Ionic Channels and Nerve Membrane Constituents Tetrodotoxin-like interaction of saxitoxin with cholesterol monolayers

Saxitoxin (STX) and tetrodotoxin (TTX) have the same striking property of blocking the Na+ channels in the axolemma. Experiments with nerve plasma membrane components of the squid Dosidicus gigas have shown that TTX interacts with cholesterol monolayers. Similar experiments were carried out with STX. The effect of STX on the surface pressure-area diagrams of lipid monolayers and on the fluorescence emission spectra of sonicated nerve membranes was studied. The results indicate a TTX-like interaction of STX with cholesterol monolayers. The expansion of the monolayers caused by 10-M STX was 2.2 A2 /cholesterol molecule at 25°C. From surface pressure measurements at constant cholesterol area (39 A2 /molecule) in media with various STX concentrations, it was calculated that the STX/cholesterol surface concentration ratio is 0.54. The apparent dissociation constant of the STX-cholesterol monolayer complex is 4.0 X 107 M. The STX/cholesterol ratio and the apparent dissociation constant are similar to those determined for TTX. The presence of other lipids in the monolayers affects the STX-cholesterol association. The interactions of STX and TTX with cholesterol monolayers suggest (a) that cholesterol molecules may be part of the nerve membrane Na+ channels, or (b) that the toxin receptor at the nerve membrane shares similar chemical features with the cholesterol monolayers. Following observation of the specific interaction between tetrodotoxin (TTX) and cholesterol monolayers (1), Dr. C. Y. Kao kindly suggested that we carry out similar experiments with saxitoxin (STX). STX and TTX are produced by quite different organisms and have different chemical structures, sharing only the presence of a guanidinium group (2-4). In spite of these facts, both molecules have the same striking property of blocking the Na+ channels in the axolemma (5-10). Na+ channels are the pathways normally used by this THE JOURNAL OF GENERAL PHYSIOLOGY VOLUME 59, 1972 pages 33-46 33 on July 1, 2017 jgp.rress.org D ow nladed fom THE JOURNAL OF GENERAL PHYSIOLOGY VOLUME 59 I1972 ion to move through the excitable membrane during the nervous impulse and the transient early phase of conductance increase in voltage-clamped axons. We are grateful to Dr. E. J. Schantz, who kindly provided the sample of STX employed in this work. The experiments to be described were carried out to explore the possible interaction of cholesterol and other constituents of squid nerve fiber plasma membranes with STX. The interactions were measured by the effect of STX on the surface pressure-area (r-A) diagrams of lipid monolayers and on the fluorescence emission spectra of sonicated nerve membrane suspensions. The results indicate a specific interaction between cholesterol monolayers and STX, similar to that observed with TTX (1).