A quantitative resolution of the spectra of a membrane potential indicator, diS-C3-(5), bound to cell components and to red blood cells.

Cationic cyanine dyes have been widely used to measure electrical potentials of red blood cells and other membrane preparations. A quantitative analysis of the binding of the most extensively studied of these dyes, diS-C3-(5), to red blood cells and their constituents is presented here. Absorption spectra were recorded for the dye in suspensions of isolated red cell membranes and in solutions of cell lysate. The dependence of the spectra on the concentrations of dye and cell constituents shows that the dye binds to these membranes as monomers with an absorbance maximum at 670 nm instead of 650 nm as for free aqueous dye and the dye binds to oxyhaemoglobin partly as monomer but primarily as dimer, with absorbance maxima ca. 670 and 595 nm, respectively. Quantitative estimates are derived for all binding constants and extinction coefficients. These estimates are applied to suspensions of whole cells to predict the dye binding, absorbance spectra, and calibration curves of binding and fluorescence vs. membrane voltage. Satisfactory agreement is found with binding and absorbance data for whole cells at zero membrane potential and with the binding and fluorescence data reported by Hladky and Rink (J. Physiol. (London) 263:287, 1976) for cells driven to positive and negative potentials using valinomycin. The marked tendency of oxyhaemoglobin to bind dye as dimer is not shared by some other proteins tested, including deoxyhaemoglobin and oxymyoglobin.