In cultured cortical and hippocampal neurons when intracellular pH drops from 6.6 to 6.1, yet unclear intracellular stores release micromolar amounts of Zn2+ into the cytosol. Mitochondria, acidic organelles, and/or intracellular ligands could release this Zn2+. Although exposure to the protonophore FCCP precludes reloading of the mitochondria and acidic organelles with Zn2+, FCCP failed to compromise the ability of the intracellular stores to repeatedly release Zn2+. Therefore, Zn2+-releasing stores were not mitochondria or acidic organelles but rather intracellular Zn2+ ligands. To test which ligands might be involved, the rate of acid-induced Zn2+ release from complexes with cysteine, glutathione, histidine, aspartate, glutamate, glycine, and carnosine was investigated; [Zn2+] was monitored in vitro using the ratiometric Zn2+-sensitive fluorescent probe FuraZin-1. Carnosine failed to chelate Zn2+ but did chelate Cu2+; the remaining ligands chelated Zn2+ and upon acidification were releasing it into the medium. However, when pH was decreasing from 6.6 to 6.1, only zinc–cysteine complexes rapidly accelerated the rate of Zn2+ release. The zinc–cysteine complexes also released Zn2+ when a histidine-modifying agent, diethylpyrocarbonate, was applied at pH 7.2. Since the cytosolic zinc–cysteine complexes can contain micromolar amounts of Zn2+, these complexes may represent the stores responsible for an acid-induced intracellular Zn2+ release.
This study aimed at identifying intracellular stores which release Zn2+ when pHi drops from 6.6 to 6.1. It was found that these stores are not mitochondria or acidic organelles, but rather intracellular Zn2+ ligands. When the pH was decreasing from 6.6 to 6.1, only zinc–cysteine complexes showed a rapid acceleration in the rate of Zn2+ release. Therefore, the stores responsible for an acid-induced intracellular Zn2+ release in neurons may be the cytosolic zinc–cysteine complexes.