The transport of dicarboxylates and some properties of fumarase in the muscle mitochondria of Ascaris suum

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.

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.

Intramitochondrial localization of fumarate reductase, NADPH----NAD transhydrogenase, 'malic' enzyme and fumarase in adult Hymenolepis diminuta

The intramitochondrial localization of the fumarate reductase, NADPH----NAD transhydrogenase, 'malic' enzyme and fumarase was determined in adult Hymenolepis diminuta. The distribution of marker enzymes for the inner membrane, matrix, intermembrane space and outer membrane of H. diminuta mitochondria simulated that of the corresponding ascarid and mammalian organelles. The electron transport-coupled fumarate reductase and the NADPH----NAD transhydrogenase were components of the inner membrane whereas the 'malic' enzyme and fumarase were in the matrix soluble compartment. Assessments of NADH utilization, malate-dependent NADP reduction and NADPH----NAD transhydrogenation by presumedly intact and disrupted mitochondria supported the localization data. The findings presented indicate that in H. diminuta mitochondria (a) NADPH and fumarate are accumulated within the matrix compartment; (b) transhydrogenation between NADPH and NAD is an event associated with the matrix side of the inner membrane; and (c) electron transport-dependent NADH oxidation and fumarate reduction occur at sites on the matrix side of the inner membrane.

Energy metabolism in Cotugnia digonopora and the effect of anthelmintics

Incorporation of 32Pi into organic phosphate by mitochondria of Cotugnia digonopora was supported maximally by malate. Fumarate and succinate induced lower but significant production of ATP. Pyruvate, alpha-ketoglutarate and oxalacetate proved to be poor substrates and citrate and isocitrate had no effect. A net phosphorylation of approximately 2 mol of ADP was observed for each mol of CO2 liberated from malate or succinate. In contrast, with pyruvate, in spite of a high rate of decarboxylation, the production of ATP was extremely low. 2,4-Dinitrophenol inhibited phosphorylation. All anthelmintics examined interfered with the mitochondrial phosphorylation of ADP, with maximum inhibition by salicylanilide compounds. The anticestodal activity of the latter group of compounds, niclosamide for example, may, therefore, be attributed to their ability to inhibit mitochondrial phosphorylation.

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.

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.