The Interaction of Bovine Plasma Albumin with Cationic Detergent. Studies by Binding Isotherm, Optical Rotation and Difference Spectrum

Abstract

Abstract Binding isotherms were determined at pH 6.9 for systems of bovine plasma albumin (BPA) and cationic detergents at 25°C and 5 °C. Detergents used were: hexadecyltrimethylammonium bromide (HTAB), tetradecyltrimethylammonium bromide (TTAB), dodecyltrimethylammonium bromide (DTAB) and decyltrimethylammonium bromide (DeTAB). Binding affinity of the cationic detergent to BPA increased with the increase in the carbon number of the detergent, and with the increase in temperature. The first five detergent ions were bound to BPA statistically at 25 °C, and succeeding detergent ions were bound cooperatively. Thermodynamic parameters indicated that the statistical binding was caused mainly by the hydrophobic bonding. Measurements of −[α]233 and −[α]313 at pH 5.2 revealed that the conformation of BPA changed when it complexed with the cationic detergent. The conformation of BPA changed slightly when 5–8 HTAB or TTAB’s were bound, and a second large conformational change occurred when 15–20 of these detergent ions were bound. DTAB and DeTAB caused only the first conformational change. Thus HTAB and TTAB are stronger unfolders of BPA than DTAB and DeTAB. The UV difference spectrum of the complex BPA–TTAB showed a red shift of the peak of Try residue (e.g. 292 nm), being in contrast to the blue shift of the same peak in the complex BPA–sodium dodecyl sulfate. It is suggested that BPA is unfolded, at least, in the NH2 terminal half by binding with cationic detergent.