An analysis of fumarate reductase systems in parasitic helminths and various intermediate molluscan hosts

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Some parasitic helminths and aquatic molluscs utilize glucose mainly through a phosphoenol-pyruvate carboxykinase(PEPCK)-succinate pathway; an alternative pathway of the carbohydrate metabolism with succinate as one of the resulting end-products. The terminal step of the pathway is a fumaratereducing reaction, which is catalyzed by mitochondrial electron transport enzymes and where fumarate functions as an electron acceptor. The fumarate reductase system produces ATP.In this project, fumarate reductase systems of Ascaris suum, Paragonimus westermani and molluscan intermediate hosts, which inhabit in various environments, were comparatively studied to understand their physiological significance. Previous study in our laboratory showed that a type b cytochrome, which can be distiguished from b cytochromes in complex III, participated in the fumarate reductase system in Ascaris and Paragonimus adult mitochondria. In the course of the project, the b-type cytochrome, b_<558> for Ascaris or … More b_<556> for Paragonimus, was found to be localized in complex II of the worm respiratory chain after the procedure for isolation of complex II had been established. The midpoint oxidation-reduction potential (Em) of Ascaris cytochrome b_<558> was - 34 mV. A similar b-type cytochrome exists in complex II of beef heart mitochondria though its Em is more negative (-185 mV). The b-type cytochrome in parasite complex II was easily reduced by succinate but it was not the case in beef heart complex II. Complexes II, of Ascaris and Paragonimus, were composed of four subunits like beef heart one. Studies using high performance liquid chromatography showed Ascaris cytochrome b_<558> tocorrespond to the two smaller subunits. Molecular properties of the largest subunit of complex II were similar among all three species and their immunological homology was high. However, the two smaller subunits had different properties with negligible immunological homology. Ascaris complex II had a high fumarate reductase activity. Also, an NADH-fumarate reductase activity was reconstituted with beef heart complex I, Ascaris complex II and rhodoquinone, indicating the quinone to be an essential component of the NADH-fumarate reductase system. As to studies on intermediate molluscan hosts, anaerobic immersion experiments, which were performed in the presence of molluscicides and followed by analyses of organic acids, showed the PEPCK-succinate pathway to be in function in vivo in Oncomelania nosophora and Biomphalaria glabrata snails. Less