Protective Effects of Tinospora cordifolia on Hepatic and Gastrointestinal Toxicity Induced by Chronic and Moderate Alcoholism.

AIMS: Heavy alcohol intake depletes the plasma vitamins due to hepatotoxicity and decreased intestinal absorption. However, moderate alcohol intake is often thought to be healthy. Therefore, effects of chronic moderate alcohol intake on liver and intestine were studied using urinary vitamin levels. Furthermore, effects of Tinospora cordifolia water extract (TCE) (hepatoprotective) on vitamin excretion and intestinal absorption were also studied. METHODS: In the study, asymptomatic moderate alcoholics (n = 12) without chronic liver disease and healthy volunteers (n = 14) of mean age 39 ± 2.2 (mean ± SD) were selected and divided into three groups. TCE treatment was performed for 14 days. The blood and urine samples were collected on Day 0 and 14 after treatment with TCE and analyzed. RESULTS: In alcoholics samples, a significant increase in the levels of gamma-glutamyl transferase, aspartate transaminase, alanine transaminase, Triglyceride, Cholesterol, HDL and LDL (P < 0.05) was observed but their level get downregulated after TCE intervention. Multivariate analysis of metabolites without missing values showed an increased excretion of 7-dehydrocholesterol, orotic acid, pyridoxine, lipoamide and niacin and TCE intervention depleted their levels (P < 0.05). In contrast, excretion of biotin, xanthine, vitamin D2 and 2-O-p-coumaroyltartronic acid (CA, an internal marker of intestinal absorption) were observed to be decreased in alcoholic samples; however, TCE intervention restored the CA and biotin levels. Vitamin metabolism biomarkers, i.e. homocysteine and xanthurenic acid, were also normalized after TCE intervention. CONCLUSION: Overall data depict that moderate alcohol intake is also hepatotoxic and decreases intestinal absorption. However, TCE treatment effectively increased the intestinal absorption and retaining power of liver that regulated alcohol-induced multivitamin deficiency.

Structures of asymmetric complexes of human neuron specific enolase with resolved substrate and product and an analogous complex with two inhibitors indicate subunit interaction and inhibitor cooperativity.

In the presence of magnesium, enolase catalyzes the dehydration of 2-phospho-d-glycerate (PGA) to phosphoenolpyruvate (PEP) in glycolysis and the reverse reaction in gluconeogensis at comparable rates. The structure of human neuron specific enolase (hNSE) crystals soaked in PGA showed that the enzyme is active in the crystals and produced PEP; conversely soaking in PEP produced PGA. Moreover, the hNSE dimer contains PGA bound in one subunit and PEP or a mixture of PEP and PGA in the other. Crystals soaked in a mixture of competitive inhibitors tartronate semialdehyde phosphate (TSP) and lactic acid phosphate (LAP) showed asymmetry with TSP binding in the same site as PGA and LAP in the PEP site. Kinetic studies showed that the inhibition of NSE by mixtures of TSP and LAP is stronger than predicted for independently acting inhibitors. This indicates that in some cases inhibition of homodimeric enzymes by mixtures of inhibitors ("heteroinhibition") may offer advantages over single inhibitors.

Recognition of (2S)-aminomalonyl-acyl carrier protein (ACP) and (2R)-hydroxymalonyl-ACP by acyltransferases in zwittermicin A biosynthesis.

Polyketide synthases elongate a polyketide backbone by condensing carboxylic acid precursors that are thioesterified to either coenzyme A or an acyl carrier protein (ACP). Two of the three known ACP-linked extender units, (2S)-aminomalonyl-ACP and (2R)-hydroxymalonyl-ACP, are found in the biosynthesis of the agriculturally important antibiotic zwittermicin A. We previously reconstituted the formation of (2S)-aminomalonyl-ACP and (2R)-hydroxymalonyl-ACP from the primary metabolites l-serine and 1,3-bisphospho-d-glycerate. In this report, we characterize the two acyltransferases involved in the specific transfer of the (2S)-aminomalonyl and (2R)-hydroxymalonyl moieties from the ACPs associated with extender unit formation to the ACPs integrated into the polyketide synthase. This work establishes which acyltransferase recognizes each extender unit and also provides insight into the substrate selectivity of these enzymes. These are important step toward harnessing these rare polyketide synthase extender units for combinatorial biosynthesis.

Stopped-flow studies of the reaction of D-tartronate semialdehyde-2-phosphate with human neuronal enolase and yeast enolase 1.

We determined the kinetics of the reaction of human neuronal enolase and yeast enolase 1 with the slowly-reacting chromophoric substrate D-tartronate semialdehyde phosphate (TSP), each in tris (tris (hydroxymethyl) aminomethane) and another buffer at several Mg2+ concentrations, 50 or 100 microM, 1 mM and 30 mM. All data were biphasic, and could be satisfactorily fit, assuming either two successive first-order reactions or two independent first-order reactions. Higher Mg2+ concentrations reduce the relative magnitude of the slower reaction. The results are interpreted in terms of a catalytically significant interaction between the two subunits of these enzymes.

Crystallographic studies on Ascaris suum NAD-malic enzyme bound to reduced cofactor and identification of an effector site.

The crystal structure of the mitochondrial NAD-malic enzyme from Ascaris suum, in a quaternary complex with NADH, tartronate, and magnesium has been determined to 2.0-A resolution. The structure closely resembles the previously determined structure of the same enzyme in binary complex with NAD. However, a significant difference is observed within the coenzyme-binding pocket of the active site with the nicotinamide ring of NADH molecule rotating by 198 degrees over the C-1-N-1 bond into the active site without causing significant movement of the other catalytic residues. The implications of this conformational change in the nicotinamide ring to the catalytic mechanism are discussed. The structure also reveals a binding pocket for the divalent metal ion in the active site and a binding site for tartronate located in a highly positively charged environment within the subunit interface that is distinct from the active site. The tartronate binding site, presumably an allosteric site for the activator fumarate, shows striking similarities and differences with the activator site of the human NAD-malic enzyme that has been reported recently. Thus, the structure provides additional insights into the catalytic as well as the allosteric mechanisms of the enzyme.

Structure of a closed form of human malic enzyme and implications for catalytic mechanism.

Malic enzymes are widely distributed in nature and have many biological functions. The crystal structure of human mitochondrial NAD(P)+-dependent malic enzyme in a quaternary complex with NAD+, Mn++ and oxalate has been determined at 2.2 A resolution. The structures of the quaternary complex with NAD+, Mg++, tartronate or ketomalonate have been determined at 2.6 A resolution. The structures show the enzyme in a closed form in these complexes and reveal the binding modes of the cation and the inhibitors. The divalent cation is coordinated in an octahedral fashion by six ligating oxygens, two from the substrate/inhibitor, three from Glu 255, Asp 256 and Asp 279 of the enzyme, and one from a water molecule. The structural information has significant implications for the catalytic mechanism of malic enzymes and identifies Tyr 112 and Lys 183 as possible catalytic residues. Changes in tetramer organization of the enzyme are also observed in these complexes, which might be relevant for its cooperative behavior and allosteric control.

Genetic analysis of a chromosomal region containing genes required for assimilation of allantoin nitrogen and linked glyoxylate metabolism in Escherichia coli.

Growth experiments with Escherichia coli have shown that this organism is able to use allantoin as a sole nitrogen source but not as a sole carbon source. Nitrogen assimilation from this compound was possible only under anaerobic conditions, in which all the enzyme activities involved in allantoin metabolism were detected. Of the nine genes encoding proteins required for allantoin degradation, only the one encoding glyoxylate carboligase (gcl), the first enzyme of the pathway leading to glycerate, had been identified and mapped at centisome 12 on the chromosome map. Phenotypic complementation of mutations in the other two genes of the glycerate pathway, encoding tartronic semialdehyde reductase (glxR) and glycerate kinase (glxK), allowed us to clone and map them closely linked to gcl. Complete sequencing of a 15.8-kb fragment encompassing these genes defined a regulon with 12 open reading frames (ORFs). Due to the high similarity of the products of two of these ORFs with yeast allantoinase and yeast allantoate amidohydrolase, a systematic analysis of the gene cluster was undertaken to identify genes involved in allantoin utilization. A BLASTP search predicted four of the genes that we sequenced to encode allantoinase (allB), allantoate amidohydrolase (allC), ureidoglycolate hydrolase (allA), and ureidoglycolate dehydrogenase (allD). The products of these genes were overexpressed and shown to have the predicted corresponding enzyme activities. Transcriptional fusions to lacZ permitted the identification of three functional promoters corresponding to three transcriptional units for the structural genes and another promoter for the regulatory gene allR. Deletion of this regulatory gene led to constitutive expression of the regulon, indicating a negatively acting function.

Effect of site-directed mutagenesis of His373 of yeast enolase on some of its physical and enzymatic properties.

The X-ray structure of yeast enolase shows His373 interacting with a water molecule also held by residues Glu168 and Glu211. The water molecule is suggested to participate in the catalytic mechanism (Lebioda, L. and Stec, B. (1991) Biochemistry 30, 2817-2822). Replacement of His373 with asparagine (H373N enolase) or phenylalanine (H373F enolase) reduces enzymatic activity to ca. 10% and 0.0003% of the native enzyme activity, respectively. H373N enolase exhibits a reduced Km for the substrate, 2-phosphoglycerate, and produces the same absorbance changes in the chromophoric substrate analogues TSP1 and AEP1, relative to native enolase. H373F enolase binds AEP less strongly, producing a smaller absorbance change than native enolase, and reacts very little with TSP. H373F enolase dissociates to monomers in the absence of substrate; H373N enolase subunit dissociation is less than H373F enolase but more than native enolase. Substrate and Mg2+ increase subunit association in both mutants. Differential scanning calorimetric experiments indicate that the interaction with substrate that stabilizes enolase to thermal denaturation involves His373. We suggest that the function of His373 in the enolase reaction may involve hydrogen bonding rather than acid/base catalysis, through interaction with the Glu168/Glu211/H2O system, which produces removal or addition of hydroxyl at carbon-3 of the substrate.

Transient-state kinetic analysis of the oxidative decarboxylation of D-malate catalyzed by tartrate dehydrogenase.

The oxidative decarboxylation of D-malate catalyzed by tartrate dehydrogenase has been analyzed by transient-state kinetic methods and kinetic isotope effect measurements. The reaction time courses show a burst of NADH formation prior to the attainment of the steady-state velocity. The binding of the inhibitor tartronate to the enzyme was examined by monitoring the quenching of the protein's intrinsic fluorescence; the tartronate concentration dependence of the observed rate constant for association was hyperbolic, supporting a two-step model for inhibitor binding. Analysis of the time courses for D-malate oxidation yielded values for many of the microscopic rate constants governing the reaction. The range of possible solutions for the microscopic rate constants was constrained by comparison of the time course for oxidation of unlabeled malate with that of deuterated malate; this analysis relied on the determination of the intrinsic isotope effect on hydride transfer via measurement of D(V/K), T(V/K), and the oxaloacetate partition ratio. The results of the transient-state kinetic analyses suggest that the rate of D-malate oxidation is largely limited by the rate of decarboxylation of the intermediate oxaloacetate which occurs at 11 s-1. Hydride transfer from D-malate to NAD+ occurs with a rate constant of 300 s-1, and (D)k for this step is 5.5. The agreement between experimentally measured steady-state kinetic parameters and kinetic isotope effects and their values calculated from the microscopic rate constants derived from the transient-state kinetic analyses was quite good.

Toward identification of acid/base catalysts in the active site of enolase: comparison of the properties of K345A, E168Q, and E211Q variants.

High-resolution crystallographic data show that Glu 168 and Glu 211 lie on opposite surfaces of the active site from Lys 345. Two different proposals for general base catalysis have emerged from these structural studies. In one scheme, the carboxylate side chains of Glu 168 and Glu 211 are proposed to ionize a trapped water molecule and the OH- serves as the base [Lebioda, L., & Stec, B. (1991) Biochemistry 30, 2817-2822]. In the other proposal, the epsilon-amino group of Lys 345 functions in general base catalysis [Wedekind, J. E., Poyner, R. R., Reed, G. H., & Rayment, I. (1994) Biochemistry 33, 9333-9342]. Genes encoding site specific mutations of these active site residues of yeast enolase, K345A, E168Q, and E211Q, have been prepared. The respective protein products of the wild type and mutant genes were expressed in Escherichia coli and isolated in homogeneous form. All three mutant proteins possess severely depressed activities in the overall reaction- < 1 part in 10(5) of wild type activity. Properties of the three mutant proteins in partial reactions were examined to define more clearly the roles of these residues in the catalytic cycle. The K345A variant fails to catalyze the exchange of the C-2 proton of 2-phospho-D-glycerate with deuterium in D2O, whereas both the E211Q and E168Q mutant proteins are functional in this partial reaction. For E211Q and E168Q enolases, exchange is essentially complete prior to appearance of product, and this observation provides further support for an intermediate in the normal reaction. K345A enolase is inactive in the ionization of tartronate semialdehyde phosphate (TSP), whereas both E168Q and E211Q proteins alter the tautomeric state or catalyze ionization of bound TSP. Wild type enolase catalyzes hydrolysis of (Z)-3-chloro-2-phosphoenolpyruvate by addition of OH- and elimination of Cl- at C-3. This reaction mimics the addition of OH- to C-3 of phosphoenolpyruvate in the reverse reaction with the normal product. All three mutant proteins are depressed in their abilities to carry out this reaction. In single-turnover assays, the activities vary in the order K345A > E168Q >> E211Q. These results suggest that Lys 345 functions as the base in the ionization of 2-PGA and that Glu 211 participates in the second step of the reaction.

Reversible dissociation of the catalytically active subunits of pigeon liver malic enzyme.

The pH-induced reversible dissociation of pigeon liver malic enzyme (EC 1.1.1.40) was studied by combined use of chemical cross-linking and SDS/polyacrylamide-gel electrophoresis. The tetrameric enzyme showed a pH-dependent dissociation in an acidic environment. At pH values above 8.0 most molecules existed as tetramers. The enzyme was gradually dissociated at lower pH. When the pH was below 5.0 most of the enzyme was present as the monomeric forms. Reassociation of the subunits was accomplished by adjusting the pH to neutrality. The dissociation and reassociation were almost instantaneous. No trimer was detected. The pigeon liver malic enzyme was thus shown to have a double-dimer quaternary structure with D2 symmetry. In the presence of substrates, the monomer-dimer-tetramer equilibrium favours the direction of dissociation. Tartronate, an L-malate analogue, was found to be more effective than L-malate in this process. When the monomeric forms were immobilized, the enzyme subunits were found to be fully active in catalysis. A possible arrangement of the four identical subunits of the enzyme molecule is proposed to account for the results obtained in this investigation. The origin of the half-of-the-sites reactivity of pigeon liver malic enzyme is also discussed.

Kinetic mechanism in the direction of oxidative decarboxylation for NAD-malic enzyme from Ascaris suum.

Measurement of the initial rate of the malic enzyme reaction varying the concentration of NAD at several different fixed levels of Mg2+ (0.25-1.0 mM) and a single malate concentration gave a pattern which intersects to the left of the ordinate. Repetition of this initial velocity pattern at several additional malate concentrations and treatment in terms of a terreactant mechanism suggests an ordered mechanism in which NAD adds prior to Mg2+ which must add prior to malate. On the other hand, when a broader concentration range of Mg2+ (0.25-50 mM) is used, data are consistent with a random mechanism in which Mg2+ must add prior to malate. By use of product inhibition studies, pyruvate is competitive vs. malate and noncompetitive vs. NAD, while NADH is competitive vs. NAD and noncompetitive vs. malate. These results are consistent with the random addition of substrates and further suggest rapid equilibrium random release of products. Tartronate, a dead-end analogue of malate, is competitive vs. malate and noncompetitive vs. NAD. Thio-NAD is a slow substrate which is used at 2.4% the maximum rate of NAD. When used as a dead-end analogue of NAD, thio-NAD is competitive vs. NAD and gives a complex inhibition pattern vs. malate in which competitive inhibition is apparent at low concentrations of malate (less than 12.5 mM), and this changes to uncompetitive inhibition at high concentrations of malate (greater than 12.5 mM). These data are consistent with a steady-state random mechanism in the direction of oxidative decarboxylation in which Mg2+ adds in rapid equilibrium prior to malate.(ABSTRACT TRUNCATED AT 250 WORDS)

Glyoxylate conversion by Hyphomicrobium species grown on allantoin as nitrogen source.

Glyoxylate, formed as a result of allantoin degradation, is converted by Hyphomicrobium species to glycerate via tartronate semialdehyde. Glyoxylate carboligase and tartronate semialdehyde reductase, the two enzymes involved, are present only in cells grown on allantoin as nitrogen source.