Low-affinity Fluorescent Indicator for Intracellular Calcium Ions

Abstract

A Ca²⁺ indicator, 7-amino-3-(2-benzothiazolyl)-6-[2-(2,3-di- fluoro-6-aminophenoxy)ethoxy]-coumarin-N,N,N’,N’-tetra- acetic acid was synthesized. The indicator has a large K_d value of 0.55 mM and shows an excitation spectral shift from 462 to 401 nm upon Ca²⁺ binding. The indicator, therefore, may be useful for the measurement of Ca²⁺ concentrations in the endoplasmic reticulum of living cells by ratiometric fluorescence spectroscopy with visible light excitation.