Quantitative Imaging of Labile Zn²⁺ in the Golgi Apparatus Using a Localizable Small-Molecule Fluorescent Probe

Fluorescent Zn²⁺ probes used for the quantitative analysis of labile Zn²⁺ concentration ([Zn²⁺]) in target organelles are crucial for understanding the role of Zn²⁺ in biological processes. Although several fluorescent Zn²⁺ probes have been developed to date, there is still a lack of consensus concerning the [Zn²⁺] in intracellular organelles. In this study, we describe the development of ZnDA-1H, a small-molecule fluorescent probe for Zn²⁺, which exhibits less pH sensitivity, high Zn²⁺ selectivity, and large fluorescence enhancement upon binding to Zn²⁺. Through protein labeling technology, ZnDA-1H was precisely targeted in various intracellular organelles, such as the nucleus, mitochondria, endoplasmic reticulum, and Golgi apparatus. ZnDA-1H exhibited a reversible fluorescence response toward labile Zn²⁺ in these organelles in live cells. Using this probe, the [Zn²⁺] in the Golgi apparatus was estimated to be 25 ± 1 nM, suggesting that labile Zn²⁺ plays a physiological role in the secretory pathway.