Growth Hormone Binding Protein: A Modulator of Growth Hormone Activity

Abstract

Growth hormone (GH) is present in the blood in many forms. A recent review (1) counts over 100 naturally occurring variant forms of human GH (hGH). Many of these forms of hGH are also probably present in blood bound, in varying amounts, to a specific high-affinity GH binding protein (GHBP). The activity and physiological relevance of all these hGH variants in their bound and unbound forms are ill defined. It is therefore somewhat surprising that when the biological activities of hGH are considered they are usually restricted to the actions of the predominant pituitary form of hGH, the 22-kd form. However, it has been estimated (1) that only about 20% of the GH in the blood circulates “free” as 22-kd hGH. The same reviewer estimates that another 20% of the hGH in blood is also 22-kd hGH, but this circulates bound to the GHBP. The older literature contains evidence that forms of GH in plasma other than 22-kd GH possess the majority of the bioactivity of plasma, as measured in the classical rat in vivo GH bioassays. There is a body of older work (2) indicating that there is much more bioassayable GH-like activity in plasma in high molecular weight fractions than in the lower molecular weight fractions containing free GH. We now know that this high molecular weight material would contain GH bound to the GHBP.