Human serum albumin (HSA) and serum transferrin (TF) are two drug carrier proteins in vivo. In this study it was investigated how lomefloxacin (LMF) binding affected the HSA-TF interaction using different spectroscopic, calorimetric and molecular modeling techniques. Fluorescence, circular dichroism and synchronous fluorescence revealed that LMF could bind to both proteins, resulting in protein conformational changes. Moreover, HSA and TF could interact so that some fluorescence residues were positioned at the interface and were shielded from quenching by LMF. The interaction between HSA and TF was further confirmed by fluorescence resonance energy transfer. Quantitative analyses of the far-UV CD spectra of the HSA-TF interaction in the presence and absence of LMF revealed secondary structural changes in detail. Resonance light-scattering studies demonstrated that the HSA-TF interaction resulted in a new species with a larger size, and that the presence of LMF could further favor this reaction. Isothermal titration calorimetry revealed that electrostatic interaction was dominant in the absence of LMF, whereas van der Waals forces and hydrogen bonding become significant in its presence. On the other hand, it was found that the binding constant of TF bound to HSA was stronger in the presence of LMF. ANS fluorescence further indicated that hydrophobic interactions play a minor part in the HSA-TF system. Molecular modeling studies confirmed the presence of fluorophore residues, hydrogen bonding and electrostatic interactions at the interface of the HSA-TF complex. It also suggested that the binding sites of LMF were not located there. These data indicate that LMF can modify the interaction between HSA and TF as two model proteins present in serum. The relevance to drugs' side effects, pharmacokinetic of drugs and selection of diagnostic biomarker is discussed.
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