GHBP Human
Description
Molecular Structure and Biosynthesis
GHBP is a 238-amino acid protein (or 248 in recombinant forms) with four glycosylation sites and three disulfide bonds . In humans, it is generated via metalloprotease-mediated shedding (primarily ADAM17) of the GHR extracellular domain . Key structural features include:
Recombinant GHBP (e.g., Prospec Bio’s product) lacks glycosylation and is optimized for diagnostic and research applications .
GH Regulation
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Bioavailability Modulation: GHBP binds ~50% of circulating GH, reducing free GH during secretory pulses and dampening cellular activity .
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Receptor Competition: Competes with membrane-bound GHR for GH, effectively acting as a GH antagonist .
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Half-Life Prolongation: Stabilizes GH in circulation, extending its metabolic effects .
Metabolic Interactions
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GHBP levels correlate positively with intraabdominal fat mass and are elevated in type II diabetics with hyperinsulinemia .
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Hepatic GHBP production dominates, with minimal contribution from peripheral tissues .
Biomarker Potential
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Visceral Adipose Tissue: Strong correlation with intraabdominal fat mass suggests GHBP as a biomarker for visceral obesity .
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GH Insensitivity: Undetectable GHBP levels may indicate GHR gene deletions (e.g., Laron syndrome) .
Therapeutic Monitoring
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The IGF-I/GHBP ratio predicts GH deficiency in adults, particularly women, and gauges responsiveness to GH therapy .
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Baseline GHBP, body fat, and IGFBP-3 levels accurately forecast post-treatment GHBP changes (adjusted R² = 0.97) .
Reference Values in Pediatrics
A study of 146 Chilean children (4–16 years) established age- and puberty-dependent GHBP norms :
| Tanner Stage | GHBP Activity (Mean ± SD) |
|---|---|
| Prepubertal | 12.3 ± 3.1 nM |
| Tanner II | 15.8 ± 4.2 nM |
| Tanner IV/V | 18.1 ± 4.5 nM (peak at Tanner III–IV) |
Mechanistic Studies
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Adipogenesis Inhibition: GHBP suppresses GH-induced adipogenesis in vitro (ED₅₀ = 3 nM) .
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Receptor Shedding Dynamics: GHR dimerization during GH binding inhibits ADAM17-mediated shedding, creating a feedback loop .
Pathological Correlations
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Elevated GHBP in obesity may reflect adipose tissue’s role in GHBP production or GHR regulation .
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GHBP declines with age after early adulthood, paralleling age-related reductions in GH secretion .
Recombinant GHBP in Biotechnology
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Applications: Used in ELISA kits (e.g., Mediagnost) to quantify GHBP in serum/plasma .
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Production: Expressed in E. coli (28.1 kDa) or eukaryotic systems (glycosylated forms) .
Unresolved Questions
Product Specs
Growth hormone binding protein (GHBP) is a transmembrane receptor that binds to growth hormone (GH). Upon binding, the receptor forms a dimer, initiating intracellular and intercellular signaling pathways that promote growth. A notable variant of this gene, GHRd3, lacks exon three and has been extensively studied. Mutations in the GHBP gene have been linked to Laron syndrome, also referred to as growth hormone insensitivity syndrome (GHIS), a disorder characterized by restricted growth. Although additional splice variants exist, such as one encoding a soluble form of the protein (GHRtr), they have not been fully characterized.
Recombinant human GHBP, produced in E.Coli, is a single, non-glycosylated polypeptide chain composed of 248 amino acids, with a molecular weight of 28107.01 Dalton. GHR is purified using proprietary chromatographic methods.
To reconstitute the lyophilized GHBP, it is advised to dissolve it in sterile 18MΩ-cm H2O to a concentration not lower than 100µg/ml. The solution can be further diluted in other aqueous solutions if needed.
Lyophilized GHBP, while stable at room temperature for up to 3 weeks, should ideally be stored in dry conditions below -18°C. After reconstitution, GHBP should be stored at 4°C for 2-7 days. For long-term storage, it is recommended to store it below -18°C. To enhance stability during long-term storage, consider adding a carrier protein such as HSA or BSA (0.1%). Avoid repeated freeze-thaw cycles.
(a) Size-exclusion high-performance liquid chromatography (SEC-HPLC) analysis.
(b) Sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) analysis.
GHBP demonstrates full biological activity, as evidenced by its capacity to form a 2:1 complex with growth hormone.
1. UV spectroscopy at 280 nm, employing an absorbance value of 2.6 as the extinction coefficient for a 0.1% (1mg/ml) solution. This value was determined using the PC GENE computer analysis program for protein sequences (IntelliGenetics).
2. Reverse-phase high-performance liquid chromatography (RP-HPLC) analysis, utilizing a calibrated GHBP solution as a reference standard.
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Q&A
Basic Research Questions
What experimental designs effectively capture GHBP variability in longitudinal human studies?
Longitudinal GHBP studies require monthly blood sampling over ≥12 months to account for biological rhythms . Key considerations:
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Use standardized GHBP immunoassays (e.g., activity-based chromatography or direct ELISAs)
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Pair with IGF-I, IGFBP-3, and anthropometric measurements (weight, height velocity)
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Control for weight changes (>5% body mass fluctuations significantly impact GHBP)
Example dataset from prepubertal children :
| Parameter | Mean ± SD | Correlation with GHBP (r-value) |
|---|---|---|
| GHBP monthly C.V. | 18% (6.7–33%) | — |
| IGF-I | 160 ± 45 µg/L | 0.61 (p<0.001) |
| Weight change | 3.2 ± 1.8 kg/yr | 0.57 (p<0.001) |
How do GHBP measurement methodologies influence clinical interpretations?
Two primary approaches exist:
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Activity-based assays (e.g., GH-binding/column chromatography): Detects functional GHBP but requires fresh samples
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Immunoassays: Higher throughput but may miss isoforms (e.g., fails to distinguish proteolytic vs. splice-variant GHBP)
Critical validation step: Compare lymph (4.6% GH binding) vs. plasma (14.1–14.9%) to assess interstitial relevance
Advanced Research Questions
What mechanisms resolve contradictions between GHBP levels and GH receptor (GHR) activity?
Three confounding scenarios require targeted experiments:
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Normal GHBP with GHR dysfunction: Screen for dominant-negative hGHRtr isoforms via RT-PCR (26-bp deletion in cytoplasmic domain)
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Low GHBP without GH insensitivity: Perform GH-GHR internalization assays (e.g., Ba/F3 cell proliferation with/without monoclonal antibodies hGH-25/hGH-33)
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Tissue-specific discordance: Use dual-labeling techniques (e.g., iodine-125-GH for hepatic vs. iodine-131-GH for mammary binding)
How to experimentally distinguish proteolytic vs. splice-variant GHBP generation?
A three-step validation framework:
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Transcript analysis: Amplify hGHR mRNA regions spanning exons 7–10 (PCR primers: 5’-CAGCTCATGGACTGCTAC-3’/5’-GTAGTGGCAGGTCCTTGAG-3’)
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Protease inhibition: Treat primary hepatocytes with TAPI-1 (10 µM); >50% GHBP reduction implies metalloprotease dependence
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Isoform-specific antibodies: Use hGH-33 mAb (conformational epitope on 20K hGH) to detect splice variants
What models best recapitulate GHBP’s role in modulating GH bioavailability?
Prioritize these systems:
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In vitro: COS-7 cells transfected with hGHR-tr (2.8-fold higher GHBP secretion vs. wild-type)
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In vivo: Hypophysectomized rats + 20K hGH (half-life 45 min vs. 22K’s 22 min)
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Clinical: Liver cirrhosis cohorts (GHBP ↓57% but GH resistance persists)
How do hGH isoforms (20K vs. 22K) interact with GHBP in experimental settings?
Key methodological insights:
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Use sandwich ELISA with hGH-33 (capture) and biotinylated hGH-12 (detection) for isoform-specific quantification
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20K hGH shows 3.2-fold slower clearance in perfusion models (Baumann et al. 1985 methodology)
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Competitive binding: 20K displaces 22K from GHR at IC₅₀ 12 nM vs. 8 nM
Data Contradiction Analysis
Why do some studies report GHBP as a GHR surrogate while others show discordance?
Resolve using this decision tree: 
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If GHBP normal but IGF-I ↓ → Measure hGHRtr/FL-GHR mRNA ratio (liver biopsy/qPCR)
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If GHBP ↑ with normal GHR → Test for anti-GH autoantibodies (hGH-12 epitope competition)
What technical artifacts confound GHBP/hGH complex studies?
Four common pitfalls:
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Epitope masking: hGH-33 mAb fails in Western blot (denaturation disrupts 20K conformation)
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Pre-analytical factors: Serum >24hr at 4°C increases GHBP proteolysis by 18%
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Cross-reactivity: 22K hGH contamination ≥7% invalidates 20K-specific assays
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Matrix effects: Lymph samples require 3x concentration vs. plasma for detection
Product Science Overview
Growth Hormone Binding Protein (GHBP) is a crucial component in the regulation of growth hormone (GH) activity in the human body. GHBP is derived from the extracellular domain of the Growth Hormone Receptor (GH-R) and plays a significant role in modulating the availability and activity of GH in the bloodstream.
The high-affinity human GHBP was first described in 1986 . It is a 60-kilodalton (kDa), 246-amino acid, single-chain glycoprotein. This protein corresponds to the extracellular domain of the GH-R and is believed to arise from the receptor through proteolytic cleavage . The precise site of cleavage and the enzyme responsible for this process remain unknown, although it is suggested that the sulfhydryl group of a free cysteine in the region of cleavage may play a crucial role in proteolysis .
GHBP is highly specific for human GH, with a rapid association rate and a slower dissociation rate. It circulates in the blood at nanomolar concentrations, allowing it to efficiently bind and release GH in the circulation . Approximately 45% of circulating GH is complexed with GHBP, creating a dynamic equilibrium that regulates GH availability .
The physiological significance of GHBP lies in its ability to modulate the bioavailability of GH. By binding to GH, GHBP can prolong the half-life of GH in the bloodstream, thereby enhancing its biological activity. However, studies have shown that recombinant GHBP does not enhance the in vivo bioactivity of GH in normal rats, indicating that the role of GHBP in GH bioactivity may be more complex than initially thought .
Recombinant GHBP has been characterized in great structural detail. It is produced using recombinant DNA technology, which involves inserting the gene encoding GHBP into a host organism, such as bacteria or yeast, to produce the protein in large quantities. This recombinant form of GHBP is used in various research and clinical applications to study its structure, function, and potential therapeutic uses .
GHBP has attracted considerable attention among basic scientists and clinicians due to its role in GH regulation. It is used in research to study the mechanisms of GH action and its interactions with GHBP. Clinically, GHBP levels are measured to assess GH activity and diagnose GH-related disorders. For example, low levels of GHBP may indicate GH deficiency, while high levels may be associated with conditions such as acromegaly .
