干旱区生态环境知识资源中心

Timely and effective inactivation of neurohormones is necessary to ensure appropriate biological responses. We have studied the means by which the actions of insect adipokinetic hormones (AKH I: pGlu-Leu-Asn-Phe-Thr-Pro-Asn-Trp-Gly-Thr-NH2) and AKH II (pGlu-Leu-Asn-Phe-Ser-Thr-Gly-Trp-NH2) are terminated in the circulatory system of the desert locust, Schistocerca gregaria. When injected into locusts, radiolabelled AKH I or II are rapidly degraded and radioactive metabolites can be recovered from the haemolymph. In vitro studies using haemolymph protein extracts showed that the degradative activity is not in the circulation, suggesting that the AKH-degrading enzymes are located on the surfaces of tissues bathed by the haemolymph. This was verified by incubating AKHs with intact tissues (fat body, muscle, and Malpighian tubules) in organ culture and with membrane preparations from these tissues in vitro. Analysis of the AKH fragments by RP-HPLC and by protein sequencing indicates that inactivation is achieved by an endopeptidase which cleaves the Asn-Phe bond present in both AKHs thereby producing biologically inactive AKH fragments. The AKH fragments C-terminal to the first cleavage are subsequently metabolized by aminopeptidases present both in the circulation and on the tissue surfaces. The amino acid sequence specificity of the activating endopeptidase and its sensitivity to phosphoramidon, an inhibitor of mammalian endopeptidase 24.11, indicates that circulating AKHs are inactivated by an enzyme similar to that which degrades AKH in the nervous system and supports the notion that many vertebrate and invertebrate neurohormones are inactivated by a single class of cell surface endopeptidases.