Intracellular forms of iron during transferrin iron uptake by mitogen-stimulated human lymphocytes

Transferrin receptors expressed by mitogen stimulated human lymphocytes mediate the uptake of transferrin iron into haem, ferritin and a non-haem, non-ferritin component. In spite of different rates of iron uptake by cells from different individuals, the proportional incorporation of 59Fe into these components was similar, suggesting that there was an obligatory relationship between the different forms of iron in the cells. By 3 h over 60% of the iron taken up was incorporated into ferritin while less than 10% was found in haem. Initially (10 min) non-haem, non-ferritin iron comprised 70% of total iron and this diminished to 30% by 3 h. At 10 min 80% of iron in the non-haem, non-ferritin component was retained by anti-transferrin affinity columns indicating it was transferrin-bound. The proportion retained fell to reach a steady state level of 50% by 60 min. These results indicate that 10-20% of the iron in the cells was not recognized as transferrin, ferritin or haem iron. The finding that iron incorporation into this unidentified pool reached equilibrium while that into haem and ferritin increased suggests the iron may act as a precursor for functional and storage compounds.