Interactions involving the cyanine dye, diS-C3-(5), cytochrome c and liposomes and their implications for estimations of delta psi in cytochrome c oxidase-reconstituted proteoliposomes.

The interference of cytochrome c with absorption and fluorescence changes of the cyanine dye, diS-C3-(5), was investigated in the presence of liposomes and cytochrome c-oxidase reconstituted proteoliposomes. The apparent cytochrome c-dependent quenching of diS-C3-(5) fluorescence, and the associated absorbance losses in the presence of liposomes and proteoliposomes in low ionic strength media, are due to destruction of the dye caused by cytochrome c-mediated lipid peroxidation. The rate of this reaction was further enhanced in the presence of hydrogen peroxide. Even in the absence of liposomes or proteoliposomes, a cytochrome c-induced breakdown of dye was observed in the presence of hydrogen peroxide. The cytochrome c mediated absorbance and fluorescence losses of diS-C3-(5) in liposomal or proteoliposomal systems are prevented by Ca2+ and La3+ ions, by ascorbate, by high ionic strength and by the antioxidant BHT. Under these conditions, the process of lipid peroxidation mediated by cytochrome c is inhibited either directly (e.g. by BHT) or indirectly, by preventing the binding of cytochrome c to lipid vesicles. The impact of these findings upon the experimental estimation of membrane potential in aa3-reconstituted proteoliposomes is discussed.