-soluble FITC-la beled peptides was measured with an excitation wavelength of 490 nm an d an emission wavelength of 513 nm. Fluorometric results in a time cou rse assay correlated highly (r=0.985) with standard calpain assay (abs orbance of TCA-soluble peptides at 278 nm) results. Fluorescence assay results correlated highly with calpain activity (r=0.997) in an activ ity titration assay and were substantially more sensitive than the sta ndard method. Using FITC-labeled casein provides a simple, sensitive a ssay for mu- and m-calpain.