Enzyme saturation and inhibition kinetics studied from multiple progress curves recorded spectrophotometrically from single reaction mixtures for ADP-ribose pyrophosphatase

A novel fluorometric assay for ADP-ribose pyrophosphatase activity

ADP-ribose pyrophosphatase (ADPRase) hydrolyzes ADP-ribose to ribose-5-phosphate and AMP. The ADPRase activity have been assessed by coupling the reaction to alkaline phosphatase and colorimetrically measuring the amount of inorganic phosphate released from AMP that is one of the products of ADPRase. Another but less sensitive colorimetric method has been employed: the reaction mixture was treated with charcoal to adsorb the adenine-containing compounds such as AMP and ADPR and subsequently remaining ribose-5-phosphate was measured colorimetrically. However, the measurement of inorganic phosphate cannot be feasible to assay ADPRase in phosphate-containing samples and the determination of ribose-5-phosphate also is less sensitive. Here we develop a fluorescent assay for ADPRase that utilizes 1, N(6)-etheno ADP-ribose, a fluorescent analogue of ADP-ribose. This method measures fluorescent 1, N(6)-etheno adenosine that is produced by coupling the hydrolysis of 1, N(6)-etheno ADP-ribose to dephosphorylation with alkaline phosphatase. The fluorometric assay is comparable in sensitivity and useful for ADPRase assay in phosphate-containing samples.

Human placenta hydrolases active on free ADP-ribose: an ADP-sugar pyrophosphatase and a specific ADP-ribose pyrophosphatase

Free ADP-ribose has a reducing ribose moiety and it is hazardous due to its nonenzymic reactivity toward protein side chains. ADP-ribose hydrolases are putative protective agents to avoid the intracellular accumulation of ADP-ribose. In mammalian sources, two types of enzymes with ADP-ribose hydrolase activity are known: (i) highly specific ADP-ribose pyrophosphatases, which in a Mg(2+)-dependent fashion hydrolyse only ADP-ribose and the nonphysiological analogue IDP-ribose, and (ii) less specific nucleoside diphosphosugar or diphosphoalcohol (NDP-X) pyrophosphatases, which besides A(I)DP-ribose hydrolyse also some nonreducing NDP-X substrates. So far, of these two enzyme types only the less specific one has been reported in human sources: an ADP-sugar pyrophosphatase purified from erythrocytes or expressed from cDNA clones. Here we report that human placenta extracts contain two ADP-ribose hydrolases, which were characterised after a near 1000-fold purification. One is an ADP-sugar pyrophosphatase: it hydrolysed ADP-ribose, ADP-glucose and ADP-mannose, but not e.g. UDP-glucose, at similar rates. It resembles the erythrocyte and recombinant enzyme(s), but showed a 5-20-fold lower K(m) for ADP-ribose (7 microM). The other enzyme is a highly specific ADP-ribose pyrophosphatase (the first of this kind to be reported in humans): it hydrolysed only ADP-ribose and IDP-ribose at similar rates, with a very low, 0.4 microM K(m) for the former. This is a major candidate to control the accumulation of free ADP-ribose in humans. It remains to be seen whether it belongs to the 'nudix' protein family, which includes several ADP-ribose hydrolases and other 'housecleaning' enzymes (M.J. Bessman, D.N. Frick, S.F. O'Handley, J. Biol. Chem. 271 (1996) 25059-25062).

ADP-ribose pyrophosphatase-I partially purified from livers of rats overdosed with acetaminophen reveals enzyme inhibition in vivo reverted in vitro by dithiothreitol

Free ADP-ribose reacts nonenzymatically with proteins and can lead to intracellular damage. The low-Km ADP-ribose pyrophosphatase-I (ADPRibase-I) is well suited to control free ADP-ribose and nonenzymatic ADP-ribosylation. In vitro, the acetaminophen metabolite N-acetyl-p-benzoquinoneimine (NAPQI) decreases ADPRibase-I Vmax and increases Km, effects not reverted by dithiothreitol (DTT) and attributed to enzyme arylation. The present study was conducted to test whether acetaminophen overdose affected ADPRibase-I in vivo. Rats pretreated with 3-methylcholanthrene and L-buthionine-[S,R]-sulfoximine to potentiate acetaminophen toxicity received an intraperitoneal dose of either acetaminophen (800 mg/ kg; n = 5) or vehicle (n = 3). ADPRibase-I partially purified from acetaminophen-overdosed rats showed a decreased Vmax (0.32+/-0.09 versus 0.60+/-0.03 mU/mg of liver protein; p<0.01) not reverted by DTT and an increased Km for ADP-ribose (1.39+/-0.31 versus 0.67+/-0.05 microM; p<0.01) that, contrary to the in vitro NAPQI effect, was reverted by DTT. Incubation of partially purified ADPRibase-I from normal rat liver with oxidized glutathione elicited a time- and dose-dependent, DTT-reverted increase of Km, without change of Vmax. The results indicate that the activity of ADPRibase-I can be regulated by thiol exchange and that the increase of Km, elicited by acetaminophen overdosage was related to the oxidative stress caused by the drug. It remains to be seen whether an increase of free ADP-ribose concomitant to ADPRibase-I inhibition could contribute to the hepatotoxicity of acetaminophen.

Purification and characterization of adenosine diphosphate ribose pyrophosphatase from human erythrocytes

Free ADP-ribose is a turnover product of NAD+, protein-bound polymeric and monomeric ADP-ribose, and cyclic ADP-ribose. But little is known about the specific cellular roles or metabolism of free ADP-ribose. ADP-ribose pyrophosphatase (EC 3.6.1.13), which hydrolyzes ADP-ribose into AMP and ribose-5'-phosphate, was purified from human erythrocytes. Purification was achieved to homogeneity by successive chromatographic steps, resulting in a final purification of 75,790-fold from the hemolysate. The purified enzyme showed a single band with the molecular weight of 34 kDa on SDS-PAGE both in the presence and absence of 2-mercaptoethanol. The molecular weight of the native enzyme calculated by gel filtration was 68 kDa, indicating that the active enzyme is a dimer of identical subunits. The enzyme requiring Mg2+ showed highest activity toward ADP-ribose, and about 40-70% activities with IDP-ribose, ADP-mannose and GDP-mannose. The enzyme showed a Km of 169 +/- 11 microM for ADP-ribose, broad pH optimum around pH 9.5, and pI of 5.1. ADP was a potent noncompetitive inhibitor with a Ki of 16 +/- 1.2 microM. These results suggest that our enzyme is unique, and different from the other ADP-ribose pyrophosphatases reported. ADP-ribose pyrophosphatase may play an important role in the regulation of intracellular steady-state of free ADP-ribose.

Rat liver ADP-ribose pyrophosphatase-I as an in vitro target of the acetaminophen metabolite N-acetyl-p-benzoquinoneimine

N-acetyl-p-benzoquinoneimine (NAPQI) is the metabolite responsible for acetaminophen hepatotoxicity. ADP-ribose pyrophosphatase-I (ADPRibase-I; EC 3.6.1.13) hydrolyzes protein-glycating ADP-ribose. The results show NAPQI-dependent alterations of ADPRibase-I leading to strong inhibition: a fast Km increase produced by low concentrations, and a time-dependent Vmax decrease by higher NAPQI concentrations. Both effects were prevented by thiols, but not reverted by them, nor by gel filtration of NAPQI-treated enzyme. Liver ADPRibase-I can be a target of NAPQI-dependent arylation. The inhibition or inactivation of the enzyme would contribute to increasing the free ADP-ribose concentration and nonenzymatic ADP-ribosylation, which is coherent with results linking free ADP-ribose-producing pathways to acetaminophen toxicity.

Rat liver nucleoside diphosphosugar or diphosphoalcohol pyrophosphatases different from nucleotide pyrophosphatase or phosphodiesterase I: substrate specificities of Mg(2+)-and/or Mn(2+)-dependent hydrolases acting on ADP-ribose

Three rat liver nucleotides(5') diphosphosugar (NDP-sugar) or nucleoside(5') diphosphoalcohol pyrophosphatases are described: two were previously identified in experiments measuring Mg(2+)-dependent ADP-ribose pyrophosphatase activity (Miró et al. (1989) FEBS Lett. 244, 123-126), and the other is a new, Mn(2+)-dependent ADP-ribose pyrophosphatase. They are resolved by ion-exchange chromatography, and differ by their substrate and cation specificities, KM values for ADP-ribose, pH-activity profiles, molecular weights and isoelectric points. The enzymes were tested for activity towards: reducing (ADP-ribose, IDP-ribose) and non-reducing NDP-sugars (ADP-glucose, ADP-mannose, GDP-mannose, UDP-mannose, UDP-glucose, UDP-xylose, CDP-glucose), CDP-alcohols (CDP-glycerol, CDP-ethanolamine, CDP-choline), dinucleotides (diadenosine pyrophosphate, NADH, NAD+, FAD), nucleoside(5') mono- and diphosphates (AMP, CMP, GMP, ADP, CDP) and dTMP p-nitrophenyl ester. Since the enzymes have not been purified to homogeneity, more than three pyrophosphatases may be present, but the co-purification of activities, thermal co-inactivation, and inhibition experiments give support to: (i) and ADP-ribose pyrophosphatase highly specific for ADP(IDP)-ribose in the presence of Mg2+, but active also on non-reducing ADP-hexoses and dinucleotides (not on NAD+) when Mg2+ was replaced with Mn2+; (ii) a Mn(2+)-dependent pyrophosphatase active on ADP(IDP)-ribose, dinucleotides and CDP-alcohols; (iii) a rather unspecific pyrophosphatase that, with Mg2+, was active on AMP(IMP)-containing NDP-sugars and dinucleotides (not on NAD+), and with Mn2+, was also active on non-adenine NDP-sugars and CDP-alcohols. The enzymes differ from nucleotide pyrophosphatase/phosphodiesterase-I (NPPase/PDEaseI) by their substrate specificities and by their cytosolic location and solubility in the absence of detergents. Although NPPase/PDEaseI is much more active in rat liver, its known location in the non-cytoplasmic sides of plasma and endoplasmic reticulum membranes, together with the known cytoplasmic synthesis of NDP-sugars and CDP-alcohols, permit the speculation that the pyrophosphatases studied in this work may have a cellular role.

Rat liver mitochondrial ADP-ribose pyrophosphatase in the matrix space with low Km for free ADP-ribose

A study involving markers of subcellular and submitochondrial fractions, gradient centrifugation, latency measurements and extraction with digitonin, demonstrates the association of a specific ADP-ribose pyrophosphatase with rat liver mitochondria and its localization in the matrix space. The enzyme hydrolyses ADP-ribose to AMP, with a Km of 2-3 microM. The results support the occurrence of a specific turnover pathway for free ADP-ribose and its relevance in mitochondria.

Dinucleoside tetraphosphatase from human blood cells. Purification and characterization as a high specific activity enzyme recognized by an anti-rat tetraphosphatase antibody

Dinucleoside tetraphosphatase (Np4Nase; EC 3.6.1.17) has been purified 170,000-fold from a 30-60% ammonium sulfate fraction of a human blood cell extract. Purification included a dye-ligand affinity elution step using the inhibitor adenosine 5'-tetraphosphate. Human blood Np4Nase resembled rat liver Np4Nase, including recognition by anti-rat Np4Nase, but differed from homogeneous human leukemia Np4Nase in the 1000-fold lower specific activity of the latter. The results are discussed in relation to the potential role of diadenosine tetraphosphate (Ap4A) in the control of cell division and the turnover of Ap4A in blood.