gramicidin-a and 6-ketocholestanol

Photodynamic inactivation of gramicidin channels in bilayer lipid membranes induced by single flashes of the visible light in the presence of phthalocyanine has been studied. The kinetic curves of the flash-induced decrease in the gramicidin-mediated electric current are used for determination of the rate constants of formation and termination of gramicidin channels in terms of the channel dimer model. It is revealed that the kinetics of the sensitized photoinactivation of gramicidin in the membrane is altered by agents which modify the dipole potential drop at the membrane-water interface. Addition of phloretin, which is known to decrease the dipole potential drop, slows down the kinetics, whereas the addition of RH421 or 6-ketocholestanol, which increase the dipole potential drop, accelerates the kinetics. It is shown that the photoinactivation kinetics is also slowed down upon the addition of the thyroid hormone L-thyronine, which reduces the dipole potential drop similar to phloretin, as it was found earlier (M. V. Tsybulskaya, Yu. N. Antonenko, A. E. Tropsha, and L. S. Yaguzhinsky, Biofizika 29:801-805 (1984) (in Russian)). It is demonstrated that the changes in the dissociation rate constant of gramicidin dimers under the action of different dipole modifiers correlate with the changes in the dipole potential drop. It is concluded that the process of the gramicidin channel termination corresponding to the dimer dissociation is sensitive to the dipole potential drop. This conclusion is supported by the data on the effect of dipole modifiers on the lifetime of single gramicidin channels.

Topics: Gramicidin; Indoles; Isoindoles; Ketocholesterols; Kinetics; Lipid Bilayers; Phloretin; Phosphatidylcholines; Pyridinium Compounds; Radiation-Sensitizing Agents; Styrenes; Thyronines

A technique of measuring of the light-induced transients of the gramicidin-mediated electric current across a membrane in the presence of a photosensitizer has been applied for the study of the effect of agents modifying the dipole potential of a bilayer lipid membrane (phloretin, 6-ketocholestanol, and RH421) on the processes of the gramicidin channel dissociation and formation. It is shown that phloretin, known to lower the dipole potential, decelerates the flash-induced decrease in the current, whereas 6-ketocholestanol and RH421, known to raise the dipole potential, accelerate the current decrease. It is revealed that the addition of phloretin leads to a decrease in the dissociation rate constant, whereas addition of either 6-ketocholestanol or RH421 causes an increase in this constant. Single-channel data show that phloretin brings about an increase in the lifetime of the gramicidin channels, whereas RH421 produces a more complicated effect. It is conclude that the dipole potential affects the process of channel dissociation, presumably via the influence on the movement of the dipoles of gramicidin molecules through the layer of the dipole potential drop near the membrane-water interface.

Topics: Electric Conductivity; Fluorescent Dyes; Gramicidin; Ion Channels; Ketocholesterols; Kinetics; Lipid Bilayers; Membrane Potentials; Models, Biological; Phloretin; Pyridinium Compounds; Styrenes