Kinetic studies of phototransients in bacteriorhodopsin

Abstract

Aqueous suspensions of bacteriorhodopsin in purple membrane fragments from Halobacterium halobium have bben subjected to microsecond flash photometry utilizing both unpolarized and polarized light. Depletion of the ground state chromophore centered at 570 nm is accompanied by the formation of transients absorbing maximally at 410 nm and 660 nm with rise times of about 0.4 and 6 ms, respectively. Decay of both transients and reformation of the ground state chromophore occurs with identical first-order kinetics with a half life of about 6 ms. All three chromophores are polarized with dichroic ratios which remain constant throughout the transient lifetimes, indicating that Brownian rotation of the chromophore within the membrane is considerably restricted. Whereas agents which induce permeability of membranes to protons (2,4-dinitrophenol, carbonylcyanide-m-chlorophenylhydrazone) and non-specific univalent cations (gramicidin) or inhibit ATPase (ouabain) had no influence, the K+-specific ionophore valinomycin in the presence of K+ inhibited and quenched the formation of the 660 nm transient with concomitant increase in lifetime of the 410 nm transient and delay in recovery of the 570 nm chromophore. High concentrations of Na+ produced an effect similar to that of valinomycin. The relationship of these data to the mechanism of the proton pump in the intact bacterium is discussed, with the conclusion that the 410 nm transient performs a key role.