Radiomodfication of xanthine oxidoreductase system in the liver of mice by phenylmethylsulfonyl fluoride and dithiothreitol

The widely distributed xanthine oxidoreductase (XOR) system has been shown to be modulated upon exposure of animals to ionizing radiation through the conversion of xanthine dehydrogenase (XDH) into xanthine oxidase (XO). In the present work, radiomodification of the XOR system by phenylmethylsulfonyl fluoride (PMSF) and dithiothreitol (DTT) was examined using female Swiss albino mice which were irradiated with gamma rays at a dose rate 0.023 Gy s(-1). PMSF, a serine protease inhibitor, and DTT, the sulfhydryl reagent, were administered intraperitoneally prior to irradiation. The specific activities of XDH and XO as well as the XDH/XO ratio and the total activity (XDH+XO) were determined in the liver of the mice. The inhibition of XO activity, restoration of XDH activity, and increase in the XDH/XO ratio upon administration of PMSF were suggestive of irreversible conversion of XDH into XO mediated through serine proteases. The biochemical events required for the conversion were probably initiated during the early phase of irradiation, as the treatment with PMSF immediately after irradiation did not have a modulatory effect. Interestingly, DTT was not effective in modulating radiation-induced changes in the XOR system or oxidative damage in the liver of mice. The DTT treatment resulted in inhibition of the release of lactate dehydrogenase. However, the protection appears to be unrelated to the formation of TBARS. On the other hand, the presence of PMSF during irradiation inhibited radiation-induced oxidative damage and radiation-induced increases in the specific activity of lactate dehydrogenase. These findings suggest that a major effect of ionizing radiation is irreversible conversion of xanthine to xanthine oxidase.     

The contribution of vascular endothelial xanthine dehydrogenase/oxidase to oxygen-mediated cell injury.

The conversion of xanthine dehydrogenase (XDH) to xanthine oxidase (XO) and the reaction of XO-derived partially reduced oxygen species (PROS) have been suggested to be important in diverse mechanisms of tissue pathophysiology, including oxygen toxicity. Bovine aortic endothelial cells expressed variable amounts of XDH and XO activity in culture. Xanthine dehydrogenase plus xanthine oxidase specific activity increased in dividing cells, peaked after achieving confluency, and decreased in postconfluent cells. Exposure of BAEC to hyperoxia (95% O2; 5% CO2) for 0-48 h caused no change in cell protein or DNA when compared to normoxic controls. Cell XDH+XO activity decreased 98% after 48 h of 95% O2 exposure and decreased 68% after 48 h normoxia. During hyperoxia, the percentage of cell XDH+XO in the XO form increased to 100%, but was unchanged in air controls. Cell catalase activity was unaffected by hyperoxia and lactate dehydrogenase activity was minimally elevated. Hyperoxia resulted in enhanced cell detachment from monolayers, which increased 112% compared to controls. Release of DNA and preincorporated [8-14C]adenine was also used to assess hyperoxic cell injury and did not significantly change in exposed cells. Pretreatment of cells with allopurinol for 1 h inhibited XDH+XO activity 100%, which could be reversed after oxidation of cell lysates with potassium ferricyanide (K3Fe(CN)6). After 48 h of culture in air with allopurinol, cell XDH+XO activity was enhanced when assayed after reversal of inhibition with K3Fe(CN)6, and cell detachment was decreased. In contrast, allopurinol treatment of cells 1 h prior to and during 48 h of hyperoxic exposure did not reduce cell damage. After K3Fe(CN)6 oxidation, XDH+XO activity was undetectable in hyperoxic cell lysates. Thus, XO-derived PROS did not contribute to cell injury or inactivation of XDH+XO during hyperoxia. It is concluded that endogenous cell XO was not a significant source of reactive oxygen species during hyperoxia and contributes only minimally to net cell production of O2- and H2O2 during normoxia.     

Aryl hydrocarbon receptor (AhR)-mediated induction of xanthine oxidase/xanthine dehydrogenase activity by 2,3,7,8-tetrachlorodibenzo-p-dioxin

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD), an environmental contaminant, induced xanthine oxidase and xanthine dehydrogenase (XO/XDH) activities, in addition to ethoxyresorufin-O-dealkylase and methoxyresorufin-O-dealkylase activities in liver of mice. When TCDD was given to mice as a single oral dose of 40 microg/kg, the activities of XO and XDH increased about threefold within 3 days and the increased levels were maintained for 4 weeks. The treatment of mice with 3-methylcholanthrene also induced XO/XDH activities, but phenobarbital and dexamethasone had no effect. The level of aldehyde oxidase, a molybdenum flavoenzyme related to XO/XDH, in mouse liver was also enhanced about 1.5-fold by TCDD treatment. The inducing effect of TCDD and 3-methylcholanthrene was not observed in null mice (AhR(-/-)), which lack the AhR gene. XO and XDH activities were induced by TCDD in heterozygous mice (AhR(+/-)). The lipid peroxidation in liver was stimulated by TCDD. The induction of XO and XDH, which produces reactive oxygen species, may contribute to the various toxicities of TCDD.     

Modulation of radiation-induced changes in the xanthine oxidoreductase system in the livers of mice by its inhibitors

The xanthine oxidoreductase (XOD) system, which consists of xanthine dehydrogenase (XDH) and xanthine oxidase (XO), is one of the major sources of free radicals in biological systems. The XOD system is present predominantly in the normal tissues as XDH. In damaged tissues, XDH is converted into XO, the form that generates free radicals. Therefore, the XO form of the XOD system is expected to be found mainly in radiolytically damaged tissue. In this case, XO may catalyze the generation of free radicals and potentiate the effect of radiation. Inhibition of the XOD system is likely to attenuate the detrimental effects of ionizing radiation. We have examined this possibility using allopurinol and folic acid, which are known inhibitors of the XOD system. Swiss albino mice (7-8 weeks old) were given single doses of allopurinol and folic acid (12.5-50 mg/kg) intraperitoneally and irradiated with different doses of gamma radiation at a dose rate of 0.023 Gy/s. The XO and XDH activities as well as peroxidative damage and lactate dehydrogenase (LDH) were determined in the liver. An enhancement of the activity of XO and a simultaneous decrease in the activity of XDH were observed at doses above 3 Gy. The decrease in the ratio XDH/XO and the unchanged total activity (XDH + XO) suggested the conversion of XDH into XO. The enhanced activity of XO may potentiate radiation damage. The increased levels of peroxidative damage and the specific activity of LDH in the livers of irradiated mice supported this possibility. Allopurinol and folic acid inhibited the activities of XDH and XO, decreased their ratio (XDH/XO), and lowered the levels of peroxidative damage and the specific activity of LDH. These results suggested that allopurinol and folic acid have the ability to inhibit the radiation-induced changes in the activities of XDH and XO and to attenuate the detrimental effect of this conversion, as is evident from the diminished levels of peroxidative damage and the decreased activity of LDH.     

Effect of radiation on the xanthine oxidoreductase system in the liver of mice.

The xanthine oxidoreductase system is one of the major sources of free radicals in many pathophysiological conditions. Since ionizing radiations cause cell damage and death, the xanthine oxidoreductase system may contribute to the detrimental effects in irradiated systems. Therefore, modulation of the xanthine oxidoreductase system by radiation has been examined in the present study. Female Swiss albino mice (7-8 weeks old) were irradiated with gamma rays (1-9 Gy) at a dose rate of 0.023 Gy s(-1) and the specific activities of xanthine oxidase (XO) and xanthine dehydrogenase (XDH) were determined in the liver of the animals. The mode and magnitude of change in the specific activities of XO and XDH were found to depend on radiation dose. At doses above 3 Gy, the specific activity of XO increased rapidly and continued to increase with increasing dose. However, the specific activity of XDH was decreased. These findings are suggestive of an inverse relationship between the activity of XO and XDH. The ratio of the activity of XDH to that of XO decreased with radiation dose. However, the total activity (XDH + XO) remained constant at all doses. These results indicate that XDH may be converted into XO. An intermediate form, D/O, appears to be transient in the process of conversion. The enhanced specific activity of XO may cause oxidative stress that contributes to the radiation damage and its persistence in the postirradiation period. Radiation-induced peroxidative damage determined in terms of the formation of TBARS and the change in the specific activity of lactate dehydrogenase support this possibility.     

Purification and properties of milk xanthine dehydrogenase.

Milk xanthine oxidase (XO) has been prepared in a dehydrogenase form (XDH) by purifying the enzyme in the presence of 2.5 mM dithiothreitol. Unlike XO, which reacts rapidly only with oxygen and not with NAD, the XDH form of the enzyme reacts rapidly with NAD. XDH has a turnover number for the NAD-dependent conversion of xanthine to urate of 380 mol/min/mol at pH 7.5, 25 degrees C, with a Km = < or = 1 microM for xanthine and a Km = 7 microM for NAD, but has very little O2-dependent activity. There is evidence that the two forms of the enzyme have different flavin environments: XDH stabilizes the neutral form of the flavin semiquinone and XO does not. Further, XDH binds the artificial flavin 8-mercapto-FAD in its neutral form, shifting the pK of this flavin by 5 pH units, while XO binds 8-mercapto-FAD in its benzoquinoid anionic form. XDH can be converted back to the XO form by the addition of three to four equivalents of the disulfide-forming reagent 4,4'-dithiodipyridine, suggesting that, in the XDH form of the enzyme, disulfide bonds are broken; this may cause a conformational change which creates a binding site for NAD and changes the protein structure near the flavin.     

Lack of conversion of xanthine dehydrogenase to xanthine oxidase during warm renal ischemia

Irreversible transformation of xanthine dehydrogenase (XDH) to xanthine oxidase (XO) during ischemia was determined measuring XDH and total enzyme activity in kidneys before and after 60 min of clamp of the renal pedicle. Tissue levels of adenine nucleotides, xanthine and hypoxanthine were used as indicators of ischemia. After 60 min of clamping, ATP levels decreased by 72% with respect to controls whereas xanthine and hypoxanthine progressively reached tissue concentrations of 732 +/- 49 and 979 +/- 15 nmol.g tissue-1, respectively. Both total and XDH activities in ischemic kidneys (30 +/- 15 and 19 +/- 1 nmol.min-1.g tissue-1) were significantly lower than in controls when expressed on a tissue weight basis. The fraction of enzyme in the XDH form was however unchanged indicating that the reduction of the nucleotide pool is not accompanied by induction of the type-O activity of xanthine oxidase.     

从猕猴肝脏组织扩增黄嘌呤脱氢酶／氧化酶基因片段

RT-PCR扩增猕猴黄嘌呤脱氢酶／氧化酶（XDH／XO）基因片段,为进一步开展相关研究提供实验资料。方法提取猕猴新鲜肝脏组织总RNA,用RT-PCR二步法进行XDH／XO基因片段扩增,对获得的目的片段进行序列测定,与GenBank上发表的人类（Homosapiens）、小鼠（Musmusculus）、家鼠（Rattusnorvegicus）、野猪（Susscrofa）等物种XDH／XO基因进行该序列同源性比对分析,DNAMAN软件预测该段核苷酸的氨基酸序列,Inter-ProScan及SWISS-MODEL工具进行XDH／XO的编码蛋白结构域及功能预测及三维结构构建。结果RT-PCR产物电泳检测得到了与设计大小相一致的目的条带,序列测定共测到683个核苷酸,DNAMAN软件预测该段核苷酸的氨基酸序列包括了1个编码53个氨基酸的开放阅读框（ORF）,通过该软件包中Multiplealignment对目的基因片段的核苷酸序列与NCBI报道的人类、小鼠、家鼠、野猪XDH／XO基因mRNA互补的cDNA核苷酸序列同源性进行同源性比较分析,结果显示所扩增得到的目的片段与人类同源性最高,为95．6％,与小鼠、家鼠、野猪的同源性分别为85．2％、84．3％、86．1％,说明获得的基因片段是猕猴的XDH／XO基因片段,且该基因在物种间具有较高的相似性。生物信息学预测该段XDH／XO编码蛋白含有醛氧化／脱氢酶的钼喋呤结合点结构域及黄嘌呤脱氢酶结构域。结论在体外成功扩增出猕猴XDH／XO基因片段,为进一步开展高尿酸血症致病机理研究,抗高尿酸血症新药研发奠定工作基础。     

Lung redox homeostasis: emerging concepts

This symposium was organized to present some aspects of current research pertaining to lung redox function. Focuses of the symposium were on roles of pulmonary endothelial NADPH oxidase, xanthine oxidase (XO)/xanthine dehydrogenase (XDH), heme oxygenase (HO), transplasma membrane electron transport (TPMET), and the zinc binding protein metallothionein (MT) in the propagation and/or protection of the lung or other organs from oxidative injury. The presentations were chosen to reflect the roles of both intracellular (metallothionein, XO/XDH, and HO) and plasma membrane (NADPH oxidase, XO/XDH, and unidentified TPMET) redox proteins in these processes. Although the lung endothelium was the predominant cell type under consideration, at least some of the proposed mechanisms operate in or affect other cell types and organs as well.     

Mechanisms of action of malondialdehyde and 4-hydroxynonenal on xanthine oxidoreductase

Studies have been made on the possible involvement of malondialdehyde (MDA) and (E)-4-hydroxynon-2-enal (HNE), two terminal compounds of lipid peroxidation, in modifying xanthine oxidoreductase activity through interaction with the oxidase (XO) and/or dehydrogenase (XDH) forms. The effect of the two aldehydes on XO (reversible, XO(rev), and irreversible, XO(irr)) and XDH was studied using xanthine oxidase from milk and xanthine oxidoreductase partially purified from rat liver. The incubation of milk xanthine oxidase with these aldehydes resulted in the inactivation of the enzyme following pseudo-first-order kinetics: enzyme activity was completely abolished by MDA (0.5-4 mM), while residual activity (5% of the starting value) associated with an XO(irr) form was always observed when the enzyme was incubated in the presence of HNE (0.5-4 mM). The addition of glutathione to the incubation mixtures prevented enzyme inactivation by HNE. The study on the xanthine oxidoreductase partially purified from rat liver showed that MDA decreases the total enzyme activity, acting only with the XO forms. On the contrary HNE leaves the same level of total activity but causes the conversion of XDH into an XO(irr) form.     

Phosphorylation of xanthine dehydrogenase/oxidase in hypoxia

The enzyme xanthine oxidase (XO) has been implicated in the pathogenesis of several disease processes, such as ischemia-reperfusion injury, because of its ability to generate reactive oxygen species. The expression of XO and its precursor xanthine dehydrogenase (XDH) is regulated at pre- and posttranslational levels by agents such as lipopolysaccharide and hypoxia. Posttranslational modification of the protein, for example through thiol oxidation or proteolysis, has been shown to be important in converting XDH to XO. The possibility of posttranslational modification of XDH/XO through phosphorylation has not been adequately investigated in mammalian cells, and studies have reported conflicting results. The present report demonstrates that XDH/XO is phosphorylated in rat pulmonary microvascular endothelial cells (RPMEC) and that phosphorylation is greatly increased ( approximately 50-fold) in response to acute hypoxia (4 h). XDH/XO phosphorylation appears to be mediated, at least in part, by casein kinase II and p38 kinase as inhibitors of these kinases partially prevent XDH/XO phosphorylation. In addition, the results indicate that p38 kinase, a stress-activated kinase, becomes activated in response to hypoxia (an approximately 4-fold increase after 1 h of exposure of RPMEC to hypoxia) further supporting a role for this kinase in hypoxia-stimulated XDH/XO phosphorylation. Finally, hypoxia-induced XDH/XO phosphorylation is accompanied by a 2-fold increase in XDH/XO activity, which is prevented by inhibitors of phosphorylation. In summary, this study shows that XDH/XO is phosphorylated in hypoxic RPMEC through a mechanism involving p38 kinase and casein kinase II and that phosphorylation is necessary for hypoxia-induced enzymatic activation.     

Inhibition of xanthine oxidase and xanthine dehydrogenase by nitric oxide. Nitric oxide converts reduced xanthine-oxidizing enzymes into the desulfo-type inactive form

Xanthine oxidase (XO) and xanthine dehydrogenase (XDH) were inactivated by incubation with nitric oxide under anaerobic conditions in the presence of xanthine or allopurinol. The inactivation was not pronounced in the absence of an electron donor, indicating that only the reduced enzyme form was inactivated by nitric oxide. The second-order rate constant of the reaction between reduced XO and nitric oxide was determined to be 14.8 +/- 1.4 M-1 s-1 at 25 degrees C. The inactivated enzymes lacked xanthine-dichlorophenolindophenol activity, and the oxypurinol-bound form of XO was partly protected from the inactivation. The absorption spectrum of the inactivated enzyme was not markedly different from that of the normal enzyme. The flavin and iron-sulfur centers of inactivated XO were reduced by dithionite and reoxidized readily with oxygen, and inactivated XDH retained electron transfer activities from NADH to electron acceptors, consistent with the conclusion that the flavin and iron-sulfur centers of the inactivated enzyme both remained intact. Inactivated XO reduced with 6-methylpurine showed no "very rapid" spectra, indicating that the molybdopterin moiety was damaged. Furthermore, inactivated XO reduced by dithionite showed the same slow Mo(V) spectrum as that derived from the desulfo-type enzyme. On the other hand, inactivated XO reduced by dithionite exhibited the same signals for iron-sulfur centers as the normal enzyme. Inactivated XO recovered its activity in the presence of a sulfide-generating system. It is concluded that nitric oxide reacts with an essential sulfur of the reduced molybdenum center of XO and XDH to produce desulfo-type inactive enzymes.     

The role of the [2Fe-2s] cluster centers in xanthine oxidoreductase

Xanthine oxidoreductases (XOR), xanthine dehydrogenase (XDH, EC1.1.1.204) and xanthine oxidase (XO, EC1.2.3.2), are the best-studied molybdenum-containing iron-sulfur flavoproteins. The mammalian enzymes exist originally as the dehydrogenase form (XDH) but can be converted to the oxidase form (XO) either reversibly by oxidation of sulfhydryl residues of the protein molecule or irreversibly by proteolysis. The active form of the enzyme is a homodimer of molecular mass 290 kDa. Each subunit contains one molybdopterin group, two non-identical [2Fe-2S] centers, and one flavin adenine dinucleotide (FAD) cofactor. This review focuses mainly on the role of the two iron-sulfur centers in catalysis, as recently elucidated by means of X-ray crystal structure and site-directed mutagenesis studies. The arrangements of cofactors indicate that the two iron-sulfur centers provide an electron transfer pathway from molybdenum to FAD. However, kinetic and thermodynamic studies suggest that these two iron-sulfur centers have roles not only in the pathway of electron flow, but also as an electron sink to provide electrons to the FAD center so that the reactivity of FAD with the electron acceptor substrate might be thermodynamically controlled by way of one-electron-reduced or fully reduced state.     

Mammalian xanthine oxidoreductase - mechanism of transition from xanthine dehydrogenase to xanthine oxidase

Reactive oxygen species are generated by various biological systems, including NADPH oxidases, xanthine oxidoreductase, and mitochondrial respiratory enzymes, and contribute to many physiological and pathological phenomena. Mammalian xanthine dehydrogenase (XDH) can be converted to xanthine oxidase (XO), which produces both superoxide anion and hydrogen peroxide. Recent X-ray crystallographic and site-directed mutagenesis studies have revealed a highly sophisticated mechanism of conversion from XDH to XO, suggesting that the conversion is not a simple artefact, but rather has a function in mammalian organisms. Furthermore, this transition seems to involve a thermodynamic equilibrium between XDH and XO; disulfide bond formation or proteolysis can then lock the enzyme in the XO form. In this review, we focus on recent advances in our understanding of the mechanism of conversion from XDH to XO.     

N-ethylmaleimide inhibits xanthine oxidase activity with no detectable change in xanthine dehydrogenase activity in rabbit liver

The effect of an alkylating agent, N-ethylmaleimide (NEM), on the activities of xanthine oxidase (XO) and xanthine dehydrogenase (XD) in the presence and absence of Cu2+ or trypsin in the cytosolic fraction from rabbit liver was examined. At concentrations ranging from 0.25 to 2.0 microM, allopurinol, which is generally considered to be a XO inhibitor, suppressed the XD activity (41.5-93.4% inhibition) in addition to the XO activity (28.6-88.4% inhibition) under basal conditions, without the addition of Cu2+ or trypsin. In contrast, NEM (100-400 microM) inhibited the XO activity (35.7-85.7% inhibition) without affecting the XD activity. Also, NEM inhibited the Cu2+- and trypsin-induced XO activities, but did not affect the XD activity at the same concentration range. These results demonstrate that NEM can be a selective inhibitor of XO activity in rabbit liver.     

Proteolytic conversion of xanthine dehydrogenase to xanthine oxidase: evidence against a role for calcium-activated protease (calpain)

The present study tested the hypothesis that calpain is responsible for the limited proteolytic conversion of xanthine dehydrogenase (XD) to xanthine oxidase (XO). We compared the effects of various proteases on the activity and molecular weight of a purified preparation of xanthine dehydrogenase from rat liver. In agreement with previous reports, trypsin treatment produced a complete conversion of XD to XO accompanied by a limited proteolysis of XDH from an Mr of 140 kD to an Mr of 90 kD. Treatment with calpain I or calpain II did not produce a conversion from XD to XO nor did it result in partial proteolysis of the enzyme. Similarly, trypsin treatment partially degraded a reversibly oxidized form of xanthine dehydrogenase while calpain I or calpain II were ineffective. The possibility that an endogenous inhibitor prevented the proteolysis of XDH by calpain I or II was excluded by verifying that brain spectrin, a known calpain substrate, was degraded under the same incubation conditions. The results indicate that calpain is not likely to be responsible for the in vivo conversion of XD to XO under pathological conditions.     

Peroxisomal xanthine oxidoreductase: characterization of the enzyme from pea (Pisum sativum L.) leaves

The presence and properties of the enzyme xanthine oxidoreductase (XOR) in peroxisomes from pea (Pisum sativum L.) leaves were studied using biochemical and immunological methods. The activity analysis showed that, in leaf peroxisomes, the superoxide-generating XOR form, xanthine oxidase (XOD), was predominant over the xanthine dehydrogenase form (XDH), with a XDH/XOD ratio of 0.5. However, in crude extracts of pea leaves, the XDH form was more abundant, with a XDH/XOD ratio of 1.6. The native molecular mass of the peroxisomal XOR determined by polyacrylamide gel electrophoresis was 290kDa. Using western blot assays, we identified an immunoreactive band of 59kDa that was not affected by the reducing reagent DTT or endogenous proteases. The analysis of pea leaves by electron microscopy immunogold labeling with affinity-purified antibodies against rat XOD confirmed that this enzyme was localized in the matrix of peroxisomes, as well as in chloroplasts and cytosol. In pea plants subjected to abiotic stress by cadmium, the activity of the peroxisomal XOR was reduced, whereas its protein level expression increased. The results confirmed that leaf peroxisomes contain XOR, and suggest that this peroxisomal metalloflavoprotein enzyme is involved in the mechanism of response of pea plants to abiotic stress by heavy metals.     

Xanthine oxidase inhibitor tungsten prevents the development of atherosclerosis in ApoE knockout mice fed a Western-type diet

Hyperlipidemia enhances xanthine oxidase (XO) activity. XO is an important source of reactive oxygen species (ROS). Since ROS are thought to promote atherosclerosis, we hypothesized that XO is involved in the development of atherosclerosis. ApoE(-/-) mice were fed a Western-type (WD) or control diet. In subgroups, tungsten (700 mg/L) was administered to inhibit XO. XO is a secreted enzyme which is formed in the liver as xanthine dehydrogenase (XDH) and binds to the vascular endothelium. High expression of XDH was found in the liver and WD increased liver XDH mRNA and XDH protein expression. WD induced the conversion of XDH to the radical-forming XO. Moreover, WD increased the hepatic expression of CD40, demonstrating activation of hepatic cells. Aortic tissue of ApoE(-/-) mice fed a WD for 6 months exhibited marked atherosclerosis, attenuated endothelium-dependent relaxation to acetylcholine, increased vascular oxidative stress, and mRNA expression of the chemokine KC. Tungsten treatment had no effect on plasma lipids but lowered the plasma XO activity. In animals fed a control diet, tungsten had no effect on radical formation, endothelial function, or atherosclerosis development. In mice fed a WD, however tungsten attenuated the vascular superoxide anion formation, prevented endothelial dysfunction, and attenuated KC mRNA expression. Most importantly, tungsten treatment largely prevented the development of atherosclerosis in the aorta of ApoE(-/-) mice on WD. Therefore, tungsten, potentially via the inhibition of XO, prevents the development of endothelial dysfunction and atherosclerosis in ApoE(-/-) mice on WD.     

Regulation of purine hydroxylase and xanthine dehydrogenase from Clostridium purinolyticum in response to purines, selenium, and molybdenum

The discovery that two distinct enzyme catalysts, purine hydroxylase (PH) and xanthine dehydrogenase (XDH), are required for the overall conversion of hypoxanthine to uric acid by Clostridium purinolyticum was unexpected. In this reaction sequence, hypoxanthine is hydroxylated to xanthine by PH and then xanthine is hydroxylated to uric acid by XDH. PH and XDH, which contain a labile selenium cofactor in addition to a molybdenum cofactor, flavin adenine dinucleotide, and FeS centers, were purified and partially characterized as reported previously. In the present study, the activities of these two enzymes were measured in cells grown in media containing various concentrations of selenite, molybdate, and various purine substrates. The levels of PH protein in extracts were determined by immunoblot assay. The amount of PH protein, as well as the specific activities of PH and XDH, increased when either selenite or molybdate was added to the culture medium. PH levels were highest in the cells cultured in the presence of either adenine or purine. XDH activity increased dramatically in cells grown with either xanthine or uric acid. The apparent increases in protein levels and activities of PH and XDH in response to selenium, molybdenum, and purine substrates demonstrate that these enzymes are tightly regulated in response to these nutrients.