Zinquin [(2-methyl-8-p-toluenesulphonamido-6-quinolyloxy)-acetic acid], a membrane-permeant fluorophore specific for Zn(II), was used with spectrofluorimetry and video image analysis to reveal and quantify labile intracellular Zn. Zinquin labelled human chronic-lymphocytic-leukaemia lymphocytes, rat splenocytes and thymocytes with a weak diffuse fluorescence that was quenched when intracellular Zn was chelated with NNN'N'-tetrakis-(2-pyridylmethyl)ethylenediamine (TPEN) and was greatly intensified by pretreatment of cells with the Zn ionophore pyrithione and exogenous Zn. There was substantial heterogeneity of labile Zn among ionophore-treated cells, and fluorescence was largely extranuclear. The average contents of labile Zn in human leukaemic lymphocytes, rat splenocytes and rat thymocytes were approx. 20, 31 and 14 pmol/10(6) cells respectively. Morphological changes and internucleosomal DNA fragmentation indicated substantial apoptosis in these cells when the level of intracellular labile Zn was decreased by treatment with TPEN. Conversely, increasing labile Zn by pretreatment with Zn plus pyrithione suppressed both spontaneous DNA fragmentation and that induced by the potent apoptosis-induced agents colchicine and dexamethasone. These results suggest that prevention of apoptosis is a function of labile Zn, and that a reduction below a threshold concentration in this Zn pool induces apoptosis.

We used an intracellular zinc-specific fluomphore, Zinquin, in conjunction with fluorescence video image analysis, to reveal labile zinc in panaeatic islet cells, which concentrate this metal for use in synthesis, storage, and secretion of insulia. 2hquin vividly demonstrated zinc in the islet cell secip tory granules, which formed a brightly labeled crescent in the cytoplasm between one side ofthe nudeus and the plasma membrane. Lower but still appreciable amounts of zinc were detected in the remaining cytoplasm, but there was little labeling in the nudeus. Fluorescence intensity varied among islet cells, suggesting Werences in zinc Content. Their average fluorescence intensity greatly surpassed that of the surrounding pancreatic acinar cells in frozen seaions of pan-

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