Biomedical Engineering Reference
In-Depth Information
Figure 11.7
Three-dimensional structure of hGH. Structural details courtesy of the Protein Data Bank,
http://www.rcsb.org/pdb/
11.4 Humangrowthhormone
hGH (somatotrophin; Figure 11.7) is a polypeptide hormone synthesized in the anterior pituitary. It
promotes normal body growth and lactation and infl uences various aspects of cellular metabolism.
Mature hGH contains 191 amino acid residues and displays a molecular mass of 22 kDa. It also
contains two characteristic intrachain disulfi de linkages. hGH mRNA can also undergo alternate
splicing, yielding a shortened GH molecule (20 kDa), which appears to display biological activi-
ties indistinguishable from the 22 kDa species.
hGH displays signifi cant, although not absolute, species specifi city. GHs isolated from other
primates are the only preparations biologically active in humans. (This precluded the earlier use
of bovine/porcine preparations for medical use in humans.)
GH synthesis and release from the pituitary is regulated by two peptide hypothalamic factors: GH-
releasing hormone (GHRH, also known as GH-releasing factor (GHRF) or somatorelin) and GH re-
lease inhibiting hormone (GHRIH) or somatostatin. Furthermore, whereas GH directly mediates some
of its biological actions, its major infl uence on body growth is mediated indirectly via IGF-1, as dis-
cussed below. GHRH, GHRIH, GH and IGF-1 thus form a hormonal axis, as depicted in Figure 11.8.
11.4.1 The growth hormone receptor
GH induces its characteristic biological effects by binding to a specifi c cell surface receptor. The
human receptor is a single-chain 620 amino acid transmembrane polypeptide. Sequence analysis
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