Molecular Pharmacology, Vol 11, 144-152, Copyright © 1975 by the American Society for Pharmacology and Experimental Therapeutics

Molecular Aspects of the Interaction of Bromosulfophthalein with High-Affinity Binding Sites of Bovine Serum Albumin

¹ Institute of Toxicology, University of Tübingen, D-7400 Tübingen, West Germany

Binding of the amphipathic dye bromosulfophthalein (BSP) to bovine serum albumin has been studied, especially at small molar ratios. Observations with regard to binding at the first two or three sites revealed that the absorption spectrum of BSP is shifted to longer wavelengths and that the color associated with bound BSP disappears when more of the dye is added. A circular dichroism effect is produced on attachment of the first 2 or 3 molecules but is abolished on further binding. The ellipticity is independent of pH in the range 5-8.3. Finally, binding of BSP is accompanied by absorption of protons to the binding site. These findings indicate cooperative behavior for binding at the different sites, and we have proposed a model for the attachment of BSP to albumin at a binding site that may contain a tryptophan and a histidine residue. The methods used should be suitable for the characterization of other BSP-binding proteins in the liver cell membrane.

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ACKNOWLEDGMENT We thank Mrs. Sylvia Kasperek for her excellent technical assistance.