UTILIZATION OF INTRACELLULAR FERRITIN IRON FOR HEMOGLOBIN-SYNTHESIS IN DEVELOPING HUMAN ERYTHROID PRECURSORS

Ferritin (Ft) plays an important role in cellular iron metabolism. it can store substantial amounts of iron in a nontoxic soluble form. Howe ver, its ability to donate iron for cellular needs. in particular for hemoglobin (Hb) synthesis in human erythroid cells, is still controver sial. We studied the role of intracellular Ft-iron in Hb synthesis and the involvement of lysosomal proteolysis in iron release from Ft. Ft- iron release and its subsequent incorporation into heme was investigat ed in normal human erythroid precursors developing in culture. Dual st aining flow cytometry with antibody (Ab)-specific for Ft and for Hb sh owed a decrease in cellular Ft content in erythroid cells during their maturation. Cellular Ft-iron participation in heme synthesis was stud ied by labeling cells with Fe-59. Cells were incubated with Fe-59-labe led human diferric transferrin (Tf), then chased, and intracellular ra dioiron distribution between Ft and Hb was determined on subsequent da ys by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-P AGE) and/or Ft immunoprecipitation and heme extraction. On day 6, most of the Fe-59 accumulated in Ft. Thereafter, a progressive decrease of radioiron in Ft and a corresponding increase of the label in Hb was o bserved. inhibition of heme synthesis with succinylacetone caused radi oiron to remain in Ft and prevented its redistribution. Addition of un labeled diferric Tf to the culture medium did not prevent radioiron fr om appearing in Hb. Chloroquine repression of lysosomal function preve nted radio-iron redistribution between Ft and Hb. inhibition df proteo lysis by chymostatin and/or leupeptin led to Ft-protein accumulation i n the cells and also prevented radioiron transfer from Ft to Hb. The r esults of the present study suggest that intracellular Ft donates iron for heme synthesis and that proteolytic Ft degradation in a lysosomal -like compartment is necessary for iron release and its transfer to he me. (C) 1997 by The American Society of Hematology.