Desferrithiocin and desferrioxamine B. Cellular pharmacology and storage iron mobilization

3H-Desferrithiocin (DFT) has been synthesized from desmethyl desferrithiocin. The uptake and release of this 3H siderophore and of its iron complex have been studied in cultured rat hepatocytes and systematically compared to 14C desferrioxamine B (DFO). At 37 degrees, the uptake of both chelators is strictly proportional to the extracellular concentration and no toxicity is observed up to, at least, 1 mM. Uptake of 3H DFT is rapid and reaches a plateau after ca. 1 hr. The accumulation of 3H DFT attains a maximum three times that of 14C DFO and the plateau is reached much more rapidly. Upon reincubation in a drug-free medium of cells that had accumulated 3H DFT, most of the 3H label is rapidly released in the culture medium. These kinetic parameters suggest that the accumulation of these two chelators results from their diffusion across cellular membranes, as a function of the gradient of concentration between the cellular compartment and the extracellular medium. Differential centrifugation of homogenates from hepatocytes incubated with 3H DFT shows that the bulk of cell associated 3H-label (82%) is found in the cytosol, whereas a small proportion (14.5%) is present in the particulate fraction. Isopycnic centrifugation on sucrose gradients suggests that 3H-label associated with the particulate fraction is localized within mitochondria. In contrast, 14C DFO distributes in almost equal proportions between cytosol and the particulate fraction (MLP). At least part of the 14C-label in MLP is associated with lysosomes. Rat hepatocytes cultivated for long term in synthetic culture medium have been used to study iron mobilization by chelators from 59Fe loaded cells. DFT mobilizes iron more rapidly than DFO. This effect is also observed in vitro with ferritin, where, in addition, DFT is much more efficient than DFO to mobilize iron at acidic pH. These results strongly suggest that different iron mobilization from cultured hepatocytes results from differences in the cellular pharmacology of these two chelators and, in particular, in their rate of uptake, cellular accumulation levels and subcellular localizations. DFT could mobilize iron from cytosol and, possibly, to a small extent from mitochondria, whereas DFO would do so from cytosol and lysosomes.