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Iron bound to low MW ligands: interactions with mitochondria and cytosolic proteins
1Dept of Medicine, Albert Einstein College of Medicine, Bronx, N.Y. 10461.
Abstract:
The iron in the low MW pool of the cell is the precursor of iron in haem and is bound primarily to ATP. This precursor-product relationship suggested that reticulocytes might accumulate ATP-iron if their haem synthesis were blocked. However reticulocytes, treated with succinylacetone or rotenone and taking up iron from transferrin, accumulated iron in nonhaem cytosolic proteins and in mitochondria and not in the low MW pool. This was demonstrated by NMR and also by disrupting the cell with shear stress, separating the cytosol and pellet and fractionating the cytosol with ammonium sulfate. This constancy of the low MW iron pool in the face of blocked haem synthesis could not be explained by saturation of cytosolic ATP or by sluggish exchange of the low MW pool with other compartments. Rather, nonhaem cytosolic proteins and mitochondria appeared to have a higher affinity for iron and to exchange it rapidly with that in the low MW pool.
Insights
Cellular iron, crucial for heme synthesis, is primarily bound to ATP. Blocking heme production in reticulocytes did not increase ATP-iron, indicating other proteins have higher iron affinity.
Area of Science:
- Cellular metabolism
- Iron homeostasis
- Reticulocyte biology
Background:
- The low molecular weight (MW) iron pool, primarily bound to adenosine triphosphate (ATP), is considered the direct precursor for heme synthesis in cells.
- This relationship suggests that blocking heme synthesis should lead to the accumulation of iron within this ATP-bound pool.
Purpose of the Study:
- To investigate the distribution and fate of cellular iron in reticulocytes when heme synthesis is experimentally inhibited.
- To determine if the low MW ATP-iron pool accumulates under conditions of blocked heme production.
Main Methods:
- Reticulocytes were treated with inhibitors of heme synthesis (succinylacetone or rotenone) and exposed to transferrin-bound iron.
- Cellular iron distribution was assessed using Nuclear Magnetic Resonance (NMR) spectroscopy.
- Further analysis involved cell disruption via shear stress, separation into cytosolic and pellet fractions, and cytosolic fractionation using ammonium sulfate precipitation.
Main Results:
- Despite blocked heme synthesis and iron uptake, iron did not accumulate in the low MW ATP-iron pool.
- Instead, iron was predominantly found in non-heme cytosolic proteins and mitochondria.
- NMR and fractionation studies confirmed the constancy of the low MW iron pool.
Conclusions:
- The low MW iron pool's stability, even with inhibited heme synthesis, challenges the simple precursor-product model.
- Non-heme cytosolic proteins and mitochondria exhibit a higher affinity for iron and rapidly exchange iron with the low MW pool.
- These findings suggest a more complex regulatory mechanism for cellular iron distribution and utilization.