Structural aspects of gastrin/CCK-like insect leucosulfakinins and FMRF-amide

The leucosulfakinins (LSKs), isolated from head extracts of the cockroach Leucophaea maderae, are sulfated neuropeptides with homology to gastrin and cholecystokinin. The undecapeptide LSK and decapeptide LSK-II stimulate contractions of the isolated cockroach hindgut. Several structural aspects of the two gastrin/CCK-like insect leucosulfakinins (LSKs) and their relation to FMRF-amide are discussed. Replacement of the oxidation sensitive Met residue with isosteric norleucine leads to an analog with retention of biological activity. The Arg residue of the LSKs is critical for cockroach hindgut contractile stimulatory activity, as its introduction into gastrin II transforms the inactive peptide into an active analog. As demonstrated by the equipotent [His14,Arg16]gastrin II, the His8 and Asp5 residues of LSK are not critical for activity. The common C-terminal tetrapeptide of the LSKs ([8-11]LSK) is inactive. Taken together with a comparison of the two LSK structures, the data suggest that the LSK active core resides between [8-11]LSK and [4-11]LSK. This is confirmed by considerable activity displayed by the sulfate analog of LSK-II, which contains an extra sulfate group on the Ser2 residue in the N-terminal region. Homology between the LSKs and molluscan cardioacceleratory and rectum contractile neuropeptide FMRF-amide and Met-enkephalin-Arg6-Phe7 is discussed. The insect LSKs may represent a molecular evolutionary link between the vertebrate gastrin/CCK family and this mammalian enkephalin.