Isolation of schistosomin, a neuropeptide which antagonizes gonadotropic hormones in a freshwater snail

The molecular mechanisms underlying parasite‐induced inhibitory effects on host reproduction were studied in the freshwater snail, Lymnaea stagnalis, infected with the schistosome parasite Trichobilharzia ocellata. This combination is used as a model system for host‐parasite interactions involved in schistosomiasis transmission. The female gonadotropic snail neuropeptide, calfluxin, was labelled with fluorescein isothiocyanate (FITC) and used as a ligand in receptor‐binding studies on membranes of its target organ, the albumen gland. The binding of calfluxin to its receptor–guanyl‐nucleotide‐binding‐protein (G‐protein) complex was inhibited in vitro in the presence of haemolymph of schistosome‐infected snails. This inhibition appeared to be established by a peptidergic factor called schistosomin. The receptor assay was used to identify schistosomin from haemolymph during subsequent purification and characterization steps. The peptide could also be purified from the central nervous systems of non infected snails, indicating that it is produced by the snail itself and released into the haemolymph as a result of infection. Analysis by plasma‐desorption mass spectrometry revealed that purified schistosomin has a molecular mass of 8780 Da.