The same region of human growth hormone is involved in its binding to various receptors

The Fab fragments of three monoclonal antihuman GH (anti-hGH) antibodies, among five tested, inhibited the binding of the hormone to the receptors of the human lymphoid cell line IM-9 and liver membranes of the pregnant rabbit. The results were similar for the receptors of human lymphocytes and rabbit liver, suggesting that both receptors reacted with the same region of the hormone. The Fab fragments of the most inhibitory antibody also inhibited the down-regulation by the hormone of the receptors on human lymphocytes. The fragments of this antibody completely blocked the binding of the hormone to the receptors of the rabbit liver, despite the fact that this carries two or more classes of receptors. Therefore, all of these various receptors apparently interact with the same region of the hormone. Three synthetic peptides extending from residues 19-128, 73-128, and 98-128 failed to inhibit the binding of hGH to its lymphocyte or liver receptors however, these peptides reacted significantly with the monoclonal antibody, which was the strongest inhibitor of the interaction of hGH with the cellular receptor, confirming that the receptor-binding site of the hormone is in the amino-terminal part of the molecule and suggesting indirectly that the short sequences 98-128 participates in the constitution of the receptor-binding site of the hormone.