REVERSIBLE EFFECTS OF MEDIUM DIELECTRIC-CONSTANT ON STRUCTURAL TRANSFORMATION OF BETA-LACTOGLOBULIN AND ITS RETINOL BINDING

The secondary structure transformation of beta-lactoglobulin from a predominantly beta-structure into a predominantly alpha-helical one, under the influence of solvent polarity changes is reversible. Independent of the alcohol used - methanol, ethanol, or 2-propanol - the midpoints of the observed structural transformation occur around dielectric constant epsilon almost-equal-to 60. The structural change destroying the hydrophobic core formed by the beta-barrel structure leads, at room temperature, to the dissociation of the retinol/beta-lactoglobulin complex in the neighborhood of dielectric constant epsilon almost-equal-to 50. However, when the dielectric constant of the medium is raised back to epsilon almost-equal-to 70 by the decrease of the temperature, both the refolding of BLG into a beta-structure and the reassociation of the retinol/beta-lactoglobulin complex are observed. The esterification of beta-lactoglobulin carboxyl groups has two effects: on the one hand it accelerates the beta-strand <-- --> alpha-helix transition induced by alcohols. On the other hand, the esterification of beta-lactoglobulin strengthens its interaction with retinol as it may be deduced from the smaller apparent dissociation constant of retinol/methylated beta-lactoglobulin complex. The binding of retinol to modified or unmodified beta-lactoglobulin has no influence (stabilizing or destabilizing) on the folding changes induced by alcohol.