Studies on enzymes from parasitic helminths. II. Purification and properties of malic enzyme from the tapeworm, Hymenolepis diminuta

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.

Catalysis of NADH-->NADP+ transhydrogenation by adult Hymenolepis diminuta mitochondria

Hymenolepis diminuta mitochondria catalyze nonenergy-linked and energy-linked NADH-->NADP(+) transhydrogenations, with the latter driven by electron-transport dependent NADH oxidation (electron transport-driven, ETD) or ATP hydrolysis (ATP-driven, ATPD). Using submitochondrial particles, NADH-->NADP(+) transhydrogenations were characterized further. ETD and ATPD reactions were enhanced by bovine serum albumin (BSA) and were inhibited by N,N'-dicyclohexylcarbodiimide (DCCD), carbonyl cyanide 3-chlorophenylhydrazone (CCCP), carbonyl cyanide 4-(trifluoromethoxy) phenylhydrazone (FCCP), and niclosamide. The nonenergy-linked reaction was unaffected by these additives. Except for DCCD inhibition of the ATPD reaction, BSA mitigated inhibitor effects on energy-linked activities. BSA enhanced NADH oxidase (but not ATPase) activity. Although DCCD inhibited NADH oxidase and ATPase, BSA only lessened oxidase inhibition. With protonophores, an increase in NADH oxidase (but not ATPase) activity was suggested. Oxidase inhibition by rotenone was unaffected by BSA. The ATP-hydrolyzed/NADPH-formed for the ATPD reaction was almost unity. A model for H. diminuta energy-linked transhydrogenation is presented.

Immunology and biochemistry of Hymenolepis diminuta

This review is an account of modern research into the immunology and biochemistry of the rat tapeworm, Hymenolepis diminuta. The first half of the review is devoted to the immunological responses of the host to the parasite. It describes the specific responses that occur when the host is exposed to a primary infection, and the changes that occur when further infections are superimposed on the primary one. The aquisition of immunity to the tapeworm and its persistence in the absence of the infection are also discussed, as well as the non-specific responses of the host to the parasite. The second half of the review is concerned with biochemistry, summarizing the early biochemical work that has been carried out on the tapeworm and describing the metabolic pathways now thought to be characteristic of the parasite. What little information that exists on intermediary metabolism in eggs and larvae is summarized here. Much of this section is concerned with the role of mitochondria in H. diminuta, especially the control of the critical branchpoint (PK/PEPCK), which partitions carbon into either the cytosol or the mitochondrion. The role of 5-hydroxytryptamine in controlling both worm behaviour and metabolism is discussed, followed by a brief look at some other effectors that may prove in the future to have great significance in regulating the parasite. Finally, there is a detailed consideration of strain variation within H. diminuta and of the impact on the tapeworm of components of the immune system, formerly described as the 'crowding effect'. The review concludes with a brief discussion of evolutionary aspects of the rat-tapeworm relationship and a comprehensive bibliography.

Purification and characterization of a malic enzyme from the ruminal bacterium Streptococcus bovis ATCC 15352 and cloning and sequencing of its gene

Malic enzyme (EC 1.1.1.39), which catalyzes L-malate oxidative decarboxylation and pyruvate reductive carboxylation, was purified to homogeneity from Streptococcus bovis ATCC 15352, and properties of this enzyme were determined. The 2.9-kb fragment containing the malic enzyme gene was cloned, and the sequence was determined and analyzed. The enzymatic properties of the S. bovis malic enzyme were almost identical to those of other malic enzymes previously reported. However, we found that the S. bovis malic enzyme catalyzed unknown enzymatic reactions, including reduction of 2-oxoisovalerate, reduction of 2-oxoisocaproate, oxidation of D-2-hydroxyisovalerate, and oxidation of D-2-hydroxyisocaproate. The requirement for cations and the optimum pH of these unique activities were different from the requirement for cations and the optimum pH of the L-malate oxidative decarboxylating activity. A sequence analysis of the cloned fragment revealed the presence of two open reading frames that were 1,299 and 1,170 nucleotides long. The 389-amino-acid polypeptide deduced from the 1,170-nucleotide open reading frame was identified as the malic enzyme; this enzyme exhibited high levels of similarity to malic enzymes of Bacillus stearothermophilus and Haemophilus influenzae and was also similar to other malic enzymes and the malolactic enzyme of Lactococcus lactis.

Oxalacetate decarboxylase and pyruvate carboxylase activities, and effect of sulfhydryl reagents in malic enzyme from Sulfolobus solfataricus

Malic enzyme (S)-malate: NADP+ oxidoreductase (oxaloacetate-decarboxylating, EC 1.1.1.40) purified from the thermoacidophilic archaebacterium Sulfolobus solfataricus, strain MT-4, catalyzed the metal-dependent decarboxylation of oxaloacetate at optimum pH 7.6 at a rate comparable to the decarboxylation of L-malate. The oxaloacetate decarboxylase activity was stimulated about 50% by NADP but only in the presence of MgCl2, and was strongly inhibited by L-malate and NADPH which abolished the NADP activation. In the presence of MnCl2 and in the absence of NADP, the Michaelis constant and Vm for oxaloacetate were 1.7 mM and 2.3 mumol.min-1.mg-1, respectively. When MgCl2 replaced MnCl2, the kinetic parameters for oxaloacetate remained substantially unvaried, whereas the Km and Vm values for L-malate have been found to vary depending on the metal ion. The enzyme carried out the reverse reaction (malate synthesis) at about 70% of the forward reaction, at pH 7.2 and in the presence of relatively high concentrations of bicarbonate and pyruvate. Sulfhydryl residues (three cysteine residues per subunit) have been shown to be essential for the enzymatic activity of the Sulfolobus solfataricus malic enzyme. 5,5'-Dithiobis(2-nitrobenzoic acid), p-hydroxymercuribenzoate and N-ethylmaleimide caused the inactivation of the oxidative decarboxylase activity, but at different rates. The inactivation of the overall activity by p-hydroxymercuribenzoate was partially prevented by NADP singly or in combination with both L-malate and MnCl2, and strongly enhanced by the carboxylic acid substrates; NADP + malate + MnCl2 afforded total protection. The inactivation of the oxaloacetate decarboxylase activity by p-hydroxymercuribenzoate treatment was found to occur at a slower rate than that of the oxidative decarboxylase activity.

Rhodoquinone requirement of the Hymenolepis diminuta mitochondrial electron transport system

The occurrence of rhodoquinone as a mitochondrial membrane component was demonstrated in adult Hymenolepis diminuta. Chromatographic separation of pentane extracts, from lyophilized mitochondrial membranes, coupled with spectral analyses of separated material demonstrated the presence of rhodoquinone. The presence of ubiquinone was not apparent. Rhodoquinone content of membranes was about 1.2 micrograms (mg protein)-1. The rhodoquinone requirement of the H. diminuta electron transport system was demonstrated both in terms of the less active NADH oxidase and the physiologically required, NADH-dependent fumarate reductase employing lyophilized mitochondrial membranes as the source of activities. Pentane extraction of membranes virtually abolished the oxidase and fumarate reductase systems. Supplementation of pentane-treated membranes with H. diminuta rhodoquinone restored oxidase and fumarate reductase activities to levels simulating those of lyophilized membranes. Ubiquinone did not substitute for rhodoquinone. The rhodoquinone-reconstituted membranes displayed rotenone sensitivity. These findings represent the first direct demonstration of the rhodoquinone requirement of helminth electron transport-coupled oxidase and fumarate reductase.

Localization of cytochrome C oxidase and cytochrome C peroxidase in mitochondria of Hymenolepis diminuta (Cestoda)

The intramitochondrial localization of cytochrome c oxidase and cytochrome c peroxidase in adult Hymenolepis diminuta was investigated. Mitochondria were fractionated into inner membrane, outer membrane, intermembrane space and matrix and the efficacy of fractionation was monitored employing marker enzymes. Cytochrome c oxidase was associated with the mitochondrial inner membrane. Whereas 55% of the cytochrome c peroxidase activity was in the matrix, 32% of the activity was in the intermembrane space fraction. Based upon the distribution of marker enzymes, a dual compartmentalization of cytochrome c peroxidase is apparent in H. diminuta mitochondria.

Reduction and oxidation of cytochrome C by Hymenolepis diminuta (Cestoda) mitochondria

Mitochondrial membranes of adult Hymenolepis diminuta catalyzed inhibitor-sensitive ferricytochrome c reduction. Cytochrome c reductase activity was noted when NAD(P)H or succinate served as the reductant with the NADH-coupled reaction being most prominent. Both rotenone-sensitive and -insensitive reduced pyridine nucleotide-coupled activities were apparent. Ferrocytochrome c oxidase activity also was catalyzed by H. diminuta mitochondrial membranes and this reaction was sensitive to azide and cyanide. A cytochrome c peroxidase activity was associated primarily with the mitochondrial soluble fraction of adult H. diminuta. The possibility that the activities observed may contribute to the elimination of peroxide in the helminth system is considered.

Coupling of "malic" enzyme and NADPH:NAD transhydrogenase in the energetics of Hymenolepis diminuta (Cestoda)

Acetylpyridine NADP replaced NADP in promoting the Mn2+ ion-requiring mitochondrial "malic" enzyme of Hymenolepis diminuta. Disrupted mitochondria displayed low levels of an apparent oxaloacetate-forming malate dehydrogenase activity when NAD or acetylpyridine NAD served as the coenzyme. Significant malate-dependent reduction of acetylpyridine NAD by H. diminuta mitochondria required Mn2+ ion and NADP, thereby indicating the tandem operation of "malic" enzyme and NADPH:NAD transhydrogenase. Incubation of mitochondrial preparations with oxaloacetate resulted in a non-enzymatic decarboxylation reaction. Coupling of malate oxidation with electron transport via the "malic" enzyme and transhydrogenase was demonstrated by polarographic assessment of mitochondrial reduced pyridine nucleotide oxidase activity.

The anaerobic metabolism of malate of Saccharomyces bailii and the partial purification and characterization of malic enzyme

1. The main pathway of the anaerobic metabolism of L-malate in Saccharomyces bailii is catalyzed by a L-malic enzyme. 2. The enzyme was purified more than 300-fold. During the purification procedure fumarase and pyruvate decarboxylase were removed completely, and malate dehydrogenase and oxalacetate decarboxylase were removed to a very large extent. 3. Manganese ions are not required for the reaction of malic enzyme of Saccharomyces bailii, but the activity of the enzyme is increased by manganese. 4. The reaction of L-malic enzyme proceeds with the coenzymes NAD and (to a lesser extent) NADP. 5. The Km-values of the malic enzyme of Saccharomyces bailii were 10 mM for L-malate and 0.1 mM for NAD. 6. A model based on the activity and substrate affinity of malic enzyme, the intracellular concentration of malate and phosphate, and its action on fumarase, is proposed to explain the complete anaerobic degradation of malate in Saccharomyces bailii as compared with the partial decomposition of malate in Saccharomyces cerevisiae.

Oxidative decarboxylation reactions in Dirofilaria immitis glucose metabolism

1. Malic enzyme (EC 1.1.1.39) activity was demonstrated in the mitochondrial fraction of dog heartworm extracts. 2. Data published earlier together with those presented here confirm the presence of all the enzymes of the pentose shunt. 3. 14C from uniformly labelled glucose was incorporated into the nucleic acids, and methylene blue increased production of xylose-5-phosphate and of 14CO2 from [1-14C]glucose indicating a functioning pathway. 4. The oxidative decarboxylation of malate and of 6-phosphogluconate may account for the observed labelled CO2 formed when labelled glucose is metabolized since a functional tricarboxylic acid cycle is doubtful in this parasitic helminth.

Mitochondrial NADH oxidase activity of adult Hymenolepis diminuta (Cestoda)

1. Mitochondria from adult Hymenolepis diminuta displayed a membrane-associated, rotenone-sensitive NADH oxidase and NADH-dependent fumarate reductase system. 2. Both the H. diminuta oxidase and fumarate reductase were relatively insensitive to antimycin A. potassium cyanide and sodium azide, at concentrations which significantly inhibited the NADH oxidase system of rat liver. 3. Malonate effectively depressed the mitochondrial NADH oxidase activity of both H. diminuta and adult Ascaris suum (Nematoda). 4. An involvement of Mn2+ ion, in NADH utilization by the H. diminuta oxidase, was apparent. 5. The utilization of NAD(P)H by H. diminuta, mitochondrial membranes resulted in hydrogen peroxide formation. Succinate utilization also resulted in peroxide accumulation but at a much slower rate than that found for NAD(P)H.

Inhibition of NADP-linked malic enzyme from Onchocerca volvulus and Dirofilaria immitis by suramin

NADP-linked malic enzyme (malate dehydrogenase (oxaloacetate-decarboxylating) NADP+, EC 1.1.1.40) has been partially purified from adult Onchocerca volvulus and Dirofilaria immitis. Suramin was found to inhibit the activity of malic enzyme from both filarial worms. The inhibition constants for suramin were calculated to be 0.011 microM and 0.015 microM for the enzymes from O. volvulus and D. immitis, respectively. In the case of NADP-linked malic enzyme from Trypanosoma brucei and chicken liver the inhibition by suramin was less pronounced. The inhibition constants were found to be 0.8 microM and 2.5 microM for the protozoan and vertebrate enzymes, respectively. The type of inhibition was competitive with respect to malate. The Michaelis constants for malate and pyruvate were determined to be 0.9 and 4.5 mM for O. volvulus and 0.85 and 5.0 mM for D. immitis, respectively. The low Km values for malate compared to those for pyruvate and the about 15-fold greater turnover in the direction of decarboxylation compared to carboxylation indicated that malic enzyme from both filarial sources might be involved in an alternative pathway leading from phosphoenolpyruvate via oxaleacetate, malate and pyruvate to lactate. It is suggested, that the inhibition of malic enzyme activity from O. volvulus by suramin might interfere with the generation of NADPH for biosynthetic reactions.