Multiple Forms of Glucagon and Somatostatin Isolated from the Intestine of the Southern-Hemisphere LampreyGeotria australis

Abstract

Current views on Agnathan phylogeny favor the hypothesis that the genera of holarctic lampreys belong to a single family (Petromyzontidae) and form an interrelated progression in whichPetromyzonis near toIchthomyzonat the base of the phylogenetic tree andLampetrais the most derived. A stock similar to that of contemporaryIchthomyzonis considered to have given rise to the southern hemisphere lampreyGeotria australis, the sole member of the Geotriidae. In the present study, two molecular forms of glucagon were isolated from an extract ofG. australisintestine that differed in structure by six amino acid residues. One form shows two amino acid substitutions (Leu14→ Met and Ala29→ Ser) compared with the single molecular form of glucagon isolated from the sea lampreyPetromyzon marinusand the second form shows three substitutions (Asp15→ Glu, Ser16→ Ala, Ile24→ Thr) compared with the single glucagon isolated from the river lampreyLampetra fluviatilis.AsPetromyzonandLampetraglucagons differ by six amino acid residues, the data suggest that a duplication of the glucagon gene occurred prior to or early in lamprey evolution. Although both genes are strongly expressed inG. australis, the expression of one gene predominates inP. marinuswhile that of the other gene predominates inL. fluviatilis.Previous work has shown that, in the islet organ ofG. australis, preprosomatostatin is processed almost exclusively to somatostatin-33. However, the present study demonstrates that somatostatin-14 is the major molecular form inG. australisintestine with somatostatin-33 present only as a minor component. This result demonstrates a tissue-dependent pathway of posttranslational processing of preprosomatostatin in theGeotriaenteropancreatic system.