ALLOSTERIC EFFECTS OF MONOCLONAL-ANTIBODIES ON HUMAN GROWTH-HORMONE

We have previously shown that a monoclonal antibody (MAb) recognizing the human growth hormone (hGH) antigenic domain left exposed after bin ding to lactogenic receptors enhanced hGH binding probably through all osteric effects on the hormone binding site. Since receptors displayin g different specificities would not recognize exactly the same hGH reg ion, we explored whether some of our MAb could affect hGH binding to s omatogenic receptors from rabbit liver and to human liver hGH-specific receptors. The effect of MAbAE5, AC8 and F11 on hGH binding was measu red by determining the formation of I-125-MAb:hGH:receptor complexes u sing two different experimental approaches. Results from procedure A, which involved the previous binding of the hormone to microsomes befor e adding I-125-MAb, indicated that the hGH domain defined by epitopes AE5, AC8 and F11 is uncovered in the various hormone:receptor complexe s. Procedure B was devised to reveal any alteration in the hGH molecul e induced by the MAb. In this case preformed I-125-MAb:hGH complexes w ere added to microsomes. Data showed that I-125-MAb AES:hGH complexes bound better to the various receptors than I-125-MAb AE5 to hGH:recept or complexes. On the contrary, hGH previously bound to I-125-MAb AC8 o r I-125-MAb F11 was less recognized by the receptors than the free hor mone. Furthermore, binding of MAb AE5 or MAb F11 to hGH 20 K (a natura l hGH variant lacking residues 32-46) also enhanced its affinity to th e various receptors whereas MAb AC8 did not inhibit hGH 20 K binding. Results indicated that MAb recognizing the hGH antigenic area that rem ains unmasked after binding to different membrane-bound receptors are able to affect hormone binding site. MAb would induce either positive or negative allosteric changes in the hormone region involved in its b inding to lactogenic, somatogenic and hGH-specific receptors.