Circular dichroism study of the interaction between conformationally altered human serum albumin and testosterone

Binding ability of testosterone (TEST) on conformationally altered human serum albumin (HSA) in the presence of several concentrations of NaSCN, urea and KCl at various pH was examined qualitatively on the basis of the rotational strength at 303, 273 and 208 nm by means of circular dichroism (CD). The values of the binding index expressed as a ratio of [theta]303/[theta]273 at each rotational strength in the presence of various concentrations of salt at pH 7.4 were inversely proportional, parallel, and independent of the alpha-helix content based on the peptide backbone alteration of HSA by urea, KCl and NaSCN, respectively. The values in the presence of a constant concentration of 1.0 M urea or KCl at various pH were dependent on the salt, showing a significant effect of these salts on the binding sites of the amino acid chain rather than the peptide backbone of the protein. It was generally observed that the decrease in alpha-helix content caused by pH changes tends to decrease the binding ability of TEST to HSA. The decreased binding index value observed in 40 mM NaSCN causing a low alpha-helix content, suggests that the secondary conformational changes caused by the salt might not be related to the binding ability, in contrast to the results of urea or KCl at different pH. It was clearly demonstrated that the binding ability of TEST to HSA is closely associated with skeletal conformational alterations as well as changes in the binding sites of the amino acid side chains of the protein.