Abstract

A factor with erythropoietic stimulating activity but distinct from erythropoietin (Ep) has been identified in normal human and mouse serum. Like Ep this factor causes a significant increase in the amount of 59Fe incorporated into heme in short term cultures of mouse marrow cells. However, dose response studies show that it acts synergistically rather than additively with erythropoietin. This factor has been partially purified using Sephadex G150 and chromatofocusing, and appears to be a protein with a molecular weight of 130,000 daltons and an isoelectric point of 4.8. Colcemid and time course studies indicate that this stimulator acts late in erythroid differentiation, primarily on erythroblasts. Benzidine staining studies suggest it may increase 59Fe uptake in hemoglobin-producing erythroblasts by enhancing their viability. Accordingly the term erythroblast enhancing factor (EEF) is proposed. Serum EEF levels have been found to be markedly reduced in polycythemic mice and EEF injected together with Ep into such animals has a synergistic effect in stimulating new red cell production. These findings strongly suggest that EEF acts as a positive regulator of terminal erythropoiesis in vivo.