The effect of antibiotics on the photocycle and protoncycle of purple membrane suspensions

Abstract

The interrelation was studied between the phototransient absorbing maximally at 412 nm (M 412) and light-induced proton release under steady-state conditions in aqueous suspensions of ‘purple membrane’ derived from Halobacterium halobium. The decay of M 412 was slowed down by the simultaneous application of the ionophoric antibiotics valinomycin and beauvericin. The former had only slight activity alone and the latter was effective only in conjunction with valinomycin. The steady-state concentration of M 412 which was formed on illumination was a direct function of the concentration of valinomycin. Maximum stabilization of M 412 was obtained when the valinomycin was approximately equimolar with the bacteriorhodopsin. Addition of salts to the medium increased the number of protons released per molecule of M 412 without affecting the level of M 412 which was produced by continuous illumination. The effectiveness of the salts in this respect depended on the nature of the cation. Ca 2+ and their antagonists La 3+ and ruthenium red were found to have especially high affinity for the system. The extent of light-induced acidification could not be enhanced by increasing the pH of the medium from 6.5 to 7.8. The possible mechanism of action of the ionophores and of the cations on the photocycle and on the proton cycle is discussed.