2-Methylvalerate formation in mitochondria ofAscaris suum and its relationship to anaerobic energy generation

Rhodoquinone biosynthesis in C. elegans requires precursors generated by the kynurenine pathway

Parasitic helminths infect over a billion humans. To survive in the low oxygen environment of their hosts, these parasites use unusual anaerobic metabolism - this requires rhodoquinone (RQ), an electron carrier that is made by very few animal species. Crucially RQ is not made or used by any parasitic hosts and RQ synthesis is thus an ideal target for anthelmintics. However, little is known about how RQ is made and no drugs are known to block RQ synthesis. C. elegans makes RQ and can use RQ-dependent metabolic pathways - here, we use C. elegans genetics to show that tryptophan degradation via the kynurenine pathway is required to generate the key amine-containing precursors for RQ synthesis. We show that C. elegans requires RQ for survival in hypoxic conditions and, finally, we establish a high throughput assay for drugs that block RQ-dependent metabolism. This may drive the development of a new class of anthelmintic drugs. This study is a key first step in understanding how RQ is made in parasitic helminths.

Anaerobiosis in the nematode Caenorhabditis elegans

In Caenorhabditis elegans, mortality rates and changes in concentrations of carbohydrate stores and anaerobic end products were determined in anoxic (test) and normoxic (control) animals at two different temperatures (10 and 20 degrees C). The anoxic tolerance of the free-living nematode proved to be well-developed: at 10 degrees C, about 50% of animals had survived a period of 50 h of anoxia. The carbohydrate stores (approximately 30 mmol glycosyl units kg-1 freshweight (FW)) were reduced by two-thirds within 24 h of anoxia at both temperatures. L-lactate, acetate, succinate, and propionate were identified as the main anaerobic end products. The amounts and proportions of the end products were dependent on temperature. They did not accumulate very much in the tissues, but were mainly excreted. During anoxia, the metabolism of C. elegans was depressed to 3-4% of the aerobic value. The food-source Escherichia coli was found to be at least partly alive in the gut of the animals. To separate between anaerobiosis in animals and bacteria, cleaning procedures were applied, and additional control measurements were made: anaerobic end products produced either by E. coli alone or by bacteria-free (axenic) bred nematodes were quantified at identical incubation conditions.

Influence of organic acid excretion on cuticle pH and drug absorption by Haemonchus contortus

To determine if a cuticle microenvironment pH is maintained by adult Haemonchus contortus, organic acid excretion kinetics and absorption kinetics of selected model weak acids and a weak base were measured in incubation media that varied in buffer capacity (0.25-20 mM HEPES or 5 mM glycine) and initial pH (7.5 or 3.5). To evaluate the importance of the cuticle as a pathway for organic acid excretion and drug absorption the pharynx was paralyzed with 1 nM ivermectin. H. contortus changed the media pH from initial values of 7.5 or 3.25 to an asymptotic value of approximately 5.6. The rate of pH change depended on the buffer capacity, but was not affected by chemical ligation with ivermectin. The intrinsic rate of excretion of organic acids (0.045 +/- 0.016 micromol/cm2 x h) was constant during the first 8-12 h of incubation and was independent of initial pH, buffer capacity or ivermectin ligation. The rates of absorption of the model weak acids, benzoic acid and p-nitrophenol, and the model weak base, aniline, were not affected by initial pH, buffer capacity or ivermectin ligation. These results suggest that H. contortus excretes organic acid endproducts of carbohydrate metabolism across its cuticle, and that these acids maintain a microenvironment pH within the water-filled pores of the cuticle that controls the rate of adsorption of weakly acidic or basic drugs.

The pyruvate dehydrogenase complex from the parasitic nematode Ascaris suum: novel subunit composition and domain structure of the dihydrolipoyl transacetylase component

The pyruvate dehydrogenase complex (PDC) from muscle of the adult parasitic nematode Ascaris suum plays a unique role in its anaerobic mitochondrial metabolism. Resolution of the intact complex in high salt dissociates the pyruvate dehydrogenase subunit but leaves the dihydrolipoyl dehydrogenase subunit (E3) and two other proteins with apparent M(r)s of 45 and 43 kDa bound to the dihydrolipoyl transacetylase (E2) core. These proteins are not observable on Coomassie brilliant blue-stained gels of other eukaryotic PDCs, but the 45-kDa protein is similar in apparent M(r), pI, and sensitivity to trypsin to the Kb subunit of the bovine kidney PDH alpha kinase. Acetylation of the ascarid PDC with [2-14C]pyruvate under conditions designed to maximize the incorporation of label into protein yielded only a single radiolabeled subunit, E2. These results confirm earlier reports that the ascarid PDC lacks protein X, an integral component recently identified in other eukaryotic PDCs. About 1.6 to 1.8 mol of 14C was incorporated/mole of E2, suggesting that the ascarid E2 contained two lipoly-bearing domains. Domain mapping of the 14C-acetylated ascarid E2 by limited tryptic digestion identified two lipoyl-bearing fragments with apparent M(r)s of 50 and 34 kDa and two core fragments with apparent M(r)s of 46 and 30 kDa. The ascarid E2 domain structure appears to be similar to that of other E2s. However, it appears that the subunit-binding domain (E2B) of the ascarid E2 may be significantly larger or be flanked by larger than normal interdomain regions. An enlarged E2B domain may be necessary to accommodate the additional binding of E3 to the E2 subunit in the ascarid complex, in the absence of protein X.

Mechanisms of microenvironmental pH regulation in the cuticle of Ascaris suum

The excretion kinetics of various organic acids by Ascaris suum were quantified to determine if the excretion of these metabolic end-products could generate and maintain a microclimate pH within the aqueous compartment of the cuticle. Ligated and nonligated A. suum were incubated in media buffered with 0.25 or 2.5 mM Hepes (initial pH 7.5) or 0.5 or 5 mM glycine (initial pH 3.25). The concentration of organic acids and the pH of the media were followed for 24 h. Several volatile fatty acids, including acetic, 2-methylbutyric, 2-methylvaleric, n-valeric, and n-butyric, were excreted at relatively high rates. Propionic, n-caproic, 2-methylcaproic, tiglic acid, and the non-volatile organic acids, lactic and succinic, were excreted more slowly. The organic acids were excreted at a constant rate and in apparently fixed molar concentration ratios. The accumulation of organic acids was associated with changes in pH of the medium until a limiting constant pH, in the vicinity of the pKa of the volatile fatty acids, was reached. The rate of organic acid excretion was not affected by initial medium pH, buffer capacity, or parasite ligation. The rate of pH change induced by the excretion of organic acids was also insensitive to whether ligated or nonligated A. suum were used, but was dependent on the initial buffer capacity of the medium. These results suggest that A. suum excrete the end-products of carbohydrate metabolism across the cuticle. The presence of organic acids in the aqueous pores of the cuticle creates and maintains a microclimate pH of about 5.0 +/- 0.3. This pH will influence the transport properties of weak acids and bases and should be considered in the design of delivery systems for anthelmintics.

Catalase activity during the development of the parasitic nematode, Ascaris suum

1. Catalase activity was partially purified from body wall muscle of the parasitic nematode, Ascaris suum, and was similar to catalases isolated from mammalian tissues. It exhibited a broad pH optimum and was unaffected by 2 mM ethylenediaminetetra-acetate. In contrast, it was inhibited reversibly by 1 mM cyanide and irreversibly by prior incubation in 40 mM 3-amino-1:2:4-triazole for 1 hr or heating at 80 degrees C for 15 min. 2. Catalase activity was highest in the unembryonated "egg" and decreased dramatically as development proceeded. 3. Catalase activity in adult body wall muscle was similar to that in rat skeletal muscle, but dramatically lower than that in rat liver. Catalase activity was barely detectable in A. suum testis. 4. Cytochrome-c peroxidase activity did not appear to be present in adult A. suum muscle mitochondria.

In vitro induction of crawling in the amoeboid sperm of the nematode parasite, Ascaris suum

In a highly synchronous process, the immotile spermatids of Ascaris suum extend pseudopods and become rapidly crawling sperm when treated with an extract from the glandular vas deferens of the male under strict anaerobic conditions. Within 9-12 min, a pseudopod develops, elongates rapidly, and exhibits a continuous flow of membrane specializations, the villipodia, from tip toward base. When attached to acid-washed glass, the pseudopod pulls the cell body along at speeds exceeding 70 microns/min. The pseudopod length remains constant while retrograde flow of villipodia proceeds at the same rate as the sperm's forward movement. Cohorts of about 15 villipodia form at the leading edge, move rearward together, and disappear at the junction of pseudopod and cell body. These are the terminations of branched, refringent fibers, which extend the length of the pseudopod. The latter are the fiber complexes that form its cytoskeleton (Sepsenwol et al.: Journal of Cell Biology 108:55-66, 1989). Locomoting cells sometimes change direction when another crawls by and follow each other. When cells are exposed to air, forward movement ceases in a predictable pattern: the forward extension of the leading edge ceases, the pseudopod shortens from the base, and the cell body continues to be pulled forward. These data contribute to a model for Ascaris sperm amoeboid motility in which independent processes of continuous extension at the leading edge and continuous shortening at the base of the pseudopod act to propel the cell forward.

Electron-transfer flavoprotein from anaerobic Ascaris suum mitochondria and its role in NADH-dependent 2-methyl branched-chain enoyl-CoA reduction

Electron-transfer flavoprotein was purified to apparent homogeneity from mitochondria of the parasitic nematode, Ascaris suum. The native molecular weight of the enzyme was 70,000, as estimated by gel filtration, and it migrated as two bands with apparent subunit molecular weights of 37,000 and 31,500 during sodium dodecylsulfate polyacrylamide gel electrophoresis. The enzyme exhibited an absorption coefficient for the bound FAD of 13.5 mM-1.cm-1 at 436 nm and a protein/flavin (270 nm/436 nm) ratio of 5.6. While the ascarid enzyme is similar to its mammalian counterpart, physiologically it functions in the reverse direction, shuttling reducing power from the electron-transport chain to a soluble 2-methyl branched-chain enoyl CoA reductase. Indeed, when A. suum submitochondrial particles were incubated with NADH, 2-methylcrotonyl-CoA and purified A. suum 2-methyl branched-chain enoyl-CoA reductase, 2-methylbutyryl-CoA formation was proportional to the amount of electron-transfer flavoprotein added.

Succinate-dependent energy generation in Ascaris suum mitochondria

Phosphorylation in isolated Ascaris suum mitochondria was much greater in the presence of malate than succinate, but, in the absence of added adenine nucleotides, incubations in succinate resulted in substantial elevations in intramitochondrial ATP levels. Succinate-dependent phosphorylation was stimulated aerobically and this stimulation was due almost entirely to a site I, rotenone-sensitive, phosphorylation. Increased substrate level phosphorylation, coupled to propionate formation, or additional sites of electron-transport associated ATP synthesis were not significant. Under aerobic conditions, 14CO2 evolution from 1,4-[14C]succinate was stimulated and NADH/NAD+ ratios were elevated, but the formation of [14C]propionate was unchanged. It appears that succinate was metabolized to pyruvate and acetate, and NADH, generated from the decarboxylations of malate and pyruvate, was the primary source of reducing power fueling electron-transport. The terminal oxidase and final electron-acceptor are still not clearly defined. However, ferricyanide, H2O2, and 100% oxygen all stimulated succinate-dependent phosphorylation. A possible role for cytochrome c peroxidase in A. suum mitochondrial metabolism is discussed.

Anaerobic metabolism in Ascaris suum: acyl CoA intermediates in isolated mitochondria synthesizing 2-methyl branched-chain fatty acids

Freshly isolated Ascaris suum mitochondria contained CoASH, acetyl CoA, propionyl CoA, 2-methylcrotonyl CoA, 2-methylbutyryl CoA, 2-methyl-2-pentenoyl CoA, and 2-methylvaleryl CoA, as determined by high-pressure liquid chromatography. Incubation of these mitochondria aerobically in the absence of substrate resulted in the conversion of the branched-chain enoyl CoA's to acetyl CoA and propionyl CoA. With the addition of malate to the incubation medium, succinyl CoA and methylmalonyl CoA accumulated and the levels of propionyl CoA decreased dramatically. However, the branched-chain fatty acids characteristic of A. suum's fermentative metabolism were not formed and it appears that the formation of propionyl CoA may be limiting in these mitochondria. Indeed, the addition of propionate to incubations with malate increased intramitochondrial levels of propionyl CoA and 2-methyl-2-pentenoyl CoA and stimulated significant 2-methylvalerate synthesis. The exclusion of air from these incubations further increased levels of 2-methyl-2-pentenoyl CoA and stimulated 2-methylvalerate synthesis. These studies suggest that in addition to elevated NADH/NAD ratios, elevated enoyl CoA/acyl CoA ratios also are important in the regulation of branched-chain fatty acid synthesis in A. suum mitochondria.

Immunochemical characterization of the pyruvate dehydrogenase complex in adult Ascaris suum and its developing larvae

Polyclonal antibody was prepared against the pyruvate dehydrogenase complex purified from adult Ascaris suum body wall muscle. The antibody reacted with the E2, X, alpha E1 and beta E1 subunits of the complex in immunoblots of mitochondrial supernatant fractions and homogenates of adult muscle. In addition, the same subunits were observed in immunoblots of homogenates of L3 and L4 ascarid larvae, suggesting that a similar enzyme complex was present in all developmental stages despite their marked differences in energy metabolism. The phosphorylated and dephosphorylated alpha E1 peptides migrated differently during sodium dodecylsulfate polyacrylamide gel electrophoresis and both forms of the enzyme were recognized by the antibody. These results and those obtained with ELISA suggest that both phosphorylated and dephosphorylated forms of the alpha E1 subunit react equally well with the antibody. In immunoblots of adult body wall muscle, the phosphorylated alpha E1 peptide predominated, while immunoblots of L3 larvae contained predominantly the dephosphorylated form. These results reflect the in vivo activity state of the pyruvate dehydrogenase complex in these two stages and suggest that this technique may be useful for determining the activity state of enzyme complex directly from immunoblots of homogenates A. suum and other helminths.

Biochemical changes during the aerobic-anaerobic transition in Ascaris suum larvae

Ascaris suum L3 larvae isolated from rabbit lungs undergo the third ecdysis to L4 larvae after 3 days in culture under a gas phase of 85% N2/10% CO2/5% O2. The L3 larvae contain substantial malic enzyme activity and are capable of producing small amounts of the reduced organic acids characteristic of the fermentative pathways which operate in the adult. However, only a small portion of the total carbon utilized is accounted for by these reduced acids and their motility is cyanide-sensitive, suggesting that their energy-generating pathways are predominantly aerobic. In contrast, after ecdysis, the L4 larvae begin to utilize glucose at a greater rate and the proportion of total carbon utilized which is accounted for as propionate, 2-methylbutyrate and 2-methylvalerate also increases. In addition, motility becomes increasingly cyanide-insensitive, suggesting that these L4 larvae are able to utilize the anaerobic energy-generating pathways of the adult. Surprisingly, on day 10 in culture, these L4 larvae, although capable of producing reduced volatile acids, still retain substantial cyanide-sensitive cytochrome oxidase activity.

Improved purification of the pyruvate dehydrogenase complex from Ascaris suum body wall muscle and characterization of PDHa kinase activity

An improved purification scheme for the isolation of the Ascaris suum pyruvate dehydrogenase complex directly from body wall muscle has been developed which yields a fully activated pyruvate dehydrogenase complex with substantial PDHa kinase activity. The apparent Km for coenzyme A (CoA) is much lower than previously reported and can only be accurately measured in the presence of a CoA-regenerating system. The alpha-pyruvate dehydrogenase subunit of the ascarid complex is unique and its migration on sodium dodecylsulfate polyacrylamide gels is altered after phosphorylation. PDHa kinase activity is inhibited by ADP, thiamine pyrophosphate, and physiological levels of pyruvate and propionate. In contrast, PDHa kinase activity is stimulated by elevated NADH/NAD+ and acetyl CoA/CoA ratios, although it appears that the NADH/NAD+ ratios required for half-maximal stimulation are more than an order of magnitude greater than those reported for mammalian pyruvate dehydrogenase complexes.