Department of Medicine, Santa Clara Valley Medical Center, San Jose, Calif.
Department of Medicine, University of California School of Medicine, San Francisco
Institute for Medical Research of Santa Clara County, San Jose, Calif.†
Address requests for reprints to Dr. Robert A. O'Reilly, Dept. of Medicine, Santa Clara Valley Medical Center, San Jose, Calif. 95128.*
Submitted for publication October 3, 1966; accepted January 27, 1967.
Supported by grant HE-8058-04 from the National Institutes of Health.
Published May 1, 1967 - More info
In studies by continuous flow electrophoresis the coumarin anticoagulant drug warfarin sodium was found to be bound solely to the albumin fraction of the plasma proteins. The interaction was studied in detail by equilibrium dialysis of solutions of crystalline human plasma albumin and warfarin sodium. Analysis of the data showed that albumin possesses a single strong binding site for warfarin with an association constant of 154,000 at 3° C and secondary classes of several sites with a much lower affinity. The free energy of binding for the first anion determined at 3° and 37° C was -6.54 and -7.01 kcal per mole, respectively. The standard enthalpy change for the interaction was -3.48 kcal per mole, and the entropy change was +11.2 U.
The negative enthalpy change was surprisingly large and the positive entropy change small for an anion-albumin interaction, suggesting significant nonionic binding. The inability to saturate the albumin binding sites, even when high concentrations of warfarin were used, is consistent with a reversible configurational alteration of the albumin molecule during the binding process. The thermodynamic data indicate that the albumin binding sites for warfarin sodium are formed during the process of binding, rather than being performed as in antigen-antibody reactions. The strength of the binding process suggests that many of the pharmacodynamic characteristics of warfarin sodium in man are determined by its strong interaction with plasma albumin. Such correlations of the physicochemical interactions and biologic effects of the coumarin anticoagulant drugs should lead to a better understanding of their mechanisms of action.