Human growth hormone

hGH (somatotrophin; Figure 11.7) is a polypeptide hormone synthesized in the anterior pituitary. It promotes normal body growth and lactation and influences 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 disulfide linkages. hGH mRNA can also undergo alternate splicing, yielding a shortened GH molecule (20 kDa), which appears to display biological activities indistinguishable from the 22 kDa species.

hGH displays significant, although not absolute, species specificity. 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 release inhibiting hormone (GHRIH) or somatostatin. Furthermore, whereas GH directly mediates some of its biological actions, its major influence on body growth is mediated indirectly via IGF-1, as discussed below. GHRH, GHRIH, GH and IGF-1 thus form a hormonal axis, as depicted in Figure 11.8.

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