Alanine substitution and deletion analogues of Manduca sexta allatostatin: Structure–activity relationship on the spontaneous contractions of the foregut of larval Lacanobia oleracea

Manduca sexta allatostatin (Manse-AS) is a 15-residue non-amidated peptide with a blocked N-terminus and a disulphide bridge between the cysteine residues at positions 7 and 14. Analogues of Manse-AS were used to examine the structural requirements of Manse-AS for inhibitory activity on spontaneous foregut contractions of larval tomato moth ( Lacanobia oleracea). Breaking the disulphide bond between C 7 and C 14 by reduction reduced the potency of the peptide, suggesting that the conformation of Manse-AS is important for its biological activity. When either of the cysteine residues were replaced with alanine the Manse-AS analogue had no measurable bioactivity. Alanine substitution at Q 6 was as potent as Manse-AS, all other alanine substitution analogues (R 5, Y 8, F 9, N 10, P 11, I 12 and S 13), were myoinhibitory but less potent than native Manse-AS to varying degrees. Analogues with alanine substitution at amino acids with aromatic side chains (Y 8 and F 9) were the least active. Amino-terminal deletion analogues Manse-AS 6–15 and Manse-AS 7–15 were inactive whereas Manse-AS 5–15 was fully active but not as potent as Manse-AS. The results show that amino acid residues both inside and outside the disulphide ring are important for biological activity.