Multiple forms of glucagon and somatostatin isolated from the intestine ofthe southern-hemisphere lamprey Geotria australis

Current views on Agnathan phylogeny favor the hypothesis that the genera of holarctic lampreys belong to a single family (Peuomyzontidae) and form an interrelated progression in which Petromyzon is near to Ichthomyzon at the base of the phylogenetic tree and Lampetra is the most derived. A stock sim ilar to that of contemporary Ichthomyzon is considered to have given rise t o the southern hemisphere lamprey Geotria australis, the sole member of the Geotriidae. In the present study, two molecular forms of glucagon were iso lated from an extract of G. australis intestine that differed in structure by six amino acid residues. One form shows two amino acid substitutions (Le u(14) --> Met and Ala(29) --> Ser) compared with the single molecular form of glucagon isolated from the sea lamprey Petromyzon marinus and the second form shows three substitutions (Asp(15) --> Glu, Ser(16) --> Ala, IIe(24) --> Thr) compared with the single glucagon isolated from the river lamprey Lampetra fluviatilis. As Petromyzon and Lampetra glucagons differ by six am ino acid residues, the data suggest that a duplication of the glucagon gene occurred prior to or early in lamprey evolution. Although both genes are s trongly expressed in G. australis, the expression of one gene predominates in P. marinus while that of the other gene predominates in L. fluviatilis. Previous work has shown that, in the islet organ of G. australis, preprosom atostatin is processed almost exclusively to somatostatin-33. However, the present study demonstrates that somatostatin-14 is the major molecular form in G. australis intestine with somatostatin-33 present only as a minor com ponent. This result demonstrates a tissue-dependent pathway of posttranslat ional. processing of preprosomatostatin in the Geotria enteropancreatic sys tem. (C) 1999 Academic Press.