Studies on the Mechanism of Pyrophosphate‐Mediated Uptake of Iron from Transferrin by Isolated Rat‐Liver Mitochondria

Abstract

1. Respiring rat liver mitochondria accumulate iron released from transferrin by pyrophosphate. The amount of iron accumulated is 1-1.5 nmol mg protein-' h-', or approximately 60"/, of the amount of iron mobilized from transferrin.2. The uptake declines if respiration is inhibited, substrate is deleted, or the experiments are run under anaerobic conditions. Substrate depletion and respiratory inhibitors are less inhibitory under anaerobic conditions.3. More than SO"/, of the amount of iron accumulated by aerobic, actively respiring mitochondria can be chelated by bathophenanthroline sulphonate, and with deuteroporphyrin included, up to 30 % of the amount of iron accumulated is recovered as deuteroheme. Iron accumulated by respiration-inhibited mitochondria under aerobic conditions is not available for heme synthesis. 4. With time the uptake of iron increases eightfold relative to the uptake of pyrophosphate. 5.The results are compatible with a model in which ferric iron is mobilized from transferrin by pyrophosphate, ferric iron pyrophosphate is bound to the mitochondria, iron is reduced, dissociates from pyrophosphate and is taken up by the mitochondria. Ferrous iron thus formed is available for heme synthesis.

Two types of receptors for iron on mitochondria

“…Earlier work studying the effect of various phosphates on Fe delivery to mitochondria for haem synthesis (Konopka & Romslo, 1980, 1981Nilsen & Romslo, 1984a,b) used transferrin as a donor reservoir for Fe and measured Fe incorporation into haem. Thus a complex reaction which, in retrospect, probably included Fe release from transferrin, hydrolysis of the ligand-Fe complex, receptor binding and haem synthesis, was subsumed in a single measurement.…”

“…Mitochondrial uptake of Fe from phosphate-containing compounds has been studied for several years (Konopka & Romslo, 1980, 1981Nilsen & Romslo, 1984a,b). The recent finding that, when ATP-Fe is incubated with mitochondria, Fe binds with high affinity to a mitochondrial receptor (Weaver et al, 1990), at first suggested that ATP might be the important physiological intracellular donor of Fe for haem synthesis.…”