Differential binding of tetracyclines with serum albumin and induced structural alterations in drug-bound protein

Interaction of tetracycline (TC) derivatives viz. oxytetracycline, doxycycline, demeclocycline and chlorotetracycline with bovine serum albumin (BSA) and concomitant changes in protein conformation were studied using fluorescence quenching and circular dichroism measurements. Fluorescence data revealed the presence of one to three binding sites on BSA for different TC derivatives. Binding studies with the marker ligands, warfarin and bilirubin, elucidated site-I as a primary binding. site for TCs on albumin. Scatchard analysis revealed the binding affinity (K-a) and capacity (n) for these derivatives vary in the range from 0.8 to 3.2 x 10(6) l/mole and 1.3-3.4, respectively. Significant reduction (60-45%) in secondary structure (alpha-helical content) of BSA was noticed upon interaction with different TC derivatives in presence of Cu (II) ions. High affinity binding of TCs with BSA signifies drug stability. However, excessive binding at higher TC concentrations in combination with Cu (II) induces conformational change in protein structure, which may exert detrimental effect on cellular protein. (C) 2002 Elsevier Science B.V. All rights reserved.