Erythrocyte pyrimidine 5′-nucleotidase inhibition by acute lead exposure in neonatal rats

Inhibition by lead of erythrocyte pyrimidine 5′-nucleotidase (P5N) is thought to contribute to morphological abnormalities observed in red blood cells (RBC) of lead-exposed subjects. However, neither the mechanism of lead inhibition of P5N nor the relationship of this inhibition to blood lead levels attained in exposed subjects is known. In the present investigation, acute in vivo and in vitro lead acetate effects on erythrocyte P5N from 21-day-old rat pups were determined and were related to blood lead concentrations ascertained by atomic absorption spectrophotometry. Acute lead administration to rat pups resulted in a 16% to 21% reduction in erythrocyte P5N, with mean blood lead levels ranging from 77 to 108 μg/dl 24 hours later. Inhibition of erythrocyte P5N was linearly related to blood lead level (r = ?0.67, P < 0.05) following acute lead administration. Lead acetate addition to RBC preparations from 21-day-old rats resulted in concentration-dependent P5N inhibition which was comparable to that produced following acute in vivo exposure. The results indicate that acute P5N inhibition in lead-treated neonatal rats is due to noncompetitive P5N inhibition by lead. The inhibition of P5N produced by acute lead treatment is linearly related to blood lead concentrations.     

Structure of pyrimidine 5'-nucleotidase type 1. Insight into mechanism of action and inhibition during lead poisoning

Eukaryotic pyrimidine 5'-nucleotidase type 1 (P5N-1) catalyzes dephosphorylation of pyrimidine 5'-mononucleotides. Deficiency of P5N-1 activity in red blood cells results in nonspherocytic hemolytic anemia. The enzyme deficiency is either familial or can be acquired through lead poisoning. We present the crystal structure of mouse P5N-1 refined to 2.35 A resolution. The mouse P5N-1 has a 92% sequence identity to its human counterpart. The structure revealed that P5N-1 adopts a fold similar to enzymes of the haloacid dehydrogenase superfamily. The active site of this enzyme is structurally highly similar to those of phosphoserine phosphatases. We propose a catalytic mechanism for P5N-1 that is also similar to that of phosphoserine phosphatases and provide experimental evidence for the mechanism in the form of structures of several reaction cycle states, including: 1) P5N-1 with bound Mg(II) at 2.25 A, 2) phosphoenzyme intermediate analog at 2.30 A, 3) product-transition complex analog at 2.35 A, and 4) product complex at 2.1A resolution with phosphate bound in the active site. Furthermore the structure of Pb(II)-inhibited P5N-1 (at 2.35 A) revealed that Pb(II) binds within the active site in a way that compromises function of the cationic cavity, which is required for the recognition and binding of the phosphate group of nucleotides.     

Some properties of human erythrocyte pyrimidine 5'-nucleotidase

In haemolysates human erythrocyte pyrimidine 5'-nucleotidase had a single optimum at pH 7.2 with CMP and 6.75 with UMP as substrate. The purified enzyme showed two pH optima at pH 6.25 and 7.2 with UMP as substrate. The enzyme was inhibited by both its products - inorganic phosphate and pyrimidine nucleoside. The inhibition by inorganic phosphate appeared to be non-competitive with Ki = 1.5 mM. Contrary to previous reports adenosine and inosine did not inhibit the enzyme.     

Problems encountered in measuring erythrocyte pyrimidine 5'-nucleotidase activity

An improved method of two-dimensional electrophoresis that allows unequilibrated first-dimension gels to be loaded and electrophoresed on second-dimension gels twice the length used in the O'Farrell technique has been developed. Normally, the electrophoresis of unequilibrated first-dimension gels on long second-dimension gels with the resolving gel set at pH 8.8 results in poor resolution of low-molecular-weight proteins. Adjusting the pH of the resolving gel to pH 8.3 maintains the low-molecular-weight proteins in a stacked configuration during their migration through the length of the 10% acrylamide gel. Utilization of a 10 to 20% exponential polyacrylamide gradient in a resolving gel set at pH 8.3 separates these low-molecular-weight proteins with excellent resolution. Electrophoretic resolution of protein spots in resolving gels set at pH 8.8 is not as sharp as in gels set at pH 8.3, and resolution progressively deteriorates in gels set at higher pH values.     

Human erythrocyte pyrimidine 5'-nucleotidase, PN-I.

Erythrocyte maturation is accompanied by RNA degradation and release of mononucleotides. Pyrimidine 5'-nucleotidase, PN-I, has been purified and characterized. The molecular and enzymatic properties determined for the enzyme shows a 36-kDa and 5.1 pI monomeric protein with no disulfide bridges and no phosphate content. The activity is dependent on Mg(2+), while it is inactivated by heavy metals and by thiol-reactive reagents. PN-I is specific for pyrimidine nucleoside monophosphates, including the antineoplastic agents 5'-AZTMP and 5'-Ara-CMP. PN-I possess phosphotransferase activity able to exchange phosphate between pyrimidine nucleoside monophosphates and pyrimidine nucleosides, including AZT and Ara-Cyd. Amino acid sequence has been obtained from tryptic and CNBr peptides. PN-I cDNA sequence, coding for a 286-residue protein, has been retrieved from tag database, amplified by PCR, and expressed in Escherichia coli. The recombinant protein was fully active and showed identical properties with respect to PN-I. Substantial identity has been revealed with the partial sequences reported for p36, an alpha-interferon-induced protein. The significance of this identity is discussed.     

Rabbit liver dCMP phosphohydrolase: a pyrimidine 5'-nucleotidase I-like enzyme in non-erythrocytic cells

A nucleotide phosphomonoesterase activity that preferably hydrolyzed dCMP was detected in rabbit liver and purified approximately 20-fold. The enzyme was similar in the catalytic and molecular properties to pyrimidine 5'-nucleotidase subclass I (P5N-I), which distributed specifically in vertebrate erythrocytes. In addition to liver, the activity was found in rabbit kidney, spleen, heart, intestine, but was not detected in any rat or chicken tissues tested. The rabbit enzyme protein reacted with antibodies against chicken P5N-I. Its pI was estimated to be approximately 5.3, and the enzyme was concluded to consist of single polypeptide of an approximately 38 kDa based on gel filtration and Western blot analysis. The partially purified enzyme preferentially hydrolyzes dCMP, UMP and CMP, K(m) values for these substrates are approximately 0.3 mM, the optimal pH is approximately 7, and the enzyme requires Mg(2+). This nucleotidase may contribute to the regulation of intracellular pyrimidine nucleotides in the rabbit.     

Electrophoretic and kinetic studies of a new mutant red cell pyrimidine 5'-nucleotidase

Molecular characteristics of a deficient pyrimidine 5'-nucleotidase (P5N) were studied in a partially purified red cell enzyme extract. The results showed a high Michaelis constant for uridine 5'-monophosphate, an acidic shift of the optimum pH and normal heat stability. Enzyme electrophoresis using a starch gel and histidine-citrate buffer pH 7.0 showed a single band with identical mobility to that of the 'minor' band of normal enzyme. This electrophoretic pattern supports the hypothesis that P5N deficiency is, at least in some cases, a consequence of the absence of a 'major' isoenzymatic band characteristically present in normal enzyme.     

Human placental cytoplasmic 5'-nucleotidase. Kinetic properties and inhibition

The kinetic properties of highly purified human placental cytoplasmic 5'-nucleotidase were investigated. Initial velocity studies gave Michaelis constants for AMP, IMP, and CMP of 18, 30, and 2.2 microM, respectively. The enzyme shows the following relative Vmax values: CMP greater than UMP greater than dUMP greater than GMP greater than AMP greater than dCMP greater than IMP. The activity was magnesium-dependent, and this cation binds sequentially with a Km of 14 microM for AMP and an apparent Km of 6 mM for magnesium. A large variety of purine, pyrimidine, and pyridine compounds exert an inhibitory effect on enzyme activity. IMP, GMP, and NADH produce almost 100% inhibition at 1.0 mM. Nucleoside di- and triphosphates are potent inhibitors. ATP and ADP are competitive inhibitors with respect to AMP and IMP as substrates with Ki values of 100 and 15 microM, respectively. Inorganic phosphate is a noncompetitive inhibitor with Ki values of 19 and 43 mM. Nucleosides and other compounds studied produce only a modest decrease of enzyme activity at 1 mM. Our findings suggest that the enzyme is regulated under physiological conditions by the concentrations of magnesium, nucleoside 5'-monophosphates, and nucleoside di- and triphosphates. The nucleotide pool concentration regulates the enzyme possibly by a mechanism of heterogeneous metabolic pool inhibition. These properties of human placental cytoplasmic 5'-nucleotidase may be related to the control of nucleotide degradation in vivo.     

Chicken erythrocyte pyrimidine 5'-nucleotidase: purification and characterization of the subclass I enzyme

Nucleotidase activities resembling subclass I and subclass II of human pyrimidine 5'-nucleotidases (P5N) were detected in chicken red blood cells (RBCs). In chicken RBCs from untreated controls, the activity of the subclass II enzyme was about one third of that of subclass I enzyme, whereas that ratio was approximately 5:1 in rat or human RBCs. The subclass I activity in chicken RBCs was increased 5- to 6-fold upon erythropoietic induction by phenylhydrazine administration, but the subclass II activity did not increase under these conditions. The subclass I enzyme was purified to near homogeneity. Its molecular mass was about 35 kDa as estimated by gel filtration and SDS-polyacrylamide gel electrophoresis. Its N-terminal 12 amino acids, PEFQKKTVHIKD, were also determined. The catalytic properties of the subclass I enzyme were very similar to those of the human enzyme with regard to substrate (preferential hydrolysis of CMP, dCMP, UMP), Km values, optimum pH, and metal ion requirements. Antibodies against chicken P5N subclass I were raised in rats. The chicken P5N-I as well as the rat P5N-I proteins could be detected by antibodies in Western blot analyses, but not the P5N-II proteins. These findings indicate that P5N subclass I may have an important function in chicken erythropoiesis.     

Perinatal lead exposure alters locomotion induced by amphetamine analogs in rats

AimsThe precise neurochemical perturbations through which perinatal (gestation/lactation) lead exposure modifies the reinforcement efficacy of various psychoactive drugs (e.g., cocaine, opiates) are unknown. The present study considers the role of altered serotonin and dopamine functionality in perinatal lead-psychostimulant interactions.Main methodsFemale rats were administered a 16-mg lead or a control solution (p.o.) for 30 days prior to breeding with non-exposed males. Lead exposure was discontinued at weaning (postnatal day [PND] 21). Starting at PND 120, male rats born to control or lead-exposed dams were injected with either PAL-287 or PAL-353, at doses of 0, 2, 4, 8, or 16 umol/kg (i.p.) with each dose given prior to an acute (45 min) locomotion test. Whereas PAL-287 is a potent releaser of serotonin, PAL-353 is not. Each drug induces comparable release of norepinephrine (NE) and of dopamine (DA).Key findingsControl and lead rats exhibited minimal locomotion to PAL-287. PAL-353 produced a dose-dependent activation of locomotion in control rats relative to the effects of PAL-287 in control rats. Lead-exposed rats exhibited a subsensitivity to PAL-353 at doses of 4 and 8 umol/kg.SignificanceThe subsensitivity of lead rats to PAL-353 is consistent with a lead-induced diminution of dopamine function, an effect noted earlier for the reuptake inhibitor cocaine (Nation et al. 2000). The similar response of lead and control rats to PAL-287 is inconsistent with diminished serotonin function.     

Structural insights into the inhibition of cytosolic 5'-nucleotidase II (cN-II) by ribonucleoside 5'-monophosphate analogues

Cytosolic 5'-nucleotidase II (cN-II) regulates the intracellular nucleotide pools within the cell by catalyzing the dephosphorylation of 6-hydroxypurine nucleoside 5'-monophosphates. Beside this physiological function, high level of cN-II expression is correlated with abnormal patient outcome when treated with cytotoxic nucleoside analogues. To identify its specific role in the resistance phenomenon observed during cancer therapy, we screened a particular class of chemical compounds, namely ribonucleoside phosphonates to predict them as potential cN-II inhibitors. These compounds incorporate a chemically and enzymatically stable phosphorus-carbon linkage instead of a regular phosphoester bond. Amongst them, six compounds were predicted as better ligands than the natural substrate of cN-II, inosine 5'-monophosphate (IMP). The study of purine and pyrimidine containing analogues and the introduction of chemical modifications within the phosphonate chain has allowed us to define general rules governing the theoretical affinity of such ligands. The binding strength of these compounds was scrutinized in silico and explained by an impressive number of van der Waals contacts, highlighting the decisive role of three cN-II residues that are Phe 157, His 209 and Tyr 210. Docking predictions were confirmed by experimental measurements of the nucleotidase activity in the presence of the three best available phosphonate analogues. These compounds were shown to induce a total inhibition of the cN-II activity at 2 mM. Altogether, this study emphasizes the importance of the non-hydrolysable phosphonate bond in the design of new competitive cN-II inhibitors and the crucial hydrophobic stacking promoted by three protein residues.     

Ecto-5'-nucleotidase/CD73 inhibition by quercetin in the human U138MG glioma cell line

Gliomas are the most malignant of the primary brain tumors. Nucleotides represent an important class of extracellular molecules that are crucial for the normal function of the nervous system. ATP and adenosine can stimulate cell proliferation in different glioma cell lines; the events induced by extracellular adenine nucleotides are controlled by the action of ecto-nucleotidases, which hydrolyze ATP into adenosine in the extracellular space. Recent studies have shown that quercetin has an anti-proliferative effect on the U138MG glioma cell line. Since evidence suggests that purinergic signaling is involved in the growth and progression of glioma and, taking into consideration the anti-proliferative effect elicited by quercetin in this tumor type, the aim of the present study was to better investigate the extracellular metabolism of AMP and evaluate the effect of quercetin on this system in the human U138MG glioma cell line. The adenine products secreted by glioma cells were first characterized; extracellular AMP was efficiently metabolized by the glioma culture, demonstrating a very active ecto-5'-NT/CD73. Quercetin was able to inhibit the ecto-5'-NT/CD73 activity and modulate its expression. In addition, the cell treatment with APCP (alpha,beta-methyleneadenosine-5'-diphosphate), an ecto-5'-NT/CD73 inhibitor, led to a significant reduction in glioma cell proliferation. We suggest that the inhibition of ecto-5'-NT/CD73 may result in a decrease in extracellular adenosine production with a consequent reduction in tumor progression.     

Mechanisms by which neonatal testosterone exposure mediates sex differences in impulsivity in prepubertal rats

Neonatal testosterone, either acting directly or through its conversion to estradiol, can exert organizational effects on the brain and behavior. The goal of the current study was to examine sex differences and determine the role of neonatal testosterone on prefrontal cortex-dependent impulsive choice behavior in prepubertal rats. Male and female prepubertal rats were tested on the delay-based impulsive choice task. Impulsive choice was defined as choosing an immediate small food reward over a delayed large reward. In a first experiment to examine sex differences, males made significantly more impulsive choices than did females. In a second experiment to examine the organizational effects of testosterone, females treated with neonatal testosterone made significantly more impulsive choices than did control females and their performance was indistinguishable from that of control males. In a third experiment to determine if the effect of testosterone on performance is due to the actions of androgens or estrogens through its conversion to estradiol, males treated neonatally with the aromatase inhibitor formestane, which blocks the conversion of testosterone to estradiol, females treated neonatally with the non-aromatizable androgen dihydrotestosterone, and females treated neonatally with estradiol made significantly more impulsive choices than did control females and their performance was indistinguishable from that of control males. Results indicate that male pubertal rats display increased impulsive choice behavior as compared to females, that this sex difference results from organizing actions of testosterone during the neonatal period, and that this effect can result from both androgenic and estrogenic actions.     

Does lead exposure influence liver protein synthesis in rats?

Glycogenolysis was stimulated by catecholamines in in vitro cultures of hepatic tissue of Xenopus laevis. Dose response curves showed that adrenaline and isoprenaline were equally effective while noradrenaline and phenylephrine were progressively less effective in eliciting glycogen breakdown. Neither oxymetazoline nor methoxamine had any effect on glycogenolysis. Administration of adrenaline to cultures was followed within 1 min by a rise in tissue cyclic AMP concentration and within 2 min by an increase in phosphorylase a activity. Both these responses were blocked by propranolol but little affected by phenoxybenzamine. These findings suggest that catecholamines activate glycogenolysis via a beta-adrenergic receptor in X. laevis and that alpha-adrenergic receptors play no role in regulating hepatic glycogenolysis in this species.     

Regulation of cytosol 5'-nucleotidase by adenylate energy charge

In the physiological range of the adenylate energy charge in liver (0.7-0.9), th rate of AMP-hydrolysis catalysed by rat liver cytosol 5'-nucleotidase (5'-ribonucleotide phosphohydrolase, EC 3.1.3.5) increased sharply with decreasing energy charge. In addition, a decrease in the concentration of Pi caused marked acceleration of the AMP-hydrolysing activity over the physiological range of adenylate energy charge. These responses seem to serve to protect the cells against a metabolic stress which could result from sudden utilization of ATP by removal of AMP. The AMP-hydrolysing activity of this enzyme decreased sharply as the size of the adenine nucleotide pool decreased in the physiological range. This effect may be a self-limiting response to prevent excess depletion of the pool. IMP-hydrolysing activity of this enzyme increased with increasing adenylate energy charge. But no marked response to its variation within the physiological range was observed. On the basis of the data obtained in this study, the IMP-hydrolysing activity of the cytosol 5'-nucleotidase in rat liver cells seems to be comparable to that of AMP deaminase reaction, but the AMP-hydrolysing activity was estimated to be less than 10% of AMP deaminase reaction at energy charge value of about 0.7. This strongly suggests that the AMP leads to IMP leads to inosine pathway is more significant that the AMP leads to adenosine leads to inosine pathway in rat liver.     

Phosphodiesterase 4 inhibition attenuates persistent heart and lung injury by neonatal hyperoxia in rats

Phosphodiesterase (PDE) 4 inhibitors are potent anti-inflammatory drugs with antihypertensive properties, and their therapeutic role in bronchopulmonary dysplasia (BPD) is still controversial. We studied the role of PDE4 inhibition with piclamilast on normal lung development and its therapeutic value on pulmonary hypertension (PH) and right ventricular hypertrophy (RVH) in neonatal rats with hyperoxia-induced lung injury, a valuable model for premature infants with severe BPD. The cardiopulmonary effects of piclamilast treatment (5 mg·kg(-1)·day(-1)) were investigated in two models of experimental BPD: 1) daily treatment during continuous exposure to hyperoxia for 10 days; and 2) late treatment and injury-recovery in which pups were exposed to hyperoxia or room air for 9 days, followed by 9 or 42 days of recovery in room air combined with treatment started on day 6 of oxygen exposure until day 18. Prophylactic piclamilast treatment reduced pulmonary fibrin deposition, septum thickness, arteriolar wall thickness, arteriolar vascular smooth muscle cell proliferation and RVH, and prolonged survival. In the late treatment and injury-recovery model, hyperoxia caused persistent aberrant alveolar and vascular development, PH, and RVH. Treatment with piclamilast in both models reduced arteriolar wall thickness, attenuated RVH, and improved right ventricular function in the injury recovery model, but did not restore alveolarization or angiogenesis. Treatment with piclamilast did not show adverse cardiopulmonary effects in room air controls in both models. In conclusion, PDE4 inhibition attenuated and partially reversed PH and RVH, but did not advance alveolar development in neonatal rats with hyperoxic lung injury or affect normal lung and heart development.