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PubMed1995

Xanthine derivatives inhibit the increase in intracellular Ca2+ concentration induced by acetylcholine in nasal gland acinar cells of the guinea-pig.

Intracellular calcium is considered to play a major role in secretory responses of various exocrine cell types. We examined whether xanthine derivatives can inhibit Ca2+ mobilization and entry in secretory cells in the airways. Therefore, the inhibitory effect of xanthines in the intracellular Ca2+ concentration ([Ca2+]i) in the isolated submucosal acinar cells of the guinea-pig nasal septum was investigated by means of fluorescence ratio microscopy. The inhibitory effects on Ca2+ release from stores was examined in Ca(2+)-free conditions. Effects on Ca2+ entry were estimated by two different protocols; 1) the sustained phase in a long-term application of acetylcholine (ACh) and 2) the [Ca2+]i overshoot following removal of ACh in Ca(2+)-free conditions. Xanthine derivatives, 3-isobutyl-1-methyl-xanthine (IBMX), caffeine, and theophylline, significantly inhibited the increase in [Ca2+]i evoked by ACh; both mobilization from internal Ca2+ stores and Ca2+ entry from the external space. The rank order of potency of these xanthine derivatives was IBMX > theophylline > caffeine. The addition of dibutyryl-cyclic adenosine monophosphate (cAMP) and forskolin to nasal gland acinar cells failed to inhibit the ACh-evoked increase in [Ca2+]i. Furthermore, a protein kinase A inhibitor, H-89, did not affect the inhibitory effect of the xanthine derivatives. The action of xanthines on the present acinar cells did not involve Ca(2+)-induced Ca2+ release (CICR) or an interaction with purinergic receptors. Thus, xanthines have a direct inhibitory effect both on Ca2+ release and entry in nasal gland acinar cells, and might thereby have antisecretory activity within the airways.

Acetylcholine↗
PubMed1998

Inhibition of xanthine oxidase by Puerto Rican plant extracts.

OBJECTIVES: This study was conducted to search for xanthine oxidase inhibitors in natural products obtained from plants collected in Puerto Rico and to assess the influence of these extracts in the prevention of cataractogenesis. BACKGROUND: Allopurinol is currently a xanthine oxidase inhibitor used in the treatment of gout. New alternatives with increased therapeutic activity and less side effects should be investigated. Preclusion of cataractogenesis in diabetic rats is also the focus of this investigation. Natural products in the form of plant extracts from Puerto Rico offer a rich and relatively untapped source for the discovery of new drugs that may address these kind of problems. METHODS: Nineteen collections of Myrtaceae plant extracts were screened for xanthine oxidase inhibition. A spectrophotometrical method was used employing allopurinol as positive control and a blank as negative control. A protocol of the assay with slight modifications was followed from the literature. Two extracts with the highest percentages of xanthine oxidase inhibition were evaluated for possible prevention of cataractogenesis in streptozotocin diabetic rats. The animals were given to drink these plant extracts ad libitum for three months while controls received water. The appearance of cataracts was assessed physically. RESULTS: Two of the nineteen plant extracts showed high inhibition percentages of xanthine oxidase. Eucalyptus deglupta and Syzygium malaccense displayed 51% and 64% inhibitions (IC50 44.5 micrograms/ml and IC50 51 micrograms/ml), respectively. As for the cataractogenesis inhibition, laboratory animals that drank E. deglupta for three months did not develop cataracts. CONCLUSIONS: Two plant extracts provided positive results with varying degrees of inhibition of xanthine oxidase. S. malaccense demonstrated the greatest xanthine oxidase inhibitory activity whereas E. deglupta presented the best finding for cataractogenesis prevention. The procedures used in this investigation are useful for the in vitro screening of xanthine oxidase inhibition and the in vivo evaluation of cataractogenesis prevention.

Allopurinol↗
PubMed1992

Studies on the relationship between xanthine oxidase and histamine release during intestinal ischemia-reperfusion.

Recent studies have demonstrated a connection between xanthine oxidase-generated reactive oxygen intermediates and histamine release during ischemia-reperfusion. In the present work, the effect of modulation of the endogenous histamine level on the xanthine oxidase activity was examined during the reperfusion of a canine ileal segment following a 2 hr of complete ischemia. The xanthine oxidase activity and the plasma histamine level peaked simultaneously at the beginning of reperfusion, reaching mean values of 14.9 nmol/ml/min and 12.1 nmol/l, respectively. Pretreatment with aminoguanidine, a blocker of diamine oxidase (histaminase), resulted in significantly higher levels of histamine during reperfusion, but this elevation was not accompanied by a further increase in xanthine oxidase activity. Pretreatment with the mast cell stabilizer cromolyn significantly diminished the rise in plasma histamine level, with an unchanging activity of xanthine oxidase. No significant alteration could be observed in the postocclusive activity of xanthine oxidase following the intra-arterial administration of 0.5, 1, or 5 nmol of histamine during the last 10 min of the ischemic period. These data suggest that the amount of histamine liberated during reperfusion does not result in a further increase in the xanthine oxidase activity. The release of histamine is not a cause, but rather an effect of the elevated activity of intestinal xanthine oxidase.

Animals↗
PubMed1976

Imbalance of purine metabolism in hepatomas of different growth rates as expressed in behavior of xanthine oxidase (EC 1.2.3.2).

The behavior of the rate-limiting enzyme of purine catabolism, xanthine oxidase (EC 1.2.3.2); was examined in normal liver, in 17 hepatomas of different growth rates, and in rapidly growing differentiating and regenerating liver. Xanthine oxidase activity was measured in the supernatant fluid prepared by centrifugation of 5% homogenates at 100,000 X g for 30 min. There was no uricase activity in the supernatant fluid. The affinity of xanthine oxidase to xanthine was similar in normal liver and in slow- and rapidly growing hepatomas (Km=6 to 8 muM), and theoptimum pH was 8.0; at pH 7.4, the activity was 80% of that at the pH optimum. A standard assay was worked out for the liver and hepatoma systems; the enzyme activity was linear during 60-min incubation and proportionate with amounts of protein added over a range of 0.5 to 3.0 mg. Xanthine oxidase specific activity was 9 times higher in small intestine than in liver. Activities in lung, spleen, kidney, heart, testes, and thymus were 67, 59, 21, 19, 8, and 8%, and in skeletal muscle, brain, and bone marrow activities were 5% of that of the liver. In regenerating liver, xanthine oxidase activity was not changed from that of the liver of sham-operated controls up to 96 hr after operation. The activity of the average differentiating liver cell was less than 5% of that of adult liver during the first week after birth. At postnatal ages of 18, 25, 30 and 40 days, the activity rose to 18, 46, 76, and 94%, respectively, of that of the adult liver. In starvation, hepatic xanthine oxidase activity per cell was preferentially depleted as compared to the decline in protein concentration. Upon refeeding, the enzymatic activity was restored more slowly than the protein content. Since xanthine oxidase activity was decreased in all examined hepatomas, including the slowest-growing, well-differentiated neoplasms, the altered activity of this enzyme appears to be.linked with neoplastic transformatiobosyl 1-pyrophosphate amidotransferase (EC 2.4.2.14), was increassed in the hepatomas, the reprogramming of gene expression results in an imbalance that favors the synthetic over the catabolic potential. This enzymatic imbalance should confer selective advantages to the cancer cells.

Animals↗
PubMed1987

Loss of fluorescence by anthracycline antibiotics: effects of xanthine oxidase and identification of the nonfluorescent metabolites.

Rat liver cytosol and buttermilk xanthine oxidase both converted 7-deoxypyrromycinone, the 7-deoxyaglycone of marcellomycin, a new anthracycline antibiotic, to a nonfluorescent compound under anaerobic conditions and in the presence of an electron donor. Reduced nicotinamide adenine dinucleotide and reduced nicotinamide adenine dinucleotide phosphate were equally effective electron donors for liver cytosol, and xanthine was the best cofactor for xanthine oxidase. However, xanthine was inactive with liver cytosol. Reactions with xanthine oxidase obeyed Menten-Michaelis kinetics and were inhibited by allopurinol. No xanthine oxidase activity was detected in liver cytosol. Xanthine oxidase also induced a loss of fluorescence when incubated with 7-deoxydaunorubicin aglycone. The nonfluorescent metabolite of 7-deoxypyrromycinone was tentatively identified as the dihydroquinonic derivative of the parent deoxyaglycone on the basis of its spectrophotometric, fluorescent, thin layer chromatographic, and mass spectral characteristics. Our data demonstrate that more than one enzymatic activity, xanthine oxidase, and an unidentified rat liver cytosolic enzyme convert the 7-deoxyaglycones of anthracycline antibiotics to nonfluorescent metabolites.

Anaerobiosis↗
PubMed1994

Reaction of 5-ethynyluracil with rat liver xanthine oxidase.

5-Ethynyluracil is a time-dependent and tight binding inhibitor of xanthine oxidase. The maximal value of the first-order rate constant for onset of inhibition is 0.01 s-1, and the concentration of 5-ethynyluracil which gives one-half of this value is 190 microM. Because the t1/2 for formation of active enzyme from inhibited enzyme is greater than 30 h in the absence of NADH, inhibition of xanthine oxidase by 5-ethynyluracil is functionally irreversible. One equivalent of 5-[2-14C]ethynyluracil/equivalent of active enzyme is required for complete inhibition. Allopurinol (100 microM), a potent inhibitor of xanthine oxidase, and cyanide (5 mM), an inactivator of the enzyme, do not abolish the binding of 5-[2-14C]ethynyluracil to the enzyme. Because radiolabel is released from 5-[2-14C]ethynyluracil-treated enzyme by treatment with 6 M guanidine HCl, a stable covalent bond is not formed between the inhibitor and the enzyme. However, the radiolabel released from inhibited enzyme is not 5-ethynyluracil. Moreover, NADH restores catalytic activity to the inhibited enzyme and displaces the radiolabel as 5-acetyluracil. Thermal denaturation of 5-ethynyluracil-inhibited xanthine oxidase results in the release of approximately equal amounts of 5-acetyluracil and a more hydrophilic product. Consequently, the 5-ethynyluracil-xanthine oxidase complex yields different degradation products of 5-ethynyluracil under different denaturation conditions. Seven uracil analogues with 5-substituents were tested as time-dependent inhibitors of xanthine oxidase. 5-Ethynyluracil is the only uracil analogue that potently inhibited xanthine oxidase. The reactivity of these uracil derivatives with sulfite was also determined. 5-Ethynyluracil is many fold more susceptible to nonenzymatic nucleophilic addition of sulfite than are the other analogues. Thus, the potency of these uracil analogues as inhibitors of xanthine oxidase is related to the nonenzymatic reactivity of the analogues with sulfite.

Allopurinol↗
PubMedDate not supplied

Inhibitory effects of plant growth regulators on xanthine oxidase.

Several plant hormones and analogues were tested for their inhibitory effects on xanthine oxidase. The flavoprotein enzyme, xanthine oxidase, catalyses the oxidation of hypoxanthine to xanthine and then xanthine to uric acid which has lambda max 295 nm. Uric acid was thus the basis for a spectrophotometric assay of the activity of xanthine oxidase. The results showed that trans-zeatin displayed the strongest activity (IC50 = 23.5 muM) on xanthine oxidase inhibition, followed by indole-3-acrylic acid (IC50 = 136.0 muM) and then by the mixed isomers of zeatin (trans-zeatin and cis-zeatin) (IC50 = 198.65 muM). Trans-zeatin induced an uncompetitive inhibition of the enzyme with respect to the substrate xanthine and the apparent inhibition constant (Ki) was 5.09 muM. However, zeatin riboside was inactive. Since xanthine oxidase serum levels are increased in hepatitis, mild hepatic intoxication, tumours brain tissues, and DNA damage induced by cytotoxic agents, it is expected that trans-zeatin may be useful for the treatment of these diseases as well as gout which is caused by deposition of uric acid in the joints and oxidative damage of tissue caused by generation of superoxide anion radical.

Enzyme Inhibitors↗
PubMed1984

Stimulation of endothelial cells by protease activity in commercial preparations of xanthine oxidase.

The oxygen radical generating system of xanthine oxidase plus xanthine, which has been used as a model for the oxidative burst of activated granulocytes, is known to damage endothelium in vivo and in vitro. We previously observed effects (inhibited by catalase, and thus associated with the formation of H2O2) on several parameters of endothelial function, using a non-commercial preparation of xanthine oxidase. Our present study demonstrates that xanthine oxidase from two different commercial sources has additional effects on endothelial morphology and ion flux that are substrate-independent (i.e. produced in the absence of added xanthine) and are attributable to the presence of pancreatin (a crude enzyme mixture used in the commercial preparation of xanthine oxidase from milk). These effects are related to the tryptic activity of pancreatin and extend previous observations on the effects of neutral proteases on endothelial cells. Our results emphasise the practical point that studies on the effects of commercial xanthine oxidase preparations on endothelial cells must take account of their trypsin-like activity as well as their capacity to generate oxygen products.

Adenine↗
PubMed2000

Reduction of nitrite to nitric oxide catalyzed by xanthine oxidoreductase.

Xanthine oxidase (XO) was shown to catalyze the reduction of nitrite to nitric oxide (NO), under anaerobic conditions, in the presence of either NADH or xanthine as reducing substrate. NO production was directly demonstrated by ozone chemiluminescence and showed stoichiometry of approximately 2:1 versus NADH depletion. With xanthine as reducing substrate, the kinetics of NO production were complicated by enzyme inactivation, resulting from NO-induced conversion of XO to its relatively inactive desulfo-form. Steady-state kinetic parameters were determined spectrophotometrically for urate production and NADH oxidation catalyzed by XO and xanthine dehydrogenase in the presence of nitrite under anaerobic conditions. pH optima for anaerobic NO production catalyzed by XO in the presence of nitrite were 7.0 for NADH and </=6.0 for xanthine. Involvement of the molybdenum site of XO in nitrite reduction was shown by the fact that alloxanthine inhibits xanthine oxidation competitively with nitrite. Strong preference for Mo=S over Mo=O was shown by the relatively very low NADH-nitrite reductase activity shown by desulfo-enzyme. The FAD site of XO was shown not to influence nitrite reduction in the presence of xanthine, although it was clearly involved when NADH was the reducing substrate. Apparent production of NO decreased with increasing oxygen tensions, consistent with reaction of NO with XO-generated superoxide. It is proposed that XO-derived NO fulfills a bactericidal role in the digestive tract.

Anaerobiosis↗
PubMed1988

Xanthine and uric acid levels in rat brain following focal ischemia.

Changes of the xanthine and uric acid (UA) levels in rat forebrain following focal cerebral ischemia were studied by reversed-phase HPLC with electrochemical detection. Focal ischemia was induced by occluding the left middle cerebral artery in the rat. The xanthine level in the normal group was 11.50 nmol/g tissue. In the ischemic group, the xanthine concentration in the ischemic hemisphere progressively increased after occlusion and reached a maximum value of 59.42 nmol/g tissue 4 h after operation. The UA level in the normal group was 2.20 nmol/g tissue, whereas in the ischemic group the UA concentration in the ischemic hemisphere gradually increased after occlusion, reaching a value of 38.53 nmol/g tissue 24 h after ischemia. The concentration of UA remained elevated in the ischemic hemisphere until 48 h after occlusion, and reached a maximum value of 38.98 nmol/g tissue. The xanthine and UA levels in the contralateral hemisphere remained unchanged. The xanthine and UA concentrations in the sham-operated group did not show a significant increase after operation. The time course of xanthine and UA levels suggests that in ischemic forebrain UA is formed from xanthine as a product of purine metabolism.

Animals↗
PubMed1997

Exogenous xanthine promotes neutrophil adherence to cultured endothelial cells.

Oxidants generated by endothelial xanthine oxidase (XO) can help trigger free radical-mediated tissue injury. An important event in oxidant-mediated tissue injury is neutrophil-endothelial adhesion. Although activation of endothelial XO increases adhesion, little is known about xanthine in the adhesive effect of XO. This study examined administered xanthine on the adhesion of neutrophils. Endothelial [human umbilical vein endothelial cells (HUVEC)] monolayers were exposed to xanthine (15 min), and neutrophils were allowed to adhere to HUVEC in an adhesion assay. Adhesion was dose dependently increased by xanthine (3-100 microM). Either catalase (1,000 U/ml), oxypurinol (XO inhibitor; 100 microM), or platelet-activating factor (PAF) receptor antagonist (WEB 2086; 10 microM) reduced neutrophil adhesion. Superoxide dismutase (1,000 U/ml) had no effect. Pretreatment of HUVEC with 50 microM tungsten also blocked xanthine-induced adherence. Adhesion was also inhibited by preincubation with 100 U/ml heparin. Finally, anti-P-selectin antibody (PB1.3; 20 micrograms/ml) attenuated adhesion. Our results indicate that xanthine may promote neutrophil-endothelial adhesion via a hydrogen peroxide- and PAF-mediated P-selectin expression.

Antioxidants↗
PubMed2004

Xanthine, xanthosine and its nucleotides: solution structures of neutral and ionic forms, and relevance to substrate properties in various enzyme systems and metabolic pathways.

The 6-oxopurine xanthine (Xan, neutral form 2,6-diketopurine) differs from the corresponding 6-oxopurines guanine (Gua) and hypoxanthine (Hyp) in that, at physiological pH, it consists of a approximately 1:1 equilibrium mixture of the neutral and monoanionic forms, the latter due to ionization of N(3)-H, in striking contrast to dissociation of the N(1)-H in both Gua and Hyp at higher pH. In xanthosine (Xao) and its nucleotides the xanthine ring is predominantly, or exclusively, a similar monoanion at physiological pH. The foregoing has, somewhat surprisingly, been widely overlooked in studies on the properties of these compounds in various enzyme systems and metabolic pathways, including, amongst others, xanthine oxidase, purine phosphoribosyltransferases, IMP dehydrogenases, purine nucleoside phosphorylases, nucleoside hydrolases, the enzymes involved in the biosynthesis of caffeine, the development of xanthine nucleotide-directed G proteins, the pharmacological properties of alkylxanthines. We here review the acid/base properties of xanthine, its nucleosides and nucleotides, their N-alkyl derivatives and other analogues, and their relevance to studies on the foregoing. Included also is a survey of the pH-dependent helical forms of polyxanthylic acid, poly(X), its ability to form helical complexes with a broad range of other synthetic homopolynucleotides, the base pairing properties of xanthine in synthetic oligonucleotides, and in damaged DNA, as well as enzymes involved in circumventing the existence of xanthine in natural DNA.

Animals↗
PubMed1994

Studies of the reductive half-reaction of milk xanthine dehydrogenase.

The reductive half-reaction of milk xanthine dehydrogenase (XDH) with NADH and with xanthine has been studied at pH 7.5, 25 degree C. NADH reduces XDH to the two-electron reduced form at a rate of 18 s-1, independent of NADH concentration over the range studied. Further reduction by NADH to the four-electron state is inhibited by excess NADH. Subsequent binding of NADH to the four-electron reduced form of the enzyme causes the redistribution of one electron from the flavin to the molybdenum center. The four-electron reduced species reached through reduction by NADH is the same as the species obtained upon reaction of NAD with fully reduced XDH. In contrast, xanthine rapidly reduces XDH to the four-electron level; further reduction is comparatively slow and is inhibited by excess xanthine. Studies using substoichiometric xanthine show that the reaction of XDH with 1 equivalent of xanthine involves rapid substrate binding and rapid reduction of the molybdenum center of the enzyme. Before the release of urate from the molybdenum active site, an electron is transferred at 15 s-1 from the reduced molybdenum center to one of the iron-sulfur centers of XDH. Urate is then released at a rate of 13 s-1, followed by a rapid electron redistribution within the protein. The reductive half-reaction of XDH with xanthine is rate-limiting in xanthine/NAD turnover, which appears to occur between the two- and four-electron reduced enzyme species. The reduction of XDH by substoichiometric amounts of the fluorescent substrate xanthopterin was also studied. This reaction, monitored by changes in both absorbance and fluorescence, was found to involve the formation of two molybdenum complexes (an Eox.S complex and an Ered.P complex) followed by the release of the product, leucopterin.

Animals↗
PubMed1997

Characterization of a Goalpha mutant that binds xanthine nucleotides.

Several GTP binding proteins, including EF-Tu, Ypt1, rab-5, and FtsY, and adenylosuccinate synthetase have been reported to bind xanthine nucleotides when the conserved aspartate residue in the NKXD motif was changed to asparagine. However, the corresponding single Goalpha mutant protein (D273N) did not bind either xanthine nucleotides or guanine nucleotides. Interestingly, the introduction of a second mutation to generate the Goalpha subunit D273N/Q205L switched nucleotide binding specificity to xanthine nucleotide. The double mutant protein GoalphaD273N/Q205L (GoalphaX) bound xanthine triphosphate, but not guanine triphosphate. Recombinant GoalphaX (GoalphaD273N/Q205L) formed heterotrimers with betagamma complexes only in the presence of xanthine diphosphate (XDP), and the binding to betagamma was inhibited by xanthine triphosphate (XTP). Furthermore, as a result of binding to XTP, the GoalphaX protein underwent a conformational change similar to that of the activated wild-type Goalpha. In transfected COS-7 cells, we demonstrate that the interaction between GoalphaX and betagamma occurred only when cell membranes were permeabilized to allow the uptake of xanthine diphosphate. This is the first example of a switch in nucleotide binding specificity from guanine to xanthine nucleotides in a heterotrimeric G protein alpha subunit.

Amino Acid Sequence↗
PubMed1978

Studies on the stability of immobilized xanthine oxidase and urate oxidase.

The stability of immobilized preparations of xanthine oxidase and urate oxidase was studied, and optimized, because of the potential joint use of both enzymes in clinical analysis. Xanthine oxidase was immobilized on cellulose, Sepharose, hornblende, Enzacryl-TIO, and porous glass. Thehalf-lives of these preparations at 30 degree C ranged from 40 min to 5.0 hr. In this respect immobilized enzyme resembled soluble enzyme in dilute solution (0.11 mg/ml), when the half-live was about 3.5 hr. More concentrated enzyme solution (1 mg/ml) had a half-life of 64 hr, and was, therefore, considerably more stable than the untreated immobilized xanthine oxidase preparations. Inclusion of albumen in storage and assay buffer increased the half-life of bound xanthine oxidase. So also did treatment with glutaraldehyde: in the case of xanthine oxidase bound to Enzarcyl-TIO such treatment increased the half-life at 30 degree C from 3 hr to about 100 hr. Immobilized xanthine dehydrogenase was more stable than immobilized xanthine oxidase: the dehydrogenase lost no activity during continuous assay for 5 hr at 30 degree C. The stability of immobilized urate oxidase depended on the quantity of enzyme used and on the time of stirring during immobilization: thus a preparation was made (by stirring urate oxidase (48 mg/g support) with Enzacryl-TIO for 24 hr) which lost no activity during 350 hr at 30 degree C.

Enzymes, Immobilized↗
PubMed1979

Selenium requirement for active xanthine dehydrogenase from Clostridium acidiurici and Clostridium cylindrosporum.

The xanthine dehydrogenase of Clostridium acidiurici and C. cylindrosporum was assayed with methyl viologen as acceptor. In C. acidiurici the basal activity level was about 0.3 mumol/min x mg of protein. Cells grown on uric acid in the presence of 10(-7) M selenite showed a 14-fold increase in xanthine dehydrogenase activity, which decreased with higher selenite concentrations (10(-5) M). The supplementation with 10(-7) M molybdate or tungstate was without effect. High concentrations of tungstate decreased the xanthine dehydrogenase if selenite was also present. In comparison, high concentrations of molybdate affected only a small decrease in activity level at the optimal concentration for selenite and relieved to some degree the inhibitory effect of 10(-5) M selenite. With hypoxanthine and xanthine as substrates for growth again only the addition of selenite was necessary to show a similar increase in xanthine dehydrogenase activity. C. acidiurici could be grown in a mineral medium. Both xanthine dehydrogenase and formate dehydrogenase exhibited the highest level of activity if selenite and tungstate were present in that medium. In C. cyclindrosporum the basal activity level of xanthine dehydrogenase was about 0.95 mumol/min x mg of protein. The addition of 10(-7) M selenite to the growth medium increased the activity level about 3-fold, but the highest level (3.7 U/mg) was reached if 10(-7) M molybdate was also added. The presence of tungstate resulted in a decreased enzyme activity.

Cell-Free System↗
PubMed1990

Decreased xanthine oxidase activities and increased urinary oxypurines in heterozygotes for hereditary xanthinuria.

Two brothers with hereditary xanthinuria (xanthine oxidase deficiency) and several members of their family were studied. In both subjects, plasma and urinary concentrations of uric acid were low whereas xanthine and hypoxanthine concentrations were markedly elevated. Xanthine oxidase activity was virtually absent in the patients' duodenal mucosa, a finding that established the diagnosis of hereditary xanthinuria. In their parents (obligate heterozygotes), the duodenal xanthine oxidase activity was about 50% of that in control subjects (father 9.3 and mother 12.8 mU/g tissue compared with 21.3 +/- 5.0 mU/g tissue, mean +/- SD). The residual xanthine oxidase from the parents exhibited normal kinetics with respect to hypoxanthine. The parents' urinary xanthine and hypoxanthine concentrations were significantly greater than those of control subjects, while their plasma concentrations of oxypurines were normal. Similar findings were observed in at least 6 other relatives, a finding that suggested that they were heterozygotes. This study suggests that obligate hereditary xanthinuria heterozygotes have only 50% of the xanthine oxidase activity of controls; this deficiency results in a partial metabolic blockage at this enzymatic step in heterozygotes.

Adult↗
PubMed1993

The hypoxanthine-guanine-xanthine phosphoribosyltransferase from Tritrichomonas foetus has unique properties.

Tritrichomonas foetus, an anaerobic, flagellated protozoan parasite, is incapable of de novo purine nucleotide synthesis, and depends primarily on the salvage of purine bases from the host. The hypoxanthine-guanine-xanthine phosphoribosyltransferase (HGXPRTase) from this organism has been purified to homogeneity by ammonium sulfate precipitation and Sephacryl-HR100 gel filtration, followed by anion exchange FPLC. Hypoxanthine, guanine and xanthine phosphoribosyltransferase activities co-eluted in all the purification steps, suggesting that they are associated with the same enzyme protein. The molecular mass of the native protein, as estimated by gel filtration, is 24 kDa. The molecular mass estimated from sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE) is also 24 kDa. Non-denaturing polyacrylamide gel electrophoresis of the purified protein, followed by activity staining with either [14C]hypoxanthine, [14C]guanine or [14C]xanthine, also demonstrates that the enzyme is a monomer of 24 kDa. This monomeric structure is distinctive from all the other reported PRTases which are either dimers or tetramers. Furthermore, unlike the mammalian HGPRTase, which is heat stable, the T. foetus enzyme is heat labile. Kinetic studies with the purified T. foetus HGXPRTase showed that the apparent Kms for hypoxanthine, guanine and xanthine were 4.1 microM, 3.8 microM and 52.4 microM respectively. This recognition of xanthine as a substrate by the parasite enzyme with only about a 10-fold higher Km value than those for hypoxanthine and guanine distinguishes it from the mammalian HGPRTase, which cannot use xanthine as a substrate, as well as the HGXPRTases of Eimeria tenella and Plasmodium falciparum, which are dimers, with xanthine about 100-times less proficient as a substrate.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗