BINDING-INDUCED ALTERATIONS IN ULTRAVIOLET ABSSORPTION OF NATIVE SERUM ALBUMIN

When bovine serum albumin combines with up to 15-17 equiv of anionic detergents (normal aliphatic sulfonates and sulfates) on its highest energy sites characteristic ultraviolet difference spectra are produced: a blue shift with minima at λ 293 and 287 mμ, and a red shift with a maximum at λ 232 mμ. From these spectral changes it is concluded that for both sulfonates and sulfates, tryptophan residues are at or very near the highest energy binding sites. For the sulfates, with the exception of the shortest chain used, additional sites with slightly lower affinity must be at or close to available tyrosine residues. Octanol, an uncharged ligand, produced at λ 230 mμ an effect very similar to that of the anions, although it has less effect on the tyrosine absorption and hardly any effect on the tryptophans. This observation suggests strongly that at least one more chromophore (histidine or phenylalanine) is involved in the binding at high-energy sites. It is proposed that changes at this residue cause the small changes in optical rotation at 233 mμ. The shortest chain compounds, even though they produce a strong tryptophan effect, do not change the rotation. Combination with larger amounts of the detergents produces no further effects except in the case of the two longest chain sulfates (dodecyl and tetradecyl) which alone produce the same massive disorganization of the protein that is produced by low pH (1.65) and by 8 M urea. Changes in peptide absorption are involved only in the latter massive disorganization. When no massive unfolding occurs, no detectable changes in viscosity accompany any of the changes in Δє although all the ligands induce small viscosity changes at v̄ values just above those which suffice to complete the changes in є. © 1968, American Chemical Society. All rights reserved.