Porcine pancreatic procarboxypeptidase A and its tryptic peptides, carboxypeptidase A and the activation segment, have been studied by high-sensitivity differential scanning calorimetry (DSC). The thermal denaturation of the zymogen and the active enzyme has been carried out at two pH values, 7.5 and 9.0, at different ionic strengths and at different scan rates. The endothermic transitions for these two proteins were always irreversible under all conditions investigated. The denaturation behaviour of both proteins seems to fit very well with the kinetic model for the DSC study of irreversible unfolding of proteins recently proposed by one of our groups. From this model, the activation energies obtained for the denaturation of the pro- and carboxypeptidase were 300 +/- 20 kJ mol-1 and 250 +/- 14 kJ mol-1 respectively. On the other hand, the isolated activation segment appears as a thermostable piece with a highly reversible thermal unfolding which follows a two-state process. The denaturation temperature observed for the isolated segment was always at least 15 K higher than those of the zymogen and the active enzyme.
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