METABOLISM OF PHOSPHOENOLPYRUVATE AND PYRUVATE IN ADULT LIVER FLUKE FASCIOLA HEPATICA

1. 1. The role of carboxylating reactions in the formation of dicarboxylic acids in adult Fasciola hepatica was investigated. The mechanism of pyruvate formation from phosphoenolpyruvate, without the involvement of a pyruvate kinase (EC 2.7.1.40) step, was determined. 2. 2. In the production of organic acids from carbohydrate, phosphoenolpyruvate carboxylase (EC 4.1.1.32) acts as the key carboxylating enzyme, while under the same conditions malate dehydrogenase (decarboxylating) (NAD/NADP) (EC 1.1.1.39/1.1.1.40) acts primarily as a decarboxylating enzyme in the production of pyruvate. No pyruvate carboxylase (EC 6.4.1.1) activity could be detected in F. hepatica. 3. 3. CO2 fixation by phosphoenolpyruvate results in the rapid formation of a large pool of succinate. The formation of succinate from carbohydrate appears to involve the production of phosphoenolpyruvate by a glycolytic series of reactions, carboxylation to oxaloacetate, reduction to malate, fumarate formation and finally the reduction of fumarate to succinate. Fixation of labelled CO2 by phosphoenolpyruvate also yields large amounts of labelled citrate. 4. 4. Phosphoenolpyruvate carboxylase was found to be readily reversible and thus this enzyme may function either in the degradation or synthesis of carbohydrate. 5. 5. Malate dehydrogenase (decarboxylating) activity was greater in the presence of NAD+ than in the presence of NADP+. 6. 6. The activity of pyruvate dehydrogenase (EC 1.2.4.1) was determined in the mitochondrial fraction from F. hepatica. 7. 7. The low level of pyruvate kinase activity in F. hepatica precludes the formation of significant amounts of pyruvate directly from phosphoenolpyruvate. However, it is suggested that pyruvate can be formed from phosphoenolpyruvate by a pathway involving the action of phosphoenolpyruvate carboxylase, malate dehydrogenase (EC 1.1.1.37) and malate dehydrogenase (decarboxylating). The significance of such a pathway is discussed. © 1968.