Polymeric and Monomeric Human Growth Hormone Binding to Rat Liver Plasma Membranes*

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Abstract
The binding of monomeric and polymeric [125I]hGH [human growth hormone] to rat liver plasma membranes was studied to define the characteristics of hGH binding and to explore the possible physiological significance of polymeric hGH. Monomeric and polymeric hGH, isolated from clinical grade hGH by gel filtration, represent approximately 44 and 35% of the protein in clinical grade hGH, respectively. The radioimmunoreactivity of polymeric hGH to hGH antibody was approximately 3% of that of monomeric hGH. Both polymeric and monomeric hGH bound to receptors on liver plasma membranes. Specific binding of both polymeric and monomeric hGH was similar with respect to time, affinity, temperature, pH and monovalent cations. Binding of both polymeric and monomeric hGH was increased 4-fold by 100 mM monovalent cation concentration. Ca and Mg produced a 9-fold increase in monomeric hGH binding between 10-100 mM concentration. Mg produced peak binding of polymeric hGH at 10 mM concentration whereas Ca produced only minimal enhancement of binding at 1 mM concentration. When binding was studied in medium containing 100 mM NaCl, 4 mM KCl, 2 mM MgCl2 and 50 mM Hepes (pH 7.5), polymeric hGH was displaced by several polypeptide hormones (FSH) [follicle stimulating hormone], TSH [thyrotropin] and LH [luteinizing hormone], but not by monomeric hGH and PRL. Monomeric hGH was displaced by polymeric hGH and PRL. Polymeric hGH was not converted to monomeric hGH by the membranes. Polymeric and monomeric hGH bind to different receptors on the rat liver plasma membranes. The receptor for polymeric hGH is relatively non-specific but polymeric possesses characteristics of monomeric hGH as evidenced by its ability to displace monomeric hGH. In the absence of a mechanism for converting polymeric hGH to monomeric hGH, the non-specific nature of polymeric hGH binding in vitro suggests a non-specific role of polymeric hGH in vivo.