Differential scanning calorimetric studies on bovine serum albumin: II. Effects of neutral salts and urea.

Using defatted and SH-blocked bovine serum albumin (BSA), measurements of differential scanning calorimetry (d.s.c.) have been made mainly in NaSCN solution. BSA undergoes a heat-induced conformational transition in a particular range of pH and ionic strength and is separated into two thermally independent units, each of which has different thermostability in acidic and alkaline pH regions. Comparisons were made of the pH dependencies of the enthalpy of thermal denaturation (delta H) and the temperature of thermal denaturation (Td) in 0.01 M NaSCN with those in 0.01 M NaCl. It has been found that the stabilizing effect of NaSCN on BSA is larger than that of NaCl at pH 3.5-8, and that the heat-induced transition occurs by the electrostatic repulsive forces among the positively charged amino acid residues in a segment Arg 184-Arg 216 containing Trp 212 and the primary binding sites of anions. At ionic strength 0.01, the relative effectiveness of anions in suppressing the heat-induced transition and increasing the thermostability of BSA follows the order ClO4- - greater than or equal to SCN- greater than I- greater than SO4(2-) greater than Br- greater than Cl-. At ionic strength 0.1, the heat-induced transition is suppressing in all the salt solutions, and a Td increase follows the order ClO4- greater than or equal to SCN- greater than I- greater than Br- greater than Cl- greater than or equal to SO4(2-). Thus, the highly chaotropic ions thermostabilize BSA more markedly than kosmotropic ions in the low and moderate salt concentrations.(ABSTRACT TRUNCATED AT 250 WORDS)