Site-specific antibodies to human erythropoietin directed toward the NH2-terminal region

Site-specific antibodies to human erythropoietin have been raised in rabbits immunized with a synthetic polypeptide composed of the putative 26 NH2-terminal amino acids of the hormone. The immunogenic peptide was coupled to bovine serum albumin. Antibodies specific for peptide were detected by enzyme-linked immunosorbent assay. They immunoprecipitated both highly purified 125I-labeled erythropoietin and biologically active erythropoietin. The immunoprecipitation of 125I-labeled erythropoietin was inhibited by unlabeled erythropoietin and by peptide, demonstrating their crossreactivity. The antibodies did not neutralize erythropoietin's biological activity. These results indicate that a portion of the NH2-terminal region of erythropoietin is exposed on the surface of the protein at some distance from the receptor-binding domain. These antibodies will be important in further studies of the hormone and its mechanism of action.