The unique C‐mannosylated hypertrehalosemic hormone of Carausius morosus: Identity, release, and biological activity

Abstract

AbstractPrevious studies had shown that the corpora cardiaca (CC) of the Indian stick insect, Carausius morosus, synthesizes two hypertrehalosemic hormones (HrTHs)—decapeptides which differ in the way that the chromatographically less‐hydrophobic form, code‐named Carmo‐HrTH‐I, is modified by an unique C‐mannosylated tryptophan residue at position 8. The availability of milligram amounts of this modified peptide in synthetic form now makes it possible to study physico‐chemical and physiological properties. This study revealed that the synthetic peptide co‐elutes with the natural peptide from the CC chromatographically, is heat stable for at least 30 min at 100°C, and causes hyperlipemia in acceptor locusts (a heterologous bioassay) and hypertrehalosemia in ligated stick insects (conspecific bioassay). In vitro incubation of Carmo‐HrTH‐I together with stick insect hemolymph (a natural source of peptidases) demonstrated clearly via chromatographic separation that the C‐mannosylated Trp bond is stable and is not broken down to Carmo‐HrTH‐II (the more‐hydrophobic decapeptide with an unmodified Trp residue). This notwithstanding, breakdown of Carmo‐HrTH‐I did take place, and the half‐life of the compound was calculated as about 5 min. Finally, the natural peptide is releasable when CC are treated in vitro with a depolarizing saline (high potassium concentration) suggesting its role as true HrTHs in the stick insect. In conclusion, the results indicate that Carmo‐HrTH‐I which is synthesized in the CC is released into the hemolymph, binds to a HrTH receptor in the fat body, activates the carbohydrate metabolism pathway and is quickly inactivated in the hemolymph by (an) as yet unknown peptidase(s).