PHOTODYNAMIC AND RADIOLYTIC INACTIVATION OF ION CHANNELS FORMED BY GRAMICIDIN-A - OXIDATION AND FRAGMENTATION

Ion channels formed by the peptide gramicidin A in planar lipid membra nes have been reported to react very sensitively upon irradiation of t he membrane by ionizing radiation (radiolysis), by UV light (photolysi s), or by visible light in the presence of appropriate photosensitizer s (photodynamic inactivation). In all three cases the effect is due to . the presence of the four tryptophan residues of the pentadecapeptide . Modifications of these amino acids - due to an interaction with free radicals formed upon water radiolysis or due to light absorption - ha ve been found to reduce the membrane conductance by many orders of mag nitude. The present study was intended to correlate functional changes , observed at the level of single ion channels, with changes of the mo lecular structure identified by mass spectrometry. About 98% of the in activated channels showed a single-channel conductance of virtually ze ro, while about 2% of the channels present before irradiation are conv erted to a state of reduced conductance (and reduced Lifetime). On the structural level, irradiation in the presence of the photosensitizer Rose Bengal was found to produce oxidation and fragmentation of the pe ptide at the positions of the tryptophan residues. Our results provide evidence that the main effect of radiolysis, or of photodynamic treat ment, is the cleavage of the peptide backbone leading to immediate clo sure of an open ion channel.