Studies of regulatory metabolism in Moniezia expansa: general considerations.

The activities of selected enzymes in the branched metabolic pathway to succinate or lactate were determined in cytosol and mitochondrial fractions. The enzymes of lowest activity in the cytosol, and thus possibly regulatory, are phosphofructokinase and pyruvate kinase. Malic enzyme activity could scarcely be detected in either compartment; phosphoenolpyruvate carboxykinase and malate dehydrogenase occur in both. The end products of metabolism are succinate and lactate; under anaerobic conditions lactate production increases whereas succinate production shows a small decrease. The presence of glucose in the medium does not influence the change, but causes an increase in total endproduct accumulation. Levels of metabolic intermediates in worms incubated aerobically and anaerobically are presented, and ‘cross-over’ plots and calculations of apparent equilibrium constants identify hexokinase, phosphofructokinase and pyruvate kinase as regulatory. Under aerobic conditions a large increase in the size of the malate pool is observed suggesting that the depression of lactate production is produced by its inhibitory effect on pyruvate kinase. Adenine nucleotide levels are maintained whether or not the worm is incubated under anaerobic conditions.