Ascaris suum: oxidative phosphorylation in mitochondria from developing eggs and adult muscle

Cyanide resistant respiration and the alternative oxidase pathway: A journey from plants to mammals

In a large number of organisms covering all phyla, the mitochondrial respiratory chain harbors, in addition to the conventional elements, auxiliary proteins that confer adaptive metabolic plasticity. The alternative oxidase (AOX) represents one of the most studied auxiliary proteins, initially identified in plants. In contrast to the standard respiratory chain, the AOX mediates a thermogenic cyanide-resistant respiration; a phenomenon that has been of great interest for over 2 centuries in that energy is not conserved when electrons flow through it. Here we summarize centuries of studies starting from the early observations of thermogenicity in plants and the identification of cyanide resistant respiration, to the fascinating discovery of the AOX and its current applications in animals under normal and pathological conditions.

Cytochemical localization of cytochrome oxidase in tissues of parasitic nematodes

Cytochemical staining with diaminobenzidine (DAB) has been used to locate cytochrome oxidase activity in mitochondria from major tissues of three intestinal parasitic nematodes, Nippostrongylus brasiliensis, Haemonchus contortus and Ascaridia galli. Deposition of DAB reaction product was uniform and relatively intense in mitochondria from all tissues studied in N. brasiliensis. However, mitochondria from different tissues of H. contortus and A. galli varied considerably in their cytochemical staining, decreasing in the order hypodermis < muscle < gut < reproductive tissue. Mitochondria in gut and reproductive tissue of A. galli possessed little or no cytochrome oxidase activity. These results are compatible with respect to oxygen availability and the extent and localization of the mammalian-type aerobic mitochondrial electron transport pathway in intestinal parasitic nematodes.

Cyanide-resistant respiration in Taenia crassiceps metacestode (cysticerci) is explained by the H2O2-producing side-reaction of respiratory complex I with O2

The nature of the cyanide-resistant respiration of Taenia crassiceps metacestode was studied. Mitochondrial respiration with NADH as substrate was partially inhibited by rotenone, cyanide and antimycin in decreasing order of effectiveness. In contrast, respiration with succinate or ascorbate plus N,N,N',N'-tetramethyl-p-phenylenediamine (TMPD) was more sensitive to antimycin and cyanide. The saturation kinetics for O2 with NADH as substrate showed two components, which exhibited different oxygen affinities. The high-O2-affinity system (Km app=1.5 microM) was abolished by low cyanide concentration; it corresponded to cytochrome aa3. The low-O2-affinity system (Km app=120 microM) was resistant to cyanide. Similar O2 saturation kinetics, using succinate or ascorbate-TMPD as electron donor, showed only the high-O2-affinity cyanide-sensitive component. Horse cytochrome c increased 2-3 times the rate of electron flow across the cyanide-sensitive pathway and the contribution of the cyanide-resistant route became negligible. Mitochondrial NADH respiration produced significant amounts of H2O2 (at least 10% of the total O2 uptake). Bovine catalase and horse heart cytochrome c prevented the production and/or accumulation of H2O2. Production of H2O2 by endogenous respiration was detected in whole cysticerci using rhodamine as fluorescent sensor. Thus, the CN-resistant and low-O2-affinity respiration results mainly from a spurious reaction of the respiratory complex I with O2, producing H2O2. The meaning of this reaction in the microaerobic habitat of the parasite is discussed.

Energy generation in parasitic helminths

Although parasitic helminths are a very heterogeneous group of organisms, they share many interesting properties in their energy metabolism. In certain stages of their life cycle, they all have a large capacity for anaerobic functioning. In other stages, an aerobic energy metabolism prevails. Parasites have to adapt to different environments in which the availability of oxygen and food varies widely. These variations in their external conditions strongly influence their energy metabolism. Here, Louis Tielens presents an introduction to the current ideas on the bioenergetics of parasitic helminths, focusing on the differences in energy metabolism between various stages (free-living and parasitic), and paying special attention to the mechanisms involved in the transitions between the different methods of energy generation.

Developmental changes in the respiratory chain of Ascaris mitochondria

The Ascaris larval respiratory chain, particularly complex II (succinate-ubiquinone oxidoreductase), was characterized in isolated mitochondria. Low-temperature difference spectra showed the presence of substrate-reducible cytochromes aa3 of complex IV, c+c1 and b of complex III (ubiquinol-cytochrome c oxidoreductase) in mitochondria from second-stage larvae (L2 mitochondria). Quinone analysis by high-performance liquid chromatography showed that, unlike adult mitochondria, which contain only rhodoquinone-9, L2 mitochondria contain ubiquinone-9 as a major component. Complex II in L2 mitochondria was kinetically different from that in adult mitochondria. The individual oxidoreductase activities comprising succinate oxidase, and fumarate reductase were determined in mitochondria from L2 larvae, from larvae cultured to later stages, and from adult nematodes. The L2 mitochondria exhibited the highest specific activity of cytochrome c oxidase, indicating that L2 larvae have the most aerobic respiratory chain among the stages studied. The Cybs subunit of complex II in L2 and cultured-larvae mitochondria exhibited different reactivities against anti-adult Cybs antibodies. Taken together, these results indicate that the complex II of larvae is different from its adult counterpart. In parallel with this change in mitochondrial biogenesis, biosynthetic conversion of quinones occurs during development in Ascaris nematodes.

Electron-transfer complexes of Ascaris suum muscle mitochondria. III. Composition and fumarate reductase activity of complex II

Complex II of the anaerobic respiratory chain in Ascaris muscle mitochondria showed a high fumarate reductase activity when reduced methyl viologen was used as the electron donor. The maximum activity was 49 mumol/min per mg protein, which is much higher than that of the mammalian counterpart. The mitochondria of Ascaris-fertilized eggs, which require oxygen for its development, also showed fumarate reductase activity with a specific activity intermediate between those of adult Ascaris and mammals. Antibody against the Ascaris flavoprotein subunit reacted with the mammalian counterparts, whereas those against the Ascaris iron-sulfur protein subunit did not crossreact, although the amino acid compositions of the subunits in Ascaris and bovine heart were quite similar. Cytochrome b-558 of Ascaris complex II was separated from flavoprotein and iron-sulphur protein subunits by high performance liquid chromatography with a gel permeation system in the presence of Sarkosyl. Isolated cytochrome b-558 is composed of two hydrophobic polypeptides with molecular masses of 17.2 and 12.5 kDa determined by gradient gel, which correspond to the two small subunits of complex II. Amino acid compositions of these small subunits showed little similarity with those of cytochrome b-560 of bovine heart complex II. NADH-fumarate reductase, which is the final enzyme complex in the anaerobic respiratory chain in Ascaris, was reconstituted with bovine heart complex I, Ascaris complex II and phospholipids. The maximum activity was 430 nmol/min per mg protein of complex II. Rhodoquinone was essential for this reconstitution, whereas ubiquinone showed no effect. The results clearly indicate the unique role of Ascaris complex II as fumarate reductase and the indispensability of rhodoquinone as the low-potential electron carrier in the NADH-fumarate reductase system.

Nematoda: aerobic respiratory pathways of adult parasitic species

Aerobic respiratory pathways have been compared in adult parasitic nematodes, including Trichostrongylus colubriformis, Nippostrongylus brasiliensis, Ostertagia ostertagi, Cooperia oncophora, Haemonchus contortus, Oesophagostomum venulosum, Chabertia ovina, Dictyocaulus filaria, Dictyocaulus viviparus, and Ascaridia galli. Respiration was measured in both whole worm or tissue homogenates and isolated mitochondrial fractions, and delineated into the mammalian type or alternative respiratory pathways on the basis of their inhibition by antimycin A. The alternative, antimycin A-insensitive respiratory pathway was of comparable activity in all parasitic nematodes studied, irrespective of the body diameter or habitat of the worm. The mammalian-type, antimycin A-sensitive respiratory pathway showed variations; the extent of this pathway correlated with both the body diameter and habitat of the worm, being greater in thinner worms and those worms whose habitat is supposedly more aerobic.

Nippostrongylus brasiliensis: the effect of mitochondrial inhibitors on life-cycle stages

The effects of mitochondrial inhibitors on the in vitro development of Nippostrongylus brasiliensis have been studied in free-living and parasitic life-cycle stages. Mitochondrial inhibitors were chosen as being representative of established electron transport inhibitors and oxidative phosphorylation inhibitors and uncouplers of the classical mammalian respiratory chain. All mitochondrial inhibitors tested were highly effective in killing or retarding development of free-living stages of N. brasiliensis. Free-living stages were particularly susceptible to such inhibitors upon hatching of embryonated eggs to 1st-stage larvae. Concentrations of inhibitors effective against free-living stages were consistent with their level of inhibition against isolated mitochondria from embryonated eggs and 3rd-stage infective larvae. Results suggest an absolute requirement in the development of free-living stages for the mammalian-like respiratory chain and associated oxidative phosphorylation. Electron transport inhibitors were effective in retarding at least the initial development of 4th-stage larvae to adults, but only antimycin A and azide produced a lasting effect leading to worm death. Oxidative phosphorylation inhibitors and uncouplers were ineffective against developing parasitic stages of N. brasiliensis. Experiments on whole-worm respiration indicated that most electron transport inhibitors were able to penetrate the adult worm, but oxidative phosphorylation inhibitors were without effect on whole-worm respiration. Results suggest that the mammalian-like electron transport chain is a necessary requirement to adult N. brasiliensis, but oxidative phosphorylation in the adult worm may not be required for development and survival in vitro although it could be necessary to support the parasite in vivo.

Nippostrongylus brasiliensis and Ascaridia galli: mitochondrial respiration in free-living and parasitic stages

Aerobic respiratory pathways have been delineated and respiratory efficiency has been assessed in mitochondria isolated from embryonated eggs, infective larvae, and adult Nippostrongylus brasiliensis and Ascaridia galli. Mitochondrial respiration in free-living stages of N. brasiliensis is mediated mainly by a mammalian-like antimycin A- and cyanide-sensitive pathway; specific respiratory activity is high and oxidative phosphorylation efficient. In mitochondria of adult N. brasiliensis, antimycin A- and cyanide-sensitive respiration is decreased relative to respiration though an alternative pathway, and specific respiratory activity and mitochondrial efficiency are lower. Respiration in mitochondria from embryonated eggs and tissues of adult A. galli is comparable, and apparently mediated by an antimycin A- and cyanide-insensitive alternative respiratory pathway; no evidence for the presence of a mammalian-like respiratory pathway in embryonated eggs of A. galli was found. The results of this study are compared to mitochondrial respiration in eggs, larvae, and adult body wall muscle of Ascaris suum.