Determination of the membrane potential in bacterial membrane vesicles from the accumulation of N-methyldeptropine

Abstract

Active transport of solutes by cytoplasmic membrane vesicles from bacteria can be driven by electron transfer in respiratory chains, anaerobic electron transfer systems or by cyclic electron transfer systems (reviewed [ 1,2]). Electron transfer results in the generation of an electrochemical proton gradient across the membrane which is composed of an electrical gradient (A$) and a chemical proton gradient (ApH) according to the equation A&+ = A$-2.3 RT/F log ApH. The generation of a membrane potential and a pH gradient, induced by electron transfer, has been demonstrated in membrane vesicles from several bacteria [3,4]. The transmembrane pH gradient can be determined from the accumulation of radioactively labeled weak acids or bases as measured by means of flow dialysis [5]. For the determination of the membrane potential several methods have been employed [3]. A very reliable method for small cells and membrane vesicles has been introduced [6]. These investigators demonstrated that lipophilic organic cations or anions distribute across the membrane in response to the membrane potential. The membrane potential can be calculated from the concentration gradient reached at steady state level of accumulation with the Nernst equation: