β-lactoglobulin structure and retinol binding changes in presence of anionic and neutral detergents

Abstract

Bovine β-lactoglobulin (β-LG) in vivo (in milks) has been found in complexes with lipids such as butyric and oleic acids. To elucidate the still unknown structure-function relationship in this protein, the structural changes of β-lactoglobulin variant A (β-LG A) in the presence of anionic surfactant such as sodium n-dodecyl sulfate (SDS) and in the presence of nonionic surfactant such as Triton X-100 have been investigated. Subsequently, the retinol binding by β-LG has been investigated in the presence of various amounts of these surfactants as its binding indicator. The results of UV-vis and fluorescence studies show a higher denaturating effect of SDS at acid pH that can be due to greater positive charges of β-LG at this pH indicating also the nonspecific hydrophobic interactions of Triton X-100 with β-LG at all studied pHs. Isothermal titration calorimetry (ITC) measurements indicate the endothermic nature of β-LG/SDS interactions and the exothermic nature of Triton X-100/β-LG interactions. The analysis of the binding data demonstrates the absence of considerable changes in retinol binding properties of β-LG in the presence of various amounts of these surfactants. This implies that surfactant binding does not change the conformation of β-LG in the regions defining the retinol-binding site. © 2008 American Chemical Society.