The 19-amino acid domain Ala111----Pro129 of human erythropoietin was identified as an accessible surface antigen based on the binding of radio-iodinated and of unmodified hormone to antibodies prepared against a synthetic peptide of homologous sequence. The specificity and affinity of this binding was sufficient to provide for the use of anti-peptide antibodies in the preparation of an immunosorbent for the purification of urinary, and of recombinant human erythropoietin. Immobilization of anti-peptide antibodies using agarose activated either with CNBr or with N-hydroxysuccinimido groups largely inactivated binding sites for erythropoietin. In contrast, antibodies crosslinked to N-acetyl-DL-homocysteine agarose through the hetero-bifunctional reagent succinimidyl 4-(N-maleimidomethyl)cyclohexane-1-carboxylate retained their antigen-binding capacity virtually completely and provided a superior immunosorbent for hormone. Urinary erythropoietin with a specific bioactivity of 100 U/A280 was prepared initially by chromatography on phenyl-Sepharose. Subsequent immunoaffinity chromatography resulted in a 350-fold purification with 46.2% recovery yielding erythropoietin with a specific bioactivity of 35,200 U/A280 (44,300 U/mg). Radioiodination of this purified protein and subsequent SDS-polyacrylamide gel electrophoresis indicated that this preparation contained a single major component (Mr 30,000) which co-migrated in gels with unmodified biologically active hormone. Recombinant erythropoietin, which was prepared by the cloning of the human erythropoietin gene and its expression in COS cells using the SV40-derived vector pSV2, was purified by the same scheme. Chromatography on phenyl-Sepharose of medium derived from transfected cells (400 U/ml, 170 U/A280) provided for a 3.6-fold purification of recombinant hormone with an apparent recovery of 122%. This erythropoietin bound to the anti-peptide antibody gel and was purified to a specific bioactivity of 10,370 U/A280 with 55% recovery. The procedure described here for attaching antibodies to a solid support maximizes their antigen-binding capacity and is generally applicable. The development of an anti-peptide immunosorbant for human erythropoietin provides a valuable means for isolating hormone for use in studies of its receptor and its presently unresolved mechanism of action.
