The kinetics of the thermal deactivation of transglutaminase from guinea-pig liver

By incubating native (N) transglutaminase from guinea-pig liver at various temperatures and assaying it at 25 degrees C, two steps in the irreversible deactivation process to the denatured form (D) have been found. The fitting of the data to the equations of two possible models (the two-steps model and the two-isoenzymes model) is only compatible with the first one (N----X----D). It is shown that the structure of the active intermediate, X, depends on the deactivation temperature and on the thermal history of the enzyme. This may mean that transglutaminase exists in a large number of microstates. Surprisingly, the activation energy of deactivation is lower than that of activity (36.6 +/- 3.4 against 47.2 +/- 2.2 kJ.mol-1). By deactivating transglutaminase at a constant temperature (55 degrees C) and assaying it at variable temperatures, the activation energy of the intermediate, (X55), has been determined to be 40.2 +/- 5 kJ.mol-1, of the same order of magnitude as the native form. Among several agents assayed, only Ca2+ had a positive effect on the thermal stability of this enzyme. At 40 degrees C, transglutaminase was quite stable in the presence of Ca2+ (in its absence, the half-life was 65 min) and at 45 degrees C, its thermostability had been considerably increased, the half-life being raised from 47 min to 275 min.