In Reply:--The premise of Johansson's alternative interpretation is that nonanesthetics decrease energy transfer and reduce the fluorescence emission of the acetylcholine analog, Dns-C6-Chol. However, this premise is not supported by experimental data; titrations using increasingly higher nonanesthetic concentrations under conditions of energy transfer from nAcChoRs revealed no reduction in Dns-C6-chol fluorescence emission at equilibrium (Figure 1(A)). Therefore, the nonanesthetics 1,2-dichlorohexafluorocyclobutane and 2,3-dichlorooctafluorobutane do not significantly disrupt energy transfer from nAcChoR tryptophan residues to Dns-C6-Chol.
Johansson assumes that indole in methanol is an adequate experimental model for tryptophan residues in intrinsic membrane proteins, and he bases his premise on the observation that anesthetic and nonanesthetic compounds quench essentially all of the fluorescence in this model system. However, a Stern-Volmer plot of halothane quenching of nAcChoR-rich membrane fluorescence is quite different from that of halothane quenching of indole fluorescence (Figure 1(B)). It is nonlinear, with curvature toward the concentration axis, indicating that a significant fraction of protein tryptophans are either poorly quenched or not quenched at all. [1–3].
Finally, none of the kinetic parameters reported in my study are even derived from absolute amplitudes. They are derived from amplitudes that are normalized to their final intensities (at equilibrium) or from observed rates. [4,5]These values are affected little, if at all, by quenching. However, because quenching could have reduced the signal:noise ratio and made the analysis more difficult, reason dictated avoiding compounds that seemed to offer the greatest probability of quenching.
Douglas E. Raines, MD; Assistant Professor of Anesthesia, Massachusetts General Hospital, 32 Fruit Street, Boston, Massachusetts 02114–2696.
(Accepted for publication April 29, 1996.)