HCG: Research Reference

HCG, human chorionic gonadotropin, is a glycoprotein hormone, not a short peptide. It is built from two different protein subunits held together without a covalent bond, both carrying sugar chains, and it is measured in international units (IU) of activity rather than milligrams. In laboratory research it is used as a reference ligand for the LH/hCG receptor, a model for glycoprotein structure and an assay calibrant.

This reference collects what a laboratory needs to know about HCG as a material: how a two-part glycoprotein behaves in the vial, what it dissolves in, how it is handled at the bench and what to do when an order arrives. Each section below has its own page.

Molecular reference

Name	HCG (human chorionic gonadotropin)
Also known as	hCG, choriogonadotropin, chorionic gonadotrophin
Type	Glycoprotein hormone; a heterodimer of an alpha and a beta subunit joined non-covalently
Alpha subunit	92 residues, five disulfide bonds, two N-linked sugar chains; the same alpha chain is shared with LH, FSH and TSH
Beta subunit	145 residues, six disulfide bonds, two N-linked sugar chains plus four O-linked chains on its C-terminal tail
Approximate mass	About 37 kDa including the sugar chains (one supplier gives 36,700 Da); the exact figure varies with glycoform
CAS number	9002-61-3
Measured in	International units, calibrated against a WHO International Standard

Subunit lengths, disulfide bonds and glycosylation sites are from the PubChem protein records for the alpha chain (P01215) and the beta chain (P0DN86), which carry the UniProt annotation. The CAS number and synonyms are from PubChem substance records (SID 627901). The mass and the 92 plus 145 residue count are on the ProSpec data sheet for human hCG (HOR-250). HCG has no single small-molecule formula, so no formula or sequence row is given.

Three features of that structure explain most of its behaviour. Its two subunits are held together by shape and contact, not by a covalent bond, so the main way it loses activity is by coming apart; a strap of the beta subunit wraps around the alpha subunit and is fastened by a disulfide bond, which the crystal structure describes as a seat belt (Lapthorn et al., Nature, 1994). Each subunit is a cystine knot, cross-linked internally by disulfide bonds that hold the shape needed for the two halves to pair (Mishra et al., Biochimica et Biophysica Acta, 2003). And its sugar chains vary from molecule to molecule, which is why HCG is standardised by activity in IU rather than by weight (Elliott et al., Endocrine, 1997).

What it is studied for

HCG is a placental hormone that acts on the same receptor as luteinising hormone, the LH/hCG receptor. Cryo-electron microscopy structures of that receptor bound to hCG show a push-and-pull activation mechanism, with the hormone pushing on the extracellular domain while a hinge loop pulls (Duan et al., Nature, 2021). Because the hormone binds tightly and is well characterised, it is a standard ligand in receptor and cell-signalling work.

It is also a model glycoprotein. Mass spectrometry of WHO reference preparations found fourteen distinct glycoforms of the alpha subunit alone, including forms with oxidised methionine (Toll et al., Electrophoresis, 2006). Thermal studies showed that removing most of the sugar made the hormone more resistant to heat denaturation (Manjunath and Sairam, Journal of Biological Chemistry, 1983), while trimming the sugar on one alpha-subunit site lowered that subunit’s stability (van Zuylen et al., Biochemical and Biophysical Research Communications, 1997). In protein engineering, the O-glycosylated tail of the beta subunit has been fused to other proteins as a tag that raised their thermal resistance and protease stability in vitro (Ceaglio et al., Journal of Biotechnology, 2016).

In analytical work, HCG is the reference material for immunoassays. Its activity is defined against the WHO 5th International Standard, NIBSC code 07/364, which showed no significant loss of immunoreactivity or bioactivity after accelerated thermal degradation testing (NIBSC, instructions for use, 2013).

Stability

HCG is stable as a dry powder. In solution its main risks are physical: the two subunits separating, unfolding with warmth, aggregation and loss to surfaces. The HCG stability page covers those routes, freeze-thaw, light and what visible changes mean.

Solubility

HCG is a hydrophilic glycoprotein and dissolves in bacteriostatic water with gentle swirling. Acid is avoided, because low pH pulls the subunits apart. The HCG solubility guide explains why and what causes haze.

Handling

Bench practice for HCG is about keeping the solution cold and clean and treating it as a protein: no shaking, no foam, no heat. The HCG handling page lists the steps and the common mistakes.

On arrival

What a sealed vial should look like, why it travels without an ice pack and where it goes first are on the HCG on-arrival page.

Storage and reconstitution

Fridge and freezer windows, before and after mixing, are on how long HCG lasts. Water volumes and the IU per mL table for a 5000 IU vial are on how much bacteriostatic water for an HCG 5000 IU vial.

FAQ

Short answers to the questions researchers ask most about storing, mixing and handling HCG are on the HCG FAQ.

Where to get it

Vistara Labs supplies HCG 5000 IU as a lyophilised vial for laboratory research, shipped within Canada by Canada Post Xpresspost. There is no ready-mixed spray. For the peptide that acts upstream of the same hormone axis, see the Kisspeptin research reference.