REGULATION OF THE ACTIVITY OF LYSOSOMAL CYSTEINE PROTEINASES BY PH-INDUCED INACTIVATION AND/OR ENDOGENOUS PROTEIN INHIBITORS, CYSTATINS

The kinetics of pH-induced inactivation of human cathepsins B and L was studied by conventional and stopped-flow methods. The inactivation of both enzymes was found to be an irreversible, first-order process. The inactivation rate constants increased exponentially with pH for both enzymes, From log k(inac) vs pH plots, 3.0 and 1.7 protons were calculated to be desorbed for pH-induced inactivation of cathepsins L and B. Cathepsin B was thus substantially more stable than cathepsin L (similar to 15-fold at pH 7.0 and 37 degrees C). Cathepsin B was efficiently inhibited by cystatin C at pH 7.4, whereas the inhibition by stefin B and high molecular weight kininogen was only moderate, In contrast, cathepsin L was efficiently inhibited by both chicken cystatin and stefin B at this pH k(ass) similar to 3.3 x 10(7) M(-1) s(-1)).