Binding of volatile anesthetics to serum albumin: Measurements of enthalpy and solvent contributions

This study directly examines the enthalpic contributions to binding in aqueous solution of closely related anesthetic haloethers (desflurane, isoflurane, enflurane, and sevoflurane), a haloalkane (halothane), and an intravenous anesthetic (propofol) to bovine and human serum albumin (BSA and HSA) using isothermal titration calorimetry. Binding to serum albumin is exothermic, yielding enthalpies (ΔH_obs) of -3 to -6 kcal/mol for BSA with a rank order of apparent equilibrium association constants (K _a values): desflurane > isoflurane ∼ enflurane > halothane ≥ sevoflurane, with the differences being largely ascribed to entropic contributions. Competition experiments indicate that volatile anesthetics, at low concentrations, share the same sites in albumin previously identified in crystallographic and photocross-linking studies. The magnitude of the observed ΔH increased linearly with increased reaction temperature, reflecting negative changes in heat capacities (ΔC_p). These -ΔC _p values significantly exceed those calculated for burial of each anesthetic in a hydrophobic pocket. The enhanced stabilities of the albumin/anesthetic complexes and -ΔC_p are consistent with favorable solvent rearrangements that promote binding. This idea is supported by substitution of D₂O for H₂O that significantly reduces the favorable binding enthalpy observed for desflurane and isoflurane, with an opposing increase of ΔS_obs. From these results, we infer that solvent restructuring, resulting from release of water weakly bound to anesthetic and anesthetic-binding sites, is a dominant and favorable contributor to the enthalpy and entropy of binding to proteins.