Biochemical characterization of allelic forms of soluble malate dehydrogenase of Drosophila melanogaster

Genetic perturbation of key central metabolic genes extends lifespan in Drosophila and affects response to dietary restriction

There is a connection between nutrient inputs, energy-sensing pathways, lifespan variation and aging. Despite the role of metabolic enzymes in energy homeostasis and their metabolites as nutrient signals, little is known about how their gene expression impacts lifespan. In this report, we use P-element mutagenesis in Drosophila to study the effect on lifespan of reductions in expression of seven central metabolic enzymes, and contrast the effects on normal diet and dietary restriction. The major observation is that for five of seven genes, the reduction of gene expression extends lifespan on one or both diets. Two genes are involved in redox balance, and we observe that lower activity genotypes significantly extend lifespan. The hexokinases also show extension of lifespan with reduced gene activity. Since both affect the ATP/ADP ratio, this connects with the role of AMP-activated protein kinase as an energy sensor in regulating lifespan and mediating caloric restriction. These genes possess significant expression variation in natural populations, and our experimental genotypes span this level of natural activity variation. Our studies link the readout of energy state with the perturbation of the genes of central metabolism and demonstrate their effect on lifespan.

Direct evidence that genetic variation in glycerol-3-phosphate and malate dehydrogenase genes (Gpdh and Mdh1) affects adult ethanol tolerance in Drosophila melanogaster

Many studies of alcohol adaptation in Drosophila melanogaster have focused on the Adh polymorphism, yet the metabolic elimination of alcohol should involve many enzymes and pathways. Here we evaluate the effects of glycerol-3-phosphate dehydrogenase (Gpdh) and cytosolic malate dehydrogenase (Mdh1) genotype activity on adult tolerance to ethanol. We have created a set of P-element-excision-derived Gpdh, Mdh1, and Adh alleles that generate a range of activity phenotypes from full to zero activity. Comparisons of paired Gpdh genotypes possessing 10 and 60% normal activity and 66 and 100% normal activity show significant effects where higher activity increases tolerance. Mdh1 null allele homozygotes show reductions in tolerance. We use piggyBac FLP-FRT site-specific recombination to create deletions and duplications of Gpdh. Duplications show an increase of 50% in activity and an increase of adult tolerance to ethanol exposure. These studies show that the molecular polymorphism associated with GPDH activity could be maintained in natural populations by selection related to adaptation to alcohols. Finally, we examine the interactions between activity genotypes for Gpdh, Mdh1, and Adh. We find no significant interlocus interactions. Observations on Mdh1 in both Gpdh and Adh backgrounds demonstrate significant increases in ethanol tolerance with partial reductions (50%) in cytosolic MDH activity. This observation strongly suggests the operation of pyruvate-malate and, in particular, pyruvate-citrate cycling in adaptation to alcohol exposure. We propose that an understanding of the evolution of tolerance to alcohols will require a system-level approach, rather than a focus on single enzymes.

Effects of temperature on the mitochondrial malate dehydrogenase of adult muscle of Toxocara canis

Purified mitochondrial malate dehydrogenase isoenzyme (m-MDH) of Toxocara canis muscle presented maximum activity at 48 degrees C. A clear change in slope of the Arrhenius plot was observed. The energy of activation calculated for the catalytic process showed values of 3.2 kcal/mol and 10.5 kcal/mol. Thermal inactivation of m-MDH showed that it is more thermolabile than the s-isoenzyme. The inactivation of the enzyme by heat could be reduced at least in part by the addition of 0.1 mM NADH. The heat denaturation showed to be a first-order process. The rate constant (k) was calculated as being of the order of 5.28 X 10(-4) s-1 at 40 degrees C. The activation energy for the heat inactivation process was 16.45 kcal/mol between 30 degrees C and 40 degrees C and 13.79 kcal/mol between 40 degrees C and 48 degrees C.

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.

Biochemical differences between cytoplasmic malate dehydrogenase allozymes of Drosophila virilis

Two allozymes (MDHf and MDHs) of cytoplasmic malate dehydrogenase of Drosophila virilis were partially purified and their biochemical properties were compared. MDHf has a pH optimum of 9.75 and MDHs one of 9.25 for malate oxidation. Optimal pH for oxaloacetate reduction is 6.75 and 8.0 for MDHf and MDHs, respectively. The Km value for oxaloacetate of MDHs is approximately twice as that of MDHf. No differences were found with respect to thermostability and Km's for malate, NAD+, or NADH. These results are discussed in terms of the physiological role of cytoplasmic malate dehydrogenase of D. virilis.

Biochemical studies of supernatant malate dehydrogenase allozymes in Drosophila melanogaster

1. A biochemical comparison was made among cytoplasmic malate dehydrogenase allozymic variants from Drosophila melanogaster. Experiments were carried out on enzyme extracted from six different genotypes: three homozygotes and their respective heterozygotes. 2. The allozyme forms (MDH A, MDH B, MDH C) were indistinguishable in terms of NAD and L-malate optima, while they are distinguishable in terms of NADH and OAA saturation conditions. Activities were inhibited at concentrations greater than 0.36 and 0.40 mM NADH for BB and AA, CC, respectively, while in relation to OAA inhibition was observed at concentrations higher than 3 or 6 mM for the AA, CC and BB, respectively. 3. differences among genotypes were also observed in thermal stability: Km values for OAA, L-malate, NADH and NAD: and pG optima. 4. A simple method is presented for the separation of the cytoplasmic from the mitochondrial malate dehydrogenase.