Biogenesis and degradation of serotonin in Fasciola hepatica

The biogenesis and degradation of serotonin (5-HT) in liver flukes was studied by reverse-phase ion-pair HPLC and radioisotope trace technique. The concentration of 5-HT in F hepatica was found to be l.23+0.22 ug/g wet weight. Incubation of the flukes in [3H] tryptophan resulted in substantial radioactivity recovered in 5-hydroxytryptophan (5-HTP). 5-HT and 5-hydroxyindoleacetic acid (5-HIAA). After 12 h incubation of the flukes in [3H]5-HTP 14% of the total radiolabel in the tissue were also found to comigrate with 5-HT and 4% with 5-HIAA. The levels of 5-HT and 5-HTP in flukes were not influenced by the tryptophan hydroxylase inhibitor p-chlorophenylalanine and the 5-HIAA by the monoamine oxidase inhibitor tranylcypramine. but the 5-HT were increased and the 5-HIAA decreased by another monoamine oxidase inhibitor iproniazid. These resutls suggested that the enzymes which converted tryptophan to 5-HTP (TRP hydroxylase), 5-HTP to 5-HT (5-HTP decarboxylase). and 5-HT to 5-HIAA (monoamine oxidase) occurred in flukes and that the pathways of serotonin biogenesis and degradation were possibly similar to that in mammals. Some similarities and differences between the enzymes of flukes and mammals were found in the catalytic properties and cnzyme-inhibitor affinities. In addition. F hepatica could take up 5-HT from exogenous source similar to S mansoni and H diminuta etc. Cyproheptadine. a 5-HT antagonist, could suppress the uptake of 5-HT by flukes. It is possible that the 5-HT in flukes are both endogenous and exogenous.