Interactions of nicotinamide-adenine dinucleotide phosphate analogues and fragments with pigeon liver malic enzyme. Synergistic effect between the nicotinamide and adenine moieties

3'-NADP and 3'-NAADP, Two Metabolites Formed by the Bacterial Type III Effector AvrRxo1

An arsenal of effector proteins is injected by bacterial pathogens into the host cell or its vicinity to increase virulence. The commonly used top-down approaches inferring the toxic mechanism of individual effector proteins from the host's phenotype are often impeded by multiple targets of different effectors as well as by their pleiotropic effects. Here we describe our bottom-up approach, showing that the bacterial type III effector AvrRxo1 of plant pathogens is an authentic phosphotransferase that produces two novel metabolites by phosphorylating nicotinamide/nicotinic acid adenine dinucleotide at the adenosine 3′-hydroxyl group. Both products of AvrRxo1, 3′-NADP and 3′-nicotinic acid adenine dinucleotide phosphate (3′-NAADP), are substantially different from the ubiquitous co-enzyme 2′-NADP and the calcium mobilizer 2′-NAADP. Interestingly, 3′-NADP and 3′-NAADP have previously been used as inhibitors or signaling molecules but were regarded as “artificial” compounds so far. Our findings now necessitate a shift in thinking about the biological importance of 3′-phosphorylated NAD derivatives.

Lysine residues 162 and 340 are involved in the catalysis and coenzyme binding of NADP(+)-dependent malic enzyme from pigeon

Alanine-scanning site-directed mutagenesis was carried out on all conserved lysine residues of pigeon cytosolic NADP(+)-dependent malic enzyme. Only two mutant enzymes, K162A and K340A, showed significant effect on their kinetic parameters. Both mutant enzymes have K(m) values for Mn(2+) and l-malate similar to those of wild-type. The K(m) value for NADP(+) of K162A is identical to that of wild-type. However, K162A demonstrated a 235-fold decrease in the k(cat) value (0.17 +/- 0.01 vs 40.0 +/- 1.3 s(-1)). These data suggested that the side chain of K162 is important for the enzyme catalytic reaction. We propose that the epsilon-amino group of K162 may serve as a general acid to protonate the 3-carbon of enolpyruvate after decarboxylation. The K340A mutant demonstrated no effect on the k(cat) value. However, its K(m) value for NADP(+) was increased by a factor of 65 (225.7 +/- 5.07 vs 3.49 +/- 0.05 microM). We propose that the NADP(+) specificity is determined by the electrostatic interaction between the epsilon-amino group of K340 and 2'-phosphate of NADP(+).

Purification and characterization of the cytosolic NADP(+)-dependent malic enzyme from human breast cancer cell line

Cytosolic NADP(+)-dependent malic enzyme from a cultured human breast cancer cell line was purified to near homogeneity by two highly efficient chromatography systems: Pharmacia-LKB Q-Sepharose anion-exchange chromatography and adenosine-2',5'-bisphosphate-agarose affinity chromatography. The overall yield was 27%. The enzyme is presumably a tetramer composed of four probably identical subunits of Mr 65,000, which is similar to the enzyme from other sources. The pI and optimum reaction pH values for the tumor malic enzyme are 5.5 and 7.2, respectively. At pH 6.9, most of the enzyme exists as monomers. Activation energy for the enzyme-catalyzed oxidative-decarboxylation reaction is 57.4 kJ/mol. The enzyme is strictly NADP+ dependent, as NAD+ cannot support the oxidative-decarboxylation reaction. ATP at low concentration inhibits the enzyme activity. Fumarate at concentrations up to 5 mM does not affect the enzymatic reaction rate. Therefore the tumor cytosolic malic enzyme, unlike the mitochondrial malic enzyme, is not an allosteric regulatory enzyme.

Quaternary structure of pigeon liver malic enzyme

Pigeon liver malic enzyme (EC 1.1.1.40) has a double dimer quaternary structure. The NADP+ analogs, aminopyridine adenine dinucleotide phosphate and nicotinamide-1,N6-ethenoadenosine dinucleotide phosphate, bind to the enzyme anti-cooperatively. In the presence of non-cooperative competing ligand NADP+, the binding parameter Hill coefficients of these analogues changed very little. Binding of L-malate with enzyme-AADP+ complex first enhanced then reduced the nucleotide fluorescence. Two L-malate binding sites, with Kd values of 23-30 and 270-400 microM, respectively. for the tight and weak binding sites were postulated. A hybrid model between the sequential and pre-existing asymmetrical models was proposed for the pigeon liver malic enzyme.

Kinetic and Structural Properties of NADP-Malic Enzyme from Sugarcane Leaves

Oligomeric structure and kinetic properties of NADP-malic enzyme, purified from sugarcane (Saccharam officinarum L.) leaves, were determined at either pH 7.0 and 8.0. Size exclusion chromatography showed the existence of an equilibrium between the dimeric and the tetrameric forms. At pH 7.0 the enzyme was found preferentially as a 125 kilodalton homodimer, whereas the tetramer was the major form found at pH 8.0. Although free forms of l-malate, NADP(+), and Mg(2+) were determined as the true substrates and cofactors for the enzyme at the two conditions, the kinetic properties of the malic enzyme were quite different depending on pH. Higher affinity for l-malate (K(m) = 58 micromolar), but also inhibition by high substrate (K(i) = 4.95 millimolar) were observed at pH 7.0. l-Malate saturation isotherms at pH 8.0 followed hyperbolic kinetics (K(m) = 120 micromolar). At both pH conditions, activity response to NADP(+) exhibited Michaelis-Menten behavior with K(m) values of 7.1 and 4.6 micromolar at pH 7.0 and 8.0, respectively. Negative cooperativity detected in the binding of Mg(2+) suggested the presence of at least two Mg(2+) - binding sites with different affinity. The K(a) values for Mg(2+) obtained at pH 7.0 (9 and 750 micromolar) were significantly higher than those calculated at pH 8.0 (1 and 84 micromolar). The results suggest that changes in pH and Mg(2+) levels could be important for the physiological regulation of NADP-malic enzyme.

Inhibition of human cancer cell growth by periodate-oxidized 3-aminopyridine adenine dinucleotide phosphate

1. Periodate-oxidized 3-aminopyridine-adenine dinucleotide phosphate inhibited the proliferation of oral epithelium cancer and breast cancer cell lines. 2. The fast growing less differentiated embryonic kidney cell was more affected by the reagent then the embryonic lung fibroblast cell. 3. Incorporation of [3H]leucine of the treated cancer cells was inhibited. Incorporation of [3H]uridine was increased. Incorporation of [3H]thymidine was first increased and then decreased. 4. Tumor malic enzyme activity was inhibited by the reagent; but the treated cells did not show any difference in malic enzyme or glucose-6-phosphate dehydrogenase levels.

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.

Fluorescence studies on the dissociation and denaturation of pigeon liver malic enzyme

Exposure of pigeon liver malic enzyme [S)-malate:NADP+ oxidoreductase (oxaloacetate-decarboxylating), EC 1.1.1.40) in medium concentrations of guanidine-HCl at 25 degrees C and pH 7.45 caused biphasic conformational changes of the enzyme molecule. Molecular weight determination confirmed that the enzyme tetramers were dissociated to monomers in phase I transition. Enzymatic activity was completely lost in this phase. Recovery of the enzyme activity was only possible in the early stages of the phase I transition. Phase II was due to enzyme unfolding, as judged by circular dichroism and the fluorescence parameters of the enzyme. The steps of the transformation of native malic enzyme into a completely denatured state were in the following sequence: tetramer----monomer----random coil. Extensive denaturation of the enzyme molecule resulted in irreversible aggregation. Dissociation and denaturation were accompanied by a red-shift of the fluorescence spectrum (328----368 nm). Fluorescence quenching studies indicated that tryptophan residues of the enzyme molecule were buried deeply in the interior of the molecule. The tryptophan residues were only partially accessible by acrylamide and almost inaccessible by KI. Dissociation and denaturation were accompanied by exposure of the tryptophan residues, as manifested by the accessibility of the enzyme molecule toward KI or acrylamide.

High malic enzyme activity in tumor cells and its cross-reaction with anti-pigeon liver malic enzyme serum

Rabbit antibodies against pigeon liver malic enzyme (EC 1.1.1.40) were prepared. The antiserum gave single precipitation line with crude pigeon liver extract. Cross reaction was observed with partially purified malic enzyme or crude extract from chicken liver. Positive cross reaction was also observed with the concentrated cytosolic fraction of two human carcinoma cell lines which were demonstrated to contain high malic enzyme activity. All other proteins examined did not react with the antibodies. When purified pigeon liver malic enzyme was mixed with the antiserum in vitro, a time-dependent inactivation of the enzyme activity was observed. Protection of the enzyme activity against antiserum inactivation was afforded by NADP+ or L-malate. Metal Mn2+ gave little protection.