EFFECT OF SODIUM DODECYL-SULFATE AND PALMITIC ACID ON THE EQUILIBRIUM UNFOLDING OF BOVINE BETA-LACTOGLOBULIN

The unfolding of bovine beta-lactoglobulin, a small globular protein that unfolds reversibly at low pH in the presence of urea or guanidine hydrochloride, has been studied at pH 6.72 in phosphate buffer at 21 degrees C. The midpoint urea concentration for the loss of CD intensity at 220 nm, loss of CD intensity at 293 nm, quenching of intrinsic fluorescence, shift in the wavelength of the maximum of the intrinsic fluorescent emission, and loss of fluorescence intensity from 1-anilino-8-naphthalenesulfonate (ANS) (and probably the hydrophobic binding site) was close to 4.4 M. Addition of sodium dodecyl sulfate (SDS) at concentrations less than 100 mu M to the beta-lactoglobulin solutions increased the midpoint urea concentration for the CD and intrinsic fluorescence parameters to about 5.8 M. Palmitic acid had a similar effect to that shown by SDS in altering the CD intensity at 293 nm, and both SDS and palmitic acid attained a maximum effect in altering the CD at 293 nm at a 1:1 molar ratio to beta-lactoglobulin. It seems Likely that the beta-sheet structure of beta-lactoglobulin breaks down simultaneously with the loss of the hydrophobic binding site and exposure of tryptophan-19 to the external environment, supporting the view that the major hydrophobic binding site of beta-lactoglobulin is closely involved with the beta-sheet core of the protein. The increased stability of the protein toward unfolding in the presence of SDS or palmitate suggests that each of these ligands occupies the cavity of the beta-barrel of beta-lactoglobulin and stabilizes the protein against urea unfolding via strengthened hydrophobic interaction and a greater exclusion of water from the cavity. This conclusion supports the notion that bovine beta-lactoglobulin binds hydrophobic ligands in an analogous fashion to human serum retinol-binding protein (RBP).