Does the calcein-AM method assay the total cellular 'labile iron pool' or only a fraction of it?

The calcein-AM (calcein-acetoxymethyl ester) method is a widely used technique that is supposed to assay the intracellular 'labile iron pool' (LIP). When cells in culture are exposed to this ester, it passes the plasma membrane and reacts with cytosolic unspecific esterases. One of the reaction products, calcein, is a fluorochrome and a hydrophilic alcohol to which membranes are non-permeable and which, consequently, is retained within the cytosol of cells. Calcein fluorescence is quenched following chelation of low-mass labile iron, and the degree of quenching gives an estimate of the amounts of chelatable iron. However, a requirement for the assay to be able to demonstrate cellular LIP in total is that such iron be localized in the cytosol and not in a membrane-limited compartment. For some time it has been known that a major part of cellular, redox-active, labile, low-mass iron is temporarily localized in the lysosomal compartment as a result of the autophagic degradation of ferruginous materials, such as mitochondrial complexes and ferritin. Even if some calcein-AM may escape cytosolic esterases and enter lysosomes to be cleaved by lysosomal acidic esterases, the resulting calcein does not significantly chelate iron at <pH 5. In the present study we show that the calcein-AM method does not capture lysosomal low-mass iron and, therefore, that the method seriously underestimates total cellular labile iron.