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Protein Engineering vol. 4 no. 2 pp. 185-190, 1990
© 1990 Oxford University Press

RESEARCH-ARTICLE

Alcohol-induced changes of ß-lactoglobulin - retinol-binding stoichiometry

Eric Dufour and Tomasz Haertl{acute}1

L.E.I.M.A., Institut National de la Recherche Agronomique B.P. 527, 44026 Nantes Cedex 03. France

1To whom correspondence should be addressed

It has been demonstrated using CD that ethanol induces important secondary structure changes of ß-lactoglobulin. CD spectra indicate that ß-lactoglobulin secondary structure, which is mainly composed of ß-strands, becomes mostly {alpha}-helical under the influence of the solvent polarity changes. The midpoint of ß-strand/{alpha}-helix transition in ß-lactoglobulin is observed at dielectric constant {small tilde}60 (35% ethanol; v/v). According to CD measurements, the ethanol-dependent secondary structure changes are reversible. The alkylation of lysines {varepsilon}-NH2 in ß-lactoglobulin weakens the central ß-barrel structure, since the ß-strand/{alpha}-helix transition midpoint of alkylated ß-lactoglobulin is shifted to lower ethanol concentration (25% ethanol; v/v). ß-Lactoglobulin structural changes are triggering the dissociation of the ß-lactoglobulin - retinol complex as judged from complete quenching of its fluorescence in ethanol concentration >30% (v/v). However, in 20% ethanol (v/v), ß-lactoglobulin still retains most of its native secondary structure as shown by CD and, in this condition, one ß-lactoglobulin molecule binds an additional second retinol molecule. This suggests that the highly populated species observed around 20% ethanol (v/v) might represent an intermediate state able to bind two molecules of retinol.

Keywords: ß-barrel structure/folding changes/ß-lactoglobulin/ligand binding/retinol

Received June 15, 1990; accepted September 26, 1990.


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