Fast Stages of Photoelectric Processes in Biological Membranes

Abstract

The functioning of visual rhodopsin as a photoelectric generator has been demonstrated with a direct method. Photoreceptor discs were incorporated into a phospholipid-impregnated collodion film. Illumination' of the resulting system with continuous light was found to induce formation of a n electric potential (the disc-free side positive) that was measured with two electrodes separated by the film. A photopotential exceeding 40 mV was shown. It dissipated before the light source was switched off. A 15 ns 530-nm laser flash induced the formation of a photopotential of up to 35 mV whose appearance was preceded with a small oppositely directed electrogenic phase. This 'negative' photoresponse took less than 200 ns. The 'positive' photoresponse was composed of at least two phases (tllz about 500 ps and several milliseconds). The latter was shown to correlated with formation of metarhodopsin 11.A 347-nm laser flash added after a 530-nm flash resulted in a photoelectric effect similar to that initiated by 530-nm flash but of opposite direction. The 347-nni rcsponse was completely abolished by hydroxylamine preventing the accumulation of metarhodopsin 11. The response at 530 nm proved to be hydroxylamine-resistant. Both the amplitude and the decay time of the flash-induced potential were maximal in the response to the first flash, each subsequent flash being less effective than the preceding one. Flashes were found to c a w acceleration of the photopotential decay. The latter effect proved to be due to a increase of membrane conductance that dcveloped faster than in 50 ms. Addition of 1 1-cis retinal after illumination improved the amplitude of the photoresponse but not the conductance. The light-induced increase in conductance was insensitive to hydroxylamine.It is suggested that a function of visual rhodopsin consists in generating a potential difference across the photoreceptor disc membrane which responds with a increase in membrane permeability to a rise of the membrane potential. A possible role of an electric breack-down of the membrane, induced by the rhodopsin-generated local or partially delocalized electric field has been discussed.Chronologically, the first idea about the biological function of bacteriorhodopsin was photoreception [ I , 21. Such an assumption seemed to be reasonable since it was known that phototaxis is inherent in Halobacteviu containing bacteriorhodopsin. However, the whole development of the bacteriorhodopsin study clearly showed that this pigment is a lightdependent H + pump. Recently in this group it was revcaled that the role of bacteriorhodopsin in the positive phototaxis is very indirect being confined to generation of ADH the level of which is monitored by an intracellular A&-measuring system producing a signal for taxis [3].As to animal (visual) rhodopsin, its direct involvement in photoreception in unquestionable. The problem is how rhodopsin fulfils the photoreceptor function. In this paper we would like to describe experiments indicating that visual rhodopsin, like its bacteria...

Fast Stages of Photoelectric Processes in Biological Membranes

“…decane, that can dissociate and extract mobile hydrophobic co-factors from studied membrane vesicles attached to collodion film. This is not a problem for bacterial and animal rhodopsin membranes (see the firs1 two papers of the series [6,7]). However, in the experiments with chromatophores, described in this article, decane caused at least two inhibitory effects, localized just after Fe-ubiquinone and in the cytochrome h region of the cyclic photosynthetic redox chain (see also [13]).…”

“…In this paper, we would like to report the data on A $ generation by chromatophores in a system similar to that used in two previous articles dealing with rhodopsins [6,7]. Chromatophores isolated from photosynthetic bacteria were associated with the surface of a phospholipid-impregnated collodion film and illuminated with a 15-11s laser.…”