Inactivation of monazomycin-induced voltage-dependent conductance in thin lipid membranes. I. Inactivation produced by long chain quaternary ammonium ions.

Abstract

The voltage-dependent conductance induced in thin lipid membranes by monazomycin undergoes inactivation upon the introduction of quaternary ammonium ions (QA) having a long alkyl chain (e.g., dodecyltrimethylammonium [C12]) to the side containing monazomycin, i.e., in response to a step of voltage the conductance rises to a peak and then falls to a much lower steady-state value. The basis of this phenomenon is the ability of QA to pass through the stimulated membrane and bind to the opposite surface. As a consequence, the surface potential on that side becomes more positive, thus reducing the voltage across the membrane proper and turning off the monazomycin-induced conductance. Because the flux of QA through the membrane increases linearly with conductance, ions probably pass through the monazomycin channels. QA permeability increases with alkyl chain length; in spite of its much larger size, C12 is about 150 times more permanent than K+. It appears, therefore, that there is a hydrophobic region of the channel that favors the alkyl chain; this region may be formed by the hydrophobic faces of the monazomycin molecules and the phospholipid tails. A comparison of the QA inactivation of monazomycin channels in lipid bilayers to QA inactivation of K+ channels in the squid giant axon suggests that there may be a common structural feature for the 2 channels. It is possible that some of the inactivation phenomena in excitable cells may arise from local field changes not measurable by the recording electrodes.