Extraction of Erythropoietin from Normal Kidneys*

Significant amounts of active erythropoietin were extracted from the kidneys of normal rats, cattle, dogs, and rabbits by homogenization of the organs in 0.1 M phosphate buffer. The mean erythropoietin activities of the extracts, as determined by the starvedrat assay, were 0.26 U/g beef kidney, 0.41 U/g dog kidney, and 0.11 U/g rat kidney. The dog kidney extracts had a mean activity of 0.35 U/g, as measured by stimulation of hemoglobin synthesis in cultured bone marrow cells (in vitro assay) and produced a dose-dependent stimulation of 59Fe incorporation into circulating red cells when assayed in polycythemic mice. Extracts of rabbit kidney cortices had a mean activity of 2.12 U/g, as measured by stimulation of hemoglobin synthesis in cultured bone marrow cells. When the dog kidney homogenate was fractionated on DEAE-cellulose, all of the erythropoietin activity was adsorbed to the exchanger in the presence of 0.01 M acetate buffer, pH 4.5, and was completely eluted by 0.1 M Na2HPO4-0.5 M NaCl, pH 8. An antibody made against human urinary erythropoietin completely inactivated the erythropoietic factor in the dog kidney extract. Serum from a donor dog had no erythropoietin activity when assayed in the starved rat, suggesting that the factor in the extracts is intracellular erythropoietin rather than that contained in plasma trapped in the renal vasculature. The complete inactivation of the erythropoietic factor in these kidney homogenates by antierythropoietin and its behavior on DEAE-celluloso indicate that this factor is structurally similar to native plasma erythropoietin. The extracts are completely active without being incubated in the presence of serum.