Pyridine nucleotide cycle of Salmonella typhimurium: in vitro demonstration of nicotinamide adenine dinucleotide glycohydrolase, nicotinamide mononucleotide glycohydrolase, and nicotinamide adenine dinucleotide pyrophosphatase activities.

Extracts of Salmonella typhimurium were chromatographed by using Sephadex G-150 to separate the various enzymes involved with pyridine nucleotide cycle metabolism. This procedure revealed a previously unsuspected nicotinamide adenine dinucleotide (NAD) glycohydrolase (EC 3.2.2.5) activity, which was not observed in crude extracts. In contrast to NAd glycohydrolase, NAD pyrophosphatase (EC 3.6.1.22) was readily measured in crude extracts. This enzyme possessed a native molecular weight of 120,000. Other enzymes examined included nicotinamide mononucleotide (NMN) deamidase (EC 3.5.1.00), molecular weight of 43,000; NMN glycohydrolase (EC 3.2.2.14), molecular weight of 67,000; nicotinic acid phosphoribosyl transferase (EC 2.4.2.11), molecular weight of 47,000; and nicotinamide deamidase (EC 3.5.1.19), molecular weight of 35,000. NMN deamidase and NMN glycohydrolase activities were both examined for end product repression by measuring their activities in crude extracts prepared from cells grown with and without 10(-5) M nicotinic acid. No repression was observed with either activity. Both activities were also examined for feedback inhibition by NAD, reduced NAD, and NADP. NMN deamidase was unaffected by any of the compounds tested. NMN glycohydrolase was greatly inhibited by NAD and reduced NAD, whereas NADP was much less effective. Inhibition of NMN glycohydrolase was found to level off at an NAD concentration of ca. 1 mN, the approximate intracellular concentration of NAD.     

Studies on the in vitro inactivation of the Neurospora crassa assimilatory nitrite reductase in the presence of reduced pyridine nucleotides plus flavin.

In vitro inactivation of Neurospora crassa nitrite reductase (NAD(P)H: nitrite oxidoreductase, EC 1.6.6.4) can be obtained by preincubation of the enzyme with reduced pyridine nucleotide plus FAD. The presence of nitrite or hydroxylamine, electron acceptors for the N. crassa nitrite reductase, or cyanide, sulfite or arsenite, competitive inhibitors with respect to nitrite of this enzyme, protects the enzyme against this inactivation. Anaerobic experiments reveal that oxygen is required in order to obtain complete inactivation of nitrite reductase by preincubation with reduced pyridine nucleotide plus FAD. Also, inactivation is prevented if catalase is included in the preincubation mixture. The presence of hydrogen peroxide in the preincubation mixture increases the sensitivity of nitrite reductase to the in vitro FAD-dependent NAD(P)H inactivation. Neither electron acceptors, competitive inhibitors nor catalase, agents which protect the enzyme against the FAD-dependent NAD(P)H inactivation, can reverse this process once it has occurred.     

Characterization of a C21 neutral steroid hormone transforming enzyme, 21-dehydroxylase, in crude cell extracts of Eubacterium lentum.

A strain of the obligate anaerobe, Eubacterium lentum, isolated from human feces, catalyzes the 21-dehydroxylation of 11-deoxycorticosterone to progesterone. A quantitative radiochromatographic assay was developed to measure 21-dehydroxylase activity in cell extracts. Maximum enzyme activity in cell extracts required both a reduced pyridine nucleotide and an oxidized flavin coenzyme. However, photochemically reduced flavin (FMNH2) could replace the requirement for NAD(P)H plus oxidized flavin. NAD(P)H : flavin (either FMN or FAD) oxidoreductase activity was detected spectrophotometrically in cell extracts assayed under anaerobic conditions. 21-Dehydroxylase was active from pH 5.4 to 8.5 with an apparent optimum between 6.4 and 6.8 using mixtures of NADH plus FMN as coenzymes. The substrate concentration at half-maximal reaction velocity was 8.0 microM and a specific acitivity of 5.8 nmol [3H]progesterone formed . h-1 . mg-1 protein was determined using [3th]deoxycorticosterone as substrate. Atabrine, rotenone, acriflavin, and 2,4-dinitrophenol (all at 1 mM) inhibited 21-dehydroxylase activity in cell extracts by 25, 24, 35 and 84%, respectively. These results suggest that 21-dehydrogenase may be coupled to a NAD(P)H : flavin oxidoreductase system in E. lentum.     

Preparation of analogues of NAD+ as substrates for a sensitive fluorimetric assay of nucleotide pyrophosphatase.

1,N6-Etheno derivatives of pyridine analogues of NAD+ were synthesized, characterized and tested as substrates for a fluorimetric assay of nucleotide pyrophosphatase (EC 3.6.1.9). Upon cleavage of their pyrophosphate bond, the fluorescence of pyridine-1,N6-ethenoadenine dinucleotide (epsilon PdAD+) and of 4-hydrazinocarbonyl-pyridine-1,N6-ethenoadenine dinucleotide (epsilon hy4PdAD+) increased respectively 15-and 73-fold, at pH 7.4. This property allows a convenient steady-state assay of nucleotide pyrophosphatase by continuous monitoring of reaction progress. Both compounds were good substrates of this class of enzyme. The relative insensitivity of the fluorescence of epsilon PdAD+ and epsilon hy4PdAD+ to pH changes allowed assays under conditions preserving cellular integrity. epsilon PdAD+ is useful as a substrate for measuring nucleotide pyrophosphatase activity on the outside of mammalian cells because it is not a substrate for the external NAD+ glycohydrolase. epsilon Hy4PdAD+ proved useful when high sensitivity was needed.     

A bioluminescent assay for glycogen phosphorylase in cultured cells

A new method for the determination of glycogen phosphorylase (1,4 alpha-D-glucose:orthophosphate alpha-glucosyltransferase, EC 2.4.1.1) in cultured cells is described. The assay utilizes bacterial luciferase (EC 2.7) in a liquid scintillation spectrometer to measure NAD(P)H formed in a coupled enzyme reaction comprising glucose-6-phosphate dehydrogenase (EC 1.1.1.49) and phosphoglucomutase (EC 2.7.5.1). This assay is highly sensitive, easily detecting as little as 10 microU phosphorylase, fast and simple to perform. With modifications this procedure can be extended to measure other glycogenolytic enzymes and intermediates.     

Bioluminescent enzyme assay for the indication of plant stress in enclosed life support systems

The application of bioluminescent sensors for monitoring key metabolites and enzymes that are indicators of stress in plants is demonstrated. The sensitivity of bioluminescent assay for NAD(P)H and NAD(P)(+) was about 0.5 and 1 nmol, respectively. The levels of NAD(P)H and NAD(P)(+) in radish (Raphanus sativus) root extracts from controls and from stress-induced conditions were compared. To induce environmental stress, the plants were grown in enclosed environmental chambers with low pressure (9 or 32 kPa), high humidity (>80%) and low oxygen partial pressure (down to 3.3-6.5 kPa). The concentrations of NAD(P)(+) and NAD(P)H in plants varied under stress conditions. Decreasing both total pressure from 101.5 to 32 or 9 kPa and partial pressure of oxygen increased the ratio of NAD(P)(+) /NAD(P)H from 0.2 to 4 or 6, respectively. The increase in this ratio suggests that plants are undergoing stress in these hypobaric environments. The developed bioluminescent assay for quantification of pyridine nucleotides in plant tissues is rapid, low-cost and easily performed.     

NAD metabolism in Vibrio cholerae.

Extracts of Vibrio cholerae were assayed for various enzymatic activities associated with pyridine nucleotide cycle metabolism. The activities measured include NAD glycohydrolase, nicotinamide deamidase, nicotinamide mononucleotide deamidase, and nicotinic acid phosphoribosyltransferase. The results obtained demonstrate the existence in V. cholerae of the five-membered pyridine nucleotide cycle and the potential for a four-membered pyridine nucleotide cycle. The data presented also suggest that most of the NAD glycohydrolase in V. cholerae extracts is not directly related to cholera toxin.     

Multiple Forms of Pseudomonas multivorans Glucose-6-Phosphate and 6-Phosphogluconate Dehydrogenases: Differences in Size, Pyridine Nucleotide Specificity, and Susceptibility to Inhibition by Adenosine 5′-Triphosphate

Two major species of glucose-6-phosphate dehydrogenase (EC 1.1.1.49) differing in size, pyridine nucleotide specificity, and susceptibility to inhibition by adenosine 5'-triphosphate (ATP) were detected in extracts of Pseudomonas multivorans (which has recently been shown to be synonymous with the species Pseudomonas cepacia) ATCC 17616. The large species (molecular weight ca. 230,000) was active with nicotinamide adenine dinucleotide (NAD) or nicotinamide adenine dinucleotide phosphate (NADP) and was markedly inhibited by ATP, which decreased its affinity for glucose-6-phosphate and for pyridine nucleotides. This form of the enzyme exhibited homotropic effects for glucose-6-phosphate. The small species (molecular weight ca. 96,000) was active with NADP but not with NAD, was not inhibited by ATP, and exhibited no homotropic effects for glucose-6-phosphate. Under certain conditions multiplicity of 6-phosphogluconate dehydrogenase (EC 1.1.1.43) activities was also noted. One form of the enzyme (80,000 molecular weight) was active with either NAD or NADP and was inhibited by ATP, which decreased its affinity for 6-phosphogluconate. The other form (120,000 molecular weight) was highly specific for NADP and was not susceptible to inhibition by ATP. Neither form of the enzyme exhibited homotropic effects for 6-phosphogluconate. The possible relationships between the different species of glucose-6-phosphate dehydrogenase and 6-phosphogluconate dehydrogenase are discussed.     

Utilization and metabolism of NAD by Haemophilus parainfluenzae

The utilization of exogenous nicotinamide adenine dinucleotide (NAD) by Haemophilus parainfluenzae was studied in suspensions of whole cells using radiolabelled NAD, nicotinamide mononucleotide (NMN), and nicotinamide ribonucleoside (NR). The utilization of these compounds by H. parainfluenzae has the following characteristics. (1) NAD is not taken up intact, but rather is degraded to NMN or NR prior to internalization. (2) Uptake is carrier-mediated and energy-dependent with saturation kinetics. (3) There is specificity for the beta-configuration of the glycopyridine linkage. (4) An intact carboxamide groups is required on the pyridine ring. The intracellular metabolism of NAD was studied in crude cell extracts and in whole cells using carbonyl-14C-labelled NR, NMN, NAD, nicotinamide, and nicotinic acid as substrates in separate experiments. A synthetic pathway from NR through NMN to NAD that requires Mg2+ and ATP was demonstrated. Nicotinamide was found as an end-product of NAD degradation. Nicotinic acid mononucleotide and nicotinic acid adenine dinucleotide were not found as intermediates. The NAD synthetic pathway in H. parainfluenzae differs from the Preiss-Handler pathway and the pyridine nucleotide cycles described in other bacteria.     

Critical study of preanalytical and analytical phases of adenine and pyridine nucleotide assay in human whole blood

Intracellular redox and energetic status play a crucial role in cardiovascular diseases and metabolic disorders. The physiological status of reducing agents, such as NADPH and NADH, and of high-energy molecules, such as ATP, is required for antioxidant system activity. For these reasons, an accurate measurement of adenine and pyridine nucleotides is fundamental. In this study we examined the preanalytical phase of reduced pyridine (RPN) and adenine and oxidized pyridine (AOPN) nucleotide assay in human whole blood. Different experimental conditions were applied to RPN alkaline and AOPN acid extracts to find the best analytical performance. Our results show that a good RPN and AOPN linearity (r from 0.994 to 0.999), recovery (near to 100%), and precision (coefficient of variation < 5%) were obtained when supernatant from acid and ultrafiltrate from alkaline extracts were neutralized, frozen, and thawed just before HPLC injection. Since NADH decays rapidly at -80 degrees C, RPN levels must be assayed within 72 h while AOPN can be stored for 1 month at the same temperature. An accurate and quantitative method for nucleotide determination can be obtained by applying the preanalytical conditions proposed in this study.     

Uncoupled redox systems in the lumen of the endoplasmic reticulum. Pyridine nucleotides stay reduced in an oxidative environment

The redox state of the intraluminal pyridine nucleotide pool was investigated in rat liver microsomal vesicles. The vesicles showed cortisone reductase activity in the absence of added reductants, which was dependent on the integrity of the membrane. The intraluminal pyridine nucleotide pool could be oxidized by the addition of cortisone or metyrapone but not of glutathione. On the other hand, intraluminal pyridine nucleotides were slightly reduced by cortisol or glucose 6-phosphate, although glutathione was completely ineffective. Redox state of microsomal protein thiols/disulfides was not altered either by manipulations affecting the redox state of pyridine nucleotides or by the addition of NAD(P)+ or NAD(P)H. The uncoupling of the thiol/disulfide and NAD(P)+/NAD(P)H redox couples was not because of their subcompartmentation, because enzymes responsible for the intraluminal oxidoreduction of pyridine nucleotides were distributed equally in smooth and rough microsomal subfractions. Instead, the phenomenon can be explained by the negligible representation of glutathione reductase in the endoplasmic reticulum lumen. The results demonstrated the separate existence of two redox systems in the endoplasmic reticulum lumen, which explains the contemporary functioning of oxidative folding and of powerful reductive reactions.     

Identification of nicotinamide mononucleotide deamidase of the bacterial pyridine nucleotide cycle reveals a novel broadly conserved amidohydrolase family

The pyridine nucleotide cycle is a network of salvage and recycling routes maintaining homeostasis of NAD(P) cofactor pool in the cell. Nicotinamide mononucleotide (NMN) deamidase (EC 3.5.1.42), one of the key enzymes of the bacterial pyridine nucleotide cycle, was originally described in Enterobacteria, but the corresponding gene eluded identification for over 30 years. A genomics-based reconstruction of NAD metabolism across hundreds of bacterial species suggested that NMN deamidase reaction is the only possible way of nicotinamide salvage in the marine bacterium Shewanella oneidensis. This prediction was verified via purification of native NMN deamidase from S. oneidensis followed by the identification of the respective gene, termed pncC. Enzymatic characterization of the PncC protein, as well as phenotype analysis of deletion mutants, confirmed its proposed biochemical and physiological function in S. oneidensis. Of the three PncC homologs present in Escherichia coli, NMN deamidase activity was confirmed only for the recombinant purified product of the ygaD gene. A comparative analysis at the level of sequence and three-dimensional structure, which is available for one of the PncC family member, shows no homology with any previously described amidohydrolases. Multiple alignment analysis of functional and nonfunctional PncC homologs, together with NMN docking experiments, allowed us to tentatively identify the active site area and conserved residues therein. An observed broad phylogenomic distribution of predicted functional PncCs in the bacterial kingdom is consistent with a possible role in detoxification of NMN, resulting from NAD utilization by DNA ligase.     

Association and redox properties of the putidaredoxin reductase-nicotinamide adenine dinucleotide complex

Putidaredoxin reductase (PdR) is the flavin protein that carries out the first electron transfer involved in the cytochrome P450cam catalytic cycle. In PdR, the flavin adenine dinucleotide (FAD/FADH2) redox center acts as a transformer by accepting two electrons from soluble nicotinamide adenine dinucleotide (NAD+/NADH) and donating them in two separate, one-electron-transfer steps to the iron-sulfur protein putidaredoxin (Pdx). PdR, like the two more intensively studied monoflavin reductases, adrenodoxin reductase (AdR) and ferredoxin-NADP+ reductase (FNR), has no other active redox moieties (e.g., sulfhydryl groups) and can exist in three different oxidation states: (i) oxidized quinone, (ii) one-electron reduced semiquinone (stable neutral species (blue) or unstable radical anion (red)), and (iii) two-electron fully reduced hydroquinone. Here, we present reduction potential measurements for PdR in support of a thermodynamic model for the modulation of equilibria among the redox components in this initial electron-transfer step of the P450 cycle. A spectroelectrochemical technique was used to measure the midpoint oxidation-reduction potential of PdR that had been carefully purified of all residual NAD+, E0' = -369 +/- 10 mV at pH 7.6, which is more negative than previously reported and more negative than the pyridine nucleotide NADH/NAD+ (-330 mV). After addition of NAD+, the formation of the oxidized reductase-oxidized pyridine nucleotide complex was followed by the two-electron-transfer redox reaction, PdRox:NAD+ + 2e- --> PdRrd:NAD+, when the electrode potential was lowered. The midpoint potential was a hyperbolic function of increasing NAD+ concentration, such that at concentrations of pyridine nucleotide typically found in an intracellular environment, the midpoint potential would be E0' = -230 +/- 10 mV, thereby providing the thermodynamically favorable redox equilibria that enables electron transfer from NADH. This thermodynamic control of electron transfer is a shared mechanistic feature with the adrenodoxin P450 and photosynthetic electron-transfer systems but is different from the kinetic control mechanisms in the microsomal P450 systems where multiple reaction pathways draw on reducing power held by NADPH-cytochrome P450 reductase. The redox measurements were combined with protein fluorescence quenching of NAD+ binding to oxidized PdR to establish that the PdRox:NAD+ complex (KD = 230 microM) is about 5 orders of magnitude weaker than PdRrd:NAD+ binding. These results are integrated with known structural and kinetic information for PdR, as well as for AdR and FNR, in support of a compulsory ordered pathway to describe the electron-transfer processes catalyzed by all three reductases.     

Partial purification and some properties of delta1-pyrroline-5-carboxylate reductase from Escherichia coli.

delta1-Pyrroline-5-carboxylate (PCA) reductase [L-proline:NAD(P)+5-oxidoreductase, EC 1.5.1.2] has been purified over 200-fold from Escherichia coli K-12. It has a molecular weight of approximately 320,000. PCA reductase mediates the pyridine nucleotide-linked reduction of PCA to proline but not the reverse reaction (even at high substrate concentrations). The partially purified preparation is free of competing pyridine nucleotide oxidase, PCA dehydrogenase, and proline oxidase activities. The Michaelis constant (Km) values for the substrate, PCA, with reduced nicotinamide adenine dinucleotide phosphate (NADPH) or NADH as cofactor are 0.15 and 0.14 mM, respectively. The Km values determined for NADPH and NADH are 0.03 and 0.23 mM, respectively. Although either NADPH or NADH can function as cofactor, the activity observed with NADPH is severalfold greater. PCA reductase is not repressed by growth in the presence of proline, but it is inhibited by the reaction end products, proline and NADP.     

Simultaneous Single-Sample Determination of NMNAT Isozyme Activities in Mouse Tissues

A novel assay procedure has been developed to allow simultaneous activity discrimination in crude tissue extracts of the three known mammalian nicotinamide mononucleotide adenylyltransferase (NMNAT, EC 2.7.7.1) isozymes. These enzymes catalyse the same key reaction for NAD biosynthesis in different cellular compartments. The present method has been optimized for NMNAT isozymes derived from Mus musculus, a species often used as a model for NAD-biosynthesis-related physiology and disorders, such as peripheral neuropathies. Suitable assay conditions were initially assessed by exploiting the metal-ion dependence of each isozyme recombinantly expressed in bacteria, and further tested after mixing them in vitro. The variable contributions of the three individual isozymes to total NAD synthesis in the complex mixture was calculated by measuring reaction rates under three selected assay conditions, generating three linear simultaneous equations that can be solved by a substitution matrix calculation. Final assay validation was achieved in a tissue extract by comparing the activity and expression levels of individual isozymes, considering their distinctive catalytic efficiencies. Furthermore, considering the key role played by NMNAT activity in preserving axon integrity and physiological function, this assay procedure was applied to both liver and brain extracts from wild-type and Wallerian degeneration slow (Wld^S) mouse. Wld^S is a spontaneous mutation causing overexpression of NMNAT1 as a fusion protein, which protects injured axons through a gain-of-function. The results validate our method as a reliable determination of the contributions of the three isozymes to cellular NAD synthesis in different organelles and tissues, and in mutant animals such as Wld^S.     

Calcium sensitive isocitrate and 2-oxoglutarate dehydrogenase activities in rat liver and AS-30D hepatoma mitochondria

NAD+-isocitrate dehydrogenase and 2-oxoglutarate dehydrogenase in extracts of mitochondria from the highly malignant AS-30D rat hepatoma cell line demonstrate Ca2+ sensitivities and affinities for substrates similar to those of normal liver mitochondria. However, the maximal activities of NAD+- and NADP+-dependent isocitrate dehydrogenase were found to be 8 and 3.5 fold higher in hepatoma mitochondrial extracts than those of liver mitochondria, whereas maximal activities of succinate and 2-oxoglutarate dehydrogenases were similar in the two tissues. At pyridine nucleotide concentrations giving the lowest physiological NADH/NAD+ ratio, NAD+-isocitrate dehydrogenase activity in hepatoma mitochondrial extracts was completely inhibited at subsaturating concentrations of Ca2+, substrate, and NAD+, in contrast to rat liver mitochondrial extracts which retained significant activity.     

Bioluminescence analysis of NAD(P)H and NAD(P)+ preventing mutual interference by selective nucleotide destruction and enhanced specific light emission.

Improved bioluminescence analysis of pyridine nucleotides has been designed based on the fact that the luminescence intensity expresses the velocity of the light formation. The bacterial luciferase system is, in principle, composed of two reactions with two different velocities, one for energy supply by the oxidation of NAD(P)H and the other for the subsequent light generation. The rate setting can be arranged such that an emission maximum is produced 30 to 40 s after mixing the sample with the light-yielding solution, hence providing for a convenient analytical performance. The maximal intensity which is easily recorded, e.g., by a tracking volt-meter, is proportional to the concentration of the reduced nucleotide. Discriminative analysis of the various pyridine nucleotides is facilitated by selective destruction of the oxidized forms with alkali and the reduced forms with acid. Erroneous conversion of NAD(P)H to NAD(P)+ may be induced by haemoglobin in a tissue sample but this is prevented by the presence of 2 mM ascorbic acid at the instant of the acidification. Simultaneous coupling of the ongoing reduction of a pyridine nucleotide to the oxidation in the bacterial luciferase system generates a light-yielding cycle which offers important advantages. With NAD(P)+ as the analytic target compound, direct measurement replaces a preceding separate conversion to NAD(P)H. The four nucleotide forms become determinable in a sample by combining selective destruction of either the reduced or oxidized species with a nucleotide-specific reduction in the cycle. Discriminative analyses are furthermore facilitated by the enhanced emission which is due to the energy derived from the continuous specific reduction, whereas initial light signals from side reactions fade out. It is often possible to suppress disturbing analytical errors by the design of the light-yielding cycle. If the rate of the dehydrogenase reactions is kept low compared with the overall rate of the luciferase system, moderately impaired function of some of its components may only give rise to a slight and tolerable decrease in emission intensity. Kinetic evaluations and model experiments are presented and supplemented with applications to tissue samples.     

Defining the metabolic and growth responses of porcine haemophili to exogenous pyridine nucleotides and precursors

A variety of biologically important pyridine nucleotides and precursors were examined for their capacities to satisfy the V-factor requirement of 30 strains of porcine haemophili. Of the compounds tested, only NAD, NMN and nicotinamide riboside (NR) supported the growth of all strains; NADP supported the growth of only the type strain of Haemophilus parasuis. Further studies with the H. parasuis type strain and the neotype strain of H. pleuropneumoniae demonstrated that, during growth, these organisms exhibited affinities for NMN that were greater than those for NAD; the affinity of H. pleuropneumoniae for NR was similar to that for NMN, whereas H. parasuis exhibited relatively low affinity for NR. With either organism, equimolar amounts of NAD and NMN supported the production of approximately equal amounts of biomass whereas growth yields were substantially lower when NR was the pyridine nucleotide source. When either organism was grown in the presence of excess exogenous [carbonyl-14C]NAD, cessation of growth was accompanied by the apparent exhaustion of the NAD supply. Approximately 80% of the radioactivity added as [14C]NAD could be recovered as extracellular [14C]nicotinamide and the majority of the assimilated radioactive material was present intracellularly in the form of a [14C]NAD(P) pool. The results are discussed in terms of the structural features required of a pyridine compound for it to support the growth of porcine haemophili, the capacity of these organisms to compete for pyridine nucleotide sources in vivo, and possible mechanisms involved in the assimilation of such compounds.     

Isolation and characterization of NAD(P)H-dehydrogenases from seeds of the castor bean

Two pyridine nucleotide dehydrogenases have been isolated from castor bean seed extracts by a combination of ion exchange chromatography on DEAE-Sepharose and gel permeation chromatography on Sephadex G-200. The enzymes were designated D-I and D-II according to their elution position on DEAE-Sepharose. Both enzymes D-I and D-II are globular proteins which have MWs of 66 000 and 60 000, respectively. Dehydrogenation is observed with both NADH and NADPH as electron donors, while the electron acceptor specificity demonstrates that the enzymes are probably NAD(P)H: quinone oxidoreductases. Successful coupling of dehydrogenase activity with that of peroxidase indicates a possible role of the enzymes in seed germination.     

The Kinetics of Pyridine Nucleotide Reduction following the Addition of Glucose to Bakers' Yeast

Changes of pyridine nucleotide concentration in yeast followingthe addition of glucose have been measured by a fluorimetricassay applied to cell extracts. The major changes were in nicotinamideadenine dinucleotide (NAD) which, during the first minute afterglucose addition, was rapidly reduced and then more slowly reoxidized.Later, one to two minutes after adding glucose, NAD was reducedagain to a steady-state value. No NAD was reduced when acetaldehydewas added simultaneously with glucose. The oxidation of NAD,which followed the initial reduction, was slowed down by sodiumfluoride and almost completely abolished by sodium sulphite.The initial reduction of NAD is ascribed to 3-phosphoglyceraldehydedehydrogenase and its subsequent oxidation is ascribed to alcoholdehydrogenase. Probably only that NAD bound to 3-phosphoglyceraldehydedehydrogenase is reduced initially. The reasons for these viewsare discussed.