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Transient kinetics of nicotinamide-adenine dinucleotide phosphate-linked isocitrate dehydrogenase from bovine heart mitochondria.

ABSTRACT: Pre-steady-state studies of the isocitrate dehydrogenase reaction show that the rate constant for the hydride-transfer step is above 990s-1, and that both subunits of the enzyme are simulataneously active. After the fast formation of NADPH in amounts equivalent to the enzyme subunit concentration, the rate of NADPH formation is equal to the steady-state rate if the enzyme has been preincubated with isocitrate and Mg2+. If the enzyme has been preincubated with NADP+ and Mg2+, in 0.05 M-triethanolamine chloride buffer, pH 7.0, with the addition of 0.1 M-NaCl, the amount of NADPH formed in the fast phase is only 60% of the enzyme subunit concentration, and the turnover rate is at first lower than the steady-state rate. In 0.05 M-triethanolamine chloride buffer, pH 7.0, if the enzyme is preincubated with NADP+ or NADPH, the turnover rate increases 3-fold to reach the steady-state rate after about 5 s. Preincubation of the enzyme with isocitrate and Mg2+ abolishes this lag phase, the steady-state rate being reached at once. It is suggested that the enzyme exists in at least two conformational forms with different activities, and that the lag phase represents the transition (k = 0.4s-1) from a form with low activity to the fully active enzyme, induced by the binding of isocitrate and Mg2+.

SUBMITTER: Dalziel K

PROVIDER: S-EPMC1184022 | biostudies-other | 1978 Jun

REPOSITORIES: biostudies-other

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Equilibrium binding of coenzymes and substrates to nicotinamide-adenine dinucleotide phosphate-linked isocitrate dehydrogenase from bovine heart mitochondria.

Project description:1. The stoicheiometries and affinities of ligand binding to isocitrate dehydrogenase were studied at pH 7.0, mainly by measuring changes in NADPH and protein fluorescence. 2. The affinity of the enzyme for NADPH is about 100-fold greater than it is for NADP+ in various buffer/salt solutions, and the affinities for both coenzymes are decreased by Mg2+, phosphate and increase in ionic strength. 3. The maximum binding capacity of the dimeric enzyme for NADPH, from coenzyme fluorescence and protein-fluorescence measurements, and also for NADP+, by ultrafiltration, is 2 mol/mol of enzyme. Protein-fluorescence titrations of the enzyme with NADP+ are apparently inconsistent with this conclusion, indicating that the increase in protein fluorescence caused by NADP+ binding is not proportional to fractional saturation of the binding sites. 4. Changes in protein fluorescence caused by changes in ionic strength and by the binding of substrates, Mg2+ or NADP+ (but not NADPH) are relatively slow, suggesting conformation changes. 5. In the presence of Mg2+, the enzyme binds isocitrate very strongly, and 2-oxoglutarate rather weakly. 6. Evidence is presented for the formation of an abortive complex of enzyme-Mg2+-isocitrate-NADPH in which isocitrate and NADPH are bound much more weakly than in their complexes with enzyme and Mg2+ alone. 7. The results are discussed in relation to the interpretation of the kinetic properties of the enzyme and its behaviour in the mitochondrion.

| S-EPMC1184021 | biostudies-other

Nicotinamide adenine dinucleotide is transported into mammalian mitochondria.

Project description:Mitochondrial NAD levels influence fuel selection, circadian rhythms, and cell survival under stress. It has alternately been argued that NAD in mammalian mitochondria arises from import of cytosolic nicotinamide (NAM), nicotinamide mononucleotide (NMN), or NAD itself. We provide evidence that murine and human mitochondria take up intact NAD. Isolated mitochondria preparations cannot make NAD from NAM, and while NAD is synthesized from NMN, it does not localize to the mitochondrial matrix or effectively support oxidative phosphorylation. Treating cells with nicotinamide riboside that is isotopically labeled on the nicotinamide and ribose moieties results in the appearance of doubly labeled NAD within mitochondria. Analogous experiments with doubly labeled nicotinic acid riboside (labeling cytosolic NAD without labeling NMN) demonstrate that NAD(H) is the imported species. Our results challenge the long-held view that the mitochondrial inner membrane is impermeable to pyridine nucleotides and suggest the existence of an unrecognized mammalian NAD (or NADH) transporter.

| S-EPMC6013257 | biostudies-literature

The interaction of reduced nicotinamide--adenine dinucleotide phosphate with reduced nicotinamide--adenine dinucleotide--ubiquinone reductase from bovine heart mitochondria.

Project description:Reduction of the chromophores of mitochondrial NADH-ubiquinone reductase by NADPH reaches only 50% of the extent of reduction by NADH, monitored at 450 nm. This effect is due to autoxidation of an enzyme component at a higher rate than its reduction by NADPH.

| S-EPMC1163948 | biostudies-other

carba Nicotinamide Adenine Dinucleotide Phosphate: Robust Cofactor for Redox Biocatalysis.

Project description:Here we report a new robust nicotinamide dinucleotide phosphate cofactor analog (carba-NADP+ ) and its acceptance by many enzymes in the class of oxidoreductases. Replacing one ribose oxygen with a methylene group of the natural NADP+ was found to enhance stability dramatically. Decomposition experiments at moderate and high temperatures with the cofactors showed a drastic increase in half-life time at elevated temperatures since it significantly disfavors hydrolysis of the pyridinium-N-glycoside bond. Overall, more than 27 different oxidoreductases were successfully tested, and a thorough analytical characterization and comparison is given. The cofactor carba-NADP+ opens up the field of redox-biocatalysis under harsh conditions.

| S-EPMC8252718 | biostudies-literature

The isocitrate dehydrogenases of Acinetobacter lwoffi. Separation and properties of two nicotinamide-adenine dinucleotide phosphate-linked isoenzymes.

Project description:Two isoenzymes of NADP-linked isocitrate dehydrogenase have been identified in Acinetobacter lwoffi and have been termed isoenzyme-I and isoenzyme-II. The isoenzymes may be separated by ion-exchange chromatography on DEAE-cellulose, by gel filtration on Sephadex G-200, or by zonal ultracentrifugation in a sucrose gradient. Low concentrations of glyoxylate or pyruvate effect considerable stimulation of the activity of isoenzyme-II. The isoenzymes also differ in pH-dependence of activity, kinetic parameters, stability to heat or urea and molecular size. Whereas isoenzyme-I resembles the NADP-linked isocitrate dehydrogenases from other organisms in having a molecular weight under 100000, isoenzyme-II is a much larger enzyme (molecular weight around 300000) resembling the NAD-linked isocitrate dehydrogenases of higher organisms.

| S-EPMC1174318 | biostudies-other

Nicotinamide-adenine dinucleotide-specific isocitrate dehydrogenase from pea mitochondria. Purification and properties.

Project description:1. A method of stabilizing the enzyme by using glycerol is described. 2. A purification procedure is presented giving a higher purification than previously described. 3. Data showing substrate activation and activation by citrate are presented. 4. Kinetic constants for NAD(+), NADH and certain bivalent metal ions are given. 5. Pronounced inhibitory buffer effects are described. 6. A brief comparison between the NAD-specific isocitrate dehydrogenase from peas and that from other sources is made.

| S-EPMC1198371 | biostudies-other

The isocitrate dehydrogenases of Acinetobacter lwoffi. Studies on the regulation of a nicotinamide-adenine dinucleotide phosphate-linked isoenzyme.

Project description:Of the two NADP-linked isocitrate dehydrogenases in Acinetobacter lwoffi the higher-molecular-weight form, isoenzyme-II, is reversibly stimulated sixfold by low concentrations of glyoxylate or pyruvate. Kinetic results indicate that this stimulation of activity involves both an increase in V(max.) and a decrease in the apparent K(m) values for substrates, most markedly that for NADP(+). Other changes brought about by glyoxylate or pyruvate include a shift in the pH optimum for activity and an increased stability to inactivation by heat or urea. Mixtures of glyoxylate plus oxaloacetate, known to inhibit isocitrate dehydrogenases from other organisms, produce inhibition of both A. lowffi isoenzymes, and do not reflect the stimulatory specificity of glyoxylate for isoenzyme-II. Isoenzyme-II is also stimulated by AMP and ADP, but the activation by glyoxylate or pyruvate is shown to be quite independent of the adenylate activation. Differential desensitization of the enzyme by urea to the two types of activator further supports the view that the enzyme possesses two distinct allosteric regulatory sites. The metabolic significance of the activations is discussed.

| S-EPMC1177580 | biostudies-other

The effect of pH on the characteristics of the binding of nicotinamide-adenine dinucleotide by nicotinamide-adenine dinucleotide-specific isocitrate dehydrogenase from pea mitochondria.

Project description:1. The effect of pH on the V(max.) and concentration of NAD(+) at half-maximum velocity at a constant isocitrate concentration was examined, and the results were related to the requirements for binding of H(+) ions to the enzyme. 2. The effect of varying the NAD(+) concentration on the pH optimum with constant isocitrate concentration was studied. 3. A comparison has been made between the effect of isocitrate concentration on the characteristics of binding of NAD(+) and the effect of NAD(+) concentration on the characteristics of isocitrate binding at three different pH values. 4. The mechanistic and metabolic significance of these studies is considered.

| S-EPMC1185504 | biostudies-other

Nicotinamide adenine dinucleotide as a photocatalyst.

Project description:Nicotinamide adenine dinucleotide (NAD+) is a key redox compound in all living cells responsible for energy transduction, genomic integrity, life-span extension, and neuromodulation. Here, we report a new function of NAD+ as a molecular photocatalyst in addition to the biological roles. Our spectroscopic and electrochemical analyses reveal light absorption and electronic properties of two π-conjugated systems of NAD+. Furthermore, NAD+ exhibits a robust photostability under UV-Vis-NIR irradiation. We demonstrate photocatalytic redox reactions driven by NAD+, such as O2 reduction, H2O oxidation, and the formation of metallic nanoparticles. Beyond the traditional role of NAD+ as a cofactor in redox biocatalysis, NAD+ executes direct photoactivation of oxidoreductases through the reduction of enzyme prosthetic groups. Consequently, the synergetic integration of biocatalysis and photocatalysis using NAD+ enables solar-to-chemical conversion with the highest-ever-recorded turnover frequency and total turnover number of 1263.4 hour-1 and 1692.3, respectively, for light-driven biocatalytic trans-hydrogenation.

| S-EPMC6641943 | biostudies-literature

Role and cellular source of nicotinamide adenine dinucleotide phosphate oxidase in hepatic fibrosis.

Project description:Reactive oxygen species (ROS) generated by nicotinamide adenine dinucleotide phosphate oxidase (NOX) is required for liver fibrosis. This study investigates the role of NOX in ROS production and the differential contribution of NOX from bone marrow (BM)-derived and non-BM-derived liver cells. Hepatic fibrosis was induced by bile duct ligation (BDL) for 21 days or by methionine-choline-deficient (MCD) diet for 10 weeks in wild-type (WT) mice and mice deficient in p47phox (p47phox knockout [KO]), a component of NOX. The p47phox KO chimeric mice were generated by the combination of liposomal clodronate injection, irradiation, and BM transplantation of p47phox KO BM into WT recipients and vice versa. Upon BDL, chimeric mice with p47phox KO BM-derived cells, including Kupffer cells, and WT endogenous liver cells showed a ?25% reduction of fibrosis, whereas chimeric mice with WT BM-derived cells and p47phox KO endogenous liver cells, including hepatic stellate cells, showed a ?60% reduction of fibrosis. In addition, p47phox KO compared to WT mice treated with an MCD diet showed no significant changes in steatosis and hepatocellular injury, but a ?50% reduction in fibrosis. Cultured WT and p47phox KO hepatocytes treated with free fatty acids had a similar increase in lipid accumulation. Free fatty acids promoted a 1.5-fold increase in ROS production both in p47phox KO and in WT hepatocytes.NOX in both BM-derived and non-BM-derived cells contributes to liver fibrosis. NOX does not play a role in experimental steatosis and the generation of ROS in hepatocytes, but exerts a key role in fibrosis.

| S-EPMC2947612 | biostudies-literature

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