Imaging zinc: old and new tools.

As recently as 20 years ago, all zinc in biological systems was believed to be tightly bound to proteins, and the idea of imaging zinc was considered heretical. Beginning with Maske's research with dithizonate staining of the hippocampus in the 1950s, however, zinc-sensitive dyes have indicated that, in mammalian cells, free zinc can exist in at least three separate pools. These pools include vesicular zinc sequestered in presynaptic vesicles and secretory granules, zinc released from these vesicles into the extracellular space after physiological stimulation, and transient increases in zinc in cells in the regions where extracellular release of zinc has occurred. This Perspective covers the zinc-imaging tools, from dithizonate to the newest FRET-based sensors, that have galvanized biomedical science.