The heterogeneity of circulating human serum erythropoietin

The presence of more than one form of immunoreactive erythropoietin was suggested by discrepancies between the levels of immunologically detectable and biologically active erythropoietin in the sera of normal individuals and patients with end-stage renal disease. In both groups, the level of immunologically detectable hormone was significantly higher than that of bioactive hormone. Sera from end-stage renal disease patients and normal subjects were fractionated on a gel permeation column, and the immunoreactive erythropoietin in the fractions was measured by RIA. Three classes of immunoreactive erythropoietin were found in the sera: one eluting before the [125I]erythropoietin marker, one coinciding with the [125I]erythropoietin marker, and a third eluting after the labeled tracer in the region of the cytochrome c marker. The high and low mol wt components were more immunoreactive than the human urinary erythropoietin standard in the RIA. Biological activity, as determined in the in vitro rat bone marrow assay, was found only in the material eluting with the erythropoietin tracer. These studies show that immunoreactive erythropoietin in the serum consists of three components with different immunoreactivities: high and low mol wt components and a component with a mol wt similar to that of the native hormone.