Ca2+ and pH dependence of hydrophobicity of alpha-lactalbumin: affinity partitioning of proteins in aqueous two-phase systems containing poly(ethylene glycol) esters of fatty acids.

Hydrophobic affinity partitioning in an aqueous two-phase system, composed of dextran and poly(ethylene glycol), has been used to study the hydrophobic binding capacity of bovine alpha-lactalbumin. The hydrophobicity of the poly(ethylene glycol)-containing phase was adjusted by including varying amounts of fatty acids bound to the polymer via an ester linkage. The change in the logarithmic partition coefficient of the protein in such systems was used as a measure of the hydrophobic binding. This value was strongly influenced by the amount of Ca2+ present as well as the pH value. The results are discussed in terms of the exposure of hydrophobic binding sites on alpha-lactalbumin and their relation to the conformational change in this protein due to Ca(2+)-binding, chelation of Ca2+ and pH dependence.