Abstract

It is often contended that inhalation anesthetics act on proteins via perturbation of lipid membranes. However, direct interaction between anesthetics and water-soluble proteins also has been demonstrated. We postulate that the anesthetic action is directed to the interface between water and macromolecules, irrespective of lipid membranes or proteins. The present study deals with anesthetic effects upon interfacial properties of a water-soluble, crystalline delipidated bovine serum albumin. A pH-indicator dye, bromothymol blue, was used to probe the surface potential of the protein. When a pH-indicator dye binds to a macromolecule, the pH, indicated by the color of the dye, differs from the bulk pH measured by a pH meter. This is because the pH of the microscopic area, where the dye is adsorbed, differs from the bulk due to the surface electrostatic potential that interacts with hydrogen ions (electrostatic terms), and the physical property that affects the color of the dye at the bound region is different from the bulk (nonelectrostatic terms). The mismatch between the bulk pH and the color of the bound pH indicator can be used to probe the property of the dye binding site. By screening the electrostatic effects with high ionic strength, the anesthetic effects upon the nonelectrostatic term were shown to be negligible under the present experimental conditions; the pH-color mismatch was mainly caused by the anesthetic effect upon the electrostatic potential of the macromolecular surface interacting with the dye. Accordingly, the surface potential of the dye binding site was estimated from the mismatch. It was found that inhalation anesthetics decreased the surface potential. The partial pressures of diethylether, enflurane, and methoxyflurane that decreased the surface potential by 10 mV were 2.1 X 10(-2), 1.7 X 10(-2), and 0.17 X 10(-2) bar, respectively, which were in agreement with the minimal alveolar concentrations of these anesthetics to achieve surgical anesthesia.