Showing papers on "Xanthine published in 2005"

Uric acid and oxidative stress.

Abstract: Uric acid is the final product of purine metabolism in humans. The final two reactions of its production catalyzing the conversion of hypoxanthine to xanthine and the latter to uric acid are catalysed by the enzyme xanthine oxidoreductase, which may attain two inter-convertible forms, namely xanthine dehydrogenase or xanthine oxidase. The latter uses molecular oxygen as electron acceptor and generates superoxide anion and other reactive oxygen products. The role of uric acid in conditions associated with oxidative stress is not entirely clear. Evidence mainly based on epidemiological studies suggests that increased serum levels of uric acid are a risk factor for cardiovascular disease where oxidative stress plays an important pathophysiological role. Also, allopurinol, a xanthine oxidoreductase inhibitor that lowers serum levels of uric acid exerts protective effects in situations associated with oxidative stress (e.g. ischaemia-reperfusion injury, cardiovascular disease). However, there is increasing experimental and clinical evidence showing that uric acid has an important role in vivo as an antioxidant. This review presents the current evidence regarding the antioxidant role of uric acid and suggests that it has an important role as an oxidative stress marker and a potential therapeutic role as an antioxidant. Further well designed clinical studies are needed to clarify the potential use of uric acid (or uric acid precursors) in diseases associated with oxidative stress.

Caffeine Use in Sports, Pharmacokinetics in Man, and Cellular Mechanisms of Action

Abstract: Caffeine is the most widely consumed psychoactive 'drug' in the world and probably one of the most commonly used stimulants in sports This is not surprising, since it is one of the few ergogenic aids with documented efficiency and minimal side effects Caffeine is rapidly and completely absorbed by the gastrointestinal tract and is readily distributed throughout all tissues of the body Peak plasma concentrations after normal consumption are usually around 50 microM, and half-lives for elimination range between 25-10 h The parent compound is extensively metabolized in the liver microsomes to more than 25 derivatives, while considerably less than 5% of the ingested dose is excreted unchanged in the urine There is, however, considerable inter-individual variability in the handling of caffeine by the body, due to both environmental and genetic factors Evidence from in vitro studies provides a wealth of different cellular actions that could potentially contribute to the observed effects of caffeine in humans in vivo These include potentiation of muscle contractility via induction of sarcoplasmic reticulum calcium release, inhibition of phosphodiesterase isoenzymes and concomitant cyclic monophosphate accumulation, inhibition of glycogen phosphorylase enzymes in liver and muscle, non-selective adenosine receptor antagonism, stimulation of the cellular membrane sodium/potassium pump, impairment of phosphoinositide metabolism, as well as other, less thoroughly characterized actions Not all, however, seem to account for the observed effects in vivo, although a variable degree of contribution cannot be readily discounted on the basis of experimental data The most physiologically relevant mechanism of action is probably the blockade of adenosine receptors, but evidence suggests that, at least under certain conditions, other biochemical mechanisms may also be operational

The plant Mo-hydroxylases aldehyde oxidase and xanthine dehydrogenase have distinct reactive oxygen species signatures and are induced by drought and abscisic acid.

Abstract: The plant molybdenum-cofactor (Moco) and flavin-containing enzymes, xanthine dehydrogenase (XDH; EC 1.2.1.37) and aldehyde oxidase (AO; EC 1.2.3.1) are thought to play important metabolic roles in purine metabolism and hormone biosynthesis, respectively. Their animal counterparts contribute to reactive oxygen species (ROS) production in numerous pathologies and here we examined these enzymes as potential sources of ROS in plants. Novel in-gel assay techniques and Moco sulfurase mutants, lacking a sulfur ligand in their Moco active center, were employed to demonstrate that the native tomato and Arabidopsis XDHs are capable of producing O, but not H2O2, while the animal counterpart was shown to produce both, O and H2O2. Superoxide production was dependent on Moco sulfuration when using hypoxanthine/xanthine but not NADH as substrates. The activity was inhibited by diphenylene iodonium (DPI), a suicide inhibitor of FAD containing enzymes. Analysis of XDH in an Arabidopsis Atxdh1 T-DNA insertion mutant and RNA interference lines revealed loss of O activity, providing direct molecular evidence that plant XDH generates superoxides. Contrary to XDH, AO activity produced only H2O2 dissimilar to native animal AO, that can produce O as well. Surprisingly, H2O2 accumulation was not sensitive to DPI. Plant ROS production and transcript levels of AO and XDH were rapidly upregulated by application of abscisic acid and in water-stressed leaves and roots. These results, supported by in vivo measurement of ROS accumulation, indicate that plant AO and XDH are possible novel sources for ROS increase during water stress.

Theobromine inhibits sensory nerve activation and cough

Abstract: SPECIFIC AIMSCough is a condition that affects the vast majority of people at some point in their lives and is the most common complaint for which medical attention is sought. Currently, no effective treatment exists. The aim of this study was to investigate the utility of a novel antitussive called theobromine, a methylxanthine derivative present in cocoa and chocolate, on cough and airway sensory nerve function in humans.PRINCIPAL FINDINGS1. Theobromine as a potential antitussiveSeveral synthetic antitussives are characterized by the presence of a 1,2,4-oxadiazole ring in their chemical structure. With the renaissance of the methylxanthine theophylline to treat asthma in the 1970s, a series of novel compounds with an oxadiazolylalkyl substituent at the N7 atom on the basic xanthine skeleton was synthesized and investigated as potential antiasthmatic and antitussive agents.With two of these compounds selected for preclinical testing, 3,7-dihydro-3-methyl-7-/(5-methyl-1,2,4-oxadiazol-3yl)methyl/-1H-purine...

Regulation of Xanthine Oxidoreductase Protein Expression by Hydrogen Peroxide and Calcium

Abstract: Objective— We have previously demonstrated that endothelial xanthine oxidase (XO) levels are dependent on the NADPH oxidase. We postulated that H 2 O 2 may modulate the irreversible conversion of xanthine dehydrogenase (XDH) to XO and sought to examine mechanisms involved. Methods and Results— H 2 O 2 (100 μmol/L) decreased bovine aortic endothelial cell (BAEC) XDH protein expression, and metabolic labeling studies indicated that H 2 O 2 stimulated conversion of XDH to XO. The decline in XDH was mimicked by the reactive oxygen species (ROS) generating compounds SIN-1 and Menadione, as well as by stimulating BAECs with angiotensin II (200 nmol/L). BAPTA-AM prevented the decline in XDH by H 2 O 2 , indicating that it was calcium-dependent. In keeping with calcium acting downstream of H 2 O 2 , the calcium ionophore A23187 (1 μmol/L) caused XDH-to-XO conversion, and this was not prevented by the antioxidants. In addition, XDH-to-XO conversion was blocked by 2-APB and NO donors and induced by thapsigargin and M-3M3FBS, implicating phospholipase C and endoplasmic reticulum calcium stores in this process. Conclusions— Endothelial XO and XDH expression are strongly dependent on H 2 O 2 and calcium. Stimulation of XDH conversion to XO may represent a feed-forward mechanism whereby H 2 O 2 can stimulate further production of ROS.

8-[3-amino-piperidin-1-yl]-xanthine, the production thereof and the use in the form of a ddp-iv inhibitor

Abstract: The invention relates to substituted xanthines of general formula (I), wherein R is such as defined in claim 1, and to the tautomers, stereoisomers, mixtures and the salts thereof, said products exhibiting precious pharmacological properties, in particular an inhibiting effect on a dipeptidylpeptidasa-IV (DPP-IV) enzyme activity.

Pharmacology of Drugs for Hyperuricemia. Mechanisms, Kinetics and Interactions

Abstract: The pharmacological profile of drugs for hyperuricemia is reviewed. These agents may reduce the amount of uric acid in blood by means of two different ways: (1) by reducing uric acid production through the inhibition of the enzyme xanthine oxidase (as allopurinol); (2) by increasing uric acid clearance through an inhibition of its renal tubular reabsorption (as probenecid), or through its metabolic conversion to a more soluble compound (as urate oxidase). Allopurinol is rapidly converted in the body to the active metabolite oxypurinol whose total body exposure may be 20-fold greater than that of the parent compound due to a much longer elimination half-life. Allopurinol undergoes several pharmacokinetic interactions with concomitant administered drugs, some of which may be potentially hazardous (especially with mercaptopurine and azathioprine). Probenecid is an uricosuric agent which undergoes extensive hepatic metabolism and whose elimination after high doses may become dose dependent. It may inhibit renal tubular secretion of several coadministered agents, including methotrexate and sulphonylureas. Rasburicase is a recombinant form of the enzyme urate oxidase which catalyzes the conversion of uric acid to the more soluble compound allantoin. Unlike allopurinol, it does not promote accumulation of hypoxanthine and xanthine in plasma, thus preventing the risk of xanthine nephropathy. Rasburicase showed no significant accumulation in children after administration of either 0.15 or 0.20 mg/kg/daily for 5 days. Rasburicase probably undergoes peptide hydrolysis and in in vitro studies was shown neither to inhibit or induce cytochrome P450 isoenzymes nor to interact with several drugs, so that no relevant interaction is expected during cotreatment in patients.

A detailed behavioral analysis of the acute motor effects of caffeine in the rat: involvement of adenosine

Abstract: Rationale: There is no consensus on the contribution of adenosine A1 and A2A receptor blockade to motor-activating effects of caffeine. Objective: Our aim was to use a detailed and continuous observational method to compare the motor effects induced by caffeine with those induced by selective A1 and A2A receptor antagonists. Methods: The behavioral repertoire induced by systemic administration of caffeine (3, 10, and 30 mg/ kg), A1 receptor antagonist 8-cyclopentyl-1,3-dimethyl- xanthine (CPT; 1.2, 4.8 and 7.2 mg/kg), and A2A receptor antagonist 3-(3-hydroxypropyl)-8-(m-methoxystyryl)-7-meth- yl-1-propargylxanthine phosphate disodium salt (MSX-3; 1, 3, and 10 mg/kg) was analyzed. The effects of pretreatment with the selective A1 receptor agonist N 6 -cyclopentyla- denosine (CPA; 0.1 mg/g) and the selective A2A receptor agonist 2-p-(2-carboxyethyl)phenethylamino-5'-N-ethylcar- boxyamidoadenosine (CGS 21680; 0.2 mg/kg) on the pattern of motor activation induced by caffeine, CPT, or MSX-3 were also examined. Results: The pattern of be- havioral activation induced by caffeine was better mim- icked by CPT than by MSX-3. Coadministration of CPT and MSX-3 gave different results depending on the dose and the type of behavioral response. CPA was more effective at decreasing the activating effects of caffeine and CPT than those of CGS 21680. On the other hand, CGS 21680 was more effective at decreasing the activating effects of MSX-3 than those of caffeine or CPT. Factor analysis revealed a complex three-dimensional behavioral profile for caffeine that was similar to the profile for CPT and was different from the profile for MSX-3. Conclusions: The results indicate a predominant role for A1receptors in the motor-activating effects of acutely administered caffeine.

Radical scavenging and anti-lipoperoxidative activities of Smallanthus sonchifolius leaf extracts.

Abstract: Radical scavenging and anti-lipoperoxidative effects of two organic fractions and two aqueous extracts from the leaves of a neglected Andean crop-yacon (Smallanthus sonchifolius Poepp. & Endl., Asteraceae) were determined using various in vitro models. The extracts' total phenolic content was 10.7-24.6%. They exhibited DPPH (IC50 16.14-33.39 microg/mL) and HO* scavenging activities (4.49-6.51 mg/mL). The extracts did not scavenge phenylglyoxylic ketyl radicals, but they retarded their formation. In the xanthine/xanthine oxidase superoxide radical generating system, the extracts' activities were 26.10-37.67 superoxide dismutase equivalents/mg. As one of the extracts displayed xanthine oxidase inhibitory activity, the effect of the extracts on a nonenzymatically generated superoxide was determined (IC50 7.36-21.01 microg/mL). The extracts inhibited t-butyl hydroperoxide-induced lipoperoxidation of microsomal and mitochondrial membranes (IC50 22.15-465.3 microg/mL). These results make yacon leaves a good candidate for use as a food supplement in the prevention of chronic diseases involving oxidative stress.

Comparative substrate recognition by bacterial and fungal purine transporters of the NAT/NCS2 family

Abstract: We compared the interactions of purines and purine analogues with representative fungal and bacterial members of the widespread Nucleobase-Ascorbate Transporter (NAT) family. These are: UapA, a well-studied xanthine-uric acid transporter of A. nidulans, Xut1, a novel transporter from C. albicans, described for the first time in this work, and YgfO, a recently characterized xanthine transporter from E. coli. Using transport inhibition experiments with 64 different purines and purine-related analogues, we describe a kinetic approach to build models on how NAT proteins interact with their substrates. UapA, Xut1 and YgfO appear to bind several substrates via interactions with both the pyrimidine and imidazol rings. Fungal homologues interact with the pyrimidine ring of xanthine and xanthine analogues via H-bonds, principally with N1-H and =O6, and to a lower extent with =O2. The E. coli homologue interacts principally with N3-H and =O2, and less strongly with N1-H and =O6. The basic interaction with the imida...

Studies on structure activity relationship of some dihydroxy-4-methylcoumarin antioxidants based on their interaction with Fe(III) and ADP.

Abstract: Three dihydroxy-4-methylcoumarin (DHMC) derivatives, namely 7,8-DHMC, 6,7-DHMC and 5,7-DHMC alone and complexed with Fe (III) and ADP have been tested for their antioxidative potential. Chemical speciation studies and formation constants reveal the formation of strong DHMC–Fe–ADP (1:1:1) ternary complex. In vitro studies were done for their antioxidative property by scavenging the superoxide radicals (O 2 − ) generated by xanthine + xanthine oxidase (XO) reaction. The IC50 values for 7,8-DHMC, 6,7-DHMC and 5,7-DHMC and their ternary complexes with Fe (III)–ADP worked out to be 34.0 μM, 62.0 μM, 8.80 mM and 10.5, 11.5 and 148.5 μM, respectively. The results indicate that O 2 − scavenging potential of all the three DHMCs increased significantly after forming the ternary complex with Fe(III) and ADP. The structure activity relationship studies suggest that the introduction of hydroxyl group at 7th and 8th positions in the coumarins, irrespective of Fe(III)–ADP complexation, increases the antioxidative efficacy. No change in uric acid production in the reactions done for all studies further reveals that the coumarin derivatives and their complexes were the only causative factors for O 2 − scavenging and not the suppression of the enzyme, xanthine oxidase.

New Pyrrolo[2,1-f]purine-2,4-dione and Imidazo[2,1-f]purine-2,4-dione Derivatives as Potent and Selective Human A3 Adenosine Receptor Antagonists

Abstract: Compounds presenting an additional fused ring on the xanthine nucleus have been reported to exhibit antagonistic activity with various levels of affinity and selectivity toward the four adenosine receptors subtypes A1, A2A, A2B, and A3. This paper reports synthesis and biological evaluation of new 1-benzyl-3-propyl-1H,6H-pyrrolo[2,1-f]purine-2,4-diones and 1-benzyl-3-propyl-1H,8H-imidazo[2,1-f]purine-2,4-diones, among which we identified potent and selective A3 adenosine receptors antagonists. In particular, 1-benzyl-7-methyl-3-propyl-1H,8H-imidazo[2,1-f]purine-2,4-dione (11e) shows a Ki (hA3) value from binding assay of 0.8 nM.

Determination of caffeine and its metabolites in urine by capillary electrophoresis-mass spectrometry.

Abstract: The caffeine content of foods and beverages varies considerably, interfering with our ability to obtain valid interpretations in many human studies with regard to the mechanism of action(s) of caffeine and/or its metabolites. The rate of metabolism of caffeine and other xanthine drugs also varies greatly from one individual to another. Therefore, it is extremely important to develop accurate, reliable analytical methods to quantify caffeine and its metabolites in simple and complex matrixes. A simple method is described for the separation and characterization of caffeine and its major metabolites employing capillary electrophoresis (CE) coupled to ultraviolet-absorption and mass spectrometry (MS) detection. After optimization of the electrophoresis separation conditions, a reliable separation of caffeine and 11 of its major metabolites was achieved in 50 mM ammonium carbonate buffer, pH 11.0. The volatile aqueous electrolyte system used with a normal electroosmotic flow polarity also provided an optimal separation condition for the characterization of the analytes by MS. The CE method achieved baseline resolution for all 12 compounds in less than 30 min. The CE-MS method is suitable for use as a routine procedure for the rapid separation and characterization of caffeine and its metabolites. The usefulness of this method was demonstrated by the extraction, separation, and identification of caffeine and its 11 metabolites from normal urine samples. The urine specimens were first acidified to obtain optimum binding efficiency to the sorbents of the off-line, solid-phase extraction procedure employed here, and an acidified eluent solvent was employed for the desorption step to maximize the recovery of the bound analytes.

Uric acid and hypertension.

Abstract: Increased levels of uric acid are associated with cardiovascular disease and the metabolic syndrome. They may predict clinical outcomes and also the onset of hypertension, though it is less clear that hyperuricaemia can be regarded as an independent risk factor given its clustering with other well-recognised factors. Uric acid may increase as a result of pathophysiological processes such as impaired renal sodium handling but may also contribute to renal and vascular damage, particularly endothelial dysfunction. It is notable that the synthesis of uric acid may be associated with the generation of reactive oxygen species if the enzyme xanthine oxidorectase is converted to the oxidase, as may occur in ischaemia. It has been suggested that uric acid may play a role in the pathogenesis of early-onset hypertension but evidence for this is limited. There is also very limited data to suggest that in some circumstances lowering uric acid can lower blood pressure. In the metabolic syndrome, the presence of elevated uric acid concentrations is closely associated with raised triglyceride levels, for reasons that have not been clearly defined. It remains to be seen whether uric acid could or should be considered a specific therapeutic target in cardiovascular disease and especially in hypertension and if so what should be the optimal pharmacological approach to lowering serum urate levels.

Xanthine derivatives with hm74a receptor activity

Abstract: The present invention relates to therapeutically active compounds of formula (I) which are xanthine derivatives, processes for the manufacture of said derivatives, pharmaceutical formulations containing the active compounds and the use of the compounds in therapy, particularly in the treatment of diseases where under-activation of the HM74A receptor contributes to the disease or where activation of the receptor will be beneficial.

Convergent evolution of hydroxylation mechanisms in the fungal kingdom: molybdenum cofactor‐independent hydroxylation of xanthine via α‐ketoglutarate‐dependent dioxygenases

Abstract: The xanthine oxidases and dehydrogenases are among the most conserved enzymes in all living kingdoms. They contain the molybdopterin cofactor Moco. We show here that in the fungi, in addition to xanthine dehydrogenase, a completely different enzyme is able to catalyse the oxidation of xanthine to uric acid. In Aspergillus nidulans this enzyme is coded by the xanA gene. We have cloned the xanA gene and determined its sequence. A deletion of the gene has the same phenotype as the previously known xanA1 miss-sense mutation. Homologues of xanA exist only in the fungal kingdom. We have inactivated the cognate gene of Schizosaccharomyces pombe and this results in strongly impaired xanthine utilization as a nitrogen source. We have shown that the Neurospora crassa homologue is functionally equivalent to xanA. The enzyme coded by xanA is an alpha-ketoglutarate- and Fe(II)-dependent dioxygenase which shares a number of properties with other enzymes of this group. This work shows that only in the fungal kingdom, an alternative mechanism of xanthine oxidation, not involving Moco, has evolved using the dioxygenase scaffold.

Involvement of molybdenum hydroxylases in reductive metabolism of nitro polycyclic aromatic hydrocarbons in mammalian skin

Abstract: Molybdenum hydroxylases, aldehyde oxidase and xanthine oxidoreductase, were shown to be involved in the nitroreduction of 2-nitrofluorene (NF), 1-nitropyrene, and 4-nitrobiphenyl, environmental pollutants, in the skin of various mammalian species. NF was reduced to 2-aminofluorene by hamster skin cytosol in the presence of 2-hydroxypyrimidine, 4-hydroxypyrimidine, N(1)-methylnicotinamide, or benzaldehyde, but not hypoxanthine or xanthine. Inhibitors of aldehyde oxidase markedly inhibited these nitroreductase activities, but oxypurinol, an inhibitor of xanthine oxidoreductase, had little effect. In DEAE column chromatography of hamster skin cytosol, the major fraction exhibiting nitroreductase activity also showed aldehyde oxidase activity. 2-Hydroxypyrimidine-linked nitroreductase activities of skin cytosol from rabbits and guinea pigs were also inhibited by an inhibitor of aldehyde oxidase. In contrast, nitroreductase activities of skin cytosols of rats and mice were markedly inhibited by oxypurinol. When aldehyde oxidase activity was estimated in skin cytosol of various mammals using benzaldehyde oxidase activity as a marker, considerable variability of the activity was found. The highest activity was observed with hamsters, and the lowest activity with rats. On the other hand, the highest xanthine oxidoreductase activity was observed with rats, and the lowest activity with rabbits. These skin cytosols of various mammals also exhibited significant 2-hydroxypyrimidine-linked nitroreductase activities toward 1-nitropyrene and 4-nitrobiphenyl catalyzed by aldehyde oxidase and xanthine oxidoreductase. Thus, NF was mainly reduced by aldehyde oxidase and xanthine oxidoreductase in skins of animals. However, the contributions of these two molybdenum hydroxylases were considerably different among animal species.

Three-way partial least-squares regression for the simultaneous kinetic-enzymatic determination of xanthine and hypoxanthine in human urine.

Abstract: The performance of three-way principal component analysis and three-way partial least-squares regression when applied to a complex kinetic-enzymatic system is studied, in order to investigate the analytical potential of the combined use of these chemometric technologies for non-selective enzymatic systems. A enzymatic-kinetic procedure for the simultaneous determination of hypoxanthine and xanthine in spiked samples of human urine is proposed. The chemical system involves two consecutive reactions catalyzed by xanthine oxidase (EC 1.17.3.2). This enzyme catalyzes the oxidation of hypoxanthine, first to xanthine and then to uric acid, a competitive inhibitor of the reactions. The influence of uric acid during quantitative determination was considered in the design of the calibration set. The sample and enzyme solution were mixed in a stopped-flow module and the reaction was monitored using a diode array spectrophotometer. The recorded data have an intrinsical three-component structure (samples, time and wavelength). This data array was studied via three-way principal component analysis and was modeled for quantitative purposes using a three-way partial least-squares calibration procedure. Results are compared with those obtained by applying classical bilinear PLS to the previously unfolded data matrix.

NADH oxidase activity of rat liver xanthine dehydrogenase and xanthine oxidase-contribution for damage mechanisms.

Abstract: The involvement of xanthine oxidase (XO) in some reactive oxygen species (ROS) -mediated diseases has been proposed as a result of the generation of and H2O2 during hypoxanthine and xanthine oxidation. In this study, it was shown that purified rat liver XO and xanthine dehydrogenase (XD) catalyse the NADH oxidation, generating and inducing the peroxidation of liposomes, in a NADH and enzyme concentration-dependent manner. Comparatively to equimolar concentrations of xanthine, a higher peroxidation extent is observed in the presence of NADH. In addition, the peroxidation extent induced by XD is higher than that observed with XO. The in vivo-predominant dehydrogenase is, therefore, intrinsically efficient at generating ROS, without requiring the conversion to XO. Our results suggest that, in those pathological conditions where an increase on NADH concentration occurs, the NADH oxidation catalysed by XD may constitute an important pathway for ROS-mediated tissue injuries.

Febuxostat (TMX-67), a Novel, Non-Purine, Selective Inhibitor of Xanthine Oxidase, Is Safe and Decreases Serum Urate in Healthy Volunteers

Abstract: In order to evaluate the safety, pharmacological properties, and urate-lowering efficacy of febuxostat, a non-purine, selective inhibitor of xanthine oxidase, a Phase 1, 2-week, multiple-dose, placebo-controlled, dose-escalation study was conducted in 154 healthy adults of both sexes. Daily febuxostat doses in the range 10 mg to 120 mg resulted in proportional mean serum urate reductions ranging from 25% to 70% and in proportional increases in maximum febuxostat plasma concentrations and area under plasma concentration versus time curves. Accompanying the hypouricemic effect were increases in serum xanthine concentrations, decreases in urinary uric acid excretion, and increases in urinary xanthine and hypoxanthine excretion, confirming inhibition of xanthine oxidase activity by febuxostat. Hepatic conjugation and oxidative metabolism were the major pathways of elimination of febuxostat from the body, and renal elimination did not appear to play a significant role. Although not uncommon, adverse events were mild and self-limited, and no deaths or serious adverse events were observed. Febuxostat is a safe and potent hypouricemic agent in healthy humans.