Interaction of warfarin with human serum albumin. A stoichiometric description

Reversible binding of warfarin to defatted serum albumin was studied by equilibrium dialysis at pH 7.4, in a 66 mM sodium phosphate buffer at 37 degrees. The binding isotherm could be described by two stoichiometric binding constants, K1 in the range 141,000 to 192,000 M-1 and K2 at 39,000 to 57,000 M-1. At least two additional molecules could be bound but gave indeterminate binding constants. The product K3 X K4 was about 4.7 X 10(7) M-2. Different site models were possible, either one high affinity and several low affinity sites, or two high affinity sites, cooperative, independent, or anticooperative, together with two low affinity sites. Binding affinity for the first warfarin molecule did not vary with pH in the interval from 6 to 9. The affinity decreased with increasing concentrations of sodium sulfate, sodium chloride, and calcium chloride, depending upon ionic strength. Specific effects of chloride and calcium ions were not observed. Light absorption spectra indicated that the warfarin anion was bound to albumin. All observations were consistent with a binding process involving albumin and the warfarin anion, without participation of hydrogen ions and not influenced by the N-B conformational transition of albumin.