Titration calorimetry of anesthetic-protein interaction: Negative enthalpy of binding and anesthetic potency

Anesthetic potency increases at lower temperatures. In contrast, the transfer enthalpy of volatile anesthetics from water to macromolecules is usually positive. The transfer decreases at lower temperature. It was proposed that a few selective proteins bind volatile anesthetics with negative ΔH, and these proteins are involved in signal transduction. There has been no report on direct estimation of binding ΔH of anesthetics to proteins. This study used isothermal titration calorimetry to analyze chloroform binding to bovine serum albumin. The calorimetrically measured ΔH(cal) was -10.37 kJ · mol^-1. Thus the negative ΔH of anesthetic binding is not limited to signal transduction proteins. The binding was saturable following Fermi-Dirac statistics and is characterized by the Langmuir adsorption isotherms, which is interfacial. The high-affinity association constant, K₁ was 2150 ± 132 M_-1 (K(D) = 0.47 mM) with the maximum binding number, B(max) = 3.7 ± 0.2. The low-affinity K was 189 ± 3.8 M^-1 (K(D) = 5.29 mM), with a B(max) of 13.2 ± 0.3. Anesthetic potency is a function of the activity of anesthetic molecules, not the concentration. Because the sign of ΔH determines the temperature dependence of distribution of anesthetic molecules, it is irrelevant to the temperature dependence of anesthetic potency.