Nitrogen metabolism in Ostertagia (Teladorsagia) circumcincta : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Physiology at Massey University, Palmerston North, New Zealand
The aim of the experiments was to investigate some key areas of nitrogen metabolism in adult and third-stage larval Ostertagia (Teladorsagia) circumcincta, to seek enzymes either not present in mammals or with distinctive kinetic properties, which clearly differentiated the nematode and host metabolic systems. The study encompassed excretion and uptake in intact worms and determining the kinetic properties of eleven enzymes involved in the metabolism of arginine, urea, alanine aspartate and glutamate. The metabolism of O. circumcincta was different from that in mammals and more like that of microorganisms and plants. Ammonia was the main excretory product, with a little urea, both apparently crossing the cuticle through specific permeases. The excretion rate increased with temperature, but decreased as the external ammonia concentration increased, suggesting that ammonia may be a source of nitrogen additional to amino acids, which were taken up by adult worms. Ammonia could be incorporated directly into glutamate and other amino acids through the glutamine synthetase-glutamate synthase pathway, which was more active in adult worms. Glutamate dehydrogenase was able to use either NADH or NADP in the deaminating direction, which would be the predominant direction because of the low affinity of GDH for ammonia. In the aminating direction, there was greater activity with NADH than NADPH. Creatinase and arginase were probably the sources of excreted urea. There was no urease activity to convert urea to ammonia. No role could be assigned to creatinase other than to degrade host creatine, perhaps to supply sarcosine for metabolism. The unusual feature of aspartate metabolism was aspartase activity in addition to aspartate aminotransferase, which, in larvae, had the highest activity of all enzymes studied. In adult worms, which are believed to have a more anaerobic metabolism than larvae, aspartase would allow aspartate to be formed directly from fumarate in association with only a partial TCA cycle. Perhaps the most important finding was the identification in the parasites of three enzymes, creatinase, aspartase and glutamate synthase, which are not believed to be expressed in the sheep host or other mammals, making them possible candidates for developing novel anthelmintic therapies.