Potential-sensitive response mechanism of diS-C3-(5) in biological membranes

The potential-sensitive response mechanism of 3,3'-dipropylthiodicarbocyanine iodide (diS-C3-(5] was examined based on our previous model of diS-C3-(5) interaction with brush border membrane vesicles (BBMV) in the absence of a membrane potential. The model contained binding (6 msec), reorientation (30 msec), dimerization (less than 10 nsec), and translocation (1 sec) reaction steps (Cabrini & Verkman, 1986. J. Membrane Biol. 90:163-175). Transmembrane potentials (psi) were induced in BBMV by K+ gradients and valinomycin. Steady-state diS-C3-(5) fluorescence (excitation 622 nm, emission 670 nm) increased linearly with psi. The reorientation and translocation reaction steps were resolved by the stopped-flow technique as a biexponential decrease in fluorescence following mixture of diS-C3-(5) with BBMV at varying psi. The fractional amplitude of the faster exponential increased from 0.36 to 0.73 with increasing psi (-17 to 87 mV); the time constant for the faster exponential (30-35 msec) was independent of psi. There were single exponential kinetics (0.5-1.5 sec) for diS-C3-(5) fluorescence response to a rapid (less than 2 msec) change in psi in the absence of a diS-C3-(5) concentration gradient. These results, and similar findings in placental brush border vesicles, red cell vesicles, and phosphatidylcholine vesicles, support a translocation mechanism for diS-C3-(5) response, where induced membrane potentials drive diS-C3-(5) redistribution between sites at the inner and outer membrane leaflets, with secondary effects on diS-C3-(5) dimerization and solution/membrane partitioning. Fluorescence lifetime and dynamic depolarization measurements showed no significant change in diS-C3-(5) rotational characteristics or in the polarity of the diS-C3-(5) environment with changes in psi. Based on the experimental results, a mathematical model is developed to explain the quantitative changes in diS-C3-(5) fluorescence which accompany changes in psi at arbitrary dye/lipid ratios.