Positive cooperative effects between receptors induced by an anti-human growth hormone allosteric monoclonal antibody

Monoclonal antibodies (MAb) anti-human growth hormone (hGH) termed MAb AE5, AC8 and F11 recognize a cluster of epitopes left exposed after hormone bin ding to receptors. Since these MAb were able to produce either positive (MA b AES) or negative (MAb AC8 and F11) allosteric effects on hGH binding, the purpose of this work was to further characterize MAb behavior. Results ind icated a straight correlation between MAb allosteric effects and affinity c onstant values for binding of different hGH:MAb complexes to lactogenic rec eptors from rat liver. Affinity of hGH:MAb AE5 as well as hGH:Fab AE5 compl exes enhanced proportionally to the fraction of occupied receptors and Hill coefficients higher than 1 were obtained, suggesting the induction of posi tive cooperative effects between membrane-bound receptors. On the other han d, hGH:MAb AC8 and hGH:MAb F11 complexes binding affinity to lactogenic sit es could not be related to receptor occupancy degree. It is proposed that b inding of hGH:MAb AE5 complexes to receptors would elicit a conformational change on adjacent receptor molecules leading to an increase of their affin ity to bind subsequent hGH:MAb AE5 complexes.