Studies on enzymes from parasitic helminths. I. Purification and physical properties of malic enzyme from the muscle tissue of Ascaris suum

Molecular Characteristics of the Malate Dehydrogenase (MDH) Gene Family in Spirometra mansoni (Cestoda: Diphyllobothriidea)

The plerocercoid larva of Spirometra mansoni can cause a parasitic zoonosis-sparganosis. Malate dehydrogenase (MDH) plays a very important role in the life activities of parasites. However, little is known about the MDH family in S. mansoni. We identified eight new MDH members in S. mansoni in this study. Clustering analysis divided SmMDHs into two groups and revealed patterns similar to the conserved motif organization. RT-qPCR suggested that five MDHs were highly expressed in the mature proglottid and that three MDHs were highly expressed in the gravid proglottid. Phylogenetic analysis revealed that SmMDHs contain both conserved family members and members in the process of further diversification. rSmMDH has an NAD binding domain, a dimer interface and a substrate binding domain. Natural SmMDH was immunolocalized in the tissues and follicles around the uterus in the mature or gravid proglottid and eggshells. The maximum forward and reverse reaction activities of rSmMDH were observed at pH 8.5 and 9.0, respectively. The optimum temperature for enzyme activity was 37 °C in the forward reaction and 40 °C in the reverse reaction. These results lay the foundation for studying the molecular functions and mechanisms of MDHs in S. mansoni and related taxa.

Identification and characterization of cytosolic malate dehydrogenase from the liver fluke Fasciola gigantica

The liver fluke zoonoses, Fasciola spp. are parasitic helminths infecting humans and animals globally. Recent sequencing of the genome of Fasciola gigantica has provided a basis to understand the biochemistry of this parasite. Here, we identified the cytosolic malate dehydrogenase in F. gigantica (FgMDH) and characterized the enzyme biochemically and structurally. F. gigantica encodes a single cytosolic MDH, a key enzyme of the citric acid cycle. It catalyzes the reversible oxidation of malate to oxaloacetate using NAD⁺. The Fgmdh gene was amplified and cloned for expression of the recombinant protein. The purified protein showed a molecular weight of ~ 36 kDa that existed in a dimeric form in solution. The recombinant enzyme was catalytically active as it catalyzed both forward and reverse reactions efficiently. The kinetic parameters were determined for both directions. The structure of FgMDH and human MDH were modeled and validated. The superimposition of both the model structures showed overall structural similarity in the active site loop region, however, the conformation of the residues was different. Molecular docking elucidated the binding sites and affinities of the substrates and cofactors to the enzyme. Simulation of molecular dynamics and principal component analysis indicated the stability of the systems and collective motions, respectively. Understanding the structural and functional properties of MDH is important to better understand the roles of this enzyme in the biochemistry of the parasite.

Cloning, sequencing and functional expression of cytosolic malate dehydrogenase from Taenia solium: Purification and characterization of the recombinant enzyme

We report herein the complete coding sequence of a Taenia solium cytosolic malate dehydrogenase (TscMDH). The cDNA fragment, identified from the T. solium genome project database, encodes a protein of 332 amino acid residues with an estimated molecular weight of 36517Da. For recombinant expression, the full length coding sequence was cloned into pET23a. After successful expression and enzyme purification, isoelectrofocusing gel electrophoresis allowed to confirm the calculated pI value at 8.1, as deduced from the amino acid sequence. The recombinant protein (r-TscMDH) showed MDH activity of 409U/mg in the reduction of oxaloacetate, with neither lactate dehydrogenase activity nor NADPH selectivity. Optimum pH for enzyme activity was 7.6 for oxaloacetate reduction and 9.6 for malate oxidation. K(cat) values for oxaloacetate, malate, NAD, and NADH were 665, 47, 385, and 962s(-1), respectively. Additionally, a partial characterization of TsMDH gene structure after analysis of a 1.56Kb genomic contig assembly is also reported.

Energy metabolism in the developing larval stages of Ancylostoma tubaeforme and Haemonchus contortus: glycolytic and tncarboxylic acid cycle enzymes

The activities of glycolytic and related enzymes and the tricarboxylic acid cycle enzymes were measured in freshly isolated 1st- (Li), 2nd- (L2) and 3rd-stage (L3) larvae of both Ancylostoma tubaeforme and Haemonchus contortus. All enzymes of the glycolytic pathway were present in all developmental stages of both strongylid nematodes although higher levels of activities were obtained in the pre-infective 1st- and 2nd-stage larvae than in the infective 3rd stage. However, the pre-infective larvae contained lower levels of pyruvate kinase (PK) than the infective larvae. Consequently, the pyruvate kinase to phosphoenolpyruvate carboxykinase (PEPCK) ratios were 0·23 and 0·26 for the L1s and L2s for A. tubaeforme and 0·36 and 0·21 for those of H. contortus respectively. High levels of activity of glucose-6-phosphate dehydrogenase obtained in the bacteriophagous pre-infective larvae were consistent with high rates of morphogenesis and substrate synthesis characteristic of the pre-infective stages. All the tricarboxylic acid cycle enzymes were present in the infective larvae of both nematodes while in the pre-infective Li and L2 stages, the enzymes at the beginning of the cycle, namely aconitate hydratase and NAD-linked isocitrate dehydrogenase, were not detected. A scheme was proposed for the energy metabolism of these developing larvae. In this scheme, the pre-infective larvae were shown to operate an anaerobic metabolic pathway involving the carboxylation of phosphoenolpyruvate (PEP) by phosphoenolpyru vate carboxykinase (PEPCK) to form oxaloacetate (OAA), whereas in the infective larvae the metabolic pathway favouring the direct dephosphorylation of PEP, as in vertebrate tissues, was followed.

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.

Purification and partial characterization of malate dehydrogenase (decarboxylating) from Tritrichomonas foetus hydrogenosomes

Malate dehydrogenase (decarboxylating) from Tritrichomonas foetus hydrogenosomes was purified close to homogeneity by a combination of differential centrifugation, zwitterionic detergent solubilization, Red-Sepharose chromatography and anion-exchange chromatography. The enzyme with apparent subunit size of 59 kDa and native molecular mass of 308 kDa utilized NAD+ preferentially to NADP+ as a cofactor and required Mn2+ or Mg2+ for its activity. Affinity curves for malate and coenzymes were hyperbolic. Km for malate was 100 microM and 458 microM in the presence of NAD+ and NADP+, respectively. Km for NAD+ and for NADP+ in the presence of malate was 18 microM and 207 microM, respectively. The enzyme is proposed to be a tetramer with a possible physiological role in the maintenance of an appropriate NAD+/NADH ratio in hydrogenosomes.

Mechanism of activation of the NAD-malic enzyme from Ascaris suum by fumarate

The mechanism of activation of the NAD-malic enzyme from Ascaris suum by fumarate has been probed using initial velocity studies, deuterium isotope effects, and isotope partitioning of the E:Mg:malate complex. Fumarate exerts its activating effect by decreasing the off-rate for malate from the E:Mg:malate and E:NAD:Mg:malate complexes. Fumarate is a positive heterotropic effector of the NAD-malic enzyme at low concentrations (K act approximately 0.05 mM) and an inhibitor competitive against malate (Ki approximately 25 mM). The activation by fumarate results in a decrease in the Ki malate and an increase in V/K malate of about 2-fold, while the maximum velocity remains constant. Isotope partitioning studies of E:Mg:[14C]malate indicate that the presence of fumarate results in a decrease in the malate off-rate constant by about 2.2-fold. The deuterium isotope effects on V and V/K malate are both 1.6 +/- 0.1 in the absence of fumarate, while in the presence of 0.5 mM fumarate DV is 1.6 +/- 0.1 and D(V/K malate) is 1.1 +/- 0.1. These data are also consistent with a decrease in the off-rate for malate from E:NAD:Mg:malate, resulting in an increase in the forward commitment factor for malate and manifested as a lower value for D(V/K malate). There is a discrimination between active and activator sites for the binding of dicarboxylic acids, with the activator site preferring the extended configuration of 4-carbon dicarboxylic acids, while the active site prefers a configuration in which the 4-carboxyl is twisted out of the C1-C3 plane. The physiologic importance and regulatory properties of fumarate in the parasite are also discussed.

Purification of tumor mitochondrial malic enzyme by specific ligand affinity chromatography

A two-step chromatographic procedure, based on a specific ligand-binding approach, for the purification of tumor NAD(P)(+)-dependent malic enzyme is described. The enzyme was purified to near homogeneity by extraction from mitochondria, negative cellulose phosphate chromatography, ammonium sulfate precipitation, and application of specific elution from a malate-agarose column. The rationale for the use of the affinity column is also described.

Modification of an arginine residue essential for the activity of NAD-malic enzyme from Ascaris suum

Purified NAD-malic enzyme from Ascaris suum is rapidly inactivated by the arginine reagent, 2,3-butanedione, and this inactivation is facilitated by 30 mM borate. Determination of the inactivation rate as a function of butanedione concentration suggests a second-order process overall, which is first order in butanedione. A second-order rate constant of 0.6 M-1 s-1 at pH 9 is obtained for the butanedione reaction. The inactivation is reversed by removal of the excess reagent upon dialysis. The enzyme is protected against inactivation by saturating amounts of malate in the presence and absence of borate. The divalent metal Mg2+ affords protection in the presence of borate but has no effect in its absence. The nucleotide reactant NAD+ has no effect on the inactivation rate in either the presence or absence of borate. A dissociation constant of 24 mM is obtained for E:malate from the decrease in the inactivation rate as a function of malate concentration. An apparent Ki of 0.5 mM is obtained for oxalate (an inhibitor competitive vs malate) from E:Mg:oxalate while no significant binding is observed for oxalate using the butanedione modified enzyme. The pH dependence of the first-order rate of inactivation by butanedione gives a pKa of 9.4 +/- 0.1 for the residue(s) modified, and this pK is increased when NAD is bound. The arginine(s) modified is implicated in the binding of malate.

Inhibition of malate dehydrogenase enzymes by benzimidazole anthelmintics

Determinations were made of the inhibitory activities of four benzimidazole anthelmintics (Albendazole, Parbendazole, Mebendazole and Thiabendazole) on purified extracts of cytoplasmic and mitochondrial malate dehydrogenase obtained from Ascaris suum, Fasciola hepatica and Moniezia expansa. The highest percentage inhibitions were exhibited by Mebendazole. The results confirm that cytoplasmic MDH and mitochondrial MDH regulator enzymes of glycogen synthesis are the sites of mebendazole inhibitory activity, but the activity sites of the other anthelmintics in the study remain unclear.

pH dependence of kinetic parameters for oxalacetate decarboxylation and pyruvate reduction reactions catalyzed by malic enzyme

Both chicken liver NADP-malic enzyme and Ascaris suum NAD-malic enzyme catalyze the metal-dependent decarboxylation of oxalacetate. Both enzymes catalyze the reaction either in the presence or in the absence of dinucleotide. The presence of dinucleotide increases the affinity of oxalacetate for the chicken liver NADP-malic enzyme, but this information could not be obtained in the case of A. suum NAD-malic enzyme because of the low affinity of free enzyme for NAD. The kinetic mechanism for oxalacetate decarboxylation by the chicken liver NADP-malic enzyme is equilibrium ordered at pH values below 5.0 with NADP adding to enzyme first. The Ki for NADP increases by a factor of 10 per pH unit below pH 5.0. An enzyme residue is required protonated for oxalacetate decarboxylation (by both enzymes) and pyruvate reduction (by the NAD-malic enzyme), but the beta-carboxyl of oxalacetate must be unprotonated for reaction (by both enzymes). The pK of the enzyme residue of the chicken liver NADP-malic enzyme decreases from a value of 6.4 in the absence of NADP to about 5.5 with Mg2+ and 4.8 with Mn2+ in the presence of NADP. The pK value of the enzyme residue required protonated for either oxalacetate decarboxylation or pyruvate reduction for the A. suum NAD-malic enzyme is about 5.5-6.0. Although oxalacetate binds equally well to protonated and unprotonated forms of the NADP-enzyme, the NAD-enzyme requires that oxalacetate or pyruvate selectively bind to the protonated form of the enzyme. Both enzymes prefer Mn2+ over Mg2+ for oxalacetate decarboxylation.(ABSTRACT TRUNCATED AT 250 WORDS)

Progress in molecular parasitology

Substantial progress has been made in the last ten years in understanding the structural and functional organization of parasitic protozoa and helminths and the complex physiological relationships that exist between these organisms and their hosts. By employing the new powerful techniques of biochemistry, molecular biology and immunology the genomic organization in parasites, the molecular basis of parasite's variation in surface antigens and the biosynthesis, processing, transport and membrane anchoring of these and other surface proteins were extensively investigated. Significant advances have also been made in our knowledge of the specific and often peculiar strategies of intermediary metabolism, cell compartmentation, the role of oxygen for parasites and the mechanisms of antiparasitic drug action. Further major fields of interest are currently the complex processes which enables parasites to evade the host's immune defense system and other mechanisms which have resulted in the specific adaptations which enabled parasites to survive within their host environments. Various approaches in molecular and biochemical parasitology and in immunoparasitology have been proven to be of high potential for serodiagnosis, immunoprophylaxis and drug design.

Diethylpyrocarbonate inactivation of NAD-malic enzyme from Ascaris suum

Treatment with diethylpyrocarbonate results in a first-order loss of the malate oxidative decarboxylase activity of NAD-malic enzyme. First-order plots are biphasic, with about 40-50% activity loss in the first phase. The inactivation process is not saturable, and the second-order rate constant is 4.7 M-1 S-1. Malate (250 mM) provides complete protection against inactivation (as measured by a decrease in the inactivation rate), and less malate is required with Mg2+ present. Partial protection (50%) is afforded by either NAD+ (1 mM) or Mg2+ (50 mM). Treatment of modified (inactive) enzyme with hydroxylamine restores activity to 100% of the control when corrected for the effect of hydroxylamine on unmodified enzyme. A total of 10-13 histidine residues/subunit are acylated concomitant with loss of activity while 1-2 tyrosines are modified prior to any activity loss. The presence of Mg2+ and malate at saturating concentrations protect 1-2 histidine residues/subunit. The intrinsic fluorescence of the enzyme decreases with time after addition of diethylpyrocarbonate, but the rate constant for this process is at least 10-fold too low to account for the biphasicity observed in the first order plots. The histidine modified which is responsible for loss of activity has a pK of 8.3 as determined from the pH dependence of the rate of inactivation. The histidine titrated is still modified under conditions where the residue is completely protonated but at a rate 1/100 the rate of the unprotonated histidine. The results suggest that 1-2 histidines are in or near the malate binding site and are required for malate oxidative decarboxylation.

The purification and steady-state kinetic behaviour of rabbit heart mitochondrial NAD(P)+ malic enzyme

The mitochondrial NAD(P)+ malic enzyme [EC 1.1.1.39, L-malate:NAD+ oxidoreductase (decarboxylating)] was purified from rabbit heart to a specific activity of 7 units (mumol/min)/mg at 23 degrees C. A study of the reductive carboxylation reaction indicates that this enzymic reaction is reversible. The rate of the reductive carboxylation reaction appears to be completely inhibited at an NADH concentration of 0.92 mM. A substrate saturation curve of this reaction with NADH as the varied substrate describes this inhibition. The apparent kinetic parameters for this reaction are Ka(NADH) = 239 microM and Vr = 1.1 mumol/min per mg at 23 degrees C. The steady-state product-inhibition patterns for pyruvate and NADH indicate a sequential binding of the substrates: NAD+ followed by L-malate. These data also indicate that NADH is the last product released. A steady-state kinetic model is proposed that incorporates NADH-enzyme dead-end complexes.

Determination of dissociation constants for enzyme-reactant complexes for NAD-malic enzyme by modulation of the thiol inactivation rate

Incubation of NAD-malic enzyme from Ascaris suum with the sulfhydryl reagents N-ethylmaleimide (NEM), 5,5'-dithiobis(2-nitrobenzoic acid) (DTNB), or 4,4'-dithiodipyridine (4-PDS) results in rapid and complete loss of malate oxidative decarboxylase and pyruvate reductive carboxylase activities. With DTNB, this loss of activity occurs concomitantly with the modification of about 1 thiol group per subunit. The majority of the activity is lost when 0.5 thiol per subunit is modified, indicative of possible half-site reactivity with DTNB. Complete restoration of activity follows addition of dithiothreitol to enzyme inactivated by DTNB and 4-PDS but not with NEM. With the DTNB-inactivated enzyme, replacement of the thionitrobenzoate moiety with cyanide restores activity. The presence of a divalent metal ion (Mg2+ or Mn2+) results in enhancement of the inactivation rate with all sulfhydryl reagents. However, malate alone or competitors of malate provide protection which is more effective in the presence of Mg2+, while NAD provides only about 25% protection. Thus, the Ascaris suum NAD-malic enzyme has a thiol group probably located in or near the malate binding site, which is not essential for enzyme activity. The changes in the rate of inactivation in the presence of reactants were used to determine the dissociation constants for enzyme-reactant complexes. These data suggest that all three possible binary and all three possible ternary complexes form. The binding of malate to free enzyme exhibits negative cooperativity, which is eliminated by the presence of either NAD or Mg2+.(ABSTRACT TRUNCATED AT 250 WORDS)

Kinetic properties of NAD malic enzyme from cauliflower

The kinetic characteristics of NAD malic enzyme purified to homogeneity from cauliflower florets have been examined. Free NAD+ is the active form of this coenzyme. Double-reciprocal plots of data obtained by varying NAD+ and malate2- at a saturating concentration of Mg2+ or by varying Mg2+ and NAD+ at a saturating level of malate2-are of intersecting type. This indicates that NAD malic enzyme obeys a sequential mechanism. Analysis of these sets of data suggests that each of these substrate pairs binds randomly to the enzyme. However, each substrate binds tighter when others are already present on the enzyme. NAD malic enzyme cannot decarboxylate malate2- in the absence of either Mg2+ or NAD+. Arrhenius plots of the NAD-linked reaction are concave downward, indicating the existence of two rate-determining steps with activation energies of 26.5 and 14.2 kcal/mol, respectively. In addition to Mg2+, the enzyme can also use Mn2+ and Co2+. Using Co2+ in place of Mg2+ does not change Vmax or Km, malate2- but the Km for metal and NAD+ are greatly decreased. At pH 7.0 and above, Mn2+ isotherms and malate2- curves with Mn2+ are nonlinear and appear to be composed of two separate saturation curves. NAD malic enzyme is completely and irreversibly inactivated by N-ethylmaleimide. The enzyme is also irreversibly inactivated approximately 50% by KCNO.

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.

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.

Purification of malic enzyme from Ascaris suum using NAD+-agarose

Malic enzyme has been purified from Ascaris suum by polyethylene glycol precipitation, ion-exchange chromatography, ammonium sulfate precipitation, and NAD-agarose affinity chromatography to a specific activity of 80 units/mg (V/[E]t = 350 s-1). The preparation was shown to be homogeneous by SDS polyacrylamide disc gel electrophoresis. The procedure can be accomplished in a maximum of four days with a 74% yield.

Respiration and carbohydrate energy metabolism of the lung-dwelling parasite Rhabdias bufonis (Nematoda: Rhabdiasoidea)

An investigation of the carbohydrate energy metabolism of Rhabdias bufonis, the lung-dwelling nematode parasite of the African toad, Bufo regularis, indicates that the nematode stores very little glycogen (0.137 +/- 0.003% on a fresh weight basis) but does utilize oxygen in vitro. The intracellular distribution and high levels of activity observed for the enzymes phosphoenolpyruvate carboxykinase, pyruvate kinase, lactate dehydrogenase, malate dehydrogenase, malic enzyme and fumarate reductase suggest two alternative pathways of carbohydrate energy metabolism.

Ascaris suum hexokinase: purification and possible function in compartmentation of glucose 6-phosphate in muscle

Hexokinase (EC 2.7.1.1) is present in a soluble and a bound form in homogenates of Ascaris suum muscle. Cellulose acetate electrophoresis, isoelectric focusing, and ion exchange chromatography confirmed the presence of only one molecular form of hexokinase in this muscle. A procedure for purifying hexokinase from Ascaris muscle has been developed utilizing ion-exchange chromatography, ammonium sulfate fractionation and gel filtration. The enzyme is a monomer with a molecular weight of 100 000 as determined by sodium dodecyl sulfate gel filtration. The Stokes' radius, diffusion coefficient, and frictional ratio have been determined. The apparent Michaelis constants for glucose and ATP are 4.7-10(-3) M and 2.2-10(-4) M, respectively. Ascaris hexokinase also exhibits end-product inhibition by glucose 6-phosphate and ADP. It is postulated that the kinetic parameters of the enzyme are the results of its function, that of generating glucose 6-phosphate primarily for glycogen synthesis.

The nature and control of the tricarboxylate cycle in beetle flight muscle

The only exogenous substrates oxidized by mitochondria isolated from the flight muscle of the Japanese beetle (Popillia japonica) are proline, pyruvate and glycerol 3-phosphate. The highest rate of oxygen consumption is obtained with proline. The oxidation of proline leads to the production of more NH3 than alanine, indicating a functioning glutamate dehydrogenase (EC 1.4.1.2). Studies of mitochondrial extracts confirm the presence of a very active glutamate dehydrogenase, and this enzyme is found to be activated by ADP and inhibited by ATP. These extracts also show high alanine aminotransferase activity (EC 2.6.1.2) and a uniquely active "malic' enzyme (EC 1.1.1.39). The "malic' enzyme is activated by succinate and inhibited by ATP and by pyruvate. It is suggested that the input of tricarboxylate-cycle intermediate from proline oxidation is balanced by the formation of pyruvate from malate, and the complete oxidation of the majority of the pyruvate. Studies of the steady-state concentrations of mitochondrial CoASH and CoA thioesters during proline oxidation show a high succinyl (3-carboxypropionyl)-CoA content which falls on activating respiration with ADP. There is a concomitant rise in CoASH. However, the reverse transition, from state-3 to state-4 respiration, causes only very slight changes in acylation. The reasons for this are discussed. Studies of the mitochondrial content of glutamate, 2-oxoglutarate, malate, pyruvate, citrate and isocitrate during the same phases of proline oxidation give results consistent with control at the level of glutamate dehydrogenase and isocitrate dehydrogenase during proline oxidation, with the possibility of further control at "malic' enzyme. During the oxidation of pyruvate all of the tricarboxylate-cycle intermediates and NAD(P)H follow the pattern of changes described in the blowfly (Johnson & Hansford, 1975; Hansford, 1974) and isocitrate dehydrogenase is identified as the primary site of control.?2OAuthor