FIGURE 5.

Transient kinetics experiments reveal that Ca²⁺ binds to derCD23 through a two-step mechanism. Mixing 20 mm CaCl₂ with 2 mm derCD23 in assay buffer results in a biphasic fluorescence quench, with a rapid burst-like phase followed by a slower and larger amplitude transient. The red and green lines show fits to the data of double and single exponential equations, respectively; the latter fitting is significantly worse, especially over the shorter times. The x axis (time) is shown on a logarithmic scale as the two phases occur over very different time ranges. The y axis is the fluorescence amplitude in arbitrary units.