title: Kinetics of the pH-induced inactivation of human cathepsin L.
abstract: Cathepsin L is known as the most unstable lysosomal cysteine proteinase at neutral or alkaline pH. The kinetics of inactivation of human cathepsin L was studied by mixing the enzyme with a substrate and recording the release of product. The inactivation was found to be a first-order process, and the rate of the process decreased with the substrate concentration. The substrate-independent inactivation rate constant kinact was found to be 0.15 s-1 at pH 7.4 and 37 degrees C and increased 85-fold between pH 7.0 and 8.0. At pH 7.4, kinact increased 3200-fold between 5 and 37 degrees C with an energy of activation 174.7 kJ/mol. Inactive cathepsin L did not reactivate at pH 5.5. The rate of inhibition of cathepsin L by stefin B or chicken cystatin at pH 7.4 was much faster than the rate of spontaneous inactivation of the enzyme. The stefin B-cathepsin L complex incubated at pH 7.4 released active enzyme at pH 5.5, suggesting that the cysteine proteinase inhibitors might act as extracellular carriers of the cysteine proteinases.
authors.affiliation: Department of Biochemistry, J. Stefan Institute, Ljubljana, Slovenia.
authors.forename: B
authors.initials: B
authors.lastname: Turk
authors.forename: I
authors.initials: I
authors.lastname: Dolenc
authors.forename: V
authors.initials: V
authors.lastname: Turk
authors.forename: J G
authors.initials: JG
authors.lastname: Bieth
mesh_terms: D056668:Cathepsin L; D002403:Cathepsins / Q000037:antagonists & inhibitors* / Q000378:metabolism; D055313:Cystatin B; D015891:Cystatins / Q000378:metabolism / Q000494:pharmacology; D003546:Cysteine Endopeptidases; D010450:Endopeptidases; D004789:Enzyme Activation; D004793:Enzyme Reactivators; D004795:Enzyme Stability; D006801:Humans; D006863:Hydrogen-Ion Concentration; D007700:Kinetics; D013696:Temperature; D013816:Thermodynamics; D000511:alpha-Macroglobulins / Q000378:metabolism
chemical_list: C524994:CSTB protein, human; D015891:Cystatins; D004793:Enzyme Reactivators; D000511:alpha-Macroglobulins; D055313:Cystatin B; D002403:Cathepsins; D010450:Endopeptidases; D003546:Cysteine Endopeptidases; C534281:CTSL protein, human; D056668:Cathepsin L