Molecular Mechanisms of Ion Transport in Lipid Membranes

Abstract

The molecular mechanisms of ion transfer across biological membranes are as yet poorly understood. While it is thought that most, if not all, of those molecules responsible are . proteins, in only a few instances has even their molecular weight been characterized. Finer details of their structure needed to interpret the various rate constants of the ion transfer kinetics are completely lacking. The past few years, however, have seen the discovery of a number of relatively small molecules, some of them polypeptides, which are membrane­ active antibiotics ( 1-4). These molecules are synthesized biologically and act on biological membranes to produce unwanted ion fluxes. Although they are not obviously related to those molecules responsible for normal ion transfer processes, their study is extremely interesting in that it reveals the means by which small inorganic ions may cross the ordinarily impenetrable hydrocarbon barriers of lipid membranes. The studies also show that the transport processes may be highly specific for one ionic species relative to another, e.g. the permeabil­ ity to K + may be � 103 times that for Na +. Examination of ion-transporting antibiotics also stimulates the development of techniques likely to be valuable in elucidating normal biological ion transport processes. One such technique is the use of artificial lipid membranes to provide a simpler environment than the very complex, multicomponent biological mem­ branes. Much of the information now available for the antibiotics could not, in fact, have been obtained using natural membranes, although there is sufficient evidence to indicate that the antibiotics behave similarly in natural and artificial systems. For this reason it seems unliKely that the detailed physical chemistry of the normal biological processes will be elucidated without using artificial mem­ branes. While some progress is being made along these lines, there are severe