Nonapeptides

Nonapeptides are peptides composed of nine amino acids. They can be found as hormones, neurotransmitters, or signaling molecules in animals, and may be linear or cyclized.

Two prominent vertebrate nonapeptides are vasopressin and oxytocin, produced in the hypothalamus and released from the posterior pituitary. Oxytocin is a nine-amino-acid cyclic peptide with sequence Cys-Tyr-Ile-Gln-Asn-Cys-Pro-Leu-Gly; the cysteines form a disulfide bond that closes the ring between positions 1 and 6. Vasopressin’s nonapeptide sequence is Cys-Tyr-Phe-Gln-Asn-Cys-Pro-Arg-Gly, also cystine-bridged. Variants of vasopressin include arginine-vasopressin and lysine-vasopressin, which differ at the eighth residue.

Biogenesis and processing: In the hypothalamus, the nine-residue neuropeptides are produced as larger precursors (preprovasopressin or prepro-oxytocin) that are processed and packaged with carrier proteins (neurophysins) and transported to the posterior pituitary, where they are secreted into the bloodstream.

Functions: Vasopressin acts mainly on kidney V2 receptors to promote water reabsorption and on vascular V1 receptors to cause vasoconstriction, while oxytocin stimulates uterine contractions and milk ejection; both peptides also influence social and reproductive behaviors via other receptor subtypes.

Clinical notes: Vasopressin and its synthetic analogs, such as desmopressin, have therapeutic uses in conditions like central diabetes insipidus and perioperative hypotension.

Beyond vasopressin and oxytocin, numerous nonapeptides exist across species, reflecting the broad biological role of nine‑amino‑acid peptides as signaling molecules.