Primary structure and origin of schistosomin, an anti-gonadotropic neuropeptide of the pond snail Lymnaea stagnalis

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In the pond snail Lymnaea stagnalis infected with the schistosome parasite Trichobilharzia ocellata, a peptide called schistosomin is released from the central nervous system, which counteracts the bioactivity of a number of gonadotropic hormones. This leads to inhibition of the reproductive activities of the infected snail. In order to determine the structure of schistosomin, the neuropeptide was purified from the central nervous system using gel-permeation chromatography and reverse-phase h.p.l.c. The complete primary structure of the peptide was determined by N-terminal sequencing and peptide mapping. Schistosomin is a single-chain molecule of 79 amino acids with a molecular mass of 8738 Da. The peptide contains eight cysteine residues which may give rise to four intramolecular disulphide bridges that fold the peptide into a stable globular structure. A database search did not reveal any known peptides that show significant sequence similarity to schistosomin. By means of immunocytochemistry, the peptide was shown to be localized in the growth-controlling neurosecretory light green cells, which are located in the cerebral ganglia of the central nervous system of Lymnaea. In addition to schistosomin, these neurons are known to produce various insulin-related peptides.