The primary structure of sturgeon prolactin: phylogenetic implication.

The complete amino acid sequence of prolactin (PRL) from a chondrostean species, the sturgeon (Acipenser gueldenstaedti), has been determined. Sturgeon PRL was isolated from the pituitary glands by gel filtration on a Sephadex G-25 column and high-performance liquid chromatography on a reverse-phase column following acid-acetone extraction. Sturgeon PRL was identified by immunoblot reactivity using antisera against salmon and ovine PRL. It consists of 204 amino acid residues, which is the largest among known PRLs, and contains three disulfide bonds corresponding to those of tetrapod PRLs. Sequence comparison with PRLs from other vertebrates revealed that sturgeon PRL has slightly higher sequence identities (35-46%) with teleost PRLs than with tetrapod PRLs (30-40%). These structural characteristics imply that an ancestor of the ray-finned fishes had PRL with three disulfide bonds and at some point after divergence of Chondrostei, the disulfide bond in the amino-terminus of PRL was lost.