Demonstration and possible function of NADH:NAD+ transhydrogenase from ascaris muscle mitochondria

NADH→NAD⁺ Transhydrogenation in Adult Ascaris suum Mitochondria

Although lacking an NADPH→NAD(+) transhydrogenase system, the essentially energetically anaerobic mitochondria of the adult intestinal nematode Ascaris suum display an inner membrane-associated NADH→NAD(+) transhydrogenation reaction. This reaction is considered to be reflective of a mechanism(s) that acts in catalyzing a transmembrane translocation of reducing equivalents from NADH in the intermembrane space to matrix NAD(+), thereby forming matrix NADH that would serve in electron transport. Ascarid mitochondrial lipoamide dehydrogenase rather than an NADH→NAD(+) transhydrogenase system has been viewed as the predominant source of inner membrane-associated NADH→NAD(+) transhydrogenation activity. However, the present study made apparent yet another source of mitochondrial, inner membrane-associated NADH→NAD(+) activity in A. suum , viz., NADH dehydrogenase. This was made evident via comparisons of the A. suum mitochondrial NADH→NAD(+) transhydrogenation, NADH dehydrogenase, and lipoamide dehydrogenase activities in terms of pH effects, thermal labilities, the involvement of NADH dehydrogenase in the activities of mitochondrial, membrane-associated rotenone-insensitive and rotenone-sensitive NADH-dependent cytochrome c reductases, and mitochondrial membrane versus mitochondrial soluble localizations. Studies of the responses of the NADH→NAD(+) transhydrogenation, rotenone-insensitive and rotenone-sensitive cytochrome c reductases, and lipoamide dehydrogenase activities to inhibition by copper and cadmium lent additional support to the catalysis of an NADH→NAD(+) transhydrogenation activity by NADH dehydrogenase. Collectively, the data presented are consistent with an additional physiological catalysis of an NADH→NAD(+) transhydrogenation in A. suum mitochondria by an inner membrane NADH dehydrogenase component of the rotenone-sensitive cytochrome c reductase system, i.e., the NADH dehydrogenase component of the electron transport system. Comparisons of the A. suum data with those from other essentially anaerobic helminth parasites as well as free-living eukaryotic mitochondrial systems are noted.

Transhydrogenase and the anaerobic mitochondrial metabolism of adult Hymenolepis diminuta

The adult cestode, Hymenolepis diminuta, is essentially anaerobic energetically. Carbohydrate dissimilation results in acetate, lactate and succinate accumulation with succinate being the major end product. Succinate accumulation results from the anaerobic, mitochondrial, 'malic' enzyme-dependent utilization of malate coupled to ATP generation via the electron transport-linked fumarate reductase. A lesser peroxide-forming oxidase is apparent, however, fumarate reduction to succinate predominates even in air. The H. diminuta matrix-localized 'malic' enzyme is NADP-specific whereas the inner membrane (IM)-associated electron transport system prefers NADH. This dilemma is circumvented by the mitochondrial, IM-associated NADPH-->NAD+ transhydrogenase in catalyzing hydride ion transfer from NADPH to NAD+ on the IM matrix surface. Hydride transfer is reversible and phospholipid-dependent. NADP+ reduction occurs as a non energy-linked and energy-linked reaction with the latter requiring electron transport NADH utilization or ATP hydrolysis. With NAD+ reduction, the cestode transhydrogenase also engages in concomitant proton translocation from the mitochondrial matrix to the intermembrane space and supports net ATP generation. Thus, the cestode NADPH-->NAD+ system can serve not only as a metabolic connector, but an additional anaerobic phosphorylation site. Although its function(s) is unknown, a separate IM-associated NADH--> NAD+ transhydrogenation, catalyzed by the lipoamide and NADH dehydrogenases, is noted.

Hymenolepis diminuta: mitochondrial NADH --> NAD transhydrogenation and the lipoamide dehydrogenase system

The occurrence of NADH --> NAD transhydrogenation and lipoamide dehydrogenase activities was demonstrated for cysticercoids of the intestinal cestode, Hymenolepis diminuta. In addition, both activities were catalyzed by the mitochondria of 6-, 10-, and 14-day H. diminuta and by the mitochondria from immature, mature, and pregravid/gravid regions of the adult cestode. A developmentally related increase in NADH --> NAD activity was suggested and the levels of both activities in the immature region of the helminth were consistent with it being a region of high metabolic activity. Adult H. diminuta mitochondrial lipoamide dehydrogenase was purified to homogeneity. The native enzyme was a homodimer with a monomeric and dimeric molecular mass of 47 and 93 kDa, respectively. Spectral analyses revealed that the enzyme contained flavin. More importantly, the purified enzyme catalyzed appreciable NADH --> NAD transhydrogenation activity, a premier finding for the phylum Platyhelminthes. The ratio of NADH --> NAD transhydrogenation to lipoamide reduction was 1:5. Both activities were inhibited by Cu2+ and Cd2+ with the NADH --> NAD activity being more resistant to inhibition. Interestingly, aside from NADH diaphorase activity, the cestode enzyme displayed NADH-ferricyanide reductase and, to a lesser degree, NADPH --> NAD transhydrogenation activities. The partial amino acid sequence of H. diminuta lipoamide dehydrogenase indicated that this enzyme was most similar to the corresponding enzymes of other parasitic helminths. Moreover, the phenylalanine for leucine substitution found in the redox-active disulfide site of the lipoamide dehydrogenases of some anaerobic systems was noted for the H. diminuta enzyme.

Transhydrogenase activities and malate dismutation linked to fumarate reductase system in the filarial parasite Setaria digitata

Setaria digitata, a cattle filarial parasite, similar to human filarial parasites, possesses significant activities of the 4 transhydrogenases namely NADH-NAD+, NADPH-NAD+, NADH-NADP+, and NADPH-NADP+ in the sonicated mitochondria like particles. The transhydrogenases appear to regulate the metabolic pathways of the parasite in response to the presence of adenyl nucleotides and are non-energy linked. Observations on the transhydrogenase and fumarate reductase activities show the existence of a protein bound NAD in the MLP and a linkage between the fumarate reductase system and malic enzyme through transhydrogenases. The malate dismutation reaction is the result of malic and fumarase enzyme activities. Fumarase and fumarate reductase activities result in succinate formation under anaerobic conditions showing major energy production at the fumarate reductase site. The existence of acetate kinase, phosphotransacetylase, pyruvate carboxylase, propionyl CoA carboxylase and CoA transferase enzymes in the mitochondrial system shows the presence of other energy producing sites in the parasite. The transhydrogenase system, NAD+/NADP+ malic enzyme, fumarase and fumarate reductase are the key enzymes of, production of reducing power for synthetic reactions and regulation of oxidative and reductive stages of the mitochondrial system. Hence, specific drugs targeted against this interconnected complex enzyme system, will be very effective in the control of filarial parasites.

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. II. Succinate-coenzyme Q reductase (complex II) associated with substrate-reducible cytochrome b-558

A succinate-coenzyme Q reductase (complex II) was isolated in highly purified form from Ascaris muscle mitochondria by detergent solubilization, ammonium sulfate fractionation and gel filtration on a Sephadex G-200 column. The enzyme preparation catalyzes electron transfer from succinate to coenzyme Q1 with a specific activity of 1.2 mumol coenzyme Q1 reduced per min per mg protein at 25 degrees C. The isolated complex II is essentially free of NADH-ferricyanide reductase, reduced CoQ2-cytochrome c reductase and cytochrome c oxidase and consists of four major polypeptides with apparent molecular weights of 66 000, 27 000, 12 000 and 11 000 and two minor ones with Mr of 36 000 and 16 000. The complex II contained cytochrome b-558, a major constituent cytochrome of Ascaris mitochondria, at a concentration of 3.6 nmol per mg protein, but neither other cytochromes nor quinone. The cytochrome b-558 in the complex II was reduced with succinate. In the presence of Ascaris NADH-cytochrome c reductase (complex I-III) (Takamiya, S., Furushima, R. and Oya, H. (1984) Mol. Biochem. Parasitol. 13, 121-134), the cytochrome b-558 in complex II was also reduced with NADH and reoxidized with fumarate. These results suggest the cytochrome b-558 to function as an electron carrier between NADH dehydrogenase and succinate dehydrogenase in the Ascaris NADH-fumarate reductase system.

Electron transfer complexes of Ascaris suum muscle mitochondria: I. Characterization of NADH-cytochrome c reductase (complex I-III), with special reference to cytochrome localization

An NADH-cytochrome c reductase (complex I-III) was isolated from Ascaris suum muscle mitochondria. The enzyme preparation catalyzed the reduction of 1.68 mumol cytochrome c min-1 mg-1 protein at 25 degrees C with NADH but not with NADPH, and retained its sensitivity to rotenone, piericidin A and 2-heptyl-4-hydroxyquinoline-N-oxide as with the submitochondrial particles. The isolated complex I-III, essentially free of succinate-cytochrome c reductase and cytochrome c oxidase, consisted of fourteen polypeptides with apparent molecular weights ranging from 76 000 to 12 000. The complex I-III contained three cytochromes, b-559.5, b-563 and c1-550.5 and Pigment-558 at concentrations of 1.28, 0.211, 1.23 and 0.321 nmol mg-1 protein, respectively. Cytochrome b-558, a major constituent cytochrome of Ascaris mitochondria and previously suggested to participate in the fumarate reductase system, was not fractionated in the complex I-III. Localization of the cytochromes in Ascaris electron transfer complexes is discussed.

The localization of fumarase and malic enzyme in muscle mitochondria of Ascaris suum

The distribution of fumarase and malic enzyme in Ascaris suum muscle mitochondria was investigated by employing digitonin fractionation techniques. The ability of these procedures to resolve the various submitochondrial fractions (intracristal space, inner membrane, matrix and inner membrane-matrix particles) was verified by electron microscopy and the distribution of appropriate marker enzymes. From the data obtained, it is concluded that fumarase in Ascaris muscle mitochondria is located solely within the matrix compartment and is not present within the intracristal space as reported earlier by other authors (Rew and Saz (1974) J. Cell. Biol. 63, 125-135). In agreement with previous findings malic enzyme in the nematode organelle appears to be associated with both soluble compartments. The implications of these findings to the parasites' mitochondrial metabolism are discussed.

Regulation of the Ascaris suum pyruvate dehydrogenase complex by phosphorylation and dephosphorylation

The pyruvate dehydrogenase complex isolated from 'anaerobic' mitochondria of Ascaris suum has a subunit composition similar to complexes isolated from most other eukaryotic organisms and is regulated by phosphorylation and dephosphorylation. Pyruvate dehydrogenase kinase activity is stimulated by NADH and a number of physiologically important acyl-CoA intermediates and is inhibited by CoA, propionate, tiglate and pyruvate. It is suggested that the elevated levels of pyruvate observed in the ascarid organelle may be important in maintaining the pyruvate dehydrogenase complex in an active state, even in the presence of a reduced pyridine nucleotide pool.

Cytosolic malate dehydrogenase in muscle extracts of Toxocara canis

1. Malate dehydrogenase (L-malic acid:NAD+ oxydoreductase, EC 1.1.1.37) was partially purified from muscle extracts of Toxocara canis by means of gel chromatography in Sephadex G-150 and affinity chromatography in Sepharose-4B-Blue dextran. 2. The purified enzyme was very active in reducing oxalacetate and less active in oxidizing L-malate. It was inhibited by excess oxalacetate but not by L-malate. 3. The kinetic parameters of the enzyme were obtained and these included: pH and temperature optima and apparent Michaelis constants for the substrates. 4. The results suggest that the enzyme from Toxocara canis behaves like the enzyme of the model helminth Ascaris lumbricoides.

Aspects of helminth metabolism

‘Each organism must be examined as a biochemical entity before any reasonable understanding of helminth metabolism can be attained’ (Saz, 1969).

Intramitochondrial location of the molecular forms of chicken liver mitochondrial malate dehydrogenase

1. The two molecular forms of mitochondrial malate dehydrogenase are partly bound to the mitochondrial membranes. 2. The A form is located on the outer surface of the inner mitochondrial membrane and also in the intermembrane space. 3. The B form of the enzyme appears in the matrix and bound in part, probably, to the inner surface of the inner mitochondrial membrane. 4. Glutamate dehydrogenase, glutamate oxaloacetate transaminase, fumarase and lactate dehydrogenase are bound, to a greater or lesser extent, to the mitochondrial membranes, the fumarase having the highest degree of binding.

Mechanisms of respiration and phosphorylation in Ascaris muscle mitochondria

In Ascaris muscle mitochondria the major respiratory chain-linked phosphorylation activity is accomplished by a NADH-linked reduction of fumarate to succinate. Oxygen can also be employed as a terminal electron acceptor via a cyanide- and salicyl-hydroxamate-resistant terminal oxidase. As in fumarate-dependent electron transport this process appears to be coupled to energy conservation at phosphorylation site I. The branchpoint from which electrons are taken from the main respiratory chain to either the alternative oxidase or fumarate reductase is likely to be on the oxygen side of the NADH dehydrogenase segment. Malate and succinate are the only substrates which appreciably support respiration in the mitochondrion of the nematode. Regardless of the presence or absence of oxygen malate is utilized by an oxidation-reduction reaction resulting in the formation of pyruvate, acetate, succinate, propionate and CO2. In addition, aerobically, hydrogen peroxide is formed as the product of oxygen reduction. Succinate accumulation was found to be significantly higher in the anaerobic as compared to the aerobic incubation mixtures. This effect was accompanied by an increase in anaerobic malate consumption. ATP generation and the formation of pyruvate, acetate and propionate were found to be similar in the presence and absence of oxygen. In malate-supported respiration of intact Ascaris mitochondria reducing equivalents (NADH) are produced exclusively through pyruvate and acetate formation. These enzymatic reactions are functionally coupled to the electron transport-linked reductions of fumarate to succinate and oxygen to hydrogen peroxide, respectively. In accordance with the position of the redox potentials of the fumarate/succinate and O2/H2O2 couples, anaerobic and aerobic respiration was found to be associated with relatively low energy conservation efficiencies. Thus one molecule of ATP was conserved per 2e- transferred to fumarate or oxygen, respectively. No evidence could be obtained for a significant activity of energy conservation sites II and III and electron transfer through the alternative oxidase pathway was shown not to be coupled to phosphorylation.

Purification and properties of the Ascaris pyruvate dehydrogenase complex

The pyruvate dehyhdrogenase complex (pyruvate:lipoate oxidoreductase (decarboxylating and acceptor-acetylating), EC 1.2.4.1) has been isolated from Ascaris muscle mitochondria and purified to near homogeneity by differential centrifugation, (NH4)2SO4 fractionation and calcium phosphate gel-cellulose chromatography. It is similar in shape, size and physical characteristics to pyruvate dehydrogenase complexes isolated from mammalian sources. It has an absolute dependence on CoA, NAD+ and pyruvate for activity and is competitively inhibited by acetyl-CoA and NADH. However, much higher NADH/NAD+ ratios are necessary to inhibit activity, suggesting regulation by the more reduced state of the pyridine nucleotide pool in Ascaris mitochondria.