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 C7 and C14 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 Q6 was as potent as Manse-AS, all other alanine substitution analogues (R5, Y8, F9, N10, P11, I12 and S13), were myoinhibitory but less potent than native Manse-AS to varying degrees. Analogues with alanine substitution at amino acids with aromatic side chains (Y8 and F9) were the least active. Amino-terminal deletion analogues Manse-AS6-15 and Manse-AS7-15 were inactive whereas Manse-AS5-15 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.