Kathy,
Although cells contain a fair amount of zinc, only a small portion of it is in soluble form (free[Zn++] about 2 nM). Polycarboxylate indicators, such as fura-2 and APTRA-BTC (Mole Probes), appear to bind only the Zinc in solution, while quinolinium dyes, such as Zinquin and TSQ, can chelate bound zinc.
A third type of Zn dye from MPI, Newport Green, also binds to zinc in cells and is cell-permeant as the diacetate. Some preliminary work on this dye in 3T3 cells showed a large increase in fluorescence on addition of extracellular Zn. It can be excited at 488 nm and, because it has an uncharged Zn binding site (similar to the heavy-metal chelator TPEN), it should be able to bind to Zn that is already bound to protein/DNA/RNA.
There is a chart in the MPI handbook that shows relative responses of a series of indicators to a wide range of metals (about 18 metals). This might be a useful resource for comparing various dyes and interferences from other metals that might be present.
I am not aware of any study that has done a rigorous comparison of the three different types of indicators in live cells. Best of luck.
Michael Kuhn, President
Helix Research Co.
Fluorescence Chemistry
mkuhn@helixresearch.com
www.helixresearch.com
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