Energy metabolisms of parasitic helminths: adaptations to parasitism.

Many metazoans, and particularly the parasitic forms, have adapted well to their environments. This is particularly obvious in organisms that reside in anoxic surroundings, such as the large lumen-dwelling intestinal parasites. However, anaerobic energy metabolisms are not confined to parasites in microaerophilic environments. Some that reside in highly aerobic surroundings (e.g. blood or lungs) also have lost much or all of their aerobic capabilities for energy generation. Adaptations toward anaerobiosis are many and varied: homolactate fermentation (schistosomes and filarial worms), heterolacate fermentations (H. diminuta), and succinate and heterosuccinate fermentations (Ascaris). Even helminths that are obligate aerobes are not complete oxidizers. All accumulate aerobic fermentation products, indicating at best a limited terminal respiration. Some worms, particularly some of the larval stages, retain their ability to survive anaerobically but require the presence of oxygen for motility. Regardless of the type of oxygen requirement, all parasitic helminths examined are dramatically different from their mammalian hosts in regard to their energy metabolisms. Many similar adaptations have been shown to occur in nonparasitic organisms ranging from metazoans, through fish and mammals. There is much room for additional studies of these biochemical adaptations.