Title | Autolysis parallels activation of mu-calpain. |
Publication Type | Journal Article |
Year of Publication | 1996 |
Authors | Baki, A., P. Tompa, A. Alexa, O. Molnár, and P. Friedrich |
Journal | Biochem J |
Volume | 318 ( Pt 3) |
Pagination | 897-901 |
Date Published | 1996 Sep 15 |
ISSN | 0264-6021 |
Keywords | Autolysis, Calpain, Enzyme Activation, Enzyme Precursors, Erythrocytes, Humans, Kinetics, Microtubule-Associated Proteins, Molecular Structure, Protein Conformation |
Abstract | The kinetics of autolysis and activation of mu-calpain were measured with microtubule-associated protein 2 (MAP2) as a very sensitive substrate. The initial rate of MAP2 hydrolysis was found to be a linear function of the autolysed 76 kDa form of mu-calpain large subunit at both 10 and 300 microM Ca2+, and both straight lines intersected the origin. This finding supports the view that native mu-calpain is an inactive proenzyme and that activation is accompanied by autolysis. The first-order rate constant of autolysis, K1(aut), was determined at different Ca2+ concentrations: the half-maximal value was at pCa2+ = 3.7 (197 microM Ca2+), whereas the maximal value was 1.52 s-1, at 30 degrees C. The Ca(2+)-induced activation process was then monitored by using our novel, continuous fluorimetric assay with labelled MAP2 as substrate. The first-order rate constant of activation, k1(act), was derived as the reciprocal of the lag phase ('transit time') at the initial part of the progress curve: half-maximum was at pCa2+ = 3.8 (158 microM Ca2+) and the maximum value was 2.15 s-1. The good agreement between the kinetic parameters of mu-calpain autolysis and activation is remarkable. We claim that this is the first kinetically correct determination of the rate constant of autolysis of mu-calpain. Pre-activated mu-calpain has a Ca2+ requirement that is almost three orders of magnitude smaller [half-maximal activation at pCa2+ = 6.22 (0.6 microM Ca2+)]. We cannot exclude the possibility that the activation process involves other mechanistic steps, e.g. the rapid dissociation of the mu-calpain heterodimer, but we state that in our conditions in vitro autolysis and activation run in close parallel. |
Alternate Journal | Biochem. J. |
PubMed ID | 8836135 |
PubMed Central ID | PMC1217702 |
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