Cycloproctolin and [α-Methyl- -Tyr]-proctolin are potent antagonists of proctolin-induced inositol phosphate production in locust foregut homogenates

Proctolin, an insect pentapeptide, causes dose-dependent production of inositol trisphosphate and inositol tetrakisphosphate from locust foregut homogenates which was antagonised by nM cycloproctolin and [α-Me- -Tyr]-proctolin, but not by [N-Me-Tyr]-proctolin. We propose that cycloproctolin exists in a conformation which resembles a β-bulge loop and contains a cis-Pro.