Growth hormone-binding proteins.
Two growth hormone-binding proteins (GH-BPs) have recently been discovered in human plasma. Both are specific for human GH; one has high affinity and the other has low affinity. The high-affinity BP corresponds to the extracellular portion of the GH receptor, whereas the low-affinity BP does not appear to be related to the receptor. About 45% of GH in plasma is complexed with the high-affinity BP, and another 5% with the low-affinity BP. Plasma BP levels are low in fetal life and rise in the first few years after birth to achieve adult levels, which stay constant. Some GH-resistant forms of short stature (Laron dwarfism, pygmies) are associated with absent, nonfunctional, or decreased levels of high-affinity BP in plasma. The BPs prolong the half-life of GH by confining its distribution and protecting it from degradation; they dampen oscillations and contribute to a circulating hormone reservoir/buffer. Certain animals, in particular the rabbit, pig, and pregnant mouse, possess similar GH-BPs.
Baumann
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Center for Endocrinology, Metabolism and Nutrition, Department of Medicine, Northwestern University Medical School, Chicago, IL 60611, USA.