Comparison of the Effect of Calpain Inhibitors on Two Extralysosomal Proteinases: The Multicatalytic Proteinase Complex and m‐Calpain

The potencies of three peptide aldehyde inhibitors of calpain (calpain inhibitors 1 and 2 and calpeptin) as inhibitors of four catalytic activities of the multicatalytic proteinase complex (MPC) were compared with their potencies as inhibitors of m‐calpain. The chymotrypsinlike activity (cleavage after hydrophobic amino acids) and the caseinolytic activity (degradation of β‐casein) of MPC were strongly inhibited by calpain inhibitors 1 and 2 (IC 50 values in the low micromolar range). Cleavage by MPC after acidic amino acids (peptidylglutamyl‐peptide bond hydrolyzing activity) and basic amino acids (trypsinlike activity) was inhibited less effectively, declining moderately with increasing concentrations of calpain inhibitors 1 and 2. Calpeptin only weakly inhibited the four MPC activities, yet was the most potent inhibitor of m‐calpain. These results indicate that caution must be exercised when calpain inhibitors 1 and 2 are used to infer calpain function. Calpeptin may be a better choice for such studies, although its effect on other cysteine or serine proteinases remains to be determined.