Growth hormone binding proteins: biochemical characterization and assays

Area of Science:

• Endocrinology
• Molecular Biology
• Biochemistry

Background:

• Circulating growth hormone binding proteins (GHBPs) are recognized in humans and animals.
• Two specific GHBPs exist in humans: one with high affinity and another with low affinity.
• The high-affinity GHBP is the extracellular part of the hepatic growth hormone receptor; the low-affinity GHBP is unrelated to the receptor.

Purpose of the Study:

• To characterize the properties of the two specific human growth hormone binding proteins.
• To describe the binding characteristics and molecular complexes formed by these proteins with growth hormone.
• To outline the methodologies for assaying these binding proteins in plasma.

Main Methods:

• Characterization of binding proteins by affinity, molecular weight, and isoelectric point.
• Analysis of growth hormone complex formation (85 kD for high-affinity, 125 kD for low-affinity).
• Assay development using radiolabeled human growth hormone and separation techniques (size exclusion, ion exchange chromatography, charcoal adsorption).

Main Results:

• Two distinct GHBPs identified: high-affinity (85 kD, pI ~5.0) and low-affinity (125 kD, pI ~7.1).
• High-affinity GHBP is associated with the hepatic growth hormone receptor.
• Most circulating growth hormone binds to the high-affinity GHBP in plasma.

Conclusions:

• Human plasma contains two distinct growth hormone binding proteins with different affinities and properties.
• The high-affinity GHBP plays a significant role in circulating growth hormone binding.
• Standardized assays utilizing radiolabeled growth hormone and chromatographic separation are effective for quantifying GHBPs.