Succinylacetone (SA) is an inhibitor of heme synthesis that acts on the enzyme delta-aminolevulinic acid dehydratase. When reticulocytes are incubated with 59Fe-transferrin (59Fe-Tf) in the presence of SA, there is an accumulation of 59Fe 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 59Fe 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 SA. 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 59Fe-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 59Fe-containing molecule in the cytosol of SA-treated reticulocytes (band Y) that is not present in control cells. However, when cells were incubated with 59Fe-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 (Mr) Fe in reticulocytes, and our studies suggest that the low-Mr Fe present does not behave as an intermediate. Moreover, after inhibition of heme synthesis with SA, 59Fe in the low-Mr compartment was markedly decreased, suggesting that this component may be heme or a low-Mr 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-Mr Fe complexes.

Lady Davis Institute for Medical Research Sir Mortimer B Davis Jewish General Hospital Montreal Quebec Canada