Enzyme levels in cercariae and adult Schistosoma mansoni

Sexual Preferences in Nutrient Utilization Regulate Oxygen Consumption and Reactive Oxygen Species Generation in Schistosoma mansoni: Potential Implications for Parasite Redox Biology

Schistosoma mansoni, one of the causative agents of human schistosomiasis, has a unique antioxidant network that is key to parasite survival and a valuable chemotherapeutic target. The ability to detoxify and tolerate reactive oxygen species increases along S. mansoni development in the vertebrate host, suggesting that adult parasites are more exposed to redox challenges than young stages. Indeed, adult parasites are exposed to multiple redox insults generated from blood digestion, activated immune cells, and, potentially, from their own parasitic aerobic metabolism. However, it remains unknown how reactive oxygen species are produced by S. mansoni metabolism, as well as their biological effects on adult worms. Here, we assessed the contribution of nutrients and parasite gender to oxygen utilization pathways, and reactive oxygen species generation in whole unpaired adult S. mansoni worms. We also determined the susceptibilities of both parasite sexes to a pro-oxidant challenge. We observed that glutamine and serum importantly contribute to both respiratory and non-respiratory oxygen utilization in adult worms, but with different proportions among parasite sexes. Analyses of oxygen utilization pathways revealed that respiratory rates were high in male worms, which contrast with high non-respiratory rates in females, regardless nutritional sources. Interestingly, mitochondrial complex I-III activity was higher than complex IV specifically in females. We also observed sexual preferences in substrate utilization to sustain hydrogen peroxide production towards glucose in females, and glutamine in male worms. Despite strikingly high oxidant levels and hydrogen peroxide production rates, female worms were more resistant to a pro-oxidant challenge than male parasites. The data presented here indicate that sexual preferences in nutrient metabolism in adult S. mansoni worms regulate oxygen utilization and reactive oxygen species production, which may differently contribute to redox biology among parasite sexes.

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.

Amino acid metabolism in helminths

Amino acids are major constituents of biological material. Chemically they are extremely stable and combine a relatively simple molecular structure with a wide range of properties and functions. In general, amino acid metabolism in helminths has been relatively neglected and the information available is often uneven and of uncertain quality. However, the search for new target sites for anthelmintic development has led to a renewed interest in this area. The amino acid composition of helminths is similar to that of other invertebrates and no unique amino acids have been reported. With the possible addition of tyrosine, helminths seem to require the same 10 essential amino acids as mammals and, where studied in detail, the pathways of amino acid synthesis in helminths are similar to those of mammals. Although amino acids are not a significant energy source in parasites, helminths are able to catabolize amino acids by pathways which, again, appear identical to those found in mammals. Helminths have also been shown to carry out a number of oxidative reactions associated with amino acid metabolism, including cysteine dioxygenase, proline hydroxylase and tryptophan hydroxylase. There are, however, differences in detail between the pathways of amino acid metabolism in helminths and mammals, particularly in the metabolism of the sulphur amino acids and arginine and proline. These differences may be exploitable in anthelmintic design and proline analogues and proline biosynthesis inhibitors show some potential as fasciolicides (Sheers et al., 1982). Differences in metabolism between parasites and their hosts may be the result of parasitic adaptation or they may merely reflect general features of the invertebrate phyla as a whole. Thus a comparison of amino acid metabolism in parasitic helminths with that of their free-living relatives may give some insight into the biochemical basis of parasitism.

Schistosoma mansoni and Schistosoma japonicum: comparison of glucose-6-phosphate dehydrogenase and 6-phosphogluconate dehydrogenase activities in adults

Glucose-6-phosphate dehydrogenase activity in paired adult Schistosoma mansoni is about twice as great as in paired adult Schistosoma japonicum. 2. 6-phosphogluconate dehydrogenase activity accounts for 25.8% of the measured production of reduced nicotinamide adenine dinucleotide phosphate (NADPH) in S. japonicum but only 8.6% of the measured production of NADPH in S. mansoni. 3. These data suggest a species difference in 6-phosphogluconate metabolism.

Tricarboxylic acid cycle enzyme activities in adult Schistosoma mansoni and Schistosoma japonicum

Reduced tricarboxylic acid cycle enzyme activities were found in paired adult Schistosoma mansoni and S. japonicum in comparison to those reported for S. mansoni cercariae (Coles, 1973). Succinate dehydrogenase activity, which has not been previously reported for either adult blood fluke, is more than twice as great in paired adult S. japonicum as in paired adult S. mansoni, but the activity is low in both species of adult worms. The similar low levels of tricarboxylic acid cycle enzyme activities which occur in adult S. mansoni and S. japonicum further substantiate the fact that anaerobic glycolysis is the major energy source in adults of both species.