Distribution of iron in reticulocytes after inhibition of heme synthesis with succinylacetone: Examination of the intermediates involved in iron metabolism

Succinylacetone (SA) is an inhibitor of heme synthesis that acts on the enzyme delta-aminolevulinic acid dehydratase. When reticulocytes are incubated with Fe-59-transferrin (Fe-59-Tf) in the presence of SA, there is an accumulation of Fe-59 in the mitochondrion and in a cytosolic non-heme intermediate that has been described as a putative Fe transporter (Adams et al, Biochim Biophys Acta 1012:243, 1989). Considering these observations, the present study was designed to examine the intermediates of Fe metabolism in control and SA-treated reticulocytes. This investigation showed that in the cytosol of control cells, most Fe-59 was incorporated into hemoglobin (Hb) with a minor amount entering ferritin. In addition, a previously unrecognized cytosolic intermediate was identified (band X) that was absent when heme synthesis was inhibited with CA. Upon reincubation of SA-treated reticulocytes with protoporphyrin IX, band X initially increased in intensity and then decreased later in the incubation. In contrast, when Fe-59-labeled control cells were reincubated in the presence of SA and unlabeled diferric Tf, there was a marked decrease in the intensity of band X. These experiments suggest that component X may be an intermediate involved in the transfer of heme in the cytosol. Alternatively, these data could also be interpreted as indicating that band X may be a short-lived hemoprotein. We have confirmed the presence of an Fe-59-containing molecule in the cytosol of SA-treated reticulocytes (band Y) that is not present in control cells. However, when cells were incubated with Fe-59-Tf plus SA and then chased in the presence of SA and unlabeled diferric Tf, there was no decrease in this cytosolic pool of Fe, suggesting that it was not a intermediate supplying Fe for either ferritin or heme synthesis. Finally, there is little low molecular weight (M(r)) Fe in reticulocytes, and our studies suggest that the low-M(r) Fe present does not behave as an intermediate. Moreover, after inhibition of heme synthesis with CA, Fe-59 in the low-M(r) compartment was markedly decreased, suggesting that this component may be heme or a low-M(r) heme-containing molecule. Considering the apparent lack of a cytosolic Fe transporter in rabbit reticulocytes, an alternative model of intracellular Fe transport is proposed that does not implicate a potentially toxic intermediate pool of low-M(r) Fe complexes. (C) 1996 by The American Society of Hematology.