A novel,
hydroxyproline-containing
neuropeptide,
Gly-Pro-Hyp-Tyr-Asp-Phe-Gly-Met-NH2, designated [HYP3]
Met-callatostatin, has been identified from extracts of heads of the blowfly Calliphora vomitoria. The
peptide is a naturally occurring hydroxylate analogue of
Met-callatostatin, a previously identified
allatostatin-like
peptide, and is present to the extent of 20% of the nonhydroxylated form. In bioassays, both forms of the
peptide show allatostatic activity by inhibiting
juvenile hormone synthesis and release in the cockroaches Periplaneta americana, Diploptera punctata, and Blattella germanica (IC50 = 100 pM-10 nM). They do not, however, influence
juvenile hormone bisepoxide synthesis and release in the blowfly. In flies, [Hyp3]
Met-callatostatin inhibits the peristaltic movements of the hindgut, showing a biphasic response (IC50 = 0.5 pM and 0.5 microM) compared with the monophasic response of
Met-callatostatin (IC50 = 100 nM). Immunocytochemical studies with
Met-callatostatin antisera provide the cytological basis for a myoinhibitory role in the gut since the axons of immunoreactive neurons in the abdominal
ganglion project to the ileum. There are also endocrine cells in the midgut that, by releasing the
peptides into the hemolymph, would allow the Met-
callatostatins to fulfill a neurohormonal role on muscles of the gut and heart. In contrast, there are no
Met-callatostatin neural pathways from the brain to the corpus allatum, the gland that produces
juvenile hormone. NH2-terminal degradation of Met-
callatostatins incubated with the hemolymph of P. americana results in cleavage of the
Pro-Tyr bond giving the pentapeptide Tyr-Asp-Phe-Gly-Met-NH2 as a degradation product. In contrast, the Hyp-Tyr bond resists cleavage. With hemolymph from C. vomitoria, no immunoassayable degradation product has been observed with either
peptide.