Zn2+ is a necessary cofactor for thousands of mammalian proteins. Research has suggested that transient fluxes of cellular Zn2+ are also involved in processes such as apoptosis. Observations of Zn2+ trafficking have been collected using Zn2+ responsive fluorescent dyes. A commonly used Zn2+ fluorophore is 6-methoxy-8-p-toluenesulfonamido-quinoline (TSQ). The chemical species responsible for TSQ's observed fluorescence in resting or activated cells have not been characterized. Parallel fluorescence microscopy and spectrofluorometry of LLC-PK1 cells incubated with TSQ demonstrated punctate staining that concentrated around the nucleus and was characterized by an emission maximum near 470 nm. Addition of cell permeable Zn-pyrithione resulted in greatly increased, diffuse fluorescence that shifted the emission peak to 490 nm, indicative of the formation of Zn(TSQ)(2). TPEN (N,N,N',N'-tetrakis(-)[2-pyridylmethyl]-ethylenediamine), a cell permeant Zn2+ chelator, largely quenched TSQ fluorescence returning the residual fluorescence to the 470 rim emission maximum. Gel filtration chromatography of cell supernatant from LLC-PK1 cells treated with TSQ revealed that TSQ fluorescence (470 nm emission) eluted with the proteome fractions. Similarly, addition of TSQ to proteome prior to chromatography resulted in 470 nm fluorescence emission that was not observed in smaller molecular weight fractions. It is hypothesized that Zn-TSQ fluorescence, blue-shifted from the 490 nm emission maximum of Zn(TSQ)(2), results from ternary complex, TSQ-Zn-protein formation. As an example, Zn-carbonic anhydrase formed a ternary adduct with TSQ characterized by a fluorescence emission maximum of 470 nm and a dissociation constant of 1.55 x 10(-7) M. Quantification of TSQ-Zn-proteome fluorescence indicated that approximately 8% of cellular Zn2+ was imaged by TSQ, These results were generalized to other cell types and model Zn-proteins.