Uptake of Iron from Transferrin by Isolated Rat‐Liver Mitochondria Mediated by Phosphate Compounds

Abstract

1. Isolated rat-liver mitochondria accumulate iron from transferrin at neutral pH by a mechanism which is markedly stimulated by small-molecular-weight polyphosphate compounds. The efficiency of the phosphate compounds decreases in the order: pyrophosphate > ATP > GTP > 2,3-bis-(phospho)glycerate > phosphate.2. The uptake has a very low energy dependence, and it does not depend on the hydrolysis of ATP or the saturation of transferrin, but it increases in parallel to the concentration of iron to reach a saturation level of 800-1200 pmol iron/mg protein.3 . Following a chase with unlabelled transferrin, ('251)-labelled transferrin bound to the mitochondria remains constant, whereas the progressive uptake of 59Fe levels off.4. During reincubation of iron-loaded mitochondria up to 30% of the iron is mobilized in the presence of ascorbate and ATP (or apotransferrin).5. The results suggest that iron is mobilized from transferrin by the polyphosphate compounds outside the mitochondria and taken up by the mitochondria in a subsequent reaction.The ferrochelatase reaction represents the ultimate destination for the major part of iron entering the cell. Although it has been claimed that the ferrochelatase resides in part [l] or exclusively in the cytosol [2] most investigators agree that the ferrochelatase is a particulate enzyme confined to the mitochondria [3-51, tentatively on the M-side of the inner membrane [6-81 and depending on unsaturated phospholipids for full activity [9]. Hence, to reach the ferrochelatase iron has to penetrate the mitochondria1 membrane systems.The mechanism(s) by means of which iron is transported across the cytosol, from the plasma membrane to the mitochondria is largely unknown, as is the nature of the cytosol iron-binding ligands. Controversy exists as to whether iron is released from transferrin at the plasma membrane [lo, to operate on the main pathway of iron to the niitochondria in vivo. The uptake of iron by isolated mitochondria has been described, using iron-ATP [18], iron-hydroxyquinoline [19] or iron-sucrose [20] as the iron donor compound. A coherent picture, however, has not emerged.The importance of transferrin as an iron-donor directly to the mitochondria has been emphasized [14,16]. As yet, however, there are only a few studies dealing with the utilization of transferrin iron by isolated mitochondria. Barnes et al. [21] concluded from experiments with sonicated mitochondria that neither transferrin not ferritin functioned as substrate to the ferrochelatase reaction. Ulvik et al. [22] and Koller et al. [23] found that iron was mobilized from transferrin by a mechanism sensitive to uncoupler, high pH and haemin. The release process operated at a rate below that necessary to cope with the ferrochelatase and also at a pH outside that occurring in vivo. It was concluded that transferrin was not a suitable donor of iron to mitochondria. More recently, Konopka [24] and Konopka and Turska [25] reported that rat liver mitochondria accumulated iron from transferrin at a signifi...

Pyrophosphate-Mediated Iron Acquisition from Transferrin in Neisseria meningitidis Does Not Require TonB Activity

“…Transportation of iron from human transferrin requires the activity of the TonB-dependent outer membrane transporter TbpAB [2] . In vitro experiments demonstrated the role of pyrophosphate as a mediator of iron transfer from transferrin to ferritin [36] . According to these results, pyrophosphate can bind iron loaded on transferrin and deliver it to ferritin [26] .…”