Kinetics of Calcium Binding to Fluo-3 Determined by Stopped-flow Fluorescence

The kinetics of Ca2+ dissociation from fluo-3 was measured using stopped flow fluorimetry. Analysis of dissociation revealed, in contrast to other commonly used fluorescent Ca2+ indicators, a biexponential behaviour with two distinct dissociation rates of 550 s-1 and 200 s-1 at physiological pH and room temperature. The dissociation rate constant of the fast phase increases to 700 s-1 at physiological temperature, whereas that of the slow phase does not change markedly. While the rate constants do not depend on pH between 6.6 and 7.8, the dissociation turns out to be monoexponential at pH 5.86. The association rate of Ca2+ to fluo-3 could not be measured within the mixing dead time and is estimated to be above 10(9) M-1 s-1. Since the rate constants of fluo-3 are larger than those of other fluorescent Ca2+ indicators, fluo-3 is well suited for investigations of Ca2+ oscillations in biological systems.