Does the serum binding protein for growth hormone have a functional role?

The presence in the serum of several animal species of highly specific binding proteins for growth hormone is now accepted. The major binding protein is a truncated, variant form of the target tissue GH receptor and is synthesized by all tissues expressing the full-length GH receptor. The GH receptor and GH binding protein are not co-ordinately expressed, being produced in variable ratios between tissues. Such independence of expression suggests that the synthesis of the GH binding protein is regulated and therefore may be biologically relevant. The GH binding protein has been shown to increase the half-life of circulating GH by decreasing the metabolic clearance rate and degradation rate; to limit the volume of distribution of bound GH to approximately twice the intravascular space; and to inhibit receptor binding and biological actions of GH in in vitro model systems. These observations suggest that circulating GH binding protein does have a functional role in modulating both the circulating concentrations of GH and its availability and effectiveness for target tissue interaction. Other possible, but as yet unproven, functions for the GH binding protein are briefly discussed. The ability to produce GH binding protein by recombinant means should allow additional, more definitive studies to be performed, thereby providing greater opportunity to define the true biological role(s) of the GH binding protein in the metabolism and function of GH.