Proinsulin and Somatostatin from the Islet Organ of the Southern-Hemisphere Lamprey Geotria australis

An extract of the islet organ of the southern-hemisphere lamprey Geotria australis contained a high concentration of somatostatin like immunoreactivity (34 nmol/g) but only trace amounts of insulin-like immunoreactivity. The primary structure of Geotria somatostatin-33 (AVQEAGGAAM¹⁰PPPGQRDRKA²⁰ GCKNFFWKTF³⁰SSC) shows almost no similarity to somatostatins from the holarctic lampreys Petromyzon marinus and Lampetra fluviatilis in the N-terminal region but the functionally important C-terminal region, including the substitution Thr³¹ → Ser, is the same. Insulin was not identified in the extract but proinsulin and an incompletely processed form with an intact A-chain/C-peptide junction were purified and partially characterized. The primary structure of the insulin region of Geotria proinsulin was established as A-chain; GIVEKCCHNR¹⁰CSIYQ MESYC²⁰N; B-chain: SALTGSGGNY¹⁰LCGSYLVDAL²⁰YLACGPRGFF³⁰YTS TPV. This sequence contains 17 amino acid substitutions compared with the identical insulins from P. marinus and L. fluviatilis but the unusual extension to the N-terminus of the B-chain (SALTG) is present. Compared with mammalian insulins, Geotria insulin contains several substitutions of strongly conserved residues such as Gin^A5 → Lys in the putative receptor-binding region, Glu^B26 → Pro important in dimerization, and Leu^A13 → Ile, His^B10 → Tyr, and His^B21 → Tyr important in hexamerization. Geotria proinsulin contains an Arg-Arg processing site at the B-chain/C-peptide junction but we speculate either that the Lys-Arg processing site at the C-peptide/A-chain junction is absent or that the Geotria pancreas is unable to synthesize a SPC2-type prohormone convertase. Our results are consistent with the view that G. australis and holarctic lampreys arose from a common stock but have been separated for a considerable period.