C. Bryant

The role of host generated free radicals in helminth infections: Nippostrongylus brasiliensis and Nematospiroides dubius compared

Abstract Smith N.C. and Bryant C. 1986. The role of host generated free radicals in helminth infections: Nippostrongylus brasiliensis and Nematospiroides dubius compared. International Journal for Parasitology16: 617–622. The possibility that free radicals are involved in the expulsion of Nippostrongylus brasiliensis from the small intestine of its mice host was explored by comparing the susceptibilities to free radicals, and levels of protective enzymes, of adult N. brasiliensis and Nematospiroides dubius, a closely related intestinal parasite of mice. Nippostrongylus brasiliensis was markedly more susceptible to in vitro free radical damage than N. dubius. The difference in susceptibility is probably related to differences in enzymatic protection against free oxygen radicals as N. dubius had roughly twice as much Superoxide dismutase, about 3 times as much catalase and about 4 times as much glutathione reductase as N. brasiliensis. This result may indicate that N. dubius persists in the rodent small intestine, whilst N. brasiliensis is spontaneously expelled, because of a more efficient enzymatic defence system against host-generated free oxygen radicals.

Carbon dioxide utilisation, and the regulation of respiratory metabolic pathways in parasitic helminths.

Publisher Summary This chapter discusses carbon dioxide utilization and the regulation of respiratory metabolic pathways in parasitic helminths. Metabolic regulation is one aspect of biochemistry, which today occupies the attention of many workers. The fixation of carbon dioxide (CO2) by many parasitic helminths, and the subsequent involvement of carbon from this source in intermediary metabolism, has been well established for many years, although the actual route of fixation has been resolved only relatively recently. There are several enzymes that might be involved in CO2 fixation in parasites. They have been well characterized in vertebrate, plant, and microbial systems, but only in the past few years, parasitologists have looked for them in helminths. The chapter distinguishes among the enzymes, when each of the major groups of parasitic helminths is considered from the point of view of CO2 utilization. It summarizes the roles that phosphoenolpyruvate carboxykinase (PEPCK), malic enzyme (ME), and pyruvate carboxylase play in intermediary metabolism. The enzymes that have been implicated in respiratory metabolic pathways, and that have been formally studied with a view to elucidate their importance in metabolic control, are discussed.

Biochemical Strain Variation in Parasitic Helminths

Publisher Summary This chapter discusses strain variations in parasitic helminthes. These variants are the raw material upon which selection acts if a speciation event subsequently occurs. Naturally, for two populations genetically separated, events that induce speciation may have different end products given that the gene pools are even ever so slightly different. Organisms are adapted to their environments at all stages of their life cycles. However, helminth parasites, such as insects, often have a number of discrete stages in their life histories. A life cycle may require that successive life stages are adapted to very different environmental conditions or hosts. Each stage requires its own, specific developmental program, and each is, acted on by forces that may induce speciation. Each host is effectively an island and is subject to the same constraints of colonization. Very often infection is by a relatively small number of organisms that cannot possibly represent all the possible genetic combinations of the population at large, and so a founder effect operates with a very high probability of producing a distinct strain.

Free radical generation during primary infections with Nippostrongylus brasiliensis

Summary The production of free radicals during infection of the rat with Nippostrongylus brasiliensis was investigated. Lipid peroxidation, which is the best documented effect of free radicals, was monitored in the small intestines of infected rats by measurement of malonyldialdehyde and was found to be increased at the time of worm rejection. The capacity of peritoneal leucocytes to produce free radicals, as measured by chemiluminescence, was monitored in rats infected with different doses of N. brasiliensis. Rejection of N. brasiliensis from rats infected with 6000 third–stage larvae (L3) began 2 days earlier than in rats infected with only 600 L3. Maximal free radical generation also occurred 2 days earlier and was quantitatively greater in rats infected with 6000 L3. Free radical generation by leucocytes in response to live adult N. brasiliensis was enhanced by plasma from infected rats indicating the existence of a plasma–borne factor responsible for the initiation of free radical generation in response to N. brasiliensis.

Possible roles for pyruvate: Ferredoxin oxidoreductase and thiol-dependent peroxidase and reductase activities in resistance to nitroheterocyclic drugs in Giardia intestinalis

Abstract Giardia intestinalis was found to lack glutathione, though it did possess abundant quantities of unidentified thiol(s) as well as thioldependent peroxidase and reductase activities. These activities were positively correlated in 14 isolates with in vitro resistance to furazolidone, but not metronidazole or tinidazole. The resistance of 17 isolates to metronidazole was, however, negatively correlated with pyruvate: ferredoxin oxidoreductase activity as was drug resistance in a line of G. intestinalis selected for resistance to metronidazole. This suggests different modes of action for nitrofurans and 5-nitroimidazoles.

Changes in energy metabolism due to anthelmintics in Fasciola hepatica maintained in vitro

Abstract The effects of rafoxanide (RFX), nitroscanate (NSC) and mebendazole (MBZ) on oxidative pathways in whole F. hepatica maintained in a simple salt solution have been examined. The anthelmintics did not alter glucose uptake or glycogen mobilization. NSC and RFX depressed ATP and increased AMP levels. MBZ behaved similarly at first, but later depressed the total adenine nucleotides. All three drugs influenced end product formation, increasing it initially, although by different mechanisms. With NSC, early increases in lactate and acetate excretion were later abolished. With RFX, there was an initial increased production of acetate and propionate. Later, excretion of propionate was reduced and that of succinate was increased. MBZ also increased succinate excretion, but to a much greater extent. In addition, it inhibited lactate production. A number of effects of the drugs on the internal concentrations of metabolic intermediates are described. The mechanisms of action of the drugs are discussed.

Studies of regulatory metabolism in Moniezia expansa: General considerations

Abstract 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.

Studies of regulatory metabolism in Moniezia expansa: Effects of cambendazole and mebendazole

Abstract Effects of cambendazole and mebendazole on the respiratory metabolism of the anterior portion of Moniezia expansa were investigated in vitro . Anaerobically and in the presence of glucose, both drugs inhibited glucose uptake and increased glycogen utilisation. They reduced succinate production, by inhibiting fumarate reductase and (in the case of cambendazole) phosphoenolypyruvate carboxykinase activities, and increased lactate production. The additional lactate formed was accumulated in the worms. The drugs diminished ATP synthesis and/or turnover of adenine nucleotides. Aerobically, the drugs exerted similar effects on glucose uptake, glycogen utilisation and adenine nucleotides but the formation of end products was unaffected. Hexokinase and phosphofructokinase activities were inhibited by the drugs in vitro , but were not inhibited in extracts of parasites preincubated with the drugs.

Novel sources of anthelmintics

The G-series of naturally occurring compounds from Eucalyptus species contain representatives that are endoperoxides and Mannich bases. Mannich bases have been shown to be effective in killing protozoa in vitro. Previous studies of the role of cells of the immune system reveal that helminth parasites are susceptible to free radical attack. A 3-year research project studied the anthelmintic potential of endoperoxides and/or Mannich bases derived from G-compounds. Results from 1 series of synthetic analogues which show a positive relationship between chemical structure and anthelmintic activity are reported here. A G-Mannich base with an aromatic ring side chain as well as a long aliphatic side chain was the most active analogue in the series with an LD 50 < 0.1 mM in an in vitro larval motility assay. This work showed that Mannich bases are a potential source of compounds for parasite control. More work on formulation and pharmacokinetics is required to advance these compounds to the threshold of commercial production.

The effect of antioxidants on the rejection of Nippostrongylus brasiliensis

Summary The effect of the antioxidants, butylated hydroxy anisole (BHA) and vitamin E, on the rejection of Nippostrongylus brasiliensis from the small intestine of the rat was investigated. Worm expulsion was inhibited by BHA. Malonyldial–dehyde production in the small intestines and free radical generation by peritoneal leucocytes from infected rats were also inhibited by BHA. Vitamin E, although inhibiting malonyldialdehyde production, did not prevent worm expulsion. Significantly, vitamin E was much less effective than BHA at reducing free radical generation by rat leucocytes in response to N. brasiliensis.