Autosomal recessive hypotrichosis with loose anagen hairs associated with TKFC mutations.

BACKGROUND: Loose anagen hair is a rare form of impaired hair anchorage in which anagen hairs that lack inner and outer root sheaths can be gently and painlessly plucked from the scalp. This condition usually occurs in children and is often self-limiting. A genetic basis for the disorder has been suggested but not proven. A better understanding the aetiology of loose anagen hair may improve prevention and treatment strategies. OBJECTIVES: To identify a possible genetic basis of loose anagen hair using next-generation DNA sequencing and functional analysis of variants identified. METHODS: In this case study, whole-exome sequencing analysis of a pedigree with one affected individual with features of loose anagen hair was performed. RESULTS: The patient was found to be compound heterozygous for two single-nucleotide substitutions in TKFC resulting in the following missense mutations: c.574G> C (p.Gly192Arg) and c.682C> T (p.Arg228Trp). Structural analysis of human TKFC showed that both mutations are located near the active site cavity. Kinetic assays of recombinant proteins bearing either of these amino acid substitutions showed almost no dihydroxyacetone kinase or D-glyceraldehyde kinase activity, and FMN cyclase activity reduced to just 10% of wildtype catalytic activity. CONCLUSIONS: TKFC missense mutations may predispose to the development of loose anagen hairs. Identification of this new biochemical pathobiology expands the metabolic and genetic basis of hypotrichosis.

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).

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.

Particulate diadenosine 5',5"'-P1,P3-triphosphate hydrolases in rat brain: two specific dinucleoside triphosphatases and two phosphodiesterase I-like hydrolases.

Rat liver and brain differ in the distribution pattern of the total hydrolytic activity on diadenosine 5',5"'-P1,P3-triphosphate (Ap3A) between the soluble and particulate fractions. The Ap3A-hydrolase activity in both the soluble and particulate liver fractions and in the brain soluble fraction had been previously studied in detail. We report now on the brain particulate fraction which, unlike liver, showed a low unspecific phosphodiesterase I-like (PDEaseI, EC 3.1.4.1) activity relative to the specific dinucleoside triphosphatase (Ap3Aase, EC 3.6.1.29). Two PDEaseI-like forms (PDEaseI-A and PDEaseI-B), with different apparent Mrs and kinetic properties, and two Ap3Aases (Ap3Aase-alpha and Ap3Aase-beta) were solubilized with 0.5% Triton X-100 from the particulate fraction. Ap3Aase-alpha resembled the cytosolic Ap3Aase (Ap3Aase-c), a known situation in liver. Comparative to Ap3Aase-alpha, Ap3Aase-beta showed a slightly higher Km (35 vs. 15 micron) and lower isoelectric point (5.25 vs. 5.45); Ap3Aase-beta was absent from the soluble fraction, and its recovery was unaffected by proteinase inhibitors, strongly arguing for distinct soluble and particulate turnover pathways for dinucleoside polyphosphates.

Specific ADP-ribose pyrophosphatase from Artemia cysts and rat liver: effects of nitroprusside, fluoride and ionic strength.

One specific ADP-ribose pyrophosphatase (ADPRibase) has been identified in Artemia cysts, following a protocol that in rat liver allows the identification of three ADPRibases. Artemia ADPRibase resulted similar, but not identical, to rat liver ADPRibase-I with respect to known and novel properties disclosed in this work. In the presence of Mg2+, Artemia ADPRibase was highly specific for ADP-ribose and showed a low, 0.7 microM Km. Preincubation with the nitric oxide donor nitroprusside and dithiothreitol, elicited dose- and time-dependent, severalfold increase of Km and decrease of Vmax. At saturating ADP-ribose concentrations, fluoride was a strong inhibitor (IC50 approximately equal to 10-20 microM), whereas bringing ionic strength to 0.3-1.3 mol/l doubled the activity measured at lower or higher strengths. The novel fluoride and ionic strength effects were studied also with rat liver ADPRibase-I. Differences between the Artemia enzyme and ADPRibase-I concerned molecular weight (31,000 versus 38,500, respectively), Mn2+ ability to substitute for Mg2+ as the activating cation (better for the rat enzyme), and Vmax decrease by nitroprusside (not seen with the rat enzyme). The results are discussed in relation with the role of specific ADPRibases as protective factors limiting free ADP-ribose accumulation and protein glycation, and as targets for cytotoxic agents.

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.

Location of dinucleoside triphosphatase in the matrix space of rat liver mitochondria.

The submitochondrial location of dinucleoside triphosphatase (EC 3.6.1.29), previously shown to be in part associated with mitochondria, has been studied in rat liver. The precipitability and latency of activity in organelle suspensions, and the profile of solubilization by digitonin, were like those of the matrix space marker glutamate dehydrogenase, and differed from those of other submitochondrial fractions. This, and the synthesis of diadenosine polyphosphates by mitochondrial aminoacyl-tRNA synthetases, suggest the occurrence of a pathway for the intramitochondrial turnover of diadenosine 5',5'''-P1,P3-triphosphate (Ap3A).

Detection of specific glucose-3-phosphatase activity in rat liver.

Sugar-3-phosphates are related to aspects of diabetes which depend on protein glycosylation events. Sorbitol-3-phosphate and fructose-3-phosphate occur in normal and diabetic individuals, and glucose-3-phosphate is a potential intermediate in their biosynthesis. Almost nothing is known about enzyme pathways for their metabolic turnover. We have found that part of the phosphohydrolytic activity on glucose-3-phosphate in rat liver supernatants corresponds to a specific, Mg(2+)-dependent, glucose-3-phosphatase much less or not active on other phosphate esters, including glucose-1-phosphate, glucose-6-phosphate, fructose-1-phosphate, fructose-6-phosphate and p-nitrophenyl-phosphate. This finding opens a route to a better understanding of the metabolism and role of sugar-3-phosphates.

A specific, low Km ADP-ribose pyrophosphatase from rat liver.

Two rat liver ADP-ribose pyrophosphatases (ADPRibases) were partially purified. ADPRibase-I hydrolyzed ADP-ribose (Km = 0.5 microM) giving AMP as a product, required Mg2+ or, less efficiently, Mn2+ (Ca2+ was not active), its activity changed little between pH 7 and 9, and was specific for ADP-ribose as it did not hydrolyze ADP-glucose, NAD+, NADH or diadenosine 5',5"'-P1,Pn-n-phosphates (Ap2A, Ap3A). ADPRibase-II showed similar properties, except that the Km for ADP-ribose was 50 microM and may be non-specific, as the same preparation hydrolyzed ADP-glucose, NADH and Ap2A. ADPRibase-I fulfills the requirements of a specific turnover pathway consistent with a cellular role for free ADP-ribose.

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.

Glycine-enhanced inhibition of rat liver nucleotide pyrophosphatase/phosphodiesterase-I by EDTA: a full account of the reported inhibition by commercial preparations of acidic fibroblast growth factor (FGF-1).

The earlier reported inhibition of rat liver nucleotide pyrophosphatase/phosphodiesterase I (EC 3.1.6.9/EC 3.1.4.1; NPP/PDE) by culture-grade acidic fibroblast growth factor (FGF-1) correlates with a low-Mr contaminant. 1H-NMR analyses revealed EDTA in the total-volume fractions of a gel-filtration experiment, where all the inhibitory activity of the FGF-1 preparation was recovered. NPP/PDE inhibition by EDTA (and by unfractionated FGF-1 or the EDTA-containing fractions) was time-dependent, blocked by the substrate p-nitrophenyl-dTMP, and strongly enhanced by glycine. The use of glycine buffers in earlier work was critical to the apparent inhibition by FGF-1. The results point to a conformational change favored by glycine that may be relevant to the biological role of NPP/PDE.

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

Saturation and inhibition kinetics data for rat liver ADP-ribose pyrophosphatase (EC 3.6.1.13) were obtained from progress curves initiated by the addition of substrate and recorded spectrophotometrically until the end point was reached. The hydrolysis of ADP-ribose was coupled to either alkaline phosphatase and adenosine deaminase or AMP deaminase. The validity of the approach was shown because: (i) the coupled hydrolysis of ADP-ribose was essentially irreversible; (ii) ADP-ribose pyrophosphate was stable at 37 degrees C in the conditions needed for the assay; and (iii) accumulated reaction products did not inhibit detectably in the conditions of the assay. In addition, several identical progress curves could be successively recorded by repetition of the addition of substrate. In that way it was possible to carry out complete inhibition studies by increasing the inhibitor concentration between successive substrate additions. Studying the inhibition by high D-ribose concentrations, meaningful results could be obtained at four different inhibitor concentrations in a single reaction mixture, which represented a great saving of enzyme preparation with respect to what would be needed in an equivalent initial rate study.

Purification to homogeneity of rat liver dinucleoside tetraphosphatase by affinity elution with adenosine 5'-tetraphosphate.

Starting from a partially purified dinucleoside tetraphosphatase (Np4Nase; EC 3.6.1.17), we developed an affinity elution purification protocol involving the strong competitive inhibitor adenosine 5'-tetraphosphate. Np4Nase bound to Cibacron Blue F3G-A-Sepharose 4B or to Reactive Blue 2-Sepharose CL-6B was specifically eluted with 10 microM adenosine 5'-tetraphosphate and 5 mM MgCl2, but not by either of them separately. The final Np4Nase preparation was homogeneous by sodium dodecyl sulfate polyacrylamide gel electrophoresis followed by Coomassie blue or silver staining. The protein band showed an apparent 18 kDa molecular mass. The specific activity of the homogeneous Np4Nase was about 150 units/mg, meaning a 45,000-fold increase and a 10% recovery with respect to the crude extract. After preparative polyacrylamide gel electrophoresis, protein visualization with KCl, fragmentation of the gel lane, and extraction, all the renatured Np4Nase activity was found associated to the 18 kDa band. The renatured enzyme showed the same Km value for diadenosine 5',5"'-P1,P4-tetraphosphate as the partially purified or the native homogeneous Np4Nase.

Binding of zinc(II) to beta-D-fructose 2,6-bisphosphate.

The formation of a complex between Zn(II) and beta-D-fructose 2,6-bisphosphate was shown because the latter compound: activated bis(5'-guanosyl)tetraphosphatase (EC 3.6.1.17) and dinucleoside triphosphatase (EC 3.6.1.29) only to the extent that they could be inhibited by Zn(II); increased the consumption of Zn(II) necessary to titrate to an end point a solution of the metallochromic indicator eriochrome black T; coeluted with Zn(II) in a gel filtration column capable of resolving them if unbound. Neither of those effects was shown by D-fructose 1,6-bisphosphate under the same conditions.

The inhibition of fructose 1,6-bisphosphatase by fructose 2,6-bisphosphate is enhanced by EDTA and diminished by zinc(II).

The sensitivity of the Mg(II)-dependent activity of rabbit liver fructose 1,6-bisphosphatase (FBPase, EC 3.1.3.11) to inhibition by fructose 2,6-bisphosphate (Fru-2,6-P2) was enhanced by EDTA and diminished to negligible levels by 0.5-2 microM Zn(II) added as another FBPase inhibitor. Fru-2,6-P2 was more efficient in the presence of the synergistic effector AMP: still, the Fru-2,6-P2 concentration inhibiting 50% changed from 3 microM (with EDTA) to higher than 50 microM (with Zn(II]. On the other hand, the Zn(II)-dependent FBPase activity was inhibited by Fru-2,6-P2 to a much lesser extent than the Mg(II)-dependent activity.

Alcohol esterification reactions and mechanisms of snake venom 5'-nucleotide phosphodiesterase.

In a previous study we have shown that snake venom 5'-nucleotide phosphodiesterase (SVP) catalyzes methanol-esterification reactions [García-Díaz, M., Avalos, M. & Cameselle, J. C. (1991) Eur. J. Biochem. 196, 451-457]. Now we have demonstrated that SVP catalyzes AMP transfer from ATP to propanol, ethanol, methanol, ethylene glycol, glycerol, 2-chloroethanol or 2,2-dichloroethanol. The AMP-O-alkyl ester products were identified by HPLC, enzyme analysis, ultraviolet and NMR spectroscopy. Those results show the potential of SVP as a tool to prepare 5'-nucleotide esters and agree with the formation of a covalent 5'-nucleotidyl-SVP intermediate susceptible to nucleophilic attack by short-chain (poly)alcohols as acceptors alternative to water. To test the kinetic influence of the solvent nucleophile in SVP mechanisms, initial rates of ATP solvolysis were assayed in different water/alcohol mixtures. Relatively high alcohol concentrations inactivated SVP but lower concentrations gave proportional rates of alcoholysis. An efficiency parameter (EA), defined as the ratio of the mole fraction of AMP-O-alkyl ester as a product to that of alcohol as an acceptor in water/alcohol mixtures, made possible the comparison of alcohols and water as AMP acceptors at low concentrations, as it could be reasoned that EA = 1 for water. Rates of hydrolysis (VH) of substrates yielding AMP and different leaving groups were also assayed. The higher EA and VH values corresponded, respectively, to those acceptors and leaving-group conjugate acids with lower pKa and higher polar-substituent constants (sigma*). The results support the occurrence of general acid-base catalysis in the active center of SVP and the identification of rate-limiting steps. A model is proposed for the mechanisms of SVP-catalyzed hydrolysis and alcoholysis which accounts for the influence of the acid-base properties of alcohols on the kinetic profile of SVP reaction sequences.

Methanol esterification reactions catalyzed by snake venom and bovine intestinal 5'-nucleotide phosphodiesterases. Formation of nucleoside 5'-monophosphate methyl esters from guanosine 5'-triphosphate and other nucleoside 5'-polyphosphates.

It is not known whether the enzymes 5'-nucleotide phosphodiesterase/nucleotide pyrophosphatase (EC 3.1.4.1/EC 3.6.1.9) catalyze the transfer of nucleotides to acceptors other than water. We have investigated the action of snake venom and bovine intestinal mucosa phosphodiesterases on nucleoside 5'-polyphosphates in the presence of methanol. In those conditions, GTP was converted by snake venom phosphodiesterase to a mixture of GMP and another compound with a different retention time in reverse-phase high-performance liquid chromatography. That compound, by ultraviolet, 1H- and 13C-nuclear magnetic resonance spectroscopic analysis, and by enzyme analysis, was characterized as the methyl ester of GMP (GMP-OMe). The molar fraction [GMP-OMe]/[GMP + GMP-OMe] formed was higher than the molar fraction of methanol as a solvent in reaction mixtures. Similar reactions took place at comparable rates with snake venom and bovine intestinal mucosa phosphodiesterases using several nucleoside 5'-polyphosphates as substrates. The ability of 5'-nucleotide phosphodiesterases to catalyze transfer reactions to a non-water acceptor is relevant to the mechanism of the enzymes, to their use as analytical tools, and to their possible use/role in the preparative/in vivo synthesis of nucleotide esters.

High efficiency of glycerol 2-phosphate and sn-glycerol 3-phosphate as nucleotidyl acceptors in snake venom phosphodiesterase esterifications. Formation of primary and secondary AMP-O-glyceryl and AMP-O-glycerophosphoryl esters and evidence for an acceptor-binding enzyme site.

Snake venom phosphodiesterase (SVP) catalyzes the alcoholysis of ATP by primary R-CH2OH alcohols with uncharged R residues, yielding AMP-O-CH2R esterification products. The alcohols compete with water for an SVP-bound adenylyl intermediate. In this study, it has been shown that SVP also catalyzes the reactions of glycerol 2-phosphate and sn-glycerol 3-phosphate with ATP to yield AMP-O-glycerophosphoryl esters. The products were identified by HPLC, the dependency of the reactions on glycerol phosphates, ultraviolet spectroscopy, and conversion to AMP by phosphodiesterase, or to AMP-O-glyceryl esters by alkaline phosphatase. The results demonstrated that R-CH2OH alcohols with negatively charged R residues, as well as secondary alcohols, act as adenylyl acceptors in SVP reactions, thus extending the usefulness of SVP as a tool to produce 5'-nucleotide derivatives. The efficiencies (EA) of glycerol phosphates as adenylyl acceptors were very high at low, millimolar concentrations, but decreased abruptly when the acceptor concentration was increased and, for glycerol 2-phosphate, when Pi or NaCl was present. In contrast, glycerol EA was independent of its own concentration, Pi, and NaCl. The responses of glycerol phosphates indicate that they act as adenylyl acceptors via a mechanism different from uncharged R-CH2OH alcohols. The occurrence of an acceptor-binding enzyme site, specific for negatively charged R residues, and its potential relevance to the in vivo role of 5'-nucleotide phosphodiesterases as 5'-nucleotidyl transferases are discussed.

Mitochondrial location of rat liver dinucleoside triphosphatase.

Rat liver dinucleoside triphosphatase (EC 3.6.1.29) is associated with sucrose-gradient purified mitochondria and can be extracted by freeze and thaw treatment. The proportion of mitochondrial dinucleoside triphosphatase approaches 50% of total liver enzyme. Evidence is also presented that 10% of total liver bis(5'-guanosyl)tetraphosphatase (EC 3.6.1.17) might be equally linked to mitochondria. Those data suggest that diadenosine 5',5'''-P1,P3-triphosphate, diadenosine 5',5'''-P1,P4-tetraphosphate, or other substrates of those enzymes, might be somehow related to mitochondria or mitochondrial function(s), although the occurrence of dinucleoside polyphosphates has not been reported in that organelle.

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.