FIGURE 6.

Analysis of pH jump experiments in the absence of CaCl₂. The time course of the fluorescence decrease was fitted with the sum of two exponentials (Eq. 1), and (A) the time constants, τ_i, and (B) amplitudes, ΔF_i, were plotted against the electrolyte pH after the pH jump. The UV-flash-induced release of protons was ∼0.125 μM, independent of pH in the buffer. (A) The faster of the two rate-limiting processes, controlled by τ₁ is not significantly dependent on pH in the pH range covered by the experiments. The slower process is decelerated below pH 6 with increasing proton concentrations. (B) The fluorescence amplitude revealed a pH dependence for both processes. Since it reflects binding of protons to the Ca-ATPase, it is obvious that below pH 6 the faster process carried more protons into the membrane domain than the slower one. Above pH 6 it was vice versa.