Carbon dioxide fixation and glycogenesis in the flatworm Hymenolepis diminuta

[摘要] The cyclophyllidean cestode Hymenolepis diminuta has been found to incorporate C1402 into organic acids, amino acids, and glycogen upon in vivo incubation. This incorporation was sensitive to incubation conditions, more C1402 being incorporated aerobically than anaerobically, and was also stimulated by starvation of the worms. The fact that no ^-ketoglutarate could be detected and that no C14 could be observed in glutamic acid upon incubation with C1402 indicated the absence of a complete citric acid cycle in this worm. There did appear to be an energy dependent connecting link between the glycolytic sequence and the dicarboxylic acid shuttle.An active phosphoenolpyruvate carboxykinase, similar to the one present in avian liver, was found to be present in H. diminuta. This activity was stimulated by starvation andwas abundant enough to account for the in vivo incubation results.A low level of malic enzyme activity was also found in acetone powder extracts of the worm. It was too low in activity to be detected in spectrophotometric assays, but was studied through the measurement of C1402 incorporation into L-malate. Prom these preliminary studies the enzyme did not appear abundant enough to be physiologically significant.A third CO2 fixation activity was found in the acetone powder extracts. It appeared to be different from any enzyme yet found and required magnesium ion and coenzyme A for activity. No nucleotide requirement could be demonstrated. Fixation was found to result in malate. The enzyme has been tentatively named the ;;malate synthesizing enzyme.;;From this study it would seem very likely that carbon dioxide fixation may play a central role in glycogenesis and carbohydrate metabolism in Hymenolepis diminuta.

[发布机构] Rice University