[Temporal characteristics of bacteriorhodopsin as a molecular biological generator of current]

Generation of electric potential difference by bacteriorhodopsin proteoliposomes incorporated into the phospholipid-impregnated collodion film has been studied. It is shown that illumination of this film by continuous light gives rise to the generation of an electric potential difference across the film (plus on the bacteriorhodopsin-free side), which can be as high as 300 mV. Short unsaturating flash inducing single turn-over of bacteriorhodopsin generates the potential difference which is a function of the flash intensity (70 mV at 3 mjoule light). The flash-induced photoelectric response consists of four phases. (1) Very fast (tau less than 1 microsec) generation of a potential difference (minus in the bacteriorhodopsin-free compartment). The amplitude of this phase is rather small (1--5 mV). (2) Fast phase of positive charging of the bacteriorhodopsin-free compartment (tau = 25--50 microsec). (3) Slow phase of positive charging of this compartment (tau = 6--12 msec) Amplitude of the second phase is to that of the third as 1 : 2. (4) A very slow phase of discharge of the flash-induced potential difference (tau = 1 sec at 10(8) ohm X cm2 film resistance). The third phase was specifically inhibited by La3+. Both the second and the third phases are decelerated by substitution of D2O in 4.5--5 and 2 times, respectively, while the amplitude of the first phase increases. Prolonged storage of the system in the dark (tua = 20--25 min) causes the decrease in the amplitudes of the second and the third phases as if the amount of active bacteriorhodopsin molecules were increased by factor 2. Such an inhibition was reversed by 30--60 sec illumination of the system. The dark adaptation is accompanied by some increase in the first phase amplitude. Comparison of these data with results of other studies on bacteriorhodopsin suggests that (1) the first phase is due to the photoinduced change in the retinal dipole; (2) the second phase corresponds to H+ transfer from Schiff base to the water solution in the proteoliposome interior; 3) the third phase represents H+ transfer from the incubation mixture to Schiff base; (4) the dark adaptation is a result of transition of photoelectrochemically active all-trans-retinal to the inactive 13-cis-retinal.