Influences of urea, pH and metal ions on the interaction between cepharanthine and lysozyme by steady state fluorescence spectroscopy

• Fluorescence of native and denatured Lys was quenched by cepharanthine at pH 7.4.
• The effect of metal ions on binding constant of cepharanthine with Lys was studied.
• Binding of cepharanthine to Lys did not cause apparent change in conformation of Lys.
• Cepharanthine was harder to bind to the denatured Lys.
• The pH 7.4 is the optimal acidity in the binding reaction.

The study on the binding mode of drug with protein is important to understand the pharmacokinetics and toxicity of the drug as well as the relationship of structure and function of the protein. In the study, the interaction between cepharanthine and lysozyme (Lys) in aqueous solution was first investigated by fluorescence spectroscopic techniques at pH 7.4. The obtained quenching rate constant and binding constant indicated the static quenching mechanism and medium binding force. The effect of cepharanthine on the conformation of Lys was analyzed using synchronous fluorescence and three-dimensional (3D) fluorescence. In addition, the effect of urea on the interaction of cepharanthine with Lys was studied and the binding capacity of cepharanthine to the denatured Lys deceases dramatically, as compared with that of cepharanthine to native Lys. Moreover, influence of pH on the interaction of cepharanthine with Lys was investigated. As compared with that at pH 7.4, the binding abilities of the drug to Lys under other pH conditions (pH 9.0, 5.5, 3.5, and 1.9) deceased. Furthermore, the effect of metal ions on the binding constant of cepharanthine with Lys was investigated.