The biochemistry of glucose-6-phosphate dehydrogenase, 6-phosphogluconate dehydrogenase and glutathione reductase

Biochemical and structural insights into 6-phosphogluconate dehydrogenase from Leishmania donovani

6-phosphogluconate dehydrogenase (6PGDH) participates in pentose phosphate pathway of glucose metabolism by catalyzing oxidative decarboxylation of 6-phsophogluconate (6PG) and its absence has been lethal for several eukaryotes. Despite being a validated drug target in many organisms like Plasmodium, the enzyme has not been explored in leishmanial parasites. In the present study, 6PGDH of Leishmania donovani (Ld6PGDH) is cloned and purified followed by its characterization using biochemical and structural approaches. Ld6PGDH lacks the glycine-serine-rich sequence at its C-terminal that is present in other eukaryotes including humans. Leishmanial 6PGDH possesses more affinity for substrate (6PG) and cofactor (NADP) in comparison to that of human. The enzymatic activity is inhibited by gentamicin and cefuroxime through competitive mode with functioning more potently towards leishmanial 6PGDH than its human counterpart. CD analysis has shown higher α-helical content in the secondary structure of Ld6PGDH, while fluorescence studies revealed that tryptophan residues are not completely accessible to solvent environment. The three-dimensional structure was generated through homology modelling and docked with substrate and cofactor. The docking studies demonstrated two separate binding pockets for 6PG and NADP with higher affinity for the cofactor binding, and Asn105 is interacting with substrate as well as the cofactor. Additionally, MD simulation has shown complexes of Ld6PGDH with 6PG and NADP to be more stable than its apo form. Altogether, the present study might provide the foundation to investigate this enzyme as potential target against leishmaniasis. KEY POINTS: • Ld6PGDH enzymatic activity is competitively inhibited by gentamicin and cefuroxime. • It displays more helical contents and all structural characteristics of 6PGDH family. • Interaction studies demonstrate higher affinity of cofactor than substrate for Ld6PGDH.

The structure of a novel membrane-associated 6-phosphogluconate dehydrogenase from Gluconacetobacter diazotrophicus (Gd6PGD) reveals a subfamily of short-chain 6PGDs

The enzyme 6-phosphogluconate dehydrogenase catalyzes the conversion of 6-phosphogluconate to ribulose-5-phosphate. It represents an important reaction in the oxidative pentose phosphate pathway, producing a ribose precursor essential for nucleotide and nucleic acid synthesis. We succeeded, for the first time, to determine the three-dimensional structure of this enzyme from an acetic acid bacterium, Gluconacetobacter diazotrophicus (Gd6PGD). Active Gd6PGD, a homodimer (70 kDa), was present in both the soluble and the membrane fractions of the nitrogen-fixing microorganism. The Gd6PGD belongs to the newly described subfamily of short-chain (333 AA) 6PGDs, compared to the long-chain subfamily (480 AA; e.g., Ovis aries, Homo sapiens). The shorter amino acid sequence in Gd6PGD induces the exposition of hydrophobic residues in the C-terminal domain. This distinct structural feature is key for the protein to associate with the membrane. Furthermore, in terms of function, the short-chain 6PGD seems to prefer NAD⁺ over NADP⁺ , delivering NADH to the membrane-bound NADH dehydrogenase of the microorganisms required by the terminal oxidases to reduce dioxygen to water for energy conservation. ENZYME: ECnonbreakingspace1.1.1.343. DATABASE: Structural data are available in PDB database under the accession number 6VPB.

6-Phosphogluconate dehydrogenase fuels multiple aspects of cancer cells: From cancer initiation to metastasis and chemoresistance

Reprogrammed metabolism is key biochemical characteristic of malignant cells, which represents one of the emerging hallmarks of cancer. Currently, there is rising contemplation on oxidative pentose phosphate pathway (PPP) enzymes as potential therapeutic hits due to their affiliation with tumor metabolism. 6-Phosphogluconate dehydrogenase (6PGD), third oxidative decarboxylase of PPP, has received a great deal of attention during recent years due to its critical role in tumorigenesis and redox homeostasis. 6PGD has been reported to overexpress in number of cancer types and its hyperactivation is mediated through post-transcriptional and post-translational modifications by YTH domain family 2 (YTHDF2), Nrf2 (nuclear factor erythroid 2-related factor 2), EGFR (epidermal growth factor receptor) and via direct structural interactions with ME1 (malic enzyme 1). Upregulated expression of 6PGD provides metabolic as well as defensive advantage to cancer cells, thus, promoting their proliferative and metastatic potential. Moreover, enhanced 6PGD expression also performs key role in development of chemoresistance as well as radiation resistance in cancer. This review aims to discuss the historical timeline and cancer-specific role of 6PGD, pharmacological and genetic inhibitors of 6PGD and 6PGD as prognostic biomarker in order to explore its potential for therapeutic interventions. We anticipate that targeting this imperative supplier of NADPH might serve as tempting avenue to combat the deadly disease like cancer.

Cloning, Expression, and Characterization of a Psychrophilic Glucose 6-Phosphate Dehydrogenase from Sphingomonas sp. PAMC 26621

Glucose 6-phosphate dehydrogenase (G6PD) (EC 1.1.1.363) is a crucial regulatory enzyme in the oxidative pentose phosphate pathway that provides reductive potential in the form of NADPH, as well as carbon skeletons for the synthesis of macromolecules. In this study, we report the cloning, expression, and characterization of G6PD (SpG6PD1) from a lichen-associated psychrophilic bacterium Sphingomonas sp. PAMC 26621. SpG6PD1 was expressed in Escherichia coli as a soluble protein, having optimum activity at pH 7.5⁻8.5 and 30 °C for NADP⁺ and 20 °C for NAD⁺. SpG6PD1 utilized both NADP⁺ and NAD⁺, with the preferential utilization of NADP⁺. A high Km value for glucose 6-phosphate and low activation enthalpy (ΔH‡) compared with the values of mesophilic counterparts indicate the psychrophilic nature of SpG6PD1. Despite the secondary structure of SpG6PD1 being maintained between 4⁻40 °C, its activity and tertiary structure were better preserved between 4⁻20 °C. The results of this study indicate that the SpG6PD1 that has a flexible structure is most suited to a psychrophilic bacterium that is adapted to a permanently cold habitat.

Glucose-6-phosphate dehydrogenase in blunt snout bream Megalobrama amblycephala: molecular characterization, tissue distribution, and the responsiveness to dietary carbohydrate levels

This study aimed to characterize the full-length cDNA of glucose-6-phosphate dehydrogenase (G6PD) from Megalobrama amblycephala with its responses to dietary carbohydrate levels characterized. The cDNA obtained covered 2768 bp with an open reading frame of 1572 bp. Sequence alignment and phylogenetic analysis revealed a high degree of conservation (77-97%) among most fish and other higher vertebrates. The highest transcription of G6PD was observed in kidney followed by liver, whereas relatively low abundance was detected in eye. Then, the transcriptions and activities of G6PD as well as lipid contents were determined in the liver, muscle, and the adipose tissue of fish fed two dietary carbohydrate levels (30 and 42%) for 12 weeks. Hepatic transcriptions of fatty acid synthetase (FAS), acetyl-CoA carboxylase α (ACCα), sterol regulatory element-binding protein-1 (SREBP1), and peroxisome proliferator-activated receptor γ (PPARγ) were also measured to corroborate the lipogenesis derived from carbohydrates. The G6PD expressions and activities in both liver and the adipose tissue as well as the lipid contents in whole-body, liver, and the adipose tissue all increased significantly after high-carbohydrate feeding. Hepatic transcriptions of FAS, ACCα, SREBP1, and PPARγ were also up-regulated remarkably by the intake of a high-carbohydrate diet. These results indicated that the G6PD of M. amblycephala shared a high similarity with that of other vertebrates. Its expressions and activities in tissues were both highly inducible by high-carbohydrate feeding, as also held true for the transcriptions of other enzymes and/or transcription factors involved in lipogenesis, evidencing an enhanced lipogenesis by high dietary carbohydrate levels.

NAD Kinases: Metabolic Targets Controlling Redox Co-enzymes and Reducing Power Partitioning in Plant Stress and Development

NAD(H) and NADP(H) are essential co-enzymes which dominantly control a number of fundamental biological processes by acting as reducing power and maintaining the intracellular redox balance of all life kingdoms. As the only enzymes that catalyze NAD(H) and ATP to synthesize NADP(H), NAD Kinases (NADKs) participate in many essential metabolic reactions, redox sensitive regulation, photosynthetic performance and also reactive oxygen species (ROS) homeostasis of cells and therefore, play crucial roles in both development and stress responses of plants. NADKs are highly conserved enzymes in amino acid sequences but have multiple subcellular localization and diverse functions. They may function as monomers, dimers or multimers in cells but the enzymatic properties in plants are not well elucidated yet. The activity of plant NADK is regulated by calcium/calmodulin and plays crucial roles in photosynthesis and redox co-enzyme control. NADK genes are expressed in almost all tissues and developmental stages of plants with specificity for different members. Their transcripts can be greatly stimulated by a number of environmental factors such as pathogenic attack, irritant applications and abiotic stress treatments. Using transgenic approaches, several studies have shown that NADKs are involved in chlorophyll synthesis, photosynthetic efficiency, oxidative stress protection, hormone metabolism and signaling regulation, and therefore contribute to the growth regulation and stress tolerance of plants. In this review, the enzymatic properties and functional mechanisms of plant NADKs are thoroughly investigated based on literature and databases. The results obtained here are greatly advantageous for further exploration of NADK function in plants.

Biological and physiological role of reactive oxygen species--the good, the bad and the ugly

Reactive oxygen species (ROS) are chemically reactive molecules that are naturally produced within biological systems. Research has focused extensively on revealing the multi-faceted and complex roles that ROS play in living tissues. In regard to the good side of ROS, this article explores the effects of ROS on signalling, immune response and other physiological responses. To review the potentially bad side of ROS, we explain the consequences of high concentrations of molecules that lead to the disruption of redox homeostasis, which induces oxidative stress damaging intracellular components. The ugly effects of ROS can be observed in devastating cardiac, pulmonary, neurodegenerative and other disorders. Furthermore, this article covers the regulatory enzymes that mitigate the effects of ROS. Glutathione peroxidase, superoxide dismutase and catalase are discussed in particular detail. The current understanding of ROS is incomplete, and it is imperative that future research be performed to understand the implications of ROS in various therapeutic interventions.

Purification and characterization of glucose 6-phosphate dehydrogenase (G6PD) from grass carp (Ctenopharyngodon idella) and inhibition effects of several metal ions on G6PD activity in vitro

Glucose 6-phosphate dehydrogenase (G6PD) is a key enzyme catalyzing the first step of the pentose phosphate pathway which generates NADPH for anabolic pathways and protection systems in various organisms, including fish. In the present study, G6PD was purified from grass carp (Ctenopharyngodon idella) hepatopancreas using the methods of 2',5'-ADP-Sepharose 4B affinity chromatography followed by DEAE Sepharose Fast Flow ion exchange chromatography. The characterization of G6PD and inhibition effects of several metal ions on G6PD activity in vitro were also determined. Grass carp hepatopancreas G6PD, with a specific activity of 18 U/mg protein, was purified 1,066-fold with a yield of 19.5 % and Mr of 71.85 kDa. The enzyme had a temperature optimum of 42 °C, pH optimum of 7.5 and 9.0. The K(m) values for G6-P and NADP(+) were determined to be 0.026, 0.0068 mM, respectively. The V(max) values for G6-P and NADP(+) were 2.20 and 2.27 μM min(-1) mg protein(-1), respectively. The catalytic efficiency for G6-P and NADP as the substrates was 0.085 and 0.334 × 10(-6) min(-1) mg protein(-1), respectively. Inhibition effects of metal ions on the purified G6PD activity indicated that IC50 values of Zn(+2), Mn(+2), Al(+3), Cu(+2), and Cd(+2) were 0.42, 0.54, 0.94, 1.20, and 4.17 mM, respectively. The Ki constants of Zn(+2), Al(+3), Cu(+2), and Cd(+2) were 0.52, 1.12, 0.26, and 4.8 mM, respectively. Zn(+2), Al(+3), and Cd(+2) showed competitive inhibition, while Cu(+2) inhibited the G6PD in a noncompetitive inhibition manner. Our study provided important information about the control of the grass carp liver PPP, the biosynthesis of several important related biomolecules, and the status of detoxification systems in grass carp liver in relation to metabolism.

Biomolecular filters for improved separation of output signals in enzyme logic systems applied to biomedical analysis

Biomolecular logic systems processing biochemical input signals and producing "digital" outputs in the form of YES/NO were developed for analysis of physiological conditions characteristic of liver injury, soft tissue injury, and abdominal trauma. Injury biomarkers were used as input signals for activating the logic systems. Their normal physiological concentrations were defined as logic-0 level, while their pathologically elevated concentrations were defined as logic-1 values. Since the input concentrations applied as logic 0 and 1 values were not sufficiently different, the output signals being at low and high values (0, 1 outputs) were separated with a short gap making their discrimination difficult. Coupled enzymatic reactions functioning as a biomolecular signal processing system with a built-in filter property were developed. The filter process involves a partial back-conversion of the optical-output-signal-yielding product, but only at its low concentrations, thus allowing the proper discrimination between 0 and 1 output values.

Comparative Approach of the de novo Fatty Acid Synthesis (Lipogenesis) between Ruminant and Non Ruminant Mammalian Species: From Biochemical Level to the Main Regulatory Lipogenic Genes

Over the second half of 20^th century much research on lipogenesis has been conducted, especially focused on increasing the production efficiency and improving the quality of animal derived products. However, many diferences are observed in the physiology of lipogenesis between species. Recently, many studies have also elucidated the involvement of numerous genes in this procedure, highlighting diferences not only at physiology but also at the molecular level. The main scope of this review is to point out the major differences between ruminant and non ruminant species, that are observed in key regulatory genes involved in lipogenesis. Human is used as a central reference and according to the findinggs, main differences are analysed. These findings could serve not only as basis for understanding the main physiology of lipogenesis and further basic research, but also as a basis for any animal scientist to develop new concepts and methods for use in improving animal production and modern genetic improvement.

Glucose-6-phosphate dehydrogenase of trypanosomatids: characterization, target validation, and drug discovery

In trypanosomatids, glucose-6-phosphate dehydrogenase (G6PDH), the first enzyme of the pentosephosphate pathway, is essential for the defense of the parasite against oxidative stress. Trypanosoma brucei, Trypanosoma cruzi, and Leishmania mexicana G6PDHs have been characterized. The parasites' G6PDHs contain a unique 37 amino acid long N-terminal extension that in T. cruzi seems to regulate the enzyme activity in a redox-state-dependent manner. T. brucei and T. cruzi G6PDHs, but not their Leishmania spp. counterpart, are inhibited, in an uncompetitive way, by steroids such as dehydroepiandrosterone and derivatives. The Trypanosoma enzymes are more susceptible to inhibition by these compounds than the human G6PDH. The steroids also effectively kill cultured trypanosomes but not Leishmania and are presently considered as promising leads for the development of new parasite-selective chemotherapeutic agents.

Enzymatic and mRNA Transcript Response of Ovine 6-Phosphogluconate Dehydrogenase (6PGD) in Respect to Different Milk Yield

Ovine 6-phosphogluconate dehydrogenase (6PGD) is an enzyme of the pentose phosphate pathway, providing the necessary compounds of NADPH for the synthesis of fatty acids. Much of research has been conducted both on enzymatic level and on molecular level. However, to our knowledge, any correlation between enzymatic activity and 6PGD gene expression pattern related to different physiological stages has not been yet reported. With this report, we tried to highlight if any correlation between enzymatic activity and expression of ovine 6PGD gene exists, in respect to different milk yield. According to the determined enzymatic activities and adipocytes characteristics, ewes with low milk production possessed a greater (P ≤ .001) 6PGD activity and larger adipocytes than the highly productive ewes. Although 6PGD expression pattern was higher in low milk yield ewes than in ewes with high milk production, this difference was not found statistically significant. Thus, 6PGD gene expression pattern was not followed by so rapid and great/sizeable changes as it was observed for its respective enzymatic activity, suggesting that other mechanisms such as post translation regulation may be involved in the regulation of the respective gene.

Long-term impact of an antioxidant-deficient neonatal diet on lipid and glucose metabolism

Newborn infants are at risk for oxidative stress leading to metabolic syndrome features. Oxidative stress can be induced by oxidant load such as oxygen supplementation, peroxides from intravenous nutrition, or low antioxidant defenses. We hypothesize that a modulation of antioxidant defenses during the neonatal period, without external oxidant challenge, will have a long-term influence on energy metabolism. Guinea pigs were fed between their third and their seventh day of life a regular chow leading to "mature" antioxidant defenses or a deficient chow leading to lower antioxidant defenses. Between weeks 1 and 14, the animals were fed regular chow. The hepatic oxidized redox status of glutathione associated with the deficient diet (-221 +/- 2 vs -228 +/- 1 mV, p < 0.01) was maintained until 14 weeks. At 13-14 weeks, animals fed the deficient diet presented lower plasma TG (479 +/- 57 vs 853 +/- 32 microM, p < 0.01), lower blood glucose (5.8 +/- 0.3 vs 6.9 +/- 0.3 mM, p < 0.05), and better tolerance to glucose (p < 0.05). Blood glucose correlated negatively with the redox status (r2 = 0.47, p < 0.01). Low antioxidant defenses during the neonatal period induce a better energy substrate profile associated with an oxidized redox status later in life. These findings suggest being aware of negative consequences when adopting "aggressive" antioxidant therapies in newborn infants.

Selection at 6-PGD locus in laboratory populations of Bactrocera oleae

We have previously shown that laboratory populations of the olive fruitfly Bactrocera oleae come to equilibrium with allele frequencies at the 6-phosphogluconate dehydrogenase (6-PGD) locus markedly different from those of wild populations. In this study, we present new evidence from perturbation experiments in support of the notion that the locus is under selective pressure under laboratory conditions. Eleven populations were started with frequencies at the 6-PGD locus different from the laboratory equilibrium. Over 12 generations, the populations showed a return to the previous equilibrium, indicating a direct and powerful selection pressure on the naturally occurring allozymes of this locus. That is, a marked increase of the F allele followed by a compensatory decrease of allele I. Populations were set up to minimize the effects of associative overdominance, and we discuss the possible influence of this factor. Nucleotide sequence for the 6-PGD F and I alleles revealed two missense mutations at positions 501 and 730 leading to different amino acids among the two alleles.

Crystal structure of Saccharomyces cerevisiae 6-phosphogluconate dehydrogenase Gnd1

BACKGROUND

As the third enzyme of the pentose phosphate pathway, 6-phosphogluconate dehydrogenase (6PGDH) is the main generator of cellular NADPH. Both thioredoxin reductase and glutathione reductase require NADPH as the electron donor to reduce oxidized thioredoxin or glutathione (GSSG). Since thioredoxin and GSH are important antioxidants, it is not surprising that 6PGDH plays a critical role in protecting cells from oxidative stress. Furthermore the activity of 6PGDH is associated with several human disorders including cancer and Alzheimer's disease. The 3D structural investigation would be very valuable in designing small molecules that target this enzyme for potential therapeutic applications.

RESULTS

The crystal structure of 6-phosphogluconate dehydrogenase (6PGDH/Gnd1) from Saccharomyces cerevisiae has been determined at 2.37 A resolution by molecular replacement. The overall structure of Gnd1 is a homodimer with three domains for each monomer, a Rossmann fold NADP+ binding domain, an all-alpha helical domain contributing the majority to hydrophobic interaction between the two subunits and a small C-terminal domain penetrating the other subunit. In addition, two citrate molecules occupied the 6PG binding pocket of each monomer. The intact Gnd1 had a Km of 50 +/- 9 microM for 6-phosphogluconate and of 35 +/- 6 microM for NADP+ at pH 7.5. But the truncated mutants without the C-terminal 35, 39 or 53 residues of Gnd1 completely lost their 6PGDH activity, despite remaining the homodimer in solution.

CONCLUSION

The overall tertiary structure of Gnd1 is similar to those of 6PGDH from other species. The substrate and coenzyme binding sites are well conserved, either from the primary sequence alignment, or from the 3D structural superposition. Enzymatic activity assays suggest a sequential mechanism of catalysis, which is in agreement with previous studies. The C-terminal domain of Gnd1 functions as a hook to further tighten the dimer, but it is not necessary for the dimerization. This domain also works as a lid on the substrate binding pocket to control the binding of substrate and the release of product, so it is indispensable for the 6PGDH activity. Moreover, the co-crystallized citrate molecules, which mimic the binding mode of the substrate 6-phosphogluconate, provided us a novel strategy to design the 6PDGH inhibitors.

Cloning and structural characterization of the 6-phosphogluconate dehydrogenase locus of the medfly Ceratitis capitata and the olive fruit fly Bactrocera oleae

The pentose phosphate cycle is considered as a major source of NADPH and pentose needed for nucleic acid biosynthesis. 6-Phosphogluconate dehydrogenase (6PGD), an enzyme participating in this cycle, catalyzes the oxidative decarboxylation of 6PGD to ribulose 5-phosphate with the subsequent release of CO(2) and the reduction of NADP. We have determined the genomic sequences of 6PGD of two species of Tephritidae, the medfly Ceratitis capitata and olive fruit fly Bactrocera oleae, and constructed a three-dimensional model of 6PGD of C. capitata based on the homologous known sheep structure. In a comparative study of 6PGD sequences from seven species, all the conserved and variable sites of the enzyme were analyzed and the regions of functional importance were localized, an attempt promoted also by the direct involvement of the enzyme in various human diseases. The enzymes between the two species of Tephritidae have a very high homology and further examination of the variable positions with respect to the highly conserved binding site residues enabled their grouping in three distinct categories, with possible association to dimer formation, functional specificity, and antigenicity. Moreover, placement of sequence differences on the 3-D model suggests probable sites accommodating variations appearing at the allozymic variants of both species.

Purification and Kinetic Properties of 6-Phosphogluconate Dehydrogenase from Rat Small Intestine

6-Phosphogluconate dehydrogenase (6PG) was purified from rat small intestine with 36% yield and a specific activity of 15 U/mg. On SDS/PAGE, one band with a mass of 52 kDa was found. On native PAGE three protein and two activity bands were observed. The pH optimum was 7.35. Using Arrhenius plots, Ea, DeltaH, Q10 and Tm for 6PGD were found to be 7.52 kcal/mol, 6.90 kcal/mol, 1.49 and 49.4 degrees C, respectively. The enzyme obeyed "Rapid Equilibrium Random Bi Bi" kinetic model with Km values of 595 +/- 213 microM for 6PG and 53.03+/-1.99 microM for NADP. 1/Vm versus 1/6PG and 1/NADP plots gave a Vm value of 8.91+/-1.92 U/mg protein. NADPH is the competitive inhibitor with a Ki of 31.91+/-1.31 microM. The relatively small Ki for the 6PGD:NADPH complex indicates the importance of NADPH in the regulation of the pentose phosphate pathway through G6PD and 6PGD.

Functional constraints of 6-phosphogluconate dehydrogenase (6-PGD) based on sequence and structural information

The pentose phosphate cycle is considered as a major source of NADPH and pentose needed for nucleic acid biosynthesis. 6-Phosphogluconate dehydrogenase (6PGD), an enzyme participating in this cycle, catalyzes the oxidative decarboxylation of 6PGD to ribulose 5-phosphate with the subsequent release of CO2 and the reduction of NADP. We have determined the amino acid sequence of 6PGD of Bactrocera oleae and constructed a three-dimensional model based on the homologous known sheep structure. In a comparative study of 6PGD sequences from numerous species, all the conserved and variable regions of the enzyme were analyzed and the regions of functional importance were localized, in an attempt promoted also by the direct involvement of the enzyme in various human diseases. Thus, analysis of amino acid variability of 37 6PGD sequences revealed that all regions important for the catalytic activity, such as those forming the substrate and coenzyme binding sites, are highly conserved in all species examined. Moreover, several amino acid residues responsible for substrate and coenzyme specificity were also found to be identical in all species examined. The higher percentage of protein divergence is observed at two regions that accumulate mutations, located at the distant parts of the two domains of the enzyme with respect to their interface. These peripheral regions of non-functional importance are highly variable and are predicted as antigenic, thus reflecting possible regions for antibody recognition. Furthermore, locating the differences between diptera 6PGD sequences on the three-dimensional model suggests probable positions of different amino acid residues appearing at B. oleae fast, intermediate, and slow allozymic variants.

Purification and characterization of glucose-6-phosphate dehydrogenase from rat small intestine

Glucose-6-phosphate dehydrogenase (G6PD) was purified from rat small intestine with 19.2% yield and had a specific activity of 53.8 units per miligram protein. The pH optimum was determined to be 8.1. The purified rat small intestinal G6PD gave one activity, one protein band on native PAGE. The observation of one band on SDS/PAGE with an Mr of 48 kDa and a specific activity lower than expected may suggest the proteolytically affected enzyme or different form of G6PD in the rat small intestine. The activation energy, activation enthalpy, Q10, and optimum temperature from Arrhenius plot for the rat small intestinal G6PD were found to be 8.52 kcal/mol, 7.90 kcal/mol, 1.59, and 38 degrees C, respectively. The Km values for G6P and NADP+ were 70.1 +/- 20.8 and 23.2 +/- 7.6 microM, respectively. Double-reciprocal plots of 1/Vm versus 1/G6P (at constant [NADP+]) and of 1/Vm versus 1/NADP+ at constant [G6P]) intersected at the same point on the 1/Vm axis to give Vm = 53.8 U/mg protein.

Sugar derivatives as new 6-phosphogluconate dehydrogenase inhibitors selective for the parasite Trypanosoma brucei

Sugar derivatives mimicking compounds which take part in the catalysed reaction have been assayed as alternative substrates and/or competitive inhibitors of 6-phosphogluconate dehydrogenase from Trypanosoma brucei and sheep liver. Phosphonate analogues have been synthesised and the new compound 5-deoxy-5-phosphono-D-arabinonate shows good selectivity towards the parasite enzyme. A number of 4-carbon and 5-carbon aldonates are strong inhibitors of the parasite enzyme with K(i) values below the substrate K(m) and some acyl derivatives are also potent inhibitors. At least five of the compounds showing a significant selectivity for the parasite enzyme represent leads for trypanocidal drugs against this recently validated target.

Recombinant human glucose-6-phosphate dehydrogenase. Evidence for a rapid-equilibrium random-order mechanism

Cloning and over-expression of human glucose 6-phosphate dehydrogenase (Glc6P dehydrogenase) has for the first time allowed a detailed kinetic study of a preparation that is genetically homogeneous and in which all the protein molecules are of identical age. The steady-state kinetics of the recombinant enzyme, studied by fluorimetric initial-rate measurements, gave converging linear Lineweaver-Burk plots as expected for a ternary-complex mechanism. Patterns of product and dead-end inhibition indicated that the enzyme can bind NADP+ and Glc6P separately to form binary complexes, suggesting a random-order mechanism. The Kd value for the binding of NADP+ measured by titration of protein fluorescence is 8.0 microm, close to the value of 6.8 microm calculated from the kinetic data on the assumption of a rapid-equilibrium random-order mechanism. Strong evidence for this mechanism and against either of the compulsory-order possibilities is provided by repeating the kinetic analysis with each of the natural substrates replaced in turn by structural analogues. A full kinetic analysis was carried out with deaminoNADP+ and with deoxyglucose 6-phosphate as the alternative substrates. In each case the calculated dissociation constant upon switching a substrate in a random-order mechanism (e.g. that for NADP+ upon changing the sugar phosphate) was indeed constant within experimental error as expected. The calculated rate constants for binding of the leading substrate in a compulsory-order mechanism, however, did not remain constant when the putative second substrate was changed. Previous workers, using enzyme from pooled blood, have variously proposed either compulsory-order or random-order mechanisms. Our study appears to provide unambiguous evidence for the latter pattern of substrate binding.

Dog liver glucose-6-phosphate dehydrogenase: purification and kinetic properties

Glucose-6-phosphate dehydrogenase (G6PD) catalyses the first step of the pentose phosphate pathway which generates NADPH for anabolic pathways and protection systems in liver. G6PD was purified from dog liver with a specific activity of 130 U x mg(-1) and a yield of 18%. PAGE showed two bands on protein staining; only the slower moving band had G6PD activity. The observation of one band on SDS/PAGE with M(r) of 52.5 kDa suggested the faster moving band on native protein staining was the monomeric form of the enzyme. Dog liver G6PD had a pH optimum of 7.8. The activation energy, activation enthalpy, and Q10, for the enzymatic reaction were calculated to be 8.96, 8.34 kcal x mol(-1), and 1.62, respectively.The enzyme obeyed "Rapid Equilibrium Random Bi Bi" kinetic model with Km values of 122 +/- 18 microM for glucose-6-phosphate (G6P) and 10 +/- 1 microM for NADP. G6P and 2-deoxyglucose-6-phosphate were used with catalytic efficiencies (kcat/Km) of 1.86 x 10(6) and 5.55 x 10(6) M(-1) x s(-1), respectively. The intrinsic Km value for 2-deoxyglucose-6-phosphate was 24 +/- 4mM. Deamino-NADP (d-NADP) could replace NADP as coenzyme. With G6P as cosubstrate, Km d-ANADP was 23 +/- 3mM; Km for G6P remained the same as with NADP as coenzyme (122 +/- 18 microM). The catalytic efficiencies of NADP and d-ANADP (G6P as substrate) were 2.28 x 10(7) and 6.76 x 10(6) M(-1) x s(-1), respectively. Dog liver G6PD was inhibited competitively by NADPH (K(i)=12.0 +/- 7.0 microM). Low K(i) indicates tight enzyme:NADPH binding and the importance of NADPH in the regulation of the pentose phosphate pathway.

Selective inhibition of 6-phosphogluconate dehydrogenase from Trypanosoma brucei

A number of triphenylmethane derivatives have been screened against 6-phosphogluconate dehydrogenase from Trypanosoma brucei and sheep liver. Some of these compounds show good inhibition of the enzymes and also selectivity towards the parasite enzyme. Modelling was undertaken to dock the compounds into the active sites of both enzymes. Using a combination of DOCK 3.5 and FLEXIDOCK a correlation was obtained between docking score and both activity for the enzymes and selectivity. Visualisation of the docked structures of the inhibitors in the active sites of the enzymes yielded a possible explanation of the selectivity for the parasite enzyme.

Kinetic properties of human placental glucose-6-phosphate dehydrogenase

The kinetic properties of placental glucose-6-phosphate dehydrogenase were studied, since this enzyme is expected to be an important component of the placental protection system. In this capacity it is also very important for the health of the fetus. The placental enzyme obeyed "Rapid Equilibrium Ordered Bi Bi" sequential kinetics with K(m) values of 40+/-8 microM for glucose-6-phosphate and 20+/-10 microM for NADP. Glucose-6-phosphate, 2-deoxyglucose-6-phosphate and galactose-6-phosphate were used with catalytic efficiencies (k(cat)/K(m)) of 7.4 x 10(6), 4.89 x 10(4) and 1.57 x 10(4) M(-1).s(-1), respectively. The K(m)app values for galactose-6-phosphate and for 2-deoxyglucose-6-phosphate were 10+/-2 and 0.87+/-0.06 mM. With galactose-6-phosphate as substrate, the same K(m) value for NADP as glucose-6-phosphate was obtained and it was independent of galactose-6-phosphate concentration. On the other hand, when 2-deoxyglucose-6-phosphate used as substrate, the K(m) for NADP decreased from 30+/-6 to 10+/-2 microM as the substrate concentration was increased from 0.3 to 1.5 mM. Deamino-NADP, but not NAD, was a coenzyme for placental glucose-6-phosphate dehydrogenase. The catalytic efficiencies of NADP and deamino-NADP (glucose-6-phosphate as substrate) were 1.48 x 10(7) and 4.80 x 10(6) M(-1)s(-1), respectively. With both coenzymes, a hyperbolic saturation and an inhibition above 300 microM coenzyme concentration, was observed. Human placental glucose-6-phosphate dehydrogenase was inhibited competitively by 2,3-diphosphoglycerate (K(i)=15+/-3 mM) and NADPH (K(i)=17.1+/-3.2 microM). The small dissociation constant for the G6PD:NADPH complex pointed to tight enzyme:NADPH binding and the important role of NADPH in the regulation of the pentose phosphate pathway.

Purification and some properties of human placental glucose-6-phosphate dehydrogenase

Glucose-6-phosphate dehydrogenase was purified from human placenta using DEAE-Sepharose fast flow, 2',5'-ADP Sepharose 4B column chromatography, and chromatofocusing on PBE 94 with PB 74. The enzyme was purified with 62% yield and had a specific activity of 87 units per milligram protein. The pH optimum was determined to be 7.8, using zero buffer extrapolation method. The purified placental glucose-6-phosphate dehydrogenase gave two activity bands on native PAGE: one band, constituting about 3--5% of total activity, moved slower than the remaining 95%. Among the activity bands only the faster moving band gave a band on protein staining. The slower moving band, which probably corresponded to the higher polymeric form of the G6PD with high specific activity, was not seen on native PAGE due to insufficient protein for Coomassie brilliant blue staining. The observation of one band on SDS--PAGE with an M(r) of 54 kDa and a specific activity lower than expected, suggests the presence of both forms of the G6PD, the high polymeric form at low concentration and the inactive form at high concentration, in our preparation. Measuring the activities of placental glucose-6-phosphate dehydrogenase between 20 and 50 degrees C, the activation energy, activation enthalpy, and Q(10) were calculated to be 8.16 kcal/mol, 7.55 kcal/mol, and 1.57, respectively. It was found that human placental G6PD obeys Michaelis-Menten kinetics. K(m) values were determined using the concentration ranges of 20--300 microM for G6P and 10--200 microM for NADP(+). The K(m) value for G6P was 40 microM; the K(m) value NADP(+) was found to be 20 microM. Double-reciprocal plots of 1/Vm vs 1/G6P (at constant [NADP(+)]) and of 1/Vm vs 1/NADP(+) (at constant [G6P]) intersected at the same point on the 1/V(m) axis to give V(m) = 87 U/mg protein.

Molecular characterization of the first two enzymes of the pentose-phosphate pathway of Trypanosoma brucei. Glucose-6-phosphate dehydrogenase and 6-phosphogluconolactonase

Trypanosomatids are parasitic protists that have part of their glycolytic pathway sequestered inside peroxisome-like organelles: the glycosomes. So far, at least one enzyme of the pentose-phosphate pathway has been found to be associated partially with glycosomes. Here, we describe how two genes from Trypanosoma brucei, coding for the first two enzymes of the pentose-phosphate pathway, i.e. glucose-6-phosphate dehydrogenase and 6-phosphogluconolactonase, were identified by in silico screening of trypanosome genome project data bases. These genes were cloned and sequenced. Analysis of the lactonase sequence revealed that it contained a C-terminal peroxisome targeting signal in agreement with its subcellular localization in the bloodstream form trypanosome (15% glycosomal and 85% cytosolic). However, the dehydrogenase sequence did not reveal any targeting signal, despite its localization inside glycosomes. The corresponding enzymes have been overexpressed in Escherichia coli and purified, and their biochemical characteristics have been determined.

Kinetic properties of the glucose-6-phosphate and 6-phosphogluconate dehydrogenases from Corynebacterium glutamicum and their application for predicting pentose phosphate pathway flux in vivo

The glucose-6-phosphate (Glc6P) and 6-phosphogluconate (6PG) dehydrogenases of the amino-acid-producing bacterium Corynebacterium glutamicum were purified to homogeneity and kinetically characterized. The Glc6P dehydrogenase was a heteromultimeric complex, which consists of Zwf and OpcA subunits. The product inhibition pattern of the Glc6P dehydrogenase was consistent with an ordered bi-bi mechanism. The 6PG dehydrogenase was found to operate according to a Theorell-Chance ordered bi-ter mechanism. Both enzymes were inhibited by NADPH and the 6PG dehydrogenase additionally by ATP, fructose 1,6-bisphosphate (Fru1,6P2), D-glyceraldehyde 3-phosphate (Gra3P), erythrose 4-phosphate and ribulose 5-phosphate (Rib5P). The inhibition by NADPH was considered to be most important, with inhibition constants of around 25 microM for both enzymes. Intracellular metabolite concentrations were determined in two isogenic strains of C. glutamicum with plasmid-encoded NAD- and NADP-dependent glutamate dehydrogenases. NADP+ and NADPH levels were between 130 microM and 290 microM, which is very much higher than the respective Km and Ki values. The Glc6P concentration was around 500 microM in both strains. The in vivo fluxes through the oxidative part of the pentose phosphate pathway calculated on the basis of intracellular metabolite concentrations and the kinetic constants of the purified enzymes determined in vitro were in agreement with the same fluxes determined by NMR after 13C-labelling. From the derived kinetic model thus validated, it is concluded that the oxidative pentose phosphate pathway in C. glutamicum is mainly regulated by the ratio of NADPH and NADP+ concentrations and the specific enzyme activities of both dehydrogenases.

Acute effect of H.E.L.P. treatment on radical scavenging enzyme activities, total glutathione concentrations in granulocytes, and selenium in plasma

BACKGROUND

It has been suggested that granulocytes are activated on artificial surfaces such as dialyzer membranes or by plasma separation procedures resulting in the generation of free radicals. We reported recently that free radical scavenging enzyme (FRSE) activities of red blood cells obtained from patients undergoing hemodialysis and LDL-apheresis (LA) do not reflect an acute oxidative stress. However, because mature red cells are free of DNA and RNA, enzymes cannot be regulated on the gene level. In contrast, granulocytes are nucleated cells in which genes can be regulated, e. g. by redox sensitive transcription factors activated by extracellular oxidative stress. Therefore, granulocyte FRSE may better reflect acute oxidative stress caused by extracorporeal treatment.

MATERIALS AND METHODS

Hyperlipidemic patients (n = 18) with coronary heart disease (CHD) were treated with the Heparin-induced-Extracorporeal-LDL-Precipitation (H.E.L.P.) system. Glutathione peroxidase (GSH-Px), glutathione reductase (GSSG-R), superoxide dismutase (SOD) activities, and total glutathione were determined in granulocytes before and immediately after a single LA treatment. Selenium (Se) concentrations were assessed in plasma.

RESULTS

As a result of the H.E.L.P. treatment GSSG-R activity was significantly induced (+ 20%) and the GSH concentration increased (+ 41%) in granulocytes. GSH-Px activity in granulocytes (- 19%) and Se in plasma (- 27%) were significantly reduced whereas SOD activity in granulocytes was not affected by the H.E.L.P. procedure.

CONCLUSION

These results show that the defence against oxygen radicals in granulocytes is affected but not severely compromised in patients undergoing regular H.E.L.P-LDL-apheresis treatment, which points to the safety of this system with respect to oxidative stress.

Review: the oxidant/antioxidant balance during regular low density lipoprotein apheresis

Low density lipoprotein (LDL) apheresis is a safe procedure to treat severe hypercholesterolemia in patients with chronic heart disease (CHD). However, both hypercholesterolemia and extracorporeal treatment have been associated with oxidative stress. Even though LDL lowering has been proven to reduce CHD, the oxidative modification of LDL has been suggested to render these lipoproteins more atherogenic. It is therefore important to know whether LDL apheresis is safe with respect to oxidative stress including LDL oxidation. The contact of living cells such as leukocytes with artificial surfaces during extracorporeal treatment induces the liberation of various chemokines and cytokines as well as oxygen-derived radicals also known as respiratory burst. These effects justify the consideration of leukocyte activation resulting from extracorporeal treatment as an inflammatory reaction. In extracorporeal circuits such as those used for hemodialysis, the release of oxygen radicals has been shown and depends on the fiber material used in the dialyzer membranes. Reactive oxygen radicals can interact with different cell components such as carbohydrates, DNA, proteins, and lipids. Antioxidants in the form of low molecular weight molecules such as glutathione or radical scavenging enzymes such as superoxide dismutase offer protection against the damaging effects of prooxidants. The disturbed balance between prooxidants and antioxidants is considered as oxidative stress. Therefore, either an increase in oxygen radical formation or a decrease of antioxidants will lead to oxidative stress. During LDL apheresis, a decrease of low molecular weight antioxidants has been reported. In contrast, we have observed an increase in plasma glutathione concentrations but no severe reduction in the activity of antioxidant enzymes in plasma, red cells, or granulocytes, which may explain the lack of plasma lipid peroxidation shown during this kind of extracorporeal treatment. In addition, LDL isolated at the end of apheresis procedures are more resistant to oxidation. These findings suggest that LDL apheresis is safe with respect to radical mediated injury.

A rapid method for the purification of glucose-6-phosphate dehydrogenase from bovine lens

This paper describes a simple and rapid method for the purification of glucose-6-phosphate dehydrogenase from bovine lens, together with analysis of the kinetic behaviour and some properties of the enzyme. The purification consisted of two steps, 2',5'-ADP-Sepharose 4B affinity chromatography and DEAE Sepharose Fast Flow ion exchange chromatography in procedure which took two working days. The enzyme was obtained with a yield of 13.7% and had a specific activity of 2.64 U/mg protein. The overall purification was about 19,700-fold. The molecular weight of the enzyme was found to be 62 +/- 3 kDa by Sephadex G-200 gel filtration chromatography. A protein band corresponding to a molecular weight of 69.2 +/- 3.2 kDa was obtained on SDS polyacrylamide slab gel electrophoresis. On chromatofocusing, lens glucose-6-phosphate dehydrogenase gave a single peak at pI 5.14. The activation energy of the reaction catalyzed by the enzyme was calculated from Arrhenius plot as Ea = 5.88 kcal/mol. The pH versus velocity curve had two peaks at pH 7.7 and 9.6. By the double-reciprocal plots and the product inhibition studies, it was shown that the enzyme follows 'Ordered Bi Bi' sequential kinetics. From the graphical and statistical analyses, KmNADP+, KmG-6-P, KiNADPH, Ki6-PGA were estimated to be 0.008 +/- 0.002, 0.035 +/- 0.013, 0.173 +/- 0.007 and 1.771 +/- 0.160 mM, respectively. The observed kinetic behaviour of glucose-6-phosphate dehydrogenase from bovine lens was in accordance with the enzyme from other sources.

A 2.8 A resolution structure of 6-phosphogluconate dehydrogenase from the protozoan parasite Trypanosoma brucei: comparison with the sheep enzyme accounts for differences in activity with coenzyme and substrate analogues

The three-dimensional structure of 6-phosphogluconate dehydrogenase (6PGDH) from the parasitic protozoan Trypanosoma brucei has been solved at 2.8 A resolution. This pentose phosphate pathway enzyme is NADP-dependent; NADPH generated in the reaction protects against oxidative stress. The enzyme crystallises in the space-group P3121 with a dimer in the asymmetric unit and cell dimensions a=b=135.13 A, c=116.74 A, alpha=beta=90 degrees, gamma=120 degrees. The structure has refined to R=18.6% (Rfree=27.3%) with good geometry. The amino acid sequence of T. brucei 6PGDH is only 35% identical to that of the sheep liver enzyme and significant activity differences have been observed. The active dimer assembles with the C-terminal tail of one subunit threaded through the other, forming part of the substrate binding site. The tail of T. brucei 6PGDH is shorter than that of the sheep enzyme and its terminal residues associate tightly with the second monomer. The three-dimensional structure shows this generates additional interactions between the subunits close to the active site; the coenzyme binding domain is thereby associated more tightly with the helical domain. Three residues, conserved in all other known sequences, are important in creating a salt bridge between monomers close to the substrate binding site. The differences could explain the 200-fold enhanced affinity observed for the substrate analogue 6-phospho-2-deoxy-D-gluconate and suggest targets for anti-parasite drug design. The coenzyme binding domain of 6PGDH has a beta-alpha-beta fold; while in most species the "fingerprint" sequence is GxAxxG, in the T. brucei enzyme it is GxGxxG. Additional interactions between the enzyme and the coenzyme bis-phosphate are likely in the parasite 6PGDH, accounting for greater inhibition (40-fold) of 2'5'-ADP. While the core of the T. brucei dimer was restrained during refinement, several conformational differences have been found between the monomers; those at the coenzyme binding site suggest the molecule could be asymmetric during the enzyme reaction.

Crystal structure and active site location of N-(1-D-carboxylethyl)-L-norvaline dehydrogenase

Opine dehydrogenases catalyze the NAD(P)H-dependent reversible reaction to form opines that contain two asymmetric centers exhibiting either (L,L) or (D,L) stereochemistry. The first structure of a (D,L) superfamily member, N-(1-D-carboxylethyl)-L-norvaline dehydrogenase (CENDH) from Arthrobacter sp. strain 1C, has been determined at 1.8 A resolution and the location of the bound nucleotide coenzyme has been identified. Six conserved residues cluster in the cleft between the enzyme's two domains, close to the nucleotide binding site, and are presumed to define the enzyme's catalytic machinery. Conservation of a His-Asp pair as part of this cluster suggests that the enzyme mechanism is related to the 2-hydroxy acid dehydrogenases. The pattern of sequence conservation and substitution between members of this enzyme family has permitted the tentative location of the residues that define their differential substrate specificities.

6-Phosphogluconate Dehydrogenase from Trypanosoma Brucei. Kinetic Analysis and Inhibition by Trypanocidal Drugs

The kinetics of 6-phosphogluconate dehydrogenase from Trypanosoma brucei was examined and compared to those of the same enzyme from lamb's liver. Variation of kinetic parameters as a function of pH suggests a chemical mechanism similar to other 6-phosphogluconate dehydrogenases. The comparison extended to a detailed analysis of the effect on enzyme activity by several inhibitors including the trypanocidal drugs suramin, melarsoprol and analogues of these compounds. The T. brucei enzyme differs significantly from its mammalian counterpart with respect to several inhibitors, particularly the substrate analogue 6-phospho-2-deoxygluconate and the coenzyme analogue adenosine 2',5'-bisphosphate which have respectively 170-fold and 40-fold higher affinity for the parasite enzyme.

Importance of Se-glutathione peroxidase, catalase, and Cu/Zn-SOD for cell survival against oxidative stress

Eukaryotic cells have to constantly cope with highly reactive oxygen-derived free radicals. Their defense against these free radicals is achieved by natural antioxidant molecules but also by antioxidant enzymes. In this paper, we review some of the data comparing the efficiency of three different antioxidant enzymes: Cu/Zn-superoxide dismutase (Cu/Zn-SOD), catalase, and selenium-glutathione peroxidase. We perform our comparison on one experimental model (human fibroblasts) where the activities of these three antioxidant enzymes have been modulated inside the cells, and the repercussion of these changes was investigated in different conditions. We also focus our attention on the protecting role of selenium-glutathione peroxidase, because this enzyme is very rarely studied due to the difficulties linked to its biochemical properties. These studies evidenced that all three antioxidant enzymes give protection for the cells. They show a high efficiency for selenium-glutathione peroxidase and emphasize the fact that each enzyme has a specific as well as an irreplaceable function. They are all necessary for the survival of the cell even in normal conditions. In addition, these three enzymes act in a cooperative or synergistic way to ensure a global cell protection. However, optimal protection is achieved only when an appropriate balance between the activities of these enzymes is maintained. Interpretation of the deleterious effects of free radicals has to be analyzed not only as a function of the amount of free radicals produced but also relative to the efficiency and to the activities of these enzymatic and chemical antioxidant systems. The threshold of protection can indeed vary dramatically as a function of the level of activity of these enzymes.

Crystallographic study of coenzyme, coenzyme analogue and substrate binding in 6-phosphogluconate dehydrogenase: implications for NADP specificity and the enzyme mechanism

BACKGROUND

The nicotinamide adenine dinucleotide phosphate (NADP)-dependent oxidative decarboxylase, 6-phosphogluconate dehydrogenase, is a major source of reduced coenzyme for synthesis. Enzymes later in the pentose phosphate pathway convert the reaction product, ribulose 5-phosphate, to ribose 5-phosphate. Crystallographic study of complexes with coenzyme and substrate explain the NADP dependence which determines the enzyme's metabolic role and support the proposed general base-general acid mechanism.

RESULTS

The refined structures of binary coenzyme/analogue complexes show that Arg33 is ordered by binding the 2'-phosphate, and provides one face of the adenine site. The nicotinamide, while less tightly bound, is more extended when reduced than when oxidized. All substrate binding residues are conserved; the 3-hydroxyl of 6-phosphogluconate is hydrogen bonded to N zeta of Lys183 and the 3-hydrogen points towards the oxidized nicotinamide. The 6-phosphate replaces a tightly bound sulphate in the apo-enzyme.

CONCLUSIONS

NADP specificity is achieved primarily by Arg33 which binds the 2'-phosphate but, in its absence, obscures the adenine pocket. The bound oxidized nicotinamide is syn; hydride transfer from bound substrate to the nicotinamide si- face is achieved with a small movement of the nicotinamide nucleotide. Lys183 may act as general base. A water bound to Gly130 in the coenzyme domain is the most likely acid required in decarboxylation. The dihydronicotinamide ring of NADPH competes for ligands with the 1-carboxyl of 6-phosphogluconate.

A two-step purification procedure for sheep liver 6-phosphogluconate dehydrogenase

A two-step procedure for the purification of 6-phosphogluconate dehydrogenase (EC 1.1.1.44; 6-PGDH) from sheep liver is described. The enzyme is directly bound to cellulose phosphate by batch extraction and eluted with a linear salt gradient. Purification is completed by affinity chromatography using NADP(+)-agarose. The result is 6-PGDH of high purity, greatly increased yield, and the highest specific activity yet achieved, with a significant reduction in the purification time.

Kinetics of human erythrocyte glucose-6-phosphate dehydrogenase dimers

The steady-state kinetics of human erythrocyte glucose-6-phosphate dehydrogenase (D-glucose-6-phosphate: NADP+ 1-oxidoreductase, EC 1.1.1.49) dimers were studied by initial rate measurement. These experiments gave intersecting double-reciprocal plots suggesting a ternary complex mechanism with a Km for NADP and glucose 6-phosphate of 11 microM and 43 microM, respectively. These studies were combined with rate measurements in the presence of one product (NADPH), dead-end inhibitors, as well as alternative substrates. The inhibition by NADPH was found to be competitive with respect to both substrates. Alternate substrates experiments gave linear double-reciprocal plots over a wide range of substrate concentrations. The results suggest that the dimeric enzyme follows either a random or a Theorell-Chance mechanism.

Hexose monophosphate shunt activity in erythrocytes related to cell age

Erythrocytes were separated by age using a combination of density centrifugation and counterflow centrifugation and tested for basal activity of the hexose monophosphate shunt (HMP-shunt) as well as the methylene blue-stimulated maximal capacity by measuring CO2 production. No significant differences were found in basal HMP-shunt activity, but the maximal methylene blue-stimulated activity of old erythrocytes reached only half of the activity of the total cell population. The maximal HMP-shunt activity showed a significant correlation with hexokinase activity, but not with glucose-6-phosphate dehydrogenase activity in all but the youngest cells. The sensitivity to oxidative stress was tested by measuring the kinetics of pyruvate kinase isolated from erythrocytes incubated in presence and absence of methylene blue. Pyruvate kinase kinetics were affected more in the old cell population than in the total cell population: the K0.5 for phosphoenol-pyruvate increased four times in the unseparated cells and eight times in old cells.

Early strain-related changes in enzyme activity in osteocytes following bone loading in vivo

The skeleton's architecture is matched to the changing loads to which it is subjected because mechanical loading directly or indirectly influences the activity of cell populations to deposit, maintain, or remove bone tissue as appropriate. This responsiveness to load bearing presupposes that certain cells are sensitive to load itself or to its consequences within the tissue. The nature of this effect and the cells concerned have not yet been determined. In this series of experiments, bones were exposed in vivo to a single short period of dynamic loading, which if repeated daily had been shown to result in increased new bone formation. There was an increase in the activity of glucose 6-phosphate dehydrogenase (G6PD) in the periosteal cells adjacent to the bone surface 6 min after this single period of loading. This increase was proportional to the strain magnitude in the bone tissue in the immediate vicinity of the cells. In osteocytes, although the G6PD activity in each individual cell was unchanged by loading, the number of cells displaying activity was increased. This increase was also directly proportional to the applied strain in that area of the cortex (52% compared with 26% active osteocytes at a strain of 0.002). Activation of G6PD was unaccompanied by any equivalent changes in the activities of either glyceraldehyde 3-phosphate dehydrogenase (GA3PD) or lactate dehydrogenase (LDH). This finding is consistent with loading increasing the activity of the oxidative part of the pentose monophosphate shunt pathway. It is also consistent with stimulation of a synthetic process, such as the production of RNA from ribose 5-phosphate.(ABSTRACT TRUNCATED AT 250 WORDS)

Tyrosyl-tRNA synthetase from baker's yeast. Order of substrate addition, discrimination of 20 amino acids in aminoacylation of tRNATyr-C-C-A and tRNATyr-C-C-A(3'NH2)

The order of substrate addition to tyrosyl-tRNA synthetase from baker's yeast was investigated by bisubstrate kinetics, product inhibition and inhibition by dead-end inhibitors. The kinetic patterns are consistent with a random bi-uni uni-bi ping-pong mechanism. Substrate specificity with regard to ATP analogs shows that the hydroxyl groups of the ribose moiety and the amino group in position 6 of the base are essential for recognition of ATP as substrate. Specificity with regard to amino acids is characterized by discrimination factors D which are calculated from kcat and Km values obtained in aminoacylation of tRNATyr-C-C-A. The lowest values are observed for Cys, Phe, Trp (D = 28,000-40,000), showing that, at the same amino acid concentrations, tyrosine is 28,000-40,000 times more often attached to tRNATyr-C-C-A than the noncognate amino acids. With Gly, Ala and Ser no misacylation could be detected (D greater than 500,000); D values of the other amino acids are in the range of 100,000-500,000. Lower specificity is observed in aminoacylation of the modified substrate tRNATyr-C-C-A(3'NH2) (D1 = 500-55,000). From kinetic constants and AMP-formation stoichiometry observed in aminoacylation of this tRNA species, as well as in acylating tRNATyr-C-C-A hydrolytic proof-reading factors could be calculated for a pretransfer (II 1) and a post-transfer (II 2) proof-reading step. The observed values of II 1 = 12-280 show that pretransfer proof-reading is the main correction step whereas post-transfer proof-reading is marginal for most amino acids (II 2 = 1-2). Initial discrimination factors caused by differences in Gibbs free energies of binding between tyrosine and noncognate amino acids are calculated from discrimination and proof-reading factors. Assuming a two-step binding process, two factors (I1 and I2) are determined which can be related to hydrophobic interaction forces. The tyrosine side chain is bound by hydrophobic forces and hydrogen bonds formed by its hydroxyl group. A hypothetical model of the amino acid binding site is discussed and compared with results of X-ray analysis of the enzyme from Bacillus stearothermophilus.