Radiolysis and Photolysis of Ion Channels Formed by Gramicidin A

Abstract

The conductance of planar lipid membranes induced by the presence of gramicidin A is reduced by several orders of magnitude, if the membrane is exposed either to UV-light (photolysis) or to ionizing radiation (radiolysis). Both effects are virtually negligible, if the four tryptophan residues of gramicidin A are replaced by phenylalanines, or by naphthylalanines. While the inactivation of the gramicidin channels by photolysis is a direct radiation effect based on light absorption by the tryptophan residues (Busath and Waldbillig, 1983), radiolysis is an indirect radiation effect caused by radiation induced water radicals. Experiments performed in the presence of different radical scavengers indicate that radiolysis is preferentially due to a combined action of OH (or of secondary radicals produced by OH) and of HO2 radicals. The shape of the inactivation curves, following continuous radiolysis at constant dose rate or pulse radiolysis, were found to agree with the assumption of a single site of radical attack. Experiments performed with gramicidin analogues of varying tryptophan content have shown, however, that the extreme sensitivity of the ion channels towards radiolysis is due to the presence of at least two tryptophan residues per gramicidin monomer.