Location of an epitope defined by an enhancing monoclonal antibody to growth hormone: some structural details and biological implications.

We have previously shown that a murine monoclonal antibody (MAb OA15) prepared against ovine GH (oGH) can enhance the somatogenic activity of bovine GH (bGH), as determined by increased incorporation of 35SO4(2-) into costal cartilage of hypopituitary Snell dwarf mice in vivo We have now established that MAb OA15 can also enhance the biological activity of oGH and porcine GH (pGH) in vivo. Using multiple pin peptide synthesis techniques a set of overlapping immobilized octamers representing the entire bGH sequence were synthesized and tested for their ability to bind MAb OA15 using an enzyme-linked immunosorbent assay. The pattern of binding showed that OA15 defined a functionally continuous epitope comprising residues 91-102. This region includes the C-terminal end of helix 2 plus a portion of the adjacent loop linking helices 2 and 3. Polyclonal antisera to a synthetic peptide representing this epitope mimicked the ability of MAb OA15 to enhance oGH, bGH, and pGH. Window size analysis showed that the heptapeptide 94-100 (SRVFTNS) represents the minimum unit to retain full recognition of MAb OA15. The fact that pGH, bGH, and oGH have identical sequences in this region also explains the ability of OA15 to both cross-react with and enhance the biological activity of each of these GHs. Replacement net analysis (where each residue in the heptamer is substituted with each of the 19 naturally occurring L-amino acids) demonstrated that residues R95 and T98 are critical for antibody binding and also indicated that the substitution of V96 with I, as found in rat GH, would permit the observed binding of OA15 to this hormone.(ABSTRACT TRUNCATED AT 250 WORDS)