What does the commonly used DCF-test for oxidative stress really show?
Résumé
Dihydrodichlorofluorescein (H2DCF-DA) is widely used to evaluate “cellular oxidative stress”. After passing through the plasma membrane, this lipophilic and non-fluorescent compound is de-esterified to a hydrophilic alcohol (H2DCF) that may be oxidized to fluorescent DCF by a process usually considered to involve reactive oxygen species (ROS). It is, however, not always recognized that, being a hydrophilic molecule, H2DCF does not pass membranes, except for the outer, fenestrated mitochondrial ones. It is also not generally realized that oxidation of H2DCF is dependent either on Fenton-type reactions or on unspecific enzymatic oxidation by cytochrome c, for neither superoxide, nor hydrogen peroxide, directly oxidizes H2DCF. Consequently, oxidation of H2DCF requires the presence of either cytochrome c or of both redox-active transition metals and hydrogen peroxide. Redox-active metals exist mainly within lysosomes, while cytochrome c resides bound to the outer side of the inner mitochondrial membrane. Following exposure to H2DCF-DA, weak mitochondrial fluorescence was found in both the oxidation-resistant ARPE-19 cells and the much more sensitive J774 cells. This fluorescence was only marginally enhanced following short exposure to hydrogen peroxide, showing it by itself being unable to oxidize H2DCF. Cells that were either exposed to the lysosomotropic detergent MSDH, exposed to prolonged oxidative stress, or spontaneously apoptotic showed lysosomal permeabilization and strong DCF-induced fluorescence. The results suggest that DCF-dependent fluorescence largely reflects relocation to the cytosol of lysosomal iron and/or mitochondrial cytochrome c.
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