What it is
Relaxin is a hormone made of two peptide chains, built the same way as insulin. UniProt's record for human prorelaxin H2 (P04090) — the form called relaxin-2 — gives the structure:
- an A chain of 24 amino acids: QLYSALANKCCHVGCTKRSLARFC;
- a B chain of 29 amino acids: DSWMEEVIKLCGRELVRAQIAICGMSTWS;
- two disulfide bonds — bridges between sulfur atoms — tying the chains together, and a third inside the A chain.
Both chains come from one 185-residue precursor. The cell makes the whole chain, folds it, and then cuts out a 102-residue connecting peptide, leaving the two halves bolted together. Insulin does exactly this, with C-peptide as its cut-out piece.
Humans carry three relaxin genes. Relaxin-1 and relaxin-2 share the pregnancy annotation in UniProt; relaxin-3 is annotated as a brain neuropeptide acting on different receptors (RXFP3 and RXFP4).
What it does
UniProt's function annotation describes relaxin as an ovarian hormone that acts with oestrogen to widen the birth canal in many mammals, and as possibly involved in remodelling connective tissue in pregnancy — growth of the pubic ligaments and ripening of the cervix. It acts through RXFP1 (UniProt Q9HBX9), a receptor that raises cyclic AMP inside the cell.
| What is described | How firm |
|---|---|
| Birth-canal widening with oestrogen | Described across many mammals |
| Connective-tissue remodelling in human pregnancy | UniProt: "may be involved" |
| Anti-scarring effects in heart, lung, kidney | Animal models (see B7-33) |
The drug that did not work
Relaxin's anti-scarring effects in animal models made it a heart-failure candidate. Its recombinant form, serelaxin, went into RELAX-AHF-2: 6,545 people hospitalised with acute heart failure (New England Journal of Medicine, 2019, PMID 31433919).
| Outcome at day 180 | Serelaxin | Placebo |
|---|---|---|
| Cardiovascular death | 8.7% | 8.9% |
| Worsening heart failure | not significantly reduced | — |
That is the largest human result in the relaxin family, and it is negative for that use. It does not say anything about relaxin in pregnancy, where the hormone is the body's own.
Where it stands
openFDA's Drugs@FDA returns no application for relaxin or serelaxin (2026-10-03). PubMed returns 3,982 records for relaxin, 75 tagged as randomised controlled trials. The research-market fragment B7-33 is cut from its B chain and has no human trials.
Related reading
Relaxin's two-chain build is shared with insulin, whose cut-out piece is recorded in C-peptide. For hormones that act during pregnancy and birth, see the labour hormone oxytocin; for peptides discussed for joint and ligament tissue, see peptides for joints and tendons.