New proctolin analogs modified in position 3 of peptide chain--synthesis and their biological evaluation.

Six proctolin analogs modified in position 3 of peptide chain such as Arg-Tyr-X-Pro-Thr where X = Gly (1), Val (2), Pro (3), Thr (4), Acp (1-aminocyclopentane-1-carboxylic acid residue) (5), and Ach (1-aminocyclohexane-1-carboxylic acid residue) (6) were synthesized by liquid-phase method. Biological effects of the pentapeptides (1-6) were examined in cardiostimulatory test in vitro in respect to two insect species: American cockroach (Periplaneta americana L.) and yellow mealworm (Tenebrio molitor L.). Results thus obtained pointed out that the presence of L-leucine in the position 3 of proctolin skeleton plays important role in its cardiotropic activity in insects.