Determination of the chelatable iron pool of single intact cells by laser scanning microscopy

We have previously established a method of detecting intracellular chelatable iron in viable cells based on digital fluorescence microscopy. To quantify cellular chelatable iron, it was crucial to determine the intracellular indicator concentration. In the present study, we therefore adapted the method to confocal laser scanning microscopy, which should allow the determination of the indicator concentration on the single-cell level. The fluorescent heavy-metal indicator phen green SK (PG SK), the fluorescence of which is quenched by iron, was loaded into cultured rat hepatocytes. The hepatocellular fluorescence increased when cellular chelatable iron available to PG SK was removed from the probe by an excess of the membrane-permeable transition metal chelator 2,2'-dipyridyl (2, 2'-DPD, 5 mM). We optimized the scanning parameters for quantitatively recording changes in fluorescence and determined individual intracellular PG SK concentrations from the unquenched cellular fluorescence (after 2,2'-DPD) compared with PG SK standards in a "cytosolic" medium. An ex situ calibration method based on laser scanning microscopy was set up to determine the concentration of cellular chelatable iron from the increase of PG SK fluorescence after addition of 2,2'-DPD (5 mM). As the stoichiometry of the PG SK:Fe(2+) complex was 3:1 as long as PG SK was not limiting, cellular chelatable iron was calculated directly from absolute changes in cellular fluorescence. Using this method, we found 2.5 +/- 2.2 microM chelatable iron in hepatocytes. This method makes it possible to determine the pool of chelatable iron in single vital cells independently of cellular differences (e.g., dye loading, cell volume) in heterogeneous cell populations.