Showing papers on "Xanthine published in 2006"

Determination of intracellular reactive oxygen species and high mitochondrial membrane potential in Percoll-treated viable boar sperm using fluorescence-activated flow cytometry.

Abstract: The use of frozen semen in the swine industry is limited by problems with viability and fertility compared with liquid semen. Part of the reduction in sperm motility and fertility associated with cryopreservation may be due to oxidative damage from excessive or inappropriate formation of reactive oxygen species (ROS). Chemiluminescence measurements of ROS are not possible in live cells and are problematic because of poor specificity. An alternative approach, flow cytometry, was developed to identify viable boar sperm containing ROS utilizing the dyes hydroethidine and 2', 7'-dichlorodihydrofluorescein diacetate as oxidizable substrates and impermeant DNA dyes to exclude dead sperm. The percentage of sperm with high mitochondrial transmembrane potential was determined by flow cytometry using the mitochondrial probe 5, 5', 6, 6'-tetrachloro-1, 1', 3, 3'-tetraethylbenzimidazolylcarbocyanine iodide with propidium iodide staining to exclude nonviable cells. Sperm were incubated with and without ROS generators and free radical scavengers. Basal ROS formation was low (less than 4%) and did not differ (P = 0.26) between viable fresh and frozen-thawed boar sperm. In addition, fresh and frozen-thawed viable sperm were equally susceptible (P = 0.20) to intracellular formation of ROS produced by xanthine/xanthine oxidase (94.4 and 87.9% of sperm, respectively). Menadione increased (P < 0.05) ROS formation, decreased (P < 0.05) JC-1-aggregate fluorescence intensity, and decreased (P < 0.05) motion variables by 25 to 60%. The mechanism of inhibition of motility by ROS formation may be related to a decrease in mitochondrial charge potential below a critical threshold. Catalase and superoxide dismutase treatment in the presence of xanthine/xanthine oxidase indicated that hydrogen peroxide was the primary intracellular ROS measured. Further, catalase, but not superoxide dismutase, was capable of attenuating ROS-induced inhibition of motility. Whereas basal intracellular hydrogen peroxide formation was low in viable fresh and frozen-thawed boar sperm, both were quite susceptible to external sources of hydrogen peroxide.

Xanthine Oxidoreductase Inhibition Causes Reverse Remodeling in Rats With Dilated Cardiomyopathy

Abstract: Increased reactive oxygen species (ROS) generation is implicated in cardiac remodeling in heart failure (HF). As xanthine oxidoreductase (XOR) is 1 of the major sources of ROS, we tested whether XO...

Pharmacokinetics, Pharmacodynamics and Safety of Febuxostat, a Non-Purine Selective Inhibitor of Xanthine Oxidase, in a Dose Escalation Study in Healthy Subjects

Abstract: Background Febuxostat is a novel non-purine selective inhibitor of xanthine oxidase currently being developed for the management of hyperuricemia in patients with gout.

Free radical scavenging reactions and phytochemical analysis of triphala, an ayurvedic formulation

Abstract: In order to understand the factors responsible for the potent antioxidant and radioprotecting ability of triphala, it has been evaluated for radical scavenging ability, xanthine oxidase inhibitory activity and phytochemical (phenolics) content. The radical scavenging experiments were carried out using fast reaction kinetic tools and the reactivity of triphala towards different radicals such as hydroxyl radicals, superoxide radicals, DPPH and ABTS • •was determined. When triphala was tested for superoxide radical scavenging activity using xanthine and xanthine oxidase assay, it was observed that in addition to reacting with superoxide radical, it also inhibited uric acid formation, indicative of xanthine oxidase enzyme inhibitory activity. Phytochemical ana lysis showed that triphala is rich in phenols/polyphenols (38 ± 3%) and tannins (35 ± 3%), while flavonoids were found to be absent. HPLC analysis showed that triphala contains 73 ± 5 mg gallic acid per gram of triphala, which was found to increase to 150 ± 5 mg/g upon acid hydrolysis. Relevance of these studies to the antioxidant and radio protection properties of triphala has also been discussed.

Caffeine inhibition of rat carotid body chemoreceptors is mediated by A2A and A2B adenosine receptors.

Abstract: Caffeine, an unspecific antagonist of adenosine receptors, is commonly used to treat the apnea of prematurity. We have defined the effects of caffeine on the carotid body (CB) chemoreceptors, the main peripheral controllers of breathing, and identified the adenosine receptors involved. Caffeine inhibited basal (IC50, 210 µm) and low intensity (PO2 ≈ 66 mm Hg/30 mm K+) stimulation-induced release of catecholamines from chemoreceptor cells in intact preparations of rat CB in vitro. Opposite to caffeine, 5′-(N-ethylcarboxamido)adenosine (NECA; an A2 agonist) augmented basal and low-intensity hypoxia-induced release. 2-p-(2-Carboxyethyl)phenethyl-amino-5′-N-ethylcaboxamido-adenosine hydrochloride (CGS21680), 2-hexynyl-NECA (HE-NECA) and SCH58621 (A2A receptors agents) neither affected catecholamine release nor altered the caffeine effects. The 8-cycle-1,3-dipropylxanthine (DPCPX; an A1/A2B antagonist) and 8-(4-{[(4-cyanophenyl)carbamoylmethyl]-oxy}phenyl)-1,3-di(n-propyl)xanthine (MRS1754; an A2B antagonist) mimicking of caffeine indicated that caffeine effects are mediated by A2B receptors. Immunocytochemical A2B receptors were located in tyrosine hydroxylase positive chemoreceptor cells. Caffeine reduced by 52% the chemosensory discharges elicited by hypoxia in the carotid sinus nerve. Inhibition had two components with pharmacological analysis indicating that A2A and A2B receptors mediate, respectively, the low (17 × 10−9 m) and high (160 × 10−6 m) IC50 effects. It is concluded that endogenous adenosine, via presynaptic A2B and postsynaptic A2A receptors, can exert excitatory effects on the overall output of the rat CB chemoreceptors.

Novel 1,3-disubstituted 8-(1-benzyl-1H-pyrazol-4-yl) xanthines: high affinity and selective A2B adenosine receptor antagonists.

Abstract: Adenosine has been suggested to induce bronchial hyperresponsiveness in asthmatics, which is believed to be an A(2B) adenosine receptor (AdoR) mediated pathway. We hypothesize that a selective, high-affinity A(2B) AdoR antagonist may provide therapeutic benefit in the treatment of asthma. In an attempt to identify a high-affinity, selective antagonist for the A(2B) AdoR, we synthesized 8-(C-4-pyrazolyl) xanthines. Compound 22, 8-(1H-pyrazol-4-yl)-1,3-dipropyl xanthine, is a N-1 unsubstituted pyrazole derivative that has favorable binding affinity (K(i) = 9 nM) for the A(2B) AdoR, but it is only 2-fold selective versus the A(1) AdoR. Introduction of a benzyl group at the N-1-pyrazole position of 22 resulted in 19, which had moderate selectivity. The initial focus of the SAR study was on the preparation of substituted benzyl derivatives of 19 because the corresponding phenyl, phenethyl, and phenpropyl derivatives showed a decrease in A(2B) AdoR affinity and selectivity relative to 19. The preferred substitution on the phenyl ring of 19 contains an electron-withdrawing group, specifically F or CF(3) at the m-position, as in 33 and 36 respectively, increases the selectivity while retaining the affinity for the A(2B) AdoR. Exploring disubstitutions on the phenyl ring of derivatives 33 and36 led to the 2-chloro-5-trifluoromethylphenyl derivative 50, which retained the A(2B) AdoR affinity but enhanced the selectivity relative to 36. After optimization of the substitution on the 8-pyrazole xanthine, 1,3-disubstitution of the xanthine core was explored with methyl, ethyl, butyl, and isobutyl groups. In comparison to the corresponding dipropyl analogues, the smaller 1,3-dialkyl groups (methyl and ethyl) increased the A(2B) AdoR binding selectivity of the xanthine derivatives while retaining the affinity. However, the larger 1,3-dialkyl groups (isobutyl and butyl) resulted in a decrease in both A(2B) AdoR affinity and selectivity. This final SAR optimization led to the discovery of 1,3-dimethyl derivative 60, 8-(1-(3-(trifluoromethyl) benzyl)-1H-pyrazol-4-yl)-1,3-dimethyl xanthine, a high-affinity (K(i) = 1 nM) A(2B) AdoR antagonist with high selectivity (990-, 690-, and 1,000-) for the human A(1), A(2A,) and A(3) AdoRs.

Improving potency, selectivity, and water solubility of adenosine A1 receptor antagonists: xanthines modified at position 3 and related pyrimido[1,2,3-cd]purinediones.

Abstract: The structure-activity relationships of xanthine derivatives related to the adenosine A(1) receptor antagonists 8-cyclopentyl-1,3-dipropylxanthine (DPCPX) and 1,3-dipropyl-8-(3-noradamantyl)xanthine (KW3902) were investigated by focusing on variations of the 3-substituent. Aromatic residues were well tolerated by the A(1) receptor in that position. A moderate effect of stereochemistry was found for the 3-(1-phenylethyl)-substituted analogue of DPCPX (S>R) at A(1) and A(3) receptors, whereas the opposite stereoselectivity was observed at the A(2) receptor subtypes. A 3-hydroxypropyl substituent was found to be optimal for high A(1) affinity and selectivity. The most potent compound of the present series was 1-butyl-3-(3-hydroxypropyl)-8-(3-noradamantyl)xanthine (10 c), which exhibits a K(i) value of 0.124 nM at rat, and 0.7 nM at human adenosine A(1) receptors, combined with high selectivity (>>200-fold) versus the other receptor subtypes. The similarly potent 8-cyclopentyl-3-(3-hydroxypropyl)-1-propylxanthine was converted into a water-soluble phosphate prodrug, which may become a useful pharmacological tool for in vivo studies. 8-Alkyl-2-(3-noradamantyl)pyrimido[1,2,3-cd]purine-8,10-diones, which can be envisaged as xanthine analogues with a fixed 3-propyl substituent, were identified as a new class of potent, selective adenosine A(1) receptor antagonists. For example, compound 14 (8-butyl-substituted) exhibits a K(i) value of 13.8 nM at human A(1) receptors. A selection of the most potent compounds was investigated in [(35)S]GTPgammaS binding assays and showed inverse agonistic activity. Their efficacy was generally lower than that of the full inverse agonist DPCPX, and depended on subtle structural changes. Some of the new compounds belong to the most potent and selective A(1) antagonists described to date.

An amperometric biosensor for xanthine determination prepared from xanthine oxidase immobilized in polypyrrole film.

Abstract: In order to prepare a biosensor for the determination of xanthine, electropolymerization of pyrrole on Pt surface was carried out with an electrochemical cell containing pyrrole, ferrocene (as a electron mediator) and tetrabutylamonium tetrafluoroborat in acetonitrile by cyclic voltammetry between 0.0 and 0.9V (vs SCE) at a scan rate of 50mV/s upon Pt electrode. Xanthine oxidase was immobilized by a glutaraldehyde/bovine serum albumin (BSA) crosslinking procedure on to polypyrrole film after the electropolymerization processes. The response of the biosensor against xanthine was measured after 3-4 min following the application of a constant potential of + 0.7 V (vs SCE). The resulting biosensor exhibits excellent electrocatalysis for the xanthine. The amperometric determination is based on the electrochemical detection of H202, which is generated in enzymatic reaction of xanthine. The effect of various experimental conditions was examined for the determination of the analytical performance. The sensor responds to xanthine with a detection limit of 1.0 x 10(-6)M. The response current increases linearly with xanthine concentration up to 4.0 x 10(-4) M. The sensor remains relatively stable for 45 days.

Hydrochlorides and hydrates of 1-[(3-cyanopyridin-2-yl)methyl]-3-methyl-7-(2-butyn-1-yl)-8-(3-aminopiperidin-1-yl)xanthine, their preparation and their use as medicaments

Abstract: The invention relates to salts of 1-[(3-cyanopyridin-2-yl)methyl]-3-methyl-7-(2-butyn-1-yl)-8-[3-aminopiperidin-1-yl]xanthine with hydrochloric acid, and also to their enantiomers, mixtures thereof and hydrates thereof.

Personal care compositions and methods for the beautification of mammalian skin and hair

Abstract: Personal care composition comprising from about 0.05% to about 5% of at least one aquaporin-stimulating compound selected from the group consisting of xanthine, caffeine; 2-amino-6-methyl-mercaptopurine; 1-methyl xanthine; 2-aminopurine; theophylline; theobromine; adenine; adenosine; kinetin; p-chlorophenoxyacetic acid; 2,4- dichlorophenoxyacetic acid; indole-3-butyric acid; indole-3-acetic acid methyl ester; beta-naphthoxyacetic acid; 2,3,5-triiodobenzoic acid; adenine hemisulfate; n-benzyl-9- (2-tetrahydropyranyl)adenine; 1,3-diphenylurea; l-phenyl-3-(l,2,3-thiadiazol-5-yl)urea; zeatin; indole-3-acetic acid; 6-benzylaminopurine; alpha-napthaleneacetic acid; 6-2- furoylaminopurine; green tea extract; white tea extract; menthol; tea tree oil; ginsenoside- RB l; ginsenoside-RB3; ginsenoside-RC; ginsenoside-RD; ginsenoside-RE; ginsenoside- RGl ; ginseng root extract; ginseng flower extract; pomegranate extract, extracts from Ajuga turkestanica; extracts from viola tricolor and combinations thereof; an additional ingredient selected from the group consisting of niacinamide, glycerin and mixtures thereof, and a dermatologically-acceptable carrier.

Characterization of n uricase from Bacillus fastidious A.T.C.C. 26904 and its application to serum uric acid assay by a patented kinetic uricase method

Abstract: An intracellular uricase from Bacillus fastidious A.T.C.C. 26904 was characterized and evaluated for serum uric acid assay by a patented kinetic uricase method. The active uricase was 151 kDa by gel filtration through Sephadex G-200. Both SDS/PAGE and matrix-assisted laser-desorption ionization-time-of-flight MS resolved a single polypeptide with a molecular mass of approx. 36.0 kDa. The N-terminal sequence was AERTMFYGKGDV. The optimum pH for this uricase ranged from 9.0 to 10.5. At pH 9.2, the Km (Michaelis-Menten constant) was 204+/-14 micromol/l (n=8) and the Ki (inhibition constant) for xanthine was 41+/-7 micromol/l (n=5). By analysing the data monitored within 5 min at 0.03 unit/ml uricase, this kinetic uricase method gave linear response to uric acid in reaction solution from 1.3 to 60 micromol/l. Aside from other common errors, 30 micromol/l xanthine in the reaction solution caused no error in this kinetic uricase method, while it caused negative error in the indirect equilibrium method by peroxidase-coupled assay of H2O2. Uric acid in clinical sera by this kinetic uricase method (Ck) closely and positively correlated with that from the indirect equilibrium method (Ce) (Ck = 0.008+1.081 x Ce, r>0.990, n=99). However, Bland-Altman analysis suggested inconsistency between Ck and Ce. These results indicated that this kinetic uricase method using this uricase was reliable for serum uric acid assay with enhanced resistance to xanthine besides other common errors.

Plasma concentrations and urinary excretion of purine bases (uric acid, hypoxanthine, and xanthine) and oxypurinol after rigorous exercise.

Abstract: To investigate the effects of exercise on the plasma concentrations and urinary excretion of purine bases and oxypurinol, we performed 3 experiments with 6 healthy male subjects. The first was a combination of allopurinol intake (300 mg) and exercise (VO2max, 70%) (combination experiment), the second was exercise alone (exercise-alone experiment), and the third was allopurinol intake alone (allopurinol-alone experiment). In the combination experiment, exercise increased the concentrations of purine bases and noradrenaline in plasma, as well as lactic acid in blood and the urinary excretion of oxypurines, whereas it decreased the urinary excretion of uric acid and oxypurinol as well as the fractional excretion of hypoxanthine, xanthine, uric acid, and oxypurinol. In the exercise-alone experiment, exercise increased the concentrations of purine bases and noradrenaline in plasma, lactic acid in blood, and the urinary excretion of oxypurines, whereas it decreased the urinary excretion of uric acid and fractional excretion of purine bases. In contrast, in the allopurinol-alone experiment, the plasma concentration, urinary excretion, and fractional excretion of purine bases and oxypurinol remained unchanged. These results suggest that increases in adenine nucleotide degradation and lactic acid production, as well as a release of noradrenaline caused by exercise, contribute to increases in plasma concentration and urinary excretion of oxypurines and plasma concentration of urate, as well as decreases in urinary excretion of uric acid and oxypurinol, along with fractional excretion of uric acid, oxypurinol, and xanthine. In addition, they suggest that oxypurinol does not significantly inhibit the exercise-induced increase in plasma concentration of urate.

Allopurinol Reduces Neointimal Hyperplasia in the Carotid Artery Ligation Model in Spontaneously Hypertensive Rats

Abstract: Uric acid and oxidative stress promote cardiovascular diseases, including atherosclerosis and hypertension. Xanthine oxidase, through which uric acid is generated, is a free-radical generating enzyme. The aim of the current study was to investigate whether allopurinol, an inhibitor of xanthine oxidase activity, affects vascular remodeling and vascular smooth muscle cell (VSMC) proliferation. In the carotid artery ligation model using spontaneously hypertensive rats (SHR), treatment with allopurinol induced a reduction in the neointima/media ratio by 27% (38.5+/-34.3% in the control group and 28.1 20.8% in the allopurinol-treated group, respectively, p<0.01) without alterations in vascular circumference at 3 weeks after ligation when compared to the control. Allopurinol lowered the serum uric acid concentration (147.0+/-3.6 micromol/l in the control group and 16.1+/-3.6 micromol/l in the allopurinol-treated group, respectively p<0.01) and xanthine oxidase activity, but not the blood pressure. In an in vitro study, high concentrations of uric acid (100 and 200 micromol/l) stimulated VSMC growth, but there was no stimulation of these cells by a low concentration of uric acid (50 micromol/I) or by any of three concentrations of xanthine (50, 100 and 200 micromol/l). In addition, allopurinol (5 micromol/I) had no effect on the cell growth. In conclusion, uric acid is a potent stimulator of VSMC proliferation, and allopurinol prevented vascular remodeling in SHR at least in part by inhibiting uric acid concentration.

Evaluation of a kinetic uricase method for serum uric acid assay by predicting background absorbance of uricase reaction solution with an integrated method

Abstract: A patented kinetic uricase method was evaluated for serum uric acid assay. Initial absorbance of the reaction mixture before uricase action (A0) was obtained by correcting the absorbance at 293 nm measured before the addition of uricase solution, and background absorbance (Ab) was predicted by an integrated method. Uric acid concentration in reaction solution was calculated from ΔA, the difference between A0 and Ab, using the absorptivity preset for uric acid. This kinetic uricase method exhibited CV<4.3% and recovery of 100%. Lipids, bilirubin, hemoglobin, ascorbic acid, reduced glutathione and xanthine <0.32 mmol/L in serum had no significant effects. ΔA linearly responded to 1.2 to 37.5 µmol/L uric acid in reaction solution containing 15 µl serum. The slope of linear response was consistent with the absorptivity preset for uric acid while the intercept was consistent with that for serum alone. Uric acid concentrations in clinic sera by different uricase methods positively correlated to each other. By Bland-Altman analysis, this kinetic uricase method accorded with that by quantifying the total change of UV absorbance on the completion of uricase reaction. These results demonstrated that this kinetic uricase method is reliable for serum uric acid assay with enhanced resistance to both xanthine and other common errors, wider range of linear response and much lower cost.

Xanthine derivatives as selective hm74a agonists

Abstract: The present invention relates to compounds1 of formula (I) which are xanthine derivatives, processes for the manufacture of said derivatives, pharmaceutical formulations containing these, compounds and the use of the compounds in therapy, for example, 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.

Biochemical characterization of some pyrazolopyrimidine-based inhibitors of xanthine oxidase.

Abstract: Inhibition of xanthine oxidase-catalyzed conversion of xanthine to uric acid by various pyrazolopyrimidine-based inhibitors (allopurinol derivatives) was evaluated and compared with the standard inhibitor allopurinol. Three compounds out of the seven compounds used in the study were found to be reasonably good inhibitors of xanthine oxidase (XO). 4-Amino-6-mercaptopyrazolo-3,4-d-pyrimidine was found to be the most potent inhibitor of XO (IC50 = 0.600 +/- 0.009 microM). 4-Mercapto-1H-pyrazolo-3,4-d-pyrimidine (IC50 = 1.326 +/- 0.013 microM) and 4-amino-6-hydroxypyrazolo-3,4-d-pyrimidine (IC50 = 1.564 +/- 0.065 microM) also showed comparable inhibitory activity to that of allopurinol (IC50 = 0.776 +/- 0.012 microM). All three compounds showed competitive type of inhibition with comparable Ki values. Induction of the electron transfer reaction catalyzed by XO in the presence of these compounds monitored as reduction of 2,6-dichlorophenolindophenol (DCPIP) revealed that electron transfer by 4-amino-6-mercaptopyrazolo-3,4-d-pyrimidine is comparable to that obtained by allopurinol or xanthine. However, 4-mercapto-1H-pyrazolo-3,4-d-pyrimidine and 4-amino-6-hydroxypyrazolo-3,4-d-pyrimidine did not show DCPIP reduction. On the other hand, enzymatic reduction of cytochrome c in the presence of the three compounds was found to be insignificant and much less in comparison to allopurinol and xanthine. Therefore, both 4-amino-6-hydroxypyrazolo-3,4-d-pyrimidine and 4-mercapto-1H-pyrazolo-3,4-d-pyrimidine displayed the inhibitory property and also did not produce XO-mediated reactive oxygen species (ROS). Since 4-mercapto-1H-pyrazolo-3,4-d-pyrimidine was found to have some toxicity, the effect of 4-amino-6-hydroxypyrazolo-3,4-d-pyrimidine on the enzymatic formation of uric acid and ROS was investigated and it was found that this compound was inhibiting enzymatic generation of both uric acid and ROS. It can be noted that the standard inhibitor, allopurinol, inhibits uric acid formation but produces ROS.

Regulation of the body fat percentage in developmental-stage rats by methylxanthine derivatives in a high-fat diet.

Abstract: We investigated the regulatory effects of structural differences among methylxanthine derivatives on the elevation of body fat percentage in developmental-stage rats. Caffeine, theophylline and theobromine were used as the methylxanthines. High-fat diets (20% lard) containing each methylxanthine (0.025%) were administered to male Sprague-Dawley rats for 12 weeks, with the result that the body fat percentage was generally reduced in each methylxanthine-fed group. The abdominal adipose tissue weight in the caffeine group was also significantly lower than that in the control group, the serum cholesterol and triglyceride levels in the caffeine group also being significantly lower than the levels in the control group. The study results suggest that caffeine could contribute most to preventing arteriosclerotic diseases.

Inhibitory effects of antioxidant constituents from Melothria heterophylla on matrix metalloproteinase-1 expression in UVA-irradiated human dermal fibroblasts.

Abstract: Matrix metalloproteinases (MMPs) are known to play an important role in photoaging by mediating the degradation of extracellular matrix proteins. To develop a new anti-aging agent for cosmetics from natural products, Melothria heterophylla (Lour.) Cogn. was selected for its antioxidant activity and inhibitory effect on expression of MMP-1 in UVA-irradiated human skin fibroblasts. Two compounds (compounds 1 and 2 ) were isolated from an ethyl acetate soluble fraction of the ethanolic extracts; they were identified as 1,2,4,6-tetra-O-galloyl-beta-(D)-glucopyranose (1) and 3,4,5-trihydroxybenzoic acid (2). These compounds were found to scavenge radicals and reactive oxygen species (ROS) and were measured to have SC50 values of 3.9 microM and 13.3 microM against 1,1-diphenyl-2-picrylhydrazyl (DPPH) radicals and values of 4.3 microM and 4.0 microM against superoxide radicals in the xanthine/xanthine oxidase system, respectively. Compounds 1 and 2 showed a dose-dependent inhibitory effect on the expression and activity of MMP-1 in UVA-induced human skin fibroblasts, but no inhibition of MMP-1 mRNA expression. Therefore, we concluded that compounds 1 and 2 significantly inhibited MMP-1 expression at the protein level. Also, these compounds were determined to have a potent antioxidant activity. From these results, we suggest that these compounds might be useful as a new anti-aging agent for photodamaged skin, but the in vitro findings must be verified in in vivo studies.

The extraordinary specificity of xanthine phosphoribosyltransferase from Bacillus subtilis elucidated by reaction kinetics, ligand binding, and crystallography.

Abstract: Xanthine phosphoribosyltransferase (XPRTase) from Bacillus subtilis is a representative of the highly xanthine specific XPRTases found in Gram-positive bacteria. These XPRTases constitute a distinct subclass of 6-oxopurine PRTases, which deviate strongly from the major class of H(X)GPRTases with respect to sequence, PRPP binding motif, and oligomeric structure. They are more related with the PurR repressor of Gram-positive bacteria, the adenine PRTase, and orotate PRTase. The catalytic function and high specificity for xanthine of B. subtilis XPRTase were investigated by ligand binding studies and reaction kinetics as a function of pH with xanthine, hypoxanthine, and guanine as substrates. The crystal structure of the dimeric XPRTase−GMP complex was determined to 2.05 A resolution. In a sequential reaction mechanism XPRTase binds first PRPP, stabilizing its active dimeric form, and subsequently xanthine. The XPRTase is able also to react with guanine and hypoxanthine albeit at much lower (10-4-fold) catal...