Afaltoxin B 1 transport in rat blood plasma. Binding to albumin in vivo and in vitro and spectrofluorimetric studies into the nature of the interaction

Abstract

Binding of [ 3H]aflatoxin B 1 to rat plasma was investigated in vivo and vn vitro. Column chromatographic and polyacrylamide gel electrophoretic analysis clearly demonstrated that aflatoxin B 1 bound primarily plasma albumin. Very little binding activity was shown by other plasma proteins. Spectrofluorimetric studies were undertaken to gain some insight into the nature of the aflatoxin-albumin interaction. Quenching of the lone tryptophan fluorescence intensity upon aflatoxin binding was due, at least in part, to a ligand-induced conformational change in the albumin molecule. Aflatoxin B 1 binds an apolar site with an association constant of 30 mM −1 at pH 7.4 and 20°C. Neither charcoal treatment of rat albumin nor the presence of 0.15 M NaCl had many significant effect on the interaction. The association constant was pH-dependent, increasing about 1.7-fold as the pH increased from 6.1 to 8.4. This pH dependence is ascribed to a pH-induced conformational change in the albumin molecule. Thermodynamic studies indicated that the aflatoxin-albumin interaction was exothermic ( ΔH = −29.3 kJ·mol −), with a ΔS value of −13.8 J·mol −1·K −1.