Metabolism of growth hormone (GH) and different molecular forms of GH in biological fluids.

Human growth hormone (GH) consists of a number of molecular variants which derive from 2 genes, several alternative mRNA splicing mechanisms, and post-translational modifications such as deamidation, acylation, glycosylation, and oligomerization. After secretion, GH binds to two (or possibly more) circulating GH-binding proteins, one of which is a truncated GH receptor. The resulting mixture of GH moieties in plasma is enormously complex, consisting of over 100 molecular states. The various GH forms undergo differential clearance, receptor binding, and complexing with binding proteins, each exhibiting distinct kinetics. The net effect on bioactivity is correspondingly complex, and measurements by immunoassay or receptor assays are complicated by the lack of a homogeneous analyte. GH is best seen as a family of proteins, many of which fulfill as yet unknown functions in biology.