Human Growth Hormone (Somatropin) 36IU — Recombinant hGH
Human Growth Hormone (HGH) 36IU is a recombinant somatropin preparation containing 12mg of lyophilized hGH per vial — chemically and structurally identical to endogenous pituitary-derived human growth hormone (191 amino acids, 22 kDa, single-chain polypeptide). Each vial provides 36 international units for laboratory research applications including somatotropin receptor (GHR) signaling studies, IGF-1 axis regulation, cellular proliferation assays, and metabolic pathway investigations. Supplied as lyophilized powder at 99%+ purity by HPLC. For research use only. Not for human consumption.
Molecular Architecture & Receptor Activation Mechanism
Human growth hormone is a 191-amino acid, single-chain polypeptide with two intramolecular disulfide bonds (Cys53-Cys165, Cys182-Cys189) that stabilize its characteristic four-helix bundle structure. The hormone exists as a single structural domain with the four alpha-helices arranged in an up-up-down-down topology, creating a hydrophobic core that maintains structural integrity. The molecule contains two spatially distinct receptor-binding sites: site 1 (high affinity, Kd ≈ 0.3 nM) formed by residues from helix 1, helix 4, and the connecting loop between helices 1 and 2; and site 2 (lower affinity, Kd ≈ 1 μM) formed primarily by residues from helix 3 and the N-terminal region of helix 1.
Receptor activation follows a sequential dimerization mechanism that is both kinetically and thermodynamically defined. HGH first binds one GHR monomer through site 1 with high affinity to form an inactive 1:1 complex. The hormone-receptor complex then recruits a second GHR monomer through site 2, inducing receptor homodimerization that brings two molecules of Janus kinase 2 (JAK2) — constitutively associated with the GHR intracellular domain — into proximity. This enforced proximity enables JAK2 trans-autophosphorylation on multiple tyrosine residues (particularly Y1007 and Y1008 in the activation loop), converting JAK2 from an inactive to a catalytically competent kinase. Activated JAK2 phosphorylates key tyrosine residues on the GHR cytoplasmic tail, creating docking sites for SH2 domain-containing signaling proteins.
JAK2/STAT5 Signaling & Transcriptional Regulation
The JAK2/STAT5 pathway is the primary mediator of hGH’s transcriptional effects and the most extensively characterized GHR signaling cascade. Phosphorylated tyrosine residues on GHR recruit STAT5 (signal transducer and activator of transcription 5) via its SH2 domain. GHR-associated JAK2 phosphorylates STAT5 on a conserved C-terminal tyrosine (Y694 in STAT5A, Y699 in STAT5B), inducing STAT5 dimerization through reciprocal SH2-phosphotyrosine interactions. STAT5 dimers translocate to the nucleus and bind gamma-activated sequence (GAS) elements (TTCNNNGAA) in the promoters of target genes including IGF-1, IGFBP-3, acid-labile subunit (ALS), and suppressors of cytokine signaling (SOCS) proteins that provide negative feedback.
Cross-talk between GHR and IGF-1 receptor (IGF-1R) signaling represents an important layer of signal integration. GHR and IGF-1R have been shown to physically interact at the cell surface, and this interaction modulates downstream signaling outputs from both receptors. The GHR-IGF-1R interaction appears to fine-tune the balance between the direct metabolic effects of GH (lipolysis, insulin antagonism) and the indirect anabolic effects mediated through IGF-1, with the physical receptor interaction potentially serving as a molecular switch between these functions depending on receptor expression ratios and ligand availability (PMID: 25211187).
IGF-1 Axis & Longitudinal Growth
The primary mediator of hGH’s growth-promoting effects is insulin-like growth factor-1 (IGF-1), a 70-amino acid single-chain polypeptide with approximately 48% sequence homology to proinsulin. IGF-1 is synthesized predominantly in the liver (accounting for ~75% of circulating IGF-1) in response to GHR activation, though virtually all tissues produce IGF-1 locally through paracrine and autocrine mechanisms. In circulation, IGF-1 exists primarily in ternary complexes with IGF-binding protein 3 (IGFBP-3) and the acid-labile subunit (ALS), which extend its half-life from minutes to 15-20 hours and regulate its bioavailability to peripheral tissues.
The GH/IGF-1 axis controls longitudinal bone growth through a precisely coordinated mechanism. GHR activation in growth plate chondrocytes of the proliferative and prehypertrophic zones stimulates local IGF-1 production, which drives chondrocyte proliferation, hypertrophy, and columnar organization. The critical nature of this pathway is demonstrated by genetic disruptions: deletion of Tmem263, a gene identified through genome-wide association studies for bone mineral density and fracture risk, disrupts GH/IGF-1 axis signaling and causes dwarfism with severely impaired skeletal acquisition and reduced cortical and trabecular bone mass, demonstrating the essential nature of intact GH-to-IGF-1 signal transduction for normal skeletal development (PMID: 38241182). Conversely, factors that enhance GH/IGF-1 signaling through JAK2/STAT5 activation can stimulate longitudinal bone growth in preclinical models, confirming the pathway’s central and rate-limiting role in skeletal biology (PMID: 41026801).
Metabolic Effects: Lipolysis, Protein Anabolism, and Insulin Antagonism
Beyond its growth-promoting functions, hGH exerts profound metabolic effects that are mechanistically distinct from IGF-1-mediated actions. In adipose tissue, hGH stimulates lipolysis through activation of hormone-sensitive lipase (HSL) via PKA-mediated phosphorylation and perilipin remodeling on lipid droplet surfaces. This increases circulating free fatty acids (FFAs) and glycerol, shifting whole-body substrate utilization toward lipid oxidation and away from glucose utilization — the “glucose-fatty acid cycle” or Randle cycle effect. The lipolytic action of GH is rapid (onset within 1-2 hours) and occurs at concentrations within the physiological range.
In skeletal muscle, hGH promotes net protein anabolism through coordinated stimulation of amino acid uptake, increased protein synthesis (measured by [13C]-leucine incorporation), and reduced protein oxidation. The anabolic effect is mediated by GHR signaling through the PI3K/AKT/mTOR pathway — distinct from the STAT5 pathway driving IGF-1 transcription — and results in increased lean body mass when GH is administered over sustained periods. Simultaneously, hGH induces a state of insulin antagonism, reducing insulin-stimulated glucose uptake in muscle and adipose tissue while increasing hepatic glucose output through gluconeogenesis. This insulin-antagonistic effect is physiologically important during fasting and stress states, where GH preserves glucose for obligate glucose-consuming tissues (brain, erythrocytes) while shifting peripheral tissues toward fatty acid utilization.
The clinical relevance of these metabolic actions is particularly evident in catabolic states. Burn injury and critical illness invoke a hypermetabolic response characterized by massively increased resting energy expenditure (up to 100-150% above predicted), accelerated muscle protein catabolism, and elevated counter-regulatory hormones including cortisol, catecholamines, and growth hormone. Recombinant hGH administration in these contexts has been investigated for its capacity to attenuate protein wasting, improve nitrogen balance, and accelerate wound healing through enhanced protein synthesis, IGF-1 generation, and direct metabolic effects on substrate utilization (PMID: 33909503).
Laboratory Handling & Storage
Recombinant HGH 36IU is supplied as a white lyophilized powder in a sealed glass vial. Each vial contains 12mg of recombinant somatropin (22 kDa monomeric form) equivalent to 36 international units, standardized against the WHO International Standard for somatropin. Recommended storage is at 2-8°C for unopened vials, protected from light — do not freeze the lyophilized powder. Reconstitution should be performed with bacteriostatic water (0.9% benzyl alcohol) using aseptic technique, adding the diluent slowly down the vial wall to minimize foaming and mechanical stress on the protein. Do not shake or vortex — gently swirl the vial to dissolve, which typically occurs within 2-3 minutes. Reconstituted solutions should be stored at 2-8°C and used within 14-21 days depending on the specific formulation and diluent used.
For in vitro research applications, working dilutions should be prepared in appropriate assay buffer (e.g., DMEM with 0.1% BSA, or PBS with 0.05% Tween-20) immediately before use to minimize adsorptive losses to plastic surfaces. The 22 kDa monomeric form is the biologically active species — aggregation, oxidation (particularly of Met14 and Met125), and deamidation can be monitored by analytical size-exclusion HPLC, SDS-PAGE under non-reducing conditions, or reverse-phase HPLC. Bioactivity can be confirmed by GHR phosphorylation assays in IM-9 lymphocytes or STAT5 luciferase reporter assays in HEK293 cells stably expressing GHR.
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