Fasciola hepatica: inhibition of phosphoenolpyruvate carboxykinase, and end-product formation by quinolinic acid and 3-mercaptopicolinic acid

Forty years of helminth biochemistry

This review describes some of the developments in helminth biochemistry that have taken place over the last 40 years. Since the early 1970s the main anabolic and catabolic pathways in parasitic helminths have been worked out. The mode of action of the majority of anthelmintics is now known, but in many cases the mechanisms of resistance remain elusive. Developments in helminth biochemistry have depended heavily on developments in other areas. High throughput methods such as proteomics, transcriptomics and genome sequencing are now generating vast amounts of new data. The challenge for the future is to interpret and understand the biological relevance of this new information.

Anisakis simplex: CO2-fixing enzymes and development throughout the in vitro cultivation from third larval stage to adult

We studied the effect of CO(2) on the in vitro cultivation of Anisakis simplex, an aquatic parasitic nematode of cetaceans (final hosts) and fish, squid, crustaceans and other invertebrates (intermediate/paratenic hosts), and, occasionally, of man (accidental host). The results showed that a high pCO(2), at a suitable temperature, is vital for the optimum development of these nematodes, at least from the third larval stage (L3) to adult. After 30 days cultivation in air, molting to L4 (fourth larval stage) was reduced to 1/3, while survival was about 1/3 of that when cultivated in air + 5% CO(2). The activity of the CO(2)-fixing enzymes, PEPCK and PEPC, was also studied. Throughout the development of the worms studied, PEPCK activity was much higher than that of PEPC (e.g., 305 vs. 6.8 nmol/min.mg protein, respectively, in L3 collected from the host fish). The activity of these enzymes in the worms cultivated in air + 5% CO(2) was highest during M3, and was also generally higher than that of those cultivated in air only, especially during molting from L3 to L4 (e.g., in recently molted L4, PEPCK activity was 3.7 times greater than that of PEPC 2.9 times greater than when cultivated in air).

In vitro antifilarial evaluation of phenoxycyclohexane derivatives

Numerous potential inhibitors of the phosphoenolpyruvate-carboxylase found in Molinema dessetae were synthesized and evaluated as inhibitors of the purified enzyme and as killers, in vitro, of M. dessetae adult females and infective larvae. Phosphoenolpyruvate analogs (or derivatives) appeared disappointing whereas phenoxycyclohexane derivatives inhibited the enzyme in a non-competitive manner and killed the parasites. The compounds P2281 and P2285, for example, had Ki values of 122 and 93 microM, respectively. In vivo tests of the most effective phenoxycyclohexanes will now be carried out to check their potential as antifilarial drugs.

Purification and properties of phosphoenolpyruvate carboxylase from Molinema dessetae (Nematoda: Filarioidea)

The presence of phosphoenolpyruvate (PEP) carboxylase (EC 4.1.1.31), an enzyme at the branchpoint of glycolysis and the Krebs cycle was detected in the Filaria Molinema dessetae. This enzyme has not previously been identified in Helminths, which have so far been found to only possess a phosphoenolpyruvate carboxykinase (EC 4.1.1.32). This enzyme had a level of activity comparable to that of pyruvate kinase, and was relatively less active than enzymes such as malate dehydrogenase or lactate dehydrogenase. We propose here a method of purification of M. dessetae PEP-carboxylase. When purified to electrophoretic homogeneity, the enzyme had a molecular weight of 64 kDa. Kinetic studies indicated that the carboxylation reaction had an optimal pH of 5.8. The enzyme was inhibited by cations such as Fe2+, Zn2+, Cd2+, Cu2+ but required the presence of Mg2+ or Mn2+. The enzyme was thermostable. The apparent Km value of 2.38 mmol for phosphoenolpyruvate for the carboxylation reaction was higher than previously reported values. The Km value for KHCO3 was found to be 1.6 mmol. PEP-carboxylase did not catalyse the reverse reaction.

The enigmatic presence of all gluconeogenic enzymes in Schistosoma mansoni adults

The activities of glucose-6-phosphatase (G6Pase), fructose-1,6-bisphosphatase (FBPase), phosphoenolpyruvate carboxykinase (PEPCK) and pyruvate carboxylase (PC) were determined in homogenates of adult Schistosoma mansoni worms and compared with the activities in homogenates of rat liver and rat skeletal muscle, tissues with a high and a low gluconeogenic capacity, respectively. All four gluconeogenic enzymes were present in S. mansoni. The enzymes were less active than in rat liver, but the activities of G6Pase, PEPCK and PC were at least an order of magnitude higher than in rat skeletal muscle whereas FBPase was approximately equally active in S. mansoni and in rat muscle. Experiments with 14C-labelled substrates or [14C]NaHCO3 failed to demonstrate the actual occurrence of gluconeogenesis in S. mansoni. Some possible other functions of the gluconeogenic enzymes were investigated. Experiments with inhibitors of PEPCK gave no indications that this enzyme was involved in the degradation of glucose. This was confirmed by 13C-NMR experiments which indicated that lactate was formed from phosphoenolpyruvate via the actions of pyruvate kinase and lactate dehydrogenase, and that PEPCK did not participate in the formation of lactate. Substrate cycling between fructose-6-dehydrogenase, and fructose-1,6-bisphosphate was demonstrated to occur in adult S. mansoni. This shows that FBPase participates in the glucose metabolism of this parasite.

The role of phosphoenolpyruvate carboxykinase in a marine macroalga with C4-like photosynthetic characteristics

Udotea flabellum is a marine, macroscopic green alga with C4-like photosynthetic characteristics, including little O2 inhibition of photosynthesis, a low CO2 compensation point, and minimal photorespiration; but it lacks anatomical features analogous to the Kranz compartmentation of C4 plants, and phosphoenolpyruvate carboxylase [PEPC; orthophosphate:oxaloacetate carboxy-lyase (phosphorylating), EC 4.1.1.31] activity is negligible. Phosphoenolpyruvate carboxykinase (PEPCK) activity (carboxylating) in Udotea extracts was equivalent to that of ribulose-bisphosphate carboxylase [Rubisco; 3-phospho-D-glycerate carboxy-lyase (dimerizing), EC 4.1.1.39]. When PEPCK activity was inhibited in vivo with 3-mercaptopicolinic acid (MPA), thallus photosynthesis decreased by 70% and became sensitive to O2. Codium decorticatum, a related species that lacks C4-like photosynthetic features and PEPCK activity, showed no increase in O2 inhibition upon exposure to MPA. Rubisco and PEPC activities in Udotea were not inhibited by MPA. Labeling of the early photosynthetic products malate and aspartate was reduced 66% by MPA, while intermediates of the photorespiratory carbon oxidation cycle showed a 3-fold increase. Udotea evolved O2 in the light in the absence of inorganic carbon, suggesting it had an endogenous carbon source for photosynthesis. Exogenous malate stimulated this process, while MPA inhibited it. PEPCK was not involved in Crassulacean acid metabolism or dark CO2 fixation. These MPA studies establish a direct link between PEPCK activity and the low O2 inhibition of photosynthesis and low photorespiration in Udotea. The data are consistent with carboxylation by a cytosolic PEPCK providing a C4 acid, such as malate, to the chloroplast for decarboxylation to elevate the CO2 concentration at the Rubisco fixation site. Udotea is to date the most primitive plant with a C4-like form of photosynthesis.

Energy metabolism and its regulation in the adult liver fluke Fasciola hepatica

The adult liver fluke,Fasciola hepatica, inhabits the bile duct of its final host, usually cattle or sheep. The veterinary aspects of infection withF. hepaticacan represent a major problem and consequently fascioliasis can have serious economic effects. As recently as 1972 the loss in revenue due to liver fluke infestations in the UK was estimated at an incredible £50 million per annum (Coles, 1975). Not only canF. hepaticainfect cattle and sheep, but also outbreaks of human disease have been reported. The last serious outbreak in Britain was in 1968 when at least 49 cases were identified (Ashton, Boardman, D'Sa, Everall & Houghton, 1970; Hardman, Jones & Davies, 1970).

Fasciola hepatica: carbohydrate metabolism of the adult

Glycogen (or exogenous glucose) was the only energy source utilized by adult Fasciola hepatica under a number of different incubation conditions. When exogenous glucose was present in the incubation medium, significant amounts of lactate were excreted. Anaerobically, in the presence of glucose, lactate accounted for 20% of the total end products measured. In the absence of glucose, organic acid production accounted for approximately 60% of glycogen carbon utilized; this value was reduced to 40% in the presence of exogenous glucose. There was no appreciable Pasteur effect.