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Genistein and daidzein, the major isoflavones present in soybeans, possess a wide spectrum of physiological and pharmacological functions. The bind-ing of genistein to human serum albumin (HSA) has been investigated by equilibrium dialysis, fluorescence measurements, CD and molecular visuali-zation. One mole of genistein is bound per mole of HSA with a binding constant of 1.5 ± 0.2·10 5 m )1 . Binding of genistein to HSA precludes the attachment of daidzein. | ềFEBS Journal A spectroscopic study of the interaction of isoflavones with human serum albumin H. G. Mahesha1 Sridevi A. Singh1 N. Srinivasan2 and A. G. Appu Rao1 1 Department of Protein Chemistry Technology CentralFood TechnologicalResearch Institute Mysore India 2 Molecular Biophysics unit Indian Institute of Science Bangalore India Keywords daidzein genistein serum albumin interaction studies binding pocket Correspondence Dr A.G. Appu Rao Department of Protein Chemistry Technology CentralFood Technological Research Institute Mysore 570 020 India Fax 91 821 2517233 Tel 91 821 2515331 E-mail appurao@cftri.res.in Received 5 October 2005 accepted 22 November 2005 doi 10.1111 j.1742-4658.2005.05071.x Genistein and daidzein the major isoflavones present in soybeans possess a wide spectrum of physiological and pharmacological functions. The binding of genistein to human serum albumin HSA has been investigated by equilibrium dialysis fluorescence measurements CD and molecular visualization. One mole of genistein is bound per mole of HSA with a binding constant of 1.5 0.2 X 105 M-1. Binding of genistein to HSA precludes the attachment of daidzein. The ability of HSA to bind genistein is found to be lost when the tryptophan residue of albumin is modified with N-bromosuccinimide. At 27 C pH 7.4 van t Hoff s enthalpy entropy and free energy changes that accompany the binding are found to be -13.16 kcal-mol-1 -21 cal-mol-1K-1 and -6.86 kcal-mol-1 respectively. Temperature and ionic strength dependence and competitive binding measurements of genistein with HSA in the presence of fatty acids and 8-ani-lino-1-naphthalene sulfonic acid have suggested the involvement of both hydrophobic and ionic interactions in the genistein-HSA binding. Binding measurements of genistein with BSA and HSA and those in the presence of warfarin and 2 3 5-tri-iodobenzoic acid and Forster energy transfer measurements have been used for deducing the binding pocket on HSA. Fluorescence anisotropy .