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Unfolding and Refolding of Bovine Serum Albumin at Acid pH: Ultrasound and Structural Studies

N. El Kadi ^*, N. Taulier ^*, J. Y. Le Huérou ^*, M. Gindre ^*, W. Urbach  , I. Nwigwe , P. C. Kahn  and M. Waks ^*

^* Centre National de la Recherche Scientifique, Unité Mixte de Recherche 7623, Laboratoire d'Imagerie Paramétrique, Paris F-75006, France, and Université Pierre et Marie Curie-Paris6, Paris F-75005, France; Department of Biochemistry & Microbiology, Rutgers University, New Brunswick, New Jersey 08901; UFR Biomédicale, Université René Descartes, 75006 Paris, France; and Laboratoire de Physique Statistique, UMR, 8550 CNRS, Ecole Normale Supérieure, 75231 Paris Cedex 05, France

Correspondence: Address reprint requests to W. Urbach, Laboratoire de Physique Statistique, UMR, 8550 CNRS, Ecole Normale Supérieure, 24 rue Lhomond, 75231 Paris Cedex 05, France. E-mail: urbach{at}lps.ens.fr.

Serum albumin is the most abundant protein in the circulatory system. The ability of albumins to undergo a reversible conformational transition, observed with changes in pH, is conserved in distantly related species, suggesting for it a major physiological role possibly related to the transport of small molecules including drugs. We have followed changes of bovine serum albumin (BSA) in volume by densimetry and in adiabatic compressibility during its conformational transition from pH 7–2, using ultrasound measurements. In parallel, circular dichroism was measured. The volume and adiabatic compressibility decrease from pH 4 to 2. The change in ellipticity shows a decrease over the same pH range from 70% to 40% of its -helix content. Sorbitol, at concentrations from 0 to 2 M, led to the progressive restoration of BSA volume and compressibility values, as well as a substantial recovery of its original -helix content. This finding implies that the compressibility variation observed reflects the conformational changes during the transition. The mutual interactions of the mechanical properties and structural features of BSA reported here are important in biotechnology for research in material sciences and for the design and the development of new, tailor-made drug carriers.