Binding of a variant of human growth hormone to liver plasma membranes

The binding of human growth hormone (hGH) and a naturally occurring variant of hGH (20K) to rat and rabbit liver membrane has been compared. The concentration of hormone causing half maximal displacement (ED50) of 125I-hGH from rat liver plasma membranes was 1.6 nM for hGH and 46 nM for 20K. The ED50 for hGH and 20 K against 125I-hGH binding to rabbit liver plasma membranes was 1.6 mM for hGH and 28 nM for 20K. Significant binding of 125I-20K did occur to rat and rabbit liver membranes but the relative capacities for 20K were 12 and 6 fold less in rat and rabbit liver membranes, respectively when compared to hGH. The competitive binding characteristics of 20K to pregnant rabbit liver membranes indicated lack of competition between bovine GH and 125I-20K for binding. However, significant binding of 125I-20K to male rabbit liver membranes was present and displaceable by hGH and oPrl. We conclude that deletion of a portion of the hGH molecule (defined as residues 32-46) results in significant alterations in the binding characteristics of the molecule. This indicates primary or secondary involvement of this portion of the hGH molecules in the binding process and may define a portion of the binding domain for hGH.