ETHANOL EFFECT ON THE THERMAL-DENATURATION OF PEPSINOGEN

Effects of ethanol and pH on the thermodynamic parameters and cooperat ivity of the pepsinogen thermal denaturation were studied by scanning microcalorimetry. Addition of 20% ethanol decreased the protein denatu ration temperature by 10.7 degrees C at pH 6.4 and by 15.8 degrees C a t pH 8.0, thereby the increment in denaturation heat capacity declined from 5.8 to 4.2 kcal/degrees K . mole. However, ethanol did not affec t the number of energetical domains (the molecule regions melting in a n ''all-or-none'' mode). In water as well as in 20% ethanol the depend ences of the calorimetric enthalpy of denaturation on the denaturation temperature were linear and intersected at similar to 95 degrees C, i .e., at the same temperature as for a number of proteins in water and in water-alcohol solutions. The change of pH from 5.9 to 8.2 in 20% et hanol proved to lead to a decrease in the number of pepsinogen regions melting cooperatively; neither the secondary structure of aromatic am ino acids nor their local environment did change. The conclusion was m ade that ethanol did not appreciably affect the cooperativity of pepsi nogen denaturation until the charge distribution in the molecule was s ufficiently altered by the change of pH.