S-Nitroso compounds interfere with zinc probing by Zinquin

The intracellular homeostasis of zinc is postulated to be controlled by signaling through nitric oxide (NO). Administration of the NO donor S-nitrosocysteine (SNOC) caused a rapid drop in the fluorescence of the zinc-specific fluorescence of the zinc probe zinquin in C6 glioma cells. Tentatively, a strong effect of NO on the level of mobile intracellular zinc ions was concluded. However, zinc analysis with atomic absorption spectrometry demonstrated that the total cellular zinc level was not changed under these conditions. Sodium nitrite or an NO donor devoid of sulfhydryl groups (diethylamine NONOate) exerted no degrading effect on the Zn/zinquin fluorescence, but cysteine alone evoked a similar decline as SNOC. Hence, the sulfhydryl groups of cysteine seem to compete for zinc from the Zn/zinquin complex. Analysis of the reaction products by mass spectrometry demonstrated that cysteine caused a depletion of zinc from the Zn/zinquin complex, whereas an NO donor without sulfhydryl groups (diethylamine NONOate) did not. It is concluded that great caution should be employed when using S-nitroso compounds together with zinquin in investigations of intracellular zinc homeostasis.