Ovine placental lactogen and human growth hormone bind to different regions of the same receptors

Anti-human growth hormone (hGH) polyclonal and monoclonal antibodies (MAb) failed to recognize ovine placental lactogen (oPL), indicating that the ant igenic topographies of both hormones are different. Binding assays showed t hat oPL completely inhibited hGH binding to lactogenic receptors from Nb2-c ells and to somatogenic receptors from rabbit or sheep liver; in contrast, oPL only bound to a subpopulation of rat liver receptors. Zinc ion increase d hGH and oPL binding to Nb2-cell receptors and slightly inhibited both hor mones' recognition by somatogenic receptors. However, ZnCl2 increased hGH b inding to rat liver microsomes but prevented that of oPL. Furthermore, MAb R7B4, recognizing lactogenic as well as somatogenic receptors, entirely blo cked hGH binding to the various receptor systems but not affected oPL bindi ng. Therefore, results presented in this paper suggest that oPL and hGH bin d to different regions of the same receptors. (C) 1999 Harcourt Publishers Ltd.