STRUCTURE-ACTIVITY-RELATIONSHIPS OF THE CRUSTACEAN MYOTROPIC NEUROPEPTIDE ORCOKININ

Orcokinin (OK, NFDEIDRSGFGFN) was recently identified from the crayfis h, Orconectes limosus, as a potent hindgut-stimulating factor (14). To assess the importance of structural features of the peptide involved in effective ligand-receptor interactions, synthetic analogues of orco kinin were tested in the hindgut bioassay. Tests with N- and C-termina l-truncated analogues and the C-terminal-amidated analogue (OK-NH2) de monstrate that changes at the C-terminus interfere less with biologica l activity than changes at the N-terminus. Removal of more than one am ino acid at the N-terminus resulted in a complete loss of activity, wh ereas the C-terminal deletion of three amino acids still produced an a nalogue with full intrinsic activity but with a drastic shift in thres hold concentration of activity from 1 x 10(-10) to 1 x 10(-7) M. Delet ion of four amino acids at the C-terminus resulted in a completely ina ctive analogue. The C-terminal hydroxyl group does not seem to be impo rtant because amidation (OK-NH2) resulted in almost no loss of activit y. Replacing Arg(7) with Ala produced an analogue almost equipotent to orcokinin. Replacement of Phe(2) by Tyr resulted in considerable loss of activity. An important role of Phe(2) is further suggested by the steep drop of activity after removal of this residue in the N-terminal -deleted analogues.