UPTAKE OF NON-TRANSFERRIN-BOUND IRON BY BOTH REDUCTIVE AND NONREDUCTIVE PROCESSES IS MODULATED BY INTRACELLULAR IRON

Non-transferrin-bound iron (NTBI) uptake occurs in a variety of cells by a saturable, specific and temperature-sensitive process. Our previous studies indicated that NTBI uptake by cardiac myocytes and Hep G2 cells was reversibly up regulated by iron deposition, In the present work we have characterized this up-regulation and examined its mechanism by comparing the uptake of oxidized (Fe3+) and ascorbate-reduced (Fe2+) forms of iron. Iron loading markedly enhanced the uptake of iron both in the presence and absence of ascorbate, but the increment was greater when ascorbate was absent. This up-regulation is partially inhibited by actinomycin D and cycloheximide, indicating a requirement for protein synthesis. Uptake by the iron-loaded cells was less sensitive to thiol-alkylating agents and competing metal ions, but was more sensitive to proteolysis. Iron loading causes an increase in both K-m and V-max for uptake of both Fe2+ and Fe3+, although the values differ, suggesting dis tinct rate-limiting steps for uptake of Fe2+ and Fe3+. Consistent with this idea, uptake of the two ions showed differential sensitivity to thiol reagents, competing metal ions and monensin. The Fe2+-specific chelators bathophenanthroline disulfonate and ferrozine markedly inhibited iron uptake whether ascorbate was present or not, indicating that Fe3+ uptake is dependent on reduction to the ferrous state. This requirement for reduction was independent of the iron status of the cells, demonstrating that the process of up-regulation is not due to the appearance of a new mechanism for translocation of Fe3+ without reduction. Taken together, the evidence favors a model of NTBI transport where an obligate and rate-determining reduction of Fe3+ occurs prior to or during uptake, followed by translocation through an Fe2+ carrier. The distinct translocation mechanisms of uptake in the presence and absence of ascorbate suggest that exogenous Fe2+ does not access the carrier available to the nascent ferrous ion derived from the reductase and is consistent with close coupling between the reduction and the translocation processes. In iron-loaded cells with increased rates of NTBI transport, a similar mechanism prevails.