DAO 85:217-223 (2009)  -  DOI: https://doi.org/10.3354/dao02085

ABSTRACT: CO₂ stimulates the development of many of the intestinal helminths that are able to fix CO₂ by means of phosphoenolpyruvate carboxykinase (PEPCK), such as Hysterothylacium aduncum. We determined the activity of CO₂-fixing enzymes such as PEPCK and phosphoenolpyruvate carboxylase (PEPC), although no significant activity was detected for pyruvate carboxylase or carboxylating-malic enzyme. The former act on phosphoenolpyruvate (PEP) to yield oxalacetate. In the helminths studied, PEP has a vital role in glucidic metabolism. Consequently, we determined the activity of other enzymes involved in the crossroad of PEP, such as pyruvate kinase (PK), lactate dehydrogenase and malate dehydrogenase. All enzymes detected showed significant variations in activity during the in vitro development of the parasite from the third larval stage to mature adult. Fixing of CO₂ by PEPCK decreased during development (from 228 to 115 nmol min^–1 mg^–1 protein), while that by PEPC increased (from 19 to 46 nmol min^–1 mg^–1 protein). This enzyme, which is rare in animals, could play a part in detecting levels of free phosphate, releasing it from PEP when required for processes such as glycogenolysis, glycolysis and adenosine 5’-triphosphate (ATP) synthesis. PK, which showed increasing activity during development up to immature adult (from 56 to 82 nmol min^–1 mg^–1 protein), could act in combination with PEPC to obtain energy in the cytosol (in the form of ATP) and in the mitochondria (possible destination of the pyruvate formed), compensating for the decrease in activity of PEPCK.

KEY WORDS: Fish parasite ^. Hysterothylacium aduncum ^. Nematoda ^. Carbon dioxide fixation ^. Phosphoenolpyruvate carboxykinase ^. Phosphoenolpyruvate carboxylase ^. Phosphoenolpyruvate metabolism