DIFFERENTIAL SCANNING CALORIMETRIC STUDIES ON BOVINE SERUM-ALBUMIN .3. EFFECT OF SODIUM DODECYL-SULFATE

Measurements of differential scanning calorimetry (d.s.c.) have been made on the complex bovine serum albumin (BSA)-sodium dodecyl sulphate (SDS) under various conditions. There are two peaks P1 and P2 in the d.s.c. curve for BSA at pH 7 and in the absence of NaCl, indicating the presence of the heat-induced transition of BSA. There are three peaks P1, P2 and P3 in the curve for the system with the molar mixing ratio SDS/BSA = 1. With the increase in the amount of SDS, the peak P3 grows at the expense of P1 and P2. There is only a single peak P3 in the curve for the systems SDS/BSA > 7, and no peak at SDS/BSA = 50 and 100. There is a single peak P12 in the curve for BSA at pH 7 and in the presence of 0.05 m NaCl, indicating that the heat-induced transition is suppressed. There are two peaks P12 and P3 for the systems SDS/BSA = 1-5; the area ratio of the peak P3 to P12 increases with the increase in the amount of SDS. There is only a single peak P3 when SDS/BSA > 7, and no peak at SDS/BSA = 50. It is concluded that the peak P3 is a product of SDS regardless of the presence or absence of NaCl. Values of thermal denaturation temperature (T(d)) and enthalpy (DELTAH) of thermal denaturation indicate that the complex AD12 (A = BSA, D = SDS) is in the most thermostabilized state. It is suggested that some of the higher-order structures of BSA are reorganized by the binding of 12 SDS so as to make the BSA molecule the most compact and thermostable. D.s.c. curves of AD1 have been measured in the pH range 3-11. Patterns change depending on the pH-induced conformational change of BSA. It is emphasized that the pattern is different below and above pH 5.6 or the isoionic point of BSA. The reason for this is discussed.