Xanthine Metabolism Research Articles

Rebuttal from Drs. Poole, Brown, Hudlicka

POINT-COUNTERPOINTRebuttal from Drs. Poole, Brown, HudlickaPublished Online:01 Mar 2008https://doi.org/10.1152/japplphysiol.00779.2007cMoreSectionsPDF (44 KB)Download PDF ToolsExport citationAdd to favoritesGet permissionsTrack citations ShareShare onFacebookTwitterLinkedInEmail My learned colleagues are to be congratulated for considering different techniques (1–5 below) to assess changes in the proportion of flowing capillaries in muscle. However, the resultant data must be judged in light of limitations inherent in those techniques.It is also important to define precisely what we are talking about. Professor Clark and colleagues' (1) opening statement that “The key issue here is whether resting skeletal muscle is fully perfused” is clearly erroneous. “Perfusion,” (from the Latin perfusus), infers nothing about the distribution of blood, which, along with the veracity of presumptions made to assess that distribution, is what is at issue here.Technique 1: Red blood cell occupancy in cryosections.PRESUMPTIONS.An RBC in capillary cross-section indicates capillary was flowing (4).CHALLENGES.1) Cannot discriminate stopped versus moving RBCs. 2) Low capillary hematocrit means large inter-RBC (plasma) spaces.CONCLUSION.Apparent increase in capillaries “recruited” might simply reflect elevated hematocrit during hyperemia (6, 7).Technique 2: direct intravital microscopy observation.Reports of a majority of nonperfused capillaries in resting tenuissimus muscles (8) are opposed by others (10).Technique 3: increased metabolism of 1-methyl xanthine by xanthine oxidase.PRESUMPTIONS.Xanthine oxidase (XO) found predominantly in capillary endothelial cells and increased 1-methyl xanthine (1-MX) metabolism occurs: 1) in direct proportion to the number of RBC-perfused capillaries and 2) is unaffected by capillary hemodynamics.CHALLENGES.1) XO is found in plasma (9) and smooth muscle (3), 2) increased 1-MX metabolism results from increased delivery to low-flow capillaries.Technique 4: contrast-enhanced ultrasound.PRESUMPTIONS.Microbubbles are distributed the same as RBCs.CHALLENGE.If true (doubtful), concentration in already-flowing capillaries would increase two- (or more) fold in hyperemia without obligatory capillary “recruitment” (5–7).Technique 5: laser Doppler flowmetry shows “… an increase in intramuscular hyperemia…”.PRESUMPTION.Hyperemia parallels capillary recruitment.CHALLENGE.Presumption is baseless: hyperemia reflects primarily increased RBC flux in already flowing capillaries (2, 5, 6).The famous Indian/Chinese/African legend “The Blind Men and the Elephant,” popularized by John Godfrey Saxe in 1878, bears some similarity to my opponents' approach to this debate. When several blind men seek to discover what an elephant is like, each man only touches one part and argues as follows: Hence, although all were partly right, all were wrong. This 2,000 year-old missive argues for a better integration of current techniques and knowledge if we are to understand muscle capillary exchange.Nullius in Verba!REFERENCES1 Clark MG, Rattigan S, Barrett EJ, Vincent MA. Point: There is capillary recruitment in active skeletal muscle during exercise. J Appl Physiol; doi:10.1152/japplphysiol.00779.2007.Link | ISI | Google Scholar2 Hargreaves D, Egginton S, Hudlicka O. Changes in capillary perfusion induced by different patterns of activity in rat skeletal muscle. Microvasc Res 40: 14–28, 1990.Crossref | PubMed | ISI | Google Scholar3 Hellsten-Westing Y. Immunohistochemical localization of xanthine oxidase in human cardiac and skeletal muscle. Histochemistry 100: 215–222, 1993.Crossref | PubMed | Google Scholar4 Honig CR, Odoroff CL, Frierson JL. Capillary recruitment in exercise: rate, extent, uniformity, and relation to blood flow. Am J Physiol Heart Circ Physiol 238: H31–H42, 1980.Link | ISI | Google Scholar5 Hudlicka O, Zweifach BW, Tyler KR. Capillary recruitment and flow velocity in skeletal muscle after contractions. Microvasc Res 23: 201–213, 1982.Crossref | PubMed | ISI | Google Scholar6 Kindig CA, Richardson TE, Poole DC. Skeletal muscle capillary hemodynamics from rest to contractions: implications for oxygen transfer. J Appl Physiol 92: 2513–2520, 2002.Link | ISI | Google Scholar7 Klitzman B, Duling BR. Microvascular hematocrit and red cell flow in resting and contracting striated muscle. Am J Physiol Heart Circ Physiol 237: H481–H490, 1979.Link | ISI | Google Scholar8 Lindbom L. Microvascular blood flow distribution in skeletal muscle. Acta Physiol Scand Suppl 525: 1–40, 1983.PubMed | Google Scholar9 Newaz MA, Adeeb NNN. Detection of xanthine oxidase in human plasma. Med J Malaysia 53: 70–75, 1998.Google Scholar10 Vrielink HH, Slaaf DW, Tangelder GJ, Reneman RS. Does capillary recruitment exist in young rabbit skeletal muscle? Int J Microcirc Clin Exp 6: 321–332, 1987.Google Scholar Previous Back to Top Next FiguresReferencesRelatedInformation More from this issue > Volume 104Issue 3March 2008Pages 893-894 Copyright & PermissionsCopyright © 2008 the American Physiological Societyhttps://doi.org/10.1152/japplphysiol.00779.2007cHistory Published online 1 March 2008 Published in print 1 March 2008 PDF download Metrics Downloaded 70 times

Preferential inhibition of xanthine oxidase by 2-amino-6-hydroxy-8-mercaptopurine and 2-amino-6-purine thiol

BackgroundThe anticancer drug, 6-mercaptopurine (6MP) is subjected to metabolic clearance through xanthine oxidase (XOD) mediated hydroxylation, producing 6-thiouric acid (6TUA), which is excreted in urine. This reduces the effective amount of drug available for therapeutic efficacy. Co-administration of allopurinol, a suicide inhibitor of XOD, which blocks the hydroxylation of 6MP inadvertently enhances the 6MP blood level, counters this reduction. However, allopurinol also blocks the hydroxylation of hypoxanthine, xanthine (released from dead cancer cells) leading to their accumulation in the body causing biochemical complications such as xanthine nephropathy. This necessitates the use of a preferential XOD inhibitor that selectively inhibits 6MP transformation, but leaves xanthine metabolism unaffected.ResultsHere, we have characterized two such unique inhibitors namely, 2-amino-6-hydroxy-8-mercaptopurine (AHMP) and 2-amino-6-purinethiol (APT) on the basis of IC50 values, residual activity in bi-substrate simulative reaction and the kinetic parameters like Km, Ki, kcat. The IC50 values of AHMP for xanthine and 6MP as substrate are 17.71 ± 0.29 μM and 0.54 ± 0.01 μM, respectively and the IC50 values of APT for xanthine and 6MP as substrates are 16.38 ± 0.21 μM and 2.57 ± 0.08 μM, respectively. The Ki values of XOD using AHMP as inhibitor with xanthine and 6MP as substrate are 5.78 ± 0.48 μM and 0.96 ± 0.01 μM, respectively. The Ki values of XOD using APT as inhibitor with xanthine and 6MP as substrate are 6.61 ± 0.28 μM and 1.30 ± 0.09 μM. The corresponding Km values of XOD using xanthine and 6MP as substrate are 2.65 ± 0.02 μM and 6.01 ± 0.03 μM, respectively. The results suggest that the efficiency of substrate binding to XOD and its subsequent catalytic hydroxylation is much superior for xanthine in comparison to 6MP. In addition, the efficiency of the inhibitor binding to XOD is much more superior when 6MP is the substrate instead of xanthine. We further undertook the toxicological evaluation of these inhibitors in a single dose acute toxicity study in mice and our preliminary experimental results suggested that the inhibitors were equally non-toxic in the tested doses.ConclusionWe conclude that administration of either APT or AHMP along with the major anti-leukemic drug 6MP might serve as a good combination cancer chemotherapy regimen.

Xanthine oxidase inhibitory activity of Lychnophora species from Brazil (“Arnica”)

Twenty-two extracts from five Lychnophora species and one Lychnophoriopsis species, traditionally used in Brazil as analgesic, anti-inflammatory, and to treat bruise and rheumatism were examined for the inhibition of xanthine oxidase (XO), the enzyme that catalyses the metabolism of hypoxanthine and xanthine into uric acid. Sixteen extracts were tested. All of them were found to have excellent XO inhibitory activity, with inhibitions greater than 38% at 100 μg/mL in the assay mixture. The most active plants examined were Lychnophora trichocarpha, Lychnophora ericoides, Lychnophora staavioides and Lychnophoriopsis candelabrum, with inhibitions of 77%, 78%, 66% and 63% at 100 μg/mL, respectively, and IC 50 values of 6.16, 8.28, 33.97 and 37.70 μg/mL, respectively.

Effects of sufentanil on the release and metabolism of dopamine and ascorbic acid and glutamate release in the striatum of freely moving rats

The effects of either intraperitoneally (i.p.) or intrastriatally administered sufentanil on the release and metabolism of dopamine (DA) in the rat striatum were evaluated using in vivo microdialysis. Dialysate concentrations of DA and its acidic metabolites dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA) were increased following i.p. administration of either clinical anesthetic (20 μg/kg) or clinical analgesic (1 μg/kg) sufentanil doses. In addition, sufentanil also increased uric acid concentrations. In contrast, dialysate ascorbic acid and glutamate concentrations were unaffected. Intrastriatal infusion of sufentanil (250 nM) induced only a short lasting decrease in dialysate DA. Subcutaneous naloxone (1.0 mg/kg) abolished sufentanil-induced increases in dialysate DA, DOPAC+HVA and uric acid; however, naloxone (0.1 mM) failed to affect these increases when infused intrastriatally. These results demonstrate that sufentanil, at clinical doses, increases striatal DA release and oxidative metabolism of both DA and xanthine acting at extrastriatal sites with a μ-receptor-mediated mechanism.

Effect of Clinafloxacin on the Pharmacokinetics of Theophylline and Caffeine

Several quinolones are known inhibitors of cytochrome P450-1A2 mixed-function oxidase systems, causing decreased xanthine metabolism and higher than expected plasma concentrations. These higher concentrations become a serious consideration for the use of theophylline, a drug with a narrow therapeutic index. Both in vitro P450 induction/ inhibition and in vivo studies in the rat have suggested that clinafloxacin has a high potential for xanthine metabolic interaction in humans. Structure-activity relationships with other quinolones also suggest that clinafloxacin has a very high relative potential for xanthine interaction. Therefore, studies were conducted to evaluate the effect of multiple dose clinafloxacin administration on the single-dose pharmacokinetics of theophylline and caffeine.

Xanthine metabolism in Bacillus subtilis: characterization of the xpt-pbuX operon and evidence for purine- and nitrogen-controlled expression of genes involved in xanthine salvage and catabolism

The xpt and pbuX genes from Bacillus subtilis were cloned, and their nucleotide sequences were determined. The xpt gene encodes a specific xanthine phosphoribosyltransferase, and the pbuX gene encodes a xanthine-specific purine permease. The genes have overlapping coding regions, and Northern (RNA) blot analysis indicated an operon organization. The translation of the second gene, pbuX, was strongly dependent on the translation of the first gene, xpt. Expression of the operon was repressed by purines, and the effector molecules appear to be hypoxanthine and guanine. When hypoxanthine and guanine were added together, a 160-fold repression was observed. The regulation of expression was at the level of transcription, and we propose that a transcription termination-antitermination control mechanism similar to the one suggested for the regulation of the purine biosynthesis operon exists. The expression of the xpt-pbuX operon was reduced when hypoxanthine served as the sole nitrogen source. Under these conditions, the level of the hypoxanthine- and xanthine-degrading enzyme, xanthine dehydrogenase, was induced more than 80-fold. The xanthine dehydrogenase level was completely derepressed in a glnA (glutamine synthetase) genetic background. Although the regulation of the expression of the xpt-pbuX operon was found to be affected by the nitrogen source, it was normal in a glnA mutant strain. This result suggests the existence of different signalling pathways for repression of the transcription of the xpt-pbuX operon and the induction of xanthine dehydrogenase.

Purine Metabolism in Patients with Gout: The Role of Lead

Primary gout is characterized by increased plasma and decreased urinary concentrations of hypoxanthine, xanthine and uric acid. To examine whether lead could explain the disturbance of purine metabolism in gout, we determined hypoxanthine, xanthine and uric acid metabolism and 5-day cumulative urinary lead excretion rates after an EDTA (calcium disodium edetate) test in 27 patients with primary gout and reduced creatinine clearance (C(cr)) and in 50 patients with gout and normal C(cr). The results were compared to those obtained in 26 normal subjects matched for age. All gout patients evidenced a marked renal underexcretion of hypoxanthine, xanthine and uric acid relative to their increased plasma levels. Purine metabolism was remarkably similar in both gout groups except for a significantly lower uric acid excretion in patients with reduced C(cr). Blood lead levels and cumulative lead excretion rates were significantly higher in gout patients with renal failure as compared to patients with normal C(cr). Fourteen patients (52%) with renal insufficiency and 6 (12%) with normal C(cr) showed increased lead excretion rates (95% Cl for the difference, 29-51%, p < 0.001). Mobilizable lead was not significantly correlated with serum or urinary purine concentrations. Hypoxanthine, xanthine and uric acid underexcretion was similar in gout patients with increased or normal cumulative lead excretion rates. The prevalence of atheromatosis and arterial hypertension together was significantly higher in gout patients with renal failure than in patients with normal C(cr) (81 vs. 60%, 95% Cl for the difference, 11-31%, p < 0.005). These results indicate that lead is not a significant contributor to the renal underexcretion of purines in gout. An increased mobilizable lead is not by itself evidence that lead is the cause of the renal insufficiency in patients with primary gout. Atheromatous nephropathy and/or nephroangiosclerosis may explain impaired renal function in patients with primary gout.

The proportion of xanthine oxidase activity of total xanthine oxidoreductase activity in situ remains constant in rat liver under various (patho)physiological conditions.

Activity of xanthine oxidoreductase (total xanthine dehydrogenase plus xanthine oxidase) and xanthine oxidase was determined cytophotometrically in periportal and pericentral areas of livers of rats under various (patho)physiological conditions that are known to affect the content of reduced glutathione. For this purpose, rats were either normally fed or fasted for 24 hours, fasted for 24 hours, and treated with diethylmaleate that depleted glutathione or treated by in vivo ischemia for 2 hours in the livers. Xanthine oxidoreductase activity was shown histochemically with the use of a tetrazolium salt procedure, and xanthine oxidase activity was localized with a cerium-diaminobenzidine-cobalt-hydrogen peroxide technique in unfixed cryostat sections of the livers. Cytophotometric measurements showed that total xanthine oxidoreductase activity was decreased after fasting and ischemia, whereas only ischemia caused reduced xanthine oxidase activity. Moreover, the percentage of xanthine oxidase of total xanthine oxidoreductase activity was constant in both periportal and pericentral areas at the level of approximately 4% in normally fed and 24-hour fasted and diethylmaleate-treated rats. Ischemia reduced this percentage in both areas of the liver to 2%. It was concluded that the amount of endogenous reduced glutathione did not affect the percentage of xanthine oxidase. The low percentage of xanthine oxidase as determined in the present in situ histochemical study indicates that in vivo the percentage oxidase in rat liver is lower than is assumed on the basis of biochemical assays in liver homogenates even after strictly controlled homogenization procedures. Apparently, conversion of xanthine dehydrogenase into xanthine oxidase may occur in vitro to yield percentages of xanthine oxidase of 10%-20% as are reported in the literature. The latter increase in the percentage of xanthine oxidase may be caused by changes in the local environment of the enzymes, which is left completely intact in histochemical assays. The finding of this low percentage of xanthine oxidase further stresses that the main function of xanthine oxidoreductase in the liver is not the production of superoxide anion radicals and/or hydrogen peroxide but rather the metabolism of xanthine to uric acid, which can act as a potent antioxidant.

The proportion of xanthine oxidase activity of total xanthine oxidoreductase activity in situ remains constant in rat liver under various (patho)physiological conditions

Activity of xanthine oxidoreductase (total xanthine dehydrogenase plus xanthine oxidase) and xanthine oxidase was determined cytophotometrically in periportal and pericentral areas of livers of rats under various (patho)physiological conditions that are known to affect the content of reduced glutathione. For this purpose, rats were either normally fed or fasted for 24 hours, fasted for 24 hours, and treated with diethylmaleate that depleted glutathione or treated by in vivo ischemia for 2 hours in the livers. Xanthine oxidoreductase activity was shown histochemically with the use of a tetrazolium salt procedure, and xanthine oxidase activity was localized with a cerium- diaminobenzidine-cobalt-hydrogen peroxide technique in unfixed cryostat sections of the livers. Cytophotometric measurements showed that total xanthine oxidoreductase activity was decreased after fasting and ischemia, whereas only ischemia caused reduced xanthine oxidase activity. Moreover, the percentage of xanthine oxidase of total xanthine oxidoreductase activity was constant in both periportal and pericentral areas at the level of approximately 4% in normally fed and 24-hour fasted and diethylmaleate-treated rats. Ischemia reduced this percentage in both areas of the liver to 2%. It was concluded that the amount of endogenous reduced glutathione did not affect the percentage of xanthine oxidase. The low percentage of xanthine oxidase as determined in the present in situ histochemical study indicates that in vivo the percentage oxidase in rat liver is lower than is assumed on the basis of biochemical assays in liver homogenates even after strictly controlled homogenization procedures. Apparently, conversion of xanthine dehydrogenase into xanthine oxidase may occur in vitro to yield percentages of xanthine oxidase of 10%-20% as are reported in the literature. The latter increase in the percentage of xanthine oxidase may be caused by changes in the local environment of the enzymes, which is left completely intact in histochemical assays. The finding of this low percentage of xanthine oxidase further stresses that the main function of xanthine oxidoreductase in the liver is not the production of superoxide anion radicals and/or hydrogen peroxide but rather the metabolism of xanthine to uric acid, which can act as a potent antioxidant. (Hepatology 1996 Nov;24(5):1179-84)

The influence of lipid peroxidation products (malondialdehyde, 4-hydroxynonenal) on xanthine oxidoreductase prepared from rat liver

Depending on metabolic conditions, xanthine oxidoreductase acts as either a dehydrogenase (XDH) or an oxidase (XOD). The metabolism of hypoxanthine and xanthine by the oxidase is associated with the production of reactive oxygen radicals. Reaction of reactive oxygen radicals with polyunsaturated fatty acids (lipid peroxidation) leads to the formation of malondialdehyde (MDA) and 4-hydroxynonenal (HNE), known to modify proteins by reaction with NH 2- and SH-groups. Therefore, these aldehydes could influence both the activity of xanthine oxidoreductase and the XOD/XDH ratio. We found that incubation of xanthine oxidoreductase with MDA leads to an initial increase in XDH activity and to a continuous decrease in XOD activity, whereby the total activity decreases. This was in contrast to the effects of HNE which did not alter the XDH activity; XOD was however activated. This demonstrates that the lipid peroxidation products MDA and HNE are able to modify xanthine oxidoreductase similarly to a feed-back mechanism.

Role of xanthine oxidase in ethanol-induced lipid peroxidation in rats

To investigate a possible role of free radical production by xanthine oxidase in the pathogenesis of ethanol-induced hepatic lipid peroxidation, chowfed rats were given ethanol (5 g/kg) and placed at 32 °C for 6 h, which resulted in increased hepatic malondialdehyde levels. Pretreatment with allopurinol in amounts that effectively inhibited xanthine metabolism also significantly decreased ethanolinduced lipid peroxidation, suggesting participation of free radicals produced by xanthine oxidase in the peroxidative process. Both acetaldehyde and purine can serve as substrates for xanthine oxidase. Pretreatment with cyanamide increased hepatic acetaldehyde levels 5-fold, yet this was associated with a decrease in lipid peroxidation, indicating that acetaldehyde is not the xanthine oxidase substrate involved. By contrast, ethanol increased hepatic contents of hypoxanthine and xanthine and enhanced urinary output of allantoin (a final product of xanthine metabolism), incriminating increased metabolism of purines. Ethanol administration also enhanced hepatic nicotinamide adenine dinucleotide (reduced form). A corresponding rise of nicotinamide adenine dinucleotide (reduced form) in vitro inhibited xanthine dehydrogenase activity by 60%–76%. Increased purine degradation, possibly associated with a shift from the dehydrogenase to the xanthine oxidase pathway (secondary to nicotinamide adenine dinucleotide [reduced form]-mediated inhibition of xanthine dehydrogenase activity) is proposed as a possible mechanism for ethanol-stimulated free radical production. Because allopurinol attenuates the associated lipid peroxidation, this agent might be considered for possible therapeutic use in alcoholinduced liver damage.

Drug-induced hydrogen peroxide production in isolated rat hepatocytes

A method for measuring drug-induced hydrogen peroxide production in freshly isolated rat hepatocytes is described. 3-Amino,1,2,4-triazole, an irreversible inhibitor of the enzyme catalase markedly reduces the capacity of isolated hepatocytes to metabolize hydrogen peroxide, with maximum inhibition (80%) being observed after 40 min of co-incubation. The present method is based on the observation that this inhibition of catalase by 3-amino-1,2,4-triazole is prevented by methanol and that the effect of methanol is reversed in the presence of hydrogen peroxide. Using this assay we could demonstrate increased hydrogen peroxide production during the metabolism of diquat, paraquat, xanthine, benzylamine and glycolate by hepatocytes. Inhibition of the hydrogen peroxide metabolic capacity was greatest with glycolate and diquat, whereas paraquat and benzylamine only had a minor effect.

Xanthine oxidase is not responsible for reoxygenation injury in isolated-perfused rat heart.

The massive leakage of intracellular enzymes which occurs during reoxygenation of heart tissue after hypoxic or ischemic episodes has been suggested to result from the formation of oxygen radicals. One purported source of such radicals is the xanthine oxidase-mediated metabolism of hypoxanthine and xanthine. Xanthine oxidase (O form) has been suggested to be formed in vivo by limited proteolysis of xanthine dehydrogenase (D form) during the hypoxic period (Granger et al., Gastroenterology, 81, 22 (1981)). We measured the activities of xanthine oxidase in both fresh and isolated-perfused (Langendorff) rat heart tissue. Approximately 32% of the total xanthine oxidase was in the O form in fresh and isolated-perfused rat heart. This value was unchanged following 60 min of hypoxia and 30 minutes of reoxygenation. The infusion of 250 microM allopurinol throughout the perfusion completely inhibited xanthine oxidase activity but had no effect on the massive release of lactate dehydrogenase (LDH) into the coronary effluent upon reoxygenation of heart tissue subjected to 30 or 60 min of hypoxia. Protection from 30 min of hypoxia was also not obtained when rats were pretreated for 48 h with allopurinol at a dose of 30 mg/kg/day and perfused with allopurinol containing medium. Superoxide dismutase (50 units/ml), catalase (200 units/ml), or the antioxidant cyanidanol (100 microM) also had no effect on LDH release upon reoxygenation after 60 min of hypoxia. Xanthine oxidase activity was detected in a preparation enriched in cardiac endothelial cells while no allopurinol-inhibitable activity could be measured in purified isolated cardiomyocytes. It is concluded that xanthine dehydrogenase is not converted to xanthine oxidase in hypoxic tissue of the isolated perfused rat heart, and that the release of intracellular enzymes upon reoxygenation in this experimental model is mediated by factors other than reactive oxygen generated by xanthine oxidase.

Oxidation of methionine in proteins of isolated rat heart myocytes and tissue slices by neutrophil-generated oxygen free radicals

It is now well established that reperfusion of ischemic heart tissues initiates the production of oxygen free radicals which exacerbate the damage to the myocardium (1–3). While it is clear that a large portion of the oxygen free radicals is formed in damaged tissue by endogenous reactions such as the metabolism of xanthine and arachidonic atld (4,5), it now appears likely that a major source of these free radicals are the neutrophils which migrate into the damaged tissues during the inflammatory response (6–8). At the site of damage, these neutrophils can become activated and can secrete a variety of oxidants such as superoxide anion, hydrogen peroxide, and hypochlorous acid (9).

The interaction of reduced glutathione with active oxygen species generated by xanthine-oxidase-catalyzed metabolism of xanthine

The interaction of reduced glutathione (GSH) with active oxygen species generated during xanthine-oxidase-catalyzed metabolism of xanthine was investigated. The only GSH-derived product detected in this system was oxidized glutathione (GSSG). Catalase inhibited the oxidation of GSH to GSSG by more than 80%, whereas superoxide dismutase exerted a smaller but significant inhibition of GSSG formation. Hydroxyl radical (OH) scavengers or desferrioxamine (1 mM) had no effect on GSSG formation. Using EPR spectroscopy and the spin trap 5,5-dimethylpyrroline- N-oxide (DMPO), the production of superoxide was observed by the detection of a DMPO-OOH radical adduct. This spectrum was altered by the inclusion of GSH (5–20 mM) in the reaction mixture, indicating the generation of a different radical species consistent with DMPO-glutathionyl radical adduct generation.

Sensitivity of inbred mice to methylxanthines is not determined by plasma xanthine concentration

Male CBA/J and SWR/J mice were tested with doses of caffeine, theophylline and 8- p-sulfophenyltheophylline (xanthine). Caffeine produced dose-related decreases in locomotor activity and colonic temperatures in SWR/J mice. However, caffeine produced increases in locomotor activity and failed to lower the body temperature of CBA/J mice. Theophylline produced a decrease in body temperature of SWR/J mice. Comparison of brain caffeine levels demonstrated no difference in brain pharmacokinetics. The peripherally active xanthine failed to alter body temperature at the same molar dose as that of theophylline. These data clearly demonstrate that genetic differences in the effects of methylxanthine are due to inherent differences in the central nervous system sensitivity of the two strains. The data further indicate that while differences in xanthine metabolism may occur in inbred mice, these differences are not a major factor in the acute, peak plasma level, effects of xanthines.

An investigation into the factors governing the aqueous solubility of xanthine (purine‐2,6‐dione)

AbstractThe solubility of xanthine in cold aqueous solutions has been reinvestigated in view of conflicting values given in the literature. A significant factor, not appreciated by earlier workers was that, unlike related oxopurines, only a small difference exists between the first acidic pKa of this purine and neutral pH. Thus, in view of the highly insoluble nature of the neutral molecule, compared with the very soluble anionic form, the solubility in this region is highly pH dependent. Other factors found which may have a bearing on solubility results are supersaturation effects and the formation of colloidal solutions. Some possible consequences of the solubility phenomena in pathological conditions in which xanthine metabolism is concerned are discussed.

Metabolism of xanthine and hypoxanthine in the tea plant (Thea sinensis L.).

1. The metabolism of xanthine and hypoxanthine in excised shoot tips of tea was studied with micromolar amounts of [2(-14)C]xanthine or [8(-14)C]hypoxanthine. Almost all of the radioactive compounds supplied were utilized by tea shoot tips by 30 h after their uptake. 2. The main products of [2(-14)C]xanthine and [8(-14)C]hypoxanthine metabolism in tea shoots were urea, allantoin and allantoic acid. There was also incorporation of the label into theobromine, caffeine and RNA purine nucleotides. 3. The results indicate that tea plants can catabolize purine bases by the same pathways as animals. It is also suggested that tea plants have the ability to snythesize purine nucleotides from glycine by the pathways of purine biosynthesis de novo and from hypoxanthine and xanthine by the pathway of purine salvage. 4. The results of incorporation of more radioactivity from [8(-14)C]hypoxanthine than from [2(-14)C]xanthine into RNA purine nucleotides and caffeine suggest that hypoxanthine is a more effective precursor of caffeine biosynthesis than xanthine. The formation of caffeine from hypoxanthine is a result of nucleotide synthesis via the pathway of purine salvage.

A STUDY OF XANTHINE METABOLISM IN THE RAT

A STUDY OF XANTHINE METABOLISM IN THE RAT