Binding of diuretic antihypertensive bendroflumethiazide to human serum albumin studied by 19F nuclear magnetic resonance method

Simultaneous specific and nonspecific binding of bendroflumethiazide (BFZ) to human serum albumin (HSA) and concentration profile of BFZ in HSA buffer (pH 7.40) solution were investigated by 19F nuclear magnetic resonance (NMR) method. The 19F NMR spectrum of BFZ (200 μM) in a buffer solution showed a sharp signal of its CF3 group at 17.8 ppm from the reference trifluoroethanol. Addition of 0.60 mM HSA to the sample solution caused the CF3 signal splitting into three broadened peaks at 18.4 (A), 17.9 (B) and 17.4 ppm (C). By its chemical shift and spectral behavior, B was assigned to unbound BFZ. Competition experiments with Site I and II ligands lead to C being assigned to Site II bound BFZ. However, the peak intensity (areas) of A was not reduced by these ligands, suggesting that A arises from nonspecific binding. Using the peak intensities at several total concentrations of BFZ, Scatchard plot was performed. The plot for A provided a straight line parallel to the x-axis confirming nonspecific binding and that for C was consistent with specific binding. The binding constants for nonspecific and specific Site II binding were 1.02 and 1.00 × 104 (M−1) (n = 1.1), respectively. The presence of 0.10 M Cl− in the sample solution affected the binding constant of Site II binding, but not that of nonspecific binding. The concentration profile of BFZ calculated using the binding constants revealed that nonspecific binding is more effective than Site II binding for the binding of BFZ to HSA. It was also confirmed that considerable amounts of BFZ liberated from Site II by the Site II ligands or Cl− ions bind again nonspecifically.

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