Effect of ionophoric compounds on aqueous suspensions of purple membrane

Abstract

The purple retinal-protein complex, bacteriorhodopsin, functions in Halobacterium halobium as a light dependent proton pump [I]. It is organized in the cell membrane in patches designated as ‘purple membrane’ (PM) which can be isolated [2]. When PM is reincorporated into phospholipid vesicles it is able to transpose protons on illumination [3]. In aqueous suspensions of PM rapid sequence of spectral changes can be detected by flash spectroscopy which are associated with cyclic proton release and uptake [4]. Continuous illumination of PM, suspended in salt solution saturated with ether, establishes a steady state in which most of the purple complex is present in the unprotonated form absorbing maximally at 4 12 nm [ 51. The protons released cause a measurable acidification of the medium as long as the illumination lasts. In pure aqueous suspension only a minute fraction of the purple complex is converted to the ‘412 nm complex’ and correspondingly also the extent of proton release is very small. The present report deals with a so-far unobserved response of PM to certain ionophoric substances. It is shown that valinomycin and beauvericin, when added to aqueous suspensions of PM, mimic in many respects the effect of ether-salt mixtures. They enhance on illumination the formation of the ‘412 nm-complex’ with concomitant acidification of the medium.