On the kinetics of voltage formation in purple membranes of Halobacterium salinarium

The kinetics of the bacteriorhodopsin photocycle, measured by voltage chang es in a closed membrane system using the direct electrometrical method (DEM ) of Drachev, L.A., Jasaitus, A.A., Kaulen, A.D., Kondrashin, A.A., Liberma n, E.A., Nemecek, IB., Ostroumov, S.A., Semenov, Yu, A. & Skulachev, V.P. ( 1974) Nature 249, 321-324 are sixfold slower than the kinetics obtained in optical studies with suspensions of purple membrane patches. in this study, we have investigated the reasons for this discrepancy. In the presence of the uncouplers carbonyl cyanide m-chlorophenylhydrazone or valinomycin, the rates in the DEM system are similar to the rates in suspensions of purple membrane. Two alternative explanations for the effects of uncouplers were e valuated: (a) the 'back-pressure' of the Delta<(mu)over bar>(H+) slows the kinetic steps leading to its formation, and (b) the apparent difference bet ween the two systems is due to slow major electrogenic events that produce little or no change in optical absorbance. In the latter case, the uncouple rs would decrease the RC time constant for membrane capacitance leading to a quicker discharge of voltage and concomitant decrease in photocycle turno ver time. The experimental results show that the primary cause for the slow er kinetics of voltage changes in the DEM system is thermodynamic back-pres sure as described by Westerhoff, H.V. & Dancshazy, Z. (1984) Trends Biochem . Sci. 9, 112-117.