Microplate assay for cathepsin detection in viable cells using derivatives of 4-methoxy-beta-naphthylamide

This unit describes an assay for the direct and selective detection of the four cathepsins B, H, K, and L in adherently growing cells. Cells are incubated with substrates that are peptidic derivatives of 4-methoxy-beta-naphthylamine partially selective for each cathepsin, together with 5-nitrosalicylaldehyde. The protease reaction is performed in microtiter plates and the fluorescent hydrolysis products are detected using a plate reader. The selectivity of detection is enhanced by parallel assays containing inhibitors that are also partially selective for each of the cathepsins. Individual cathepsin activities can then be calculated by the difference between the uninhibited and the inhibited assays. Detection of cathepsin H activity differs from the other assays in that other nonlysosomal aminopeptidases are inhibited by bestatin. The most common application of these assays is to compare directly cells treated with different substances, e.g., pharmaceutically interesting cathepsin inhibitors.