Urotensin I and its N-terminal flanking peptide from the flounder, Platichthys flesus

The caudal spinal cord region of teleost fish terminates in a neurosecretory organ, the urophysis. Two peptides have been purified to homogeneity from an extract of the urophysis of a teleost fish, the flounder. The primary structure of one peptide, Ser-Glu-Asp-Pro-Pro-Met-Ser-Ile-Asp-Leu¹⁰-Thr-Phe-His-Met-Leu-Arg-Asn-Met-Ile-His²⁰- Met-Ala-Lys-Met-Glu-Gly-Glu-Arg-Glu-Gln³⁰-Ala-Gln-Ile-Asn-Arg-Asn-Leu-Leu-Asp-Glu⁴⁰-Val, indicates identity with urotensin I. By analogy with other urotensins, the COOH-terminal residue is probably α-amidated. A second peptide was present in the extract in a concentration that was approximately equimolar with that of urotensin I. The amino acid composition of this peptide indicated a total of approximately 65 residues. The amino acid sequence of a fragment produced by digestion with trypsin was established as: Ala-Ala-Ala-Ala-Gly⁵-Asp-Ser-Ala-Ala-Ser¹⁰-Asp-Leu-Leu-Gly-Asp¹⁵-Asn-Ile-Leu-Arg. This sequence shows partial homology to carp prepro-urotensin I(41-59)-peptide as deduced from the nucleotide sequence of a cloned cDNA. It is concluded that the second peptide probably represents the N-terminal flanking peptide of pro-urotensin I which, it has previously been suggested, may function as a urotensin-binding peptide (urophysin) analogous to the neurophysins.