Fluorescence detection of intracellular cadmium with Leadmium Green.

Leadmium Green is a commercially available, small molecule, fluorescent probe advertised as a detector of free intracellular cadmium (Cd2+) and lead (Pb2+). Leadmium Green has been used in various paradigms, such as tracking Cd2+ sequestration in plant cells, heavy metal export in protozoa, and Pb2+ absorption by vascular endothelial cells. However very little information is available regarding its affinity and selectivity for Cd2+, Pb2+, and other metals. We evaluated the in vitro selectivity of Leadmium Green using spectrofluorimetry. Consistent with manufacturer’s claims, Leadmium Green was sensitive to Cd2+ (KD ~600 nM) and also Pb2+ (KD ~9.0 nM) in a concentration-dependent manner, and furthermore proved insensitive to Ca2+, Co2+, Mn2+ and Ni2+. Leadmium Green also responded to Zn2+ with a KD of ~82 nM. Using fluorescence microscopy, we evaluated Leadmium Green in live mouse hippocampal HT22 cells. We demonstrated that Leadmium Green detected ionophore-mediated acute elevations of Cd2+ or Zn2+ in a concentration-dependent manner. However, the maximum fluorescence produced by ionophore-delivered Zn2+ was much less than that produced by Cd2+. When tested in a model of oxidant-induced liberation of endogenous Zn2+, Leadmium Green responded weakly. We conclude that Leadmium Green is an effective probe for monitoring intracellular Cd2+, particularly in models where Cd2+ accumulates rapidly, and when concomitant fluctuations of intracellular Zn2+ are minimal.