Purification and properties of liver fructose 1,6-bisphosphatase from C57BL/KsJ normal and diabetic mice

Metal-cation regulation of enzyme dynamics is a key factor influencing the activity of S-adenosyl-L-homocysteine hydrolase from Pseudomonas aeruginosa

S-adenosyl-l-homocysteine hydrolase from Pseudomonas aeruginosa (PaSAHase) coordinates one K⁺ ion and one Zn²⁺ ion in the substrate binding area. The cations affect the enzymatic activity and substrate binding but the molecular mechanisms of their action are unknown. Enzymatic and isothermal titration calorimetry studies demonstrated that the K⁺ ions stimulate the highest activity and strongest ligand binding in comparison to other alkali cations, while the Zn²⁺ ions inhibit the enzyme activity. PaSAHase was crystallized in the presence of adenine nucleosides and K⁺ or Rb⁺ ions. The crystal structures show that the alkali ion is coordinated in close proximity of the purine ring and a ²³Na NMR study showed that the monovalent cation coordination site is formed upon ligand binding. The cation, bound in the area of a molecular hinge, orders and accurately positions the amide group of Q65 residue to allow its interaction with the ligand. Moreover, binding of potassium is required to enable unique dynamic properties of the enzyme that ensure its maximum catalytic activity. The Zn²⁺ ion is bound in the area of a molecular gate that regulates access to the active site. Zn²⁺ coordination switches the gate to a shut state and arrests the enzyme in its closed, inactive conformation.

Molecular Mechanisms of Enzyme Activation by Monovalent Cations

Regulation of enzymes through metal ion complexation is widespread in biology and underscores a physiological need for stability and high catalytic activity that likely predated proteins in the RNA world. In addition to divalent metals such as Ca²⁺, Mg²⁺, and Zn²⁺, monovalent cations often function as efficient and selective promoters of catalysis. Advances in structural biology unravel a rich repertoire of molecular mechanisms for enzyme activation by Na⁺ and K⁺ Strategies range from short-range effects mediated by direct participation in substrate binding, to more distributed effects that propagate long-range to catalytic residues. This review addresses general considerations and examples.

Aldolase-B knockout in mice phenocopies hereditary fructose intolerance in humans

The rise in fructose consumption, and its correlation with symptoms of metabolic syndrome (MBS), has highlighted the need for a better understanding of fructose metabolism. To that end, valid rodent models reflecting the same metabolism as in humans, both biochemically and physiologically, are critical. A key to understanding any type of metabolism comes from study of disease states that affect such metabolism. A serious defect of fructose metabolism is the autosomal recessive condition called hereditary fructose intolerance (HFI), caused by mutations in the human aldolase B gene (Aldob). Those afflicted with HFI experience liver and kidney dysfunction after fructose consumption, which can lead to death, particularly during infancy. With very low levels of fructose exposure, HFI patients develop non-alcoholic fatty acid liver disease and fibrosis, sharing liver pathologies also seen in MBS. A major step toward establishing that fructose metabolism in mice mimics that of humans is reported by investigating the consequences of targeting the mouse aldolase-B gene (Aldo2) for deletion in mice (Aldo2(-/-)). The Aldo2(-/-) homozygous mice show similar pathology following exposure to fructose as humans with HFI such as failure to thrive, liver dysfunction, and potential morbidity. Establishing that this mouse reflects the symptoms of HFI in humans is critical for comparison of rodent studies to the human condition, where this food source is increasing, and increasingly controversial. This animal should provide a valuable resource for answering remaining questions about fructose metabolism in HFI, as well as help investigate the biochemical mechanisms leading to liver pathologies seen in MBS from high fructose diets.

C5L2 receptor disruption enhances the development of diet-induced insulin resistance in mice

INTRODUCTION

Acylation stimulating protein (ASP) is a hormone secreted by the adipose tissue that has been shown to increase triglyceride storage and glucose transport in adipocytes. These effects are mediated by C5L2 receptor, which has also been associated with inflammatory effects. C5L2 deficient mice on a low-fat diet are hyperphagic yet lean due to increased energy expenditure. The present study assessed insulin sensitivity and metabolic and inflammatory changes in C5L2KO mice vs WT in diet-induced obesity.

METHODS

We placed C5L2KO and WT mice on a diabetogenic diet for 12 weeks and examined in vivo and ex vivo metabolism.

RESULTS

C5L2KO mice on a diabetogenic diet exhibit decreased insulin sensitivity. Whole body substrate partitioning is evidenced through increased glucose uptake by the liver and decreased uptake by adipose tissue and skeletal muscle. Lipid content of both liver and skeletal muscle was higher in C5L2KO mice vs WT. Furthermore, elevated levels of macrophage markers were found in adipose tissue, liver and skeletal muscle of C5L2KO mice vs WT. Several inflammatory cytokines such as IL-6, MIP-1α and KC were also elevated in plasma of C5L2KO mice vs WT.

CONCLUSIONS

Overall, we demonstrated that C5L2KO mice fed a diabetogenic diet develop more severe insulin resistance than WT mice through altered substrate partitioning, ectopic fat deposition and a pro-inflammatory phenotype.

Sparse linear discriminant analysis for simultaneous testing for the significance of a gene set/pathway and gene selection

MOTIVATION

Pathway and gene set-based approaches for the analysis of gene expression profiling experiments have become increasingly popular for addressing problems associated with individual gene analysis. Since most genes are not differently expressed, existing gene set tests, which consider all the genes within a gene set, are subject to considerable noise and power loss, a concern exacerbated in studies in which the degree of differential expression is moderate for truly differentially expressed genes. For a significantly differentially expressed pathway, it is also of substantial interest to select important genes that drive the differential expression of the pathway.

METHODS

We develop a unified framework to jointly test the significance of a pathway and to select a subset of genes that drive the significant pathway effect. To achieve dimension reduction and gene selection, we decompose each gene pathway into a single score by using a regularized form of linear discriminant analysis, called sparse linear discriminant analysis (sLDA). Testing for the significance of the pathway effect proceeds via permutation of the sLDA score. The sLDA-based test is compared with competing approaches with simulations and two applications: a study on the effect of metal fume exposure on immune response and a study of gene expression profiles among Type II Diabetes patients.

RESULTS

Our results show that sLDA-based testing provides a powerful approach to test for the significance of a differentially expressed pathway and gene selection.

AVAILABILITY

An implementation of the proposed sLDA-based pathway test in the R statistical computing environment is available at http://www.hsph.harvard.edu/~mwu/software/.

SUPPLEMENTARY INFORMATION

Supplementary data are available at Bioinformatics online.

Role of Na+and K+in Enzyme Function

Metal complexation is a key mediator or modifier of enzyme structure and function. In addition to divalent and polyvalent metals, group IA metals Na+and K+play important and specific roles that assist function of biological macromolecules. We examine the diversity of monovalent cation (M+)-activated enzymes by first comparing coordination in small molecules followed by a discussion of theoretical and practical aspects. Select examples of enzymes that utilize M+as a cofactor (type I) or allosteric effector (type II) illustrate the structural basis of activation by Na+and K+, along with unexpected connections with ion transporters. Kinetic expressions are derived for the analysis of type I and type II activation. In conclusion, we address evolutionary implications of Na+binding in the trypsin-like proteases of vertebrate blood coagulation. From this analysis, M+complexation has the potential to be an efficient regulator of enzyme catalysis and stability and offers novel strategies for protein engineering to improve enzyme function.

Emerging experimental therapeutics for bipolar disorder: insights from the molecular and cellular actions of current mood stabilizers

Bipolar disorder afflicts approximately 1-3% of both men and women, and is coincident with major economic, societal, medical, and interpersonal consequences. Current mediations used for its treatment are associated with variable rates of efficacy and often intolerable side effects. While preclinical and clinical knowledge in the neurosciences has expanded at a tremendous rate, recent years have seen no major breakthroughs in the development of novel types of treatment for bipolar disorder. We review here approaches to develop novel treatments specifically for bipolar disorder. Deliberate (ie not by serendipity) treatments may come from one of two general mechanisms: (1) Understanding the mechanism of action of current medications and thereafter designing novel drugs that mimics these mechanism(s); (2) Basing medication development upon the hypothetical or proven underlying pathophysiology of bipolar disorder. In this review, we focus upon the first approach. Molecular and cellular targets of current mood stabilizers include lithium inhibitable enzymes where lithium competes for a magnesium binding site (inositol monophosphatase, inositol polyphosphate 1-phosphatase, glycogen synthase kinase-3 (GSK-3), fructose 1,6-bisphosphatase, bisphosphate nucleotidase, phosphoglucomutase), valproate inhibitable enzymes (succinate semialdehyde dehydrogenase, succinate semialdehyde reductase, histone deacetylase), targets of carbamazepine (sodium channels, adenosine receptors, adenylate cyclase), and signaling pathways regulated by multiple drugs of different classes (phosphoinositol/protein kinase C, cyclic AMP, arachidonic acid, neurotrophic pathways). While the task of developing novel medications for bipolar disorder is truly daunting, we are hopeful that understanding the mechanism of action of current mood stabilizers will ultimately lead clinical trials with more specific medications and thus better treatments those who suffer from this devastating illness.

Interaction of Tl+ with product complexes of fructose-1,6-bisphosphatase

Fructose-1,6-bisphosphatase requires divalent cations (Mg2+, Mn2+, or Zn2+) for catalysis, but a diverse set of monovalent cations (K+, Tl+, Rb+, or NH(4)(+)) will further enhance enzyme activity. Here, the interaction of Tl+ with fructose-1,6-bisphosphatase is explored under conditions that support catalysis. On the basis of initial velocity kinetics, Tl+ enhances catalysis by 20% with a K(a) of 1.3 mm and a Hill coefficient near unity. Crystal structures of enzyme complexes with Mg2+, Tl+, and reaction products, in which the concentration of Tl+ is 1 mm or less, reveal Mg2+ at metal sites 1, 2, and 3 of the active site, but little or no bound Tl+. Intermediate concentrations of Tl+ (5-20 mm) displace Mg2+ from site 3 and the 1-OH group of fructose 6-phosphate from in-line geometry with respect to bound orthophosphate. Loop 52-72 appears in a new conformational state, differing from its engaged conformation by disorder in residues 61-69. Tl+ does not bind to metal sites 1 or 2 in the presence of Mg2+, but does bind to four other sites with partial occupancy. Two of four Tl+ sites probably represent alternative binding sites for the site 3 catalytic Mg2+, whereas the other sites could play roles in monovalent cation activation.

Involvement of calcium/calmodulin signaling in cercosporin toxin biosynthesis by Cercospora nicotianae

Cercosporin is a non-host-selective, perylenequinone toxin produced by many phytopathogenic Cercospora species. The involvement of Ca(2+)/calmodulin (CaM) signaling in cercosporin biosynthesis was investigated by using pharmacological inhibitors. The results suggest that maintaining endogenous Ca(2+) homeostasis is required for cercosporin biosynthesis in Cercospora nicotianae. The addition of excess Ca(2+) to the medium slightly increased fungal growth but resulted in a reduction in cercosporin production. The addition of Ca(2+) chelators [EGTA and 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid] also reduced cercosporin production. Ca(2+) channel blockers exhibited a strong inhibition of cercosporin production only at higher concentrations (>2 mM). Cercosporin production was reduced greatly by Ca(2+) ionophores (A23187 and ionomycin) and internal Ca(2+) blocker [3,4,5-trimethoxybenzoic acid 8-(diethylamino)octyl ester]. Phospholipase C inhibitors (lithium, U73122, and neomycin) led to a concentration-dependent inhibition of cercosporin biosynthesis. Furthermore, the addition of CaM inhibitors (compound 48/80, trifluoperazine, W-7, and chlorpromazine) also markedly reduced cercosporin production. In contrast to W-7, W-5, with less specificity for CaM, led to only minor inhibition of cercosporin production. The inhibitory effects of Ca(2+)/CaM inhibitors were partially or completely reversed by the addition of external Ca(2+). As assessed with Fluo-3/AM (a fluorescent Ca(2+) indicator), the Ca(2+) content in the cytoplasm decreased significantly when fungal cultures were grown in a medium containing Ca(2+)/CaM antagonists, confirming the specificity of those Ca(2+)/CaM antagonists in C. nicotianae. Taken together, the results suggest that Ca(2+)/CaM signal transduction may play a pivotal role in cercosporin biosynthesis in C. nicotianae.

Phosphoglucomutase is an in vivo lithium target in yeast

Lithium is a drug frequently used in the treatment of manic depressive disorder. We have observed that the yeast Saccharomyces cerevisiae is very sensitive to lithium when growing in galactose medium. In this work we show that lithium inhibits with high affinity yeast (IC50 approximately 0.2 mm) and human (IC50 approximately 1.5 mm) phosphoglucomutase, the enzyme that catalyzes the reversible conversion of glucose 1-phosphate to glucose 6-phosphate. Lithium inhibits the rate of fermentation when yeast are grown in galactose and induces accumulation of glucose 1-phosphate and galactose 1-phosphate. Accumulation of these metabolites was also observed when a strain deleted of the two isoforms of phosphoglucomutase was incubated in galactose medium. In glucose-grown cells lithium reduces the steady state levels of UDP-glucose, resulting in a defect on trehalose and glycogen biosynthesis. Lithium acts as a competitive inhibitor of yeast phosphoglucomutase activity by competing with magnesium, a cofactor of the enzyme. High magnesium concentrations revert lithium inhibition of growth and phosphoglucomutase activity. Lithium stress causes an increase of the phosphoglucomutase activity due to an induction of transcription of the PGM2 gene, and its overexpression confers lithium tolerance in galactose medium. These results show that phosphoglucomutase is an important in vivo lithium target.

Mechanism of fat-induced hepatic gluconeogenesis: effect of metformin

High-fat feeding has been shown to cause hepatic insulin resistance. The aims of this study were to investigate the biochemical steps responsible for enhanced gluconeogenesis as a result of increased dietary fat intake and the site or sites at which the antihyperglycemic agent metformin acts to inhibit this process. Male Hooded Wistar rats were fed either a standard chow diet (5% fat by weight) or a high-fat diet (60% fat by weight) for 14 days with or without metformin. Total endogenous glucose production and gluconeogenesis were determined using [6-(3)H]glucose and [U-(14)C]alanine, respectively. Gluconeogenic enzyme activity and, where appropriate, protein and mRNA levels were measured in liver tissues. The high-fat diet increased endogenous glucose production (21.9 +/- 4.4 vs. 32.2 +/- 4.8 micromol x kg(-1) x min(-1), P < 0.05) and alanine gluconeogenesis (4.5 +/- 0.9 vs. 9.6 +/- 1.9 micromol x kg(-1) x min(-1), P < 0.05). Metformin reduced both endogenous glucose production (32.2 +/- 4.8 vs. 16.1 +/- 2.1 micromol x kg(-1) x min(-1), P < 0.05) and alanine gluconeogenesis (9.6 +/- 1.9 vs. 4.7 +/- 0.8 micromol x kg(-1) x min(-1), P < 0.05) after high-fat feeding. These changes were reflected in liver fructose-1,6-bisphosphatase protein levels (4.5 +/- 0.9 vs. 9.6 +/- 1.9 arbitrary units, P < 0.05 chow vs. high-fat feeding; 9.5 +/- 1.9 vs. 4.7 +/- 0.8 arbitrary units, P < 0.05 high fat fed in the absence vs. presence of metformin) but not in changes to the activity of other gluconeogenic enzymes. There was a significant positive correlation between alanine gluconeogenesis and fructose-1,6-bisphosphatase protein levels (r = 0.56, P < 0.05). Therefore, excess supply of dietary fat stimulates alanine gluconeogenesis via an increase in fructose-1,6-bisphosphatase protein levels. Metformin predominantly inhibits alanine gluconeogenesis by preventing the fat-induced changes in fructose-1,6-bisphosphatase levels.

Lithium regulation of aldolase A expression in the rat frontal cortex: identification by differential display

BACKGROUND

Substantial evidence indicates that lithium may exert its therapeutic effects through progressive adaptive changes at the level of gene expression; however, the study of lithium-regulated genes has been primarily undertaken with the "candidate gene" approach based on a specific testable hypothesis. The aim of our study was to identify lithium-regulated genes that would not be predicted a priori by the candidate gene approach.

METHODS

Differential display polymerase chain reaction was used to isolate and identify messenger RNAs (mRNAs) that are differentially expressed in the frontal cortex of rats given lithium for 5 weeks to achieve plasma lithium concentrations of 0.6 to 0.9 mmol/L.

RESULTS

A putative lithium-regulated complementary DNA fragment (LRG1) was identified. Northern blot analysis revealed that 5 weeks of lithium treatment, but not 1 week, significantly reduced LRG1 mRNA levels. LRG1 mRNA levels were similarly reduced by 5 weeks of carbamazepine, but not valproate administration. Sequence analysis and search of the GenBank database revealed that LRG1 is analogous to the sequence of the gene for rat aldolase A.

CONCLUSIONS

These results demonstrate that chronic administration of lithium, but not short-term administration, down regulates the levels of aldolase A mRNA, suggesting this effect may play a role in mediating the therapeutic action of this agent.

A novel mammalian lithium-sensitive enzyme with a dual enzymatic activity, 3'-phosphoadenosine 5'-phosphate phosphatase and inositol-polyphosphate 1-phosphatase

We report the molecular cloning in Rattus norvegicus of a novel mammalian enzyme (RnPIP), which shows both 3'-phosphoadenosine 5'-phosphate (PAP) phosphatase and inositol-polyphosphate 1-phosphatase activities. This enzyme is the first PAP phosphatase characterized at the molecular level in mammals, and it represents the first member of a novel family of dual specificity enzymes. The phosphatase activity is strictly dependent on Mg2+, and it is inhibited by Ca2+ and Li+ ions. Lithium chloride inhibits the hydrolysis of both PAP and inositol-1,4-bisphosphate at submillimolar concentration; therefore, it is possible that the inhibition of the human homologue of RnPIP by lithium ions is related to the pharmacological action of lithium. We propose that the PAP phosphatase activity of RnPIP is crucial for the function of enzymes sensitive to inhibition by PAP, such as sulfotransferase and RNA processing enzymes. Finally, an unexpected connection between PAP and inositol-1,4-bisphosphate metabolism emerges from this work.

Kinetics and mechanisms of activation and inhibition of porcine liver fructose-1,6-bisphosphatase by monovalent cations

K+ and Li+ were used to study the kinetic effects of monovalent cations on porcine liver fructose-1,6-bisphosphatase (FBPase). At saturating fructose 1,6-bisphosphate (FBP) concentrations, Li+ was found to be a linear noncompetitive inhibitor with respect to Mg2+. K+ was found to activate the wild-type enzyme at low concentrations (K(m) = 17 mM) and to inhibit the enzyme at high concentrations (K(IK+) = 68mM). A steady-state random ter mechanism was proposed, and a mathematical equation was derived to account for the Mg2+ and K+ kinetics and activation of FBPase. Interestingly, when Glu280 was mutated to glutamine by site-directed mutagenesis, K+ lost the ability to activate the enzyme and became a noncompetitive inhibitor with respect to Mg2+. These kinetic data suggest that K+ has two distinct sites. One is a high-affinity activation site and the other a low-affinity inhibition site. Glu280 is essential for allowing K+ to bind at the activation site. Due to the geometric constraints and its small atomic radius, Li+ can bind only at the inhibitory site. It is postulated that monovalent cations activate FBPase by helping the Arg276 residue "deshield" the partial negative charge on the 1-phosphoryl group of the substrate so that nucleophilic attack on the 1-phosphorus atom is facilitated. In addition, the monovalent cations may, along with Mg2+ ions and surrounding residues of the protein, help orient the 1-phosphoryl group so as to achieve the optimal position required for catalysis. Monovalent cations inhibit FBPase either by distorting the geometry of the active site or by retarding turnover or product release.

Crystal structure of inositol polyphosphate 1-phosphatase at 2.3-A resolution

Bovine inositol polyphosphate 1-phosphatase (1-ptase), M(r) = 44,000, is a Mg(2+)-dependent/Li(+)-sensitive enzyme that catalyzes the hydrolysis of the 1-position phosphate from inositol 1,4-bisphosphate and inositol 1,3,4-trisphosphate. We have determined the crystal structure of recombinant bovine 1-ptase in the presence of Mg2+ by multiple isomorphous replacement. The structure is currently refined to an R value of 0.198 for 15,563 reflections within a resolution range of 8.0-2.3 A. 1-Ptase is monomeric in the crystal, consistent with biochemical data, and folds into an alternatively layered alpha/beta/alpha/beta sandwich. The central core of 1-ptase consists of a six-stranded antiparallel beta sheet perpendicular to two parallel three-turn alpha-helices. The beta sheet is flanked by two antiparallel six-turn alpha-helices aligned parallel to the beta sheet, and the central helices are flanked by a five-stranded largely parallel beta sheet. Two neighboring metal binding sites are located in adjacent acidic pockets formed by the intersection of several secondary structure elements including an unusual kink structure formed by the "DPIDST" sequence motif. The fold of 1-ptase is similar to that of two other metal-dependent/Li(+)-sensitive phosphatases, inositol monophosphate phosphatase and fructose 1,6-bisphosphatase despite minimal amino acid identity. Comparison of the active-site pockets of these proteins will likely provide insight into substrate binding and the mechanisms of metal-dependent catalysis and Li+ inhibition.

In vitro phosphorylation of fructose-1,6-bisphosphatase from rabbit and pig liver with cyclic AMP-dependent protein kinase

Homogeneous preparations of fructose-1,6-bisphosphatase from mouse, man, rabbit, pig, and rat were tested as substrates for cyclic AMP-dependent protein kinase. Up to 1 mol of [32P]phosphate per mole enzyme subunit was incorporated into fructose-1,6-bisphosphatase from pig and rabbit liver, which should be compared with 2.6 mol of phosphate per mole enzyme subunit in the case of the rat liver enzyme. The phosphorylation of fructose-1,6-bisphosphatase from the livers of man and mouse was negligible. Phosphorylation of pig and rabbit fructose-1,6-bisphosphatase decreased the apparent Km for fructose-1,6-bisphosphate, but in contrast to the case of the rat liver enzyme it did not change the inhibition constants for AMP and fructose-2,6-bisphosphate. The phosphorylation sites in rabbit and pig liver fructose-1,6-bisphosphatase were located close to the carboxyterminal of the polypeptide chains, since trypsin treatment of the phosphorylated enzyme quantitatively removed all of the protein-bound radioactivity without significantly altering the subunit molecular weight and with a maintained neutral pH optimum.

Phosphorylation- and ligand-induced conformational changes of rat liver fructose-1,6-bisphosphatase

The effects of cyclic AMP-dependent phosphorylation on the structural properties of rat liver fructose-1,6-bisphosphatase were investigated by uv difference spectroscopy and circular dichroism. The incorporation of 4 mol of phosphate per mole of fructose-1,6-bisphosphatase induces a significant increase in the alpha-helix content of the enzyme without affecting its spectrophotometric properties. The addition of fructose 1,6-bisphosphate or fructose 2,6-bisphosphate also affects the conformation of the enzyme. However, both the phosphorylated and the nonphosphorylated forms exhibit similar ligand-induced conformational changes. These results show that cyclic AMP-dependent phosphorylation of fructose-1,6-bisphosphatase induces a specific conformational change. They also suggest that this modification does not alter the interaction of the enzyme protein with fructose 1,6-bisphosphate and fructose 2,6-bisphosphate.

Peptide mapping by polyacrylamide gel electrophoresis after cleavage at aspartyl-prolyl peptide bonds in sodium dodecyl sulfate-containing buffers

Protein samples prepared for sodium dodecyl sulfate-polyacrylamide gel electrophoresis are preferentially cleaved at aspartyl-prolyl peptide bonds upon heating at 110 degrees C. The presence of aspartyl-prolyl peptide bonds in a protein can therefore be detected by gel electrophoresis of heated samples and the resulting peptides mapped. The method of heat cleavage also works well with proteins in bands cut from electrophoresed gels using modified stacking conditions in the second electrophoresis. An immunoblotting procedure for peptide mapping of nanogram quantities of specific proteins in complex mixtures is demonstrated. Peptide maps produced by aspartyl-prolyl peptide bond cleavage of fructose-1,6-bisphosphatases from different sources show the effectiveness of the above techniques and suggest a conservation of aspartyl-prolyl peptide bonds in pig kidney and mouse and rat liver fructose-1,6-bisphosphatases.