Studies on the Coumarin Anticoagulant Drugs: Interaction of Human Plasma Albumin and Warfarin Sodium*

Abstract

Summary. 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 30 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 370 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 preformed 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.

Binding of digitoxin and some related cardenolides to human plasma proteins

“…The entropy change is much larger than the 10-20 cal/mole per 'K observed for many binding reactions (27,35,(38)(39)(40). Although the processes involved in hydrophobic bonding account for the gain in entropy of many organic substances (39,40), concomitant alterations in the structure of the protein might contribute to the increase in entropy.…”

“…The binding is not as strong as that of thyroxine to albumin (k = 1.3 X 10' liter/mole) (32) or of the long-chain fatty acid anions, stearate, oleate, and palmitate, which have k values of 6 X 107-1.1 X 108 liter/mole (33). The avidity exceeds that of many pharmacologic agents (6), such as quinidine which has an association constant of 7.7 X 10' liter/mole (34), but it is in the range of warfarin sodium, which has a k of 8.7 X 10' liter/mole (35), and phenylbutazone with k of 1.25 X 10' liter/ mole (36 (26-28, 34, 35, 38-40). The slight effects of pH on the association constant indicate that ionic effects are not a significant factor in the interaction.…”

“…The endothermic nature of the reaction and the positive entropy change are characteristic of hydrophobic bonding (40), in which disruption of the water structure about the protein and the rupture of hydrogen bonds between water and both the protein and ligand consume heat and the freeing of water molecules produces more disorder and consequent increase in the entropy of the system. The binding of most substances, including steroids, to protein is facilitated by low temperatures (27,35,38,39,(41)(42)(43). These interactions are exothermic; part of the free energy change that favors formation of the protein-ligand complex is derived from the negative enthalpy.…”

Changes in drug metabolism with increasing age: 2. phenytoin clearance and protein binding.

“…All measurements were carried out in duplicate and with blank estimates. The plasma albumin levels (O'Reilly & Kowitz, 1967) in Visking sacs against calcium-free Tyrode solution (Tyrode, 1910). Unlike the warfarin study, phenytoin binding was measured using undiluted plasma.…”

In situ direct photoproduction of ketenes from substituted coumarins isolated in solid argon: The case of N-(2-oxo-2H-chromen-3-yl)acetamide

“…In particular, they exhibit different biological activities, including anticoagulant, spasmolytic, diuretic, anthelmintic and hypoglucemic actions [1][2][3][4][5][6][7][8].…”