Two distinct growth hormone-binding proteins (GHBPs) circulate in human blood, and similar GHBPs have been described in several other mammalian species. The Principal GHBP has high affinity for hGH (22K form) and represents the extracellular portion of the GH receptor. The other GHBP, which may consist of more than one component, has low affinity for hGH and is not related to the GH receptor. The GHBPs prolong the persistence of GH in the circulation, restrict its distribution in vivo, and modulate binding of GH to tissue receptors. Because of these properties, they are improtant components of the GH-IGF axis. The relationship of the high-affinity GHBP to the GH receptor has been useful in the assessment of clinical states with altered GH action (GH resistance and hypersensitivity). High-affinity GHBP levels are decreased in several conditions of absolute or relative GH resistance (Laron-type dwarfism, pygmy dwarfism, malnutrition, type I diabetes, hepatic cirrhosis, renal failure, and hypothyroidism); they are increased in obesity, a condition of GH hyperresponsivity. Thus, serum GHBP levels appear to reflect tissue GH receptor concentrations (as has been demonstrated in animals), and/or the GHBP participates directly in GH action through its effect on GH kinetics. The precise physiological role and diagnostic significance of the GHBPs is still being defined.
(C) Lippincott-Raven Publishers.