Thermal inactivation studies of normal and variant human erythrocyte carbonic anhydrases by using a sulphonamide-binding assay.

Heat-inactivation studies were carried out on the two primary erythrocyte carbonic anhydrase isoenzymes, CA I and CA II, and the secondary isoenzyme of CA I, CA I (+1). In addition, two genetic variants of human isoenzyme CA I, CA Id Michigan (100 Thr-->Lys) and CA If London (102 Glu-->Lys), and one variant of isoenzyme CA II, CA IIh (251 Asn-->Asp), were similarly analysed. The first-order rate constants and Arrhenius plots for these six enzyme forms showed that (1) isoenzyme CA II is more heat-stable than CA I, (2) isoenzyme CA I (+1) is less heat-stable than CA I, (3) the variants CA IIh and CA If London are less heat-stable than the normal enzymes, and (4) isoenzyme CA Id Michigan is more heat-stable than normal CA I. From the values of the slopes of the Arrhenius plots, the energy of activation (E(a)) for each isoenzyme and isoenzyme variant was determined, and the following thermodynamic activation parameters were calculated at 55 degrees C: the free energy of activation (DeltaG(double dagger)), the activation enthalpy (DeltaH(double dagger)) and the activation entropy (DeltaS(double dagger)). The DeltaG(double dagger) for the enzymes shows a relative constancy with compensating variation in DeltaH(double dagger) and DeltaS(double dagger). When the values for DeltaH(double dagger) are plotted against DeltaS(double dagger), an increase in DeltaH(double dagger) involves a concomitant increase in DeltaS(double dagger).