Purification of xanthine dehydrogenase from rat liver: a rapid procedure with high enzyme yields

Aldehyde oxidase functions as a superoxide generating NADH oxidase: an important redox regulated pathway of cellular oxygen radical formation

The enzyme aldehyde oxidase (AO) is a member of the molybdenum hydroxylase family that includes xanthine oxidoreductase (XOR); however, its physiological substrates and functions remain unclear. Moreover, little is known about its role in cellular redox stress. Utilizing electron paramagnetic resonance spin trapping, we measured the role of AO in the generation of reactive oxygen species (ROS) through the oxidation of NADH and the effects of inhibitors of AO on NADH-mediated superoxide (O(2)(•−)) generation. NADH was found to be a good substrate for AO with apparent K(m) and V(max) values of 29 μM and 12 nmol min(-1) mg(-1), respectively. From O(2)(•−) generation measurements by cytochrome c reduction the apparent K(m) and V(max) values of NADH for AO were 11 μM and 15 nmol min(-1) mg(-1), respectively. With NADH oxidation by AO, ≥65% of the total electron flux led to O(2)(•−) generation. Diphenyleneiodonium completely inhibited AO-mediated O(2)(•−) production, confirming that this occurs at the FAD site. Inhibitors of this NADH-derived O(2)(•−) generation were studied with amidone the most potent exerting complete inhibition at 100 μM concentration, while 150 μM menadione, raloxifene, or β-estradiol led to 81%, 46%, or 26% inhibition, respectively. From the kinetic data, and the levels of AO and NADH, O(2)(•−) production was estimated to be ~89 and ~4 nM/s in liver and heart, respectively, much higher than that estimated for XOR under similar conditions. Owing to the ubiquitous distribution of NADH, aldehydes, and other endogenous AO substrates, AO is predicted to have an important role in cellular redox stress and related disease pathogenesis.

Xanthine oxidase and aldehyde oxidase: a simple procedure for the simultaneous purification from rat liver

Aldehyde oxidase (AO) and xanthine oxidase (XO) are cytosolic enzymes that have been involved in some pathological conditions and play an important role in the biotransformation of drugs and xenobiotics. The increasing interest in these enzymes demands for a simple and rapid procedure for their purification. This paper describes for the first time a method that allows simultaneous purification of both enzymes from the same batch of rat livers. It involves few steps, is reproducible and offers high enzyme yields with high specific activities. The rat liver homogenate was fractionated by heat denaturation and by ammonium sulphate precipitation to give a crude extract containing both enzymes. This extract was chromatographed on an Hydroxyapatite column that completely separated AO from XO. Further purification of XO by anion exchange chromatography on a Q-Sepharose Fast Flow column resulted in a highly purified (1200-fold) preparation, with a specific activity of 3.64 U/mg and with a 20% yield. AO was purified about 1000-fold at a yield of 15%, with a specific activity of 3.48 U/mg, by affinity chromatography on Benzamidine-Sepharose 6B. The purified enzymes gave single bands of approximately 300 kDa on a polyacrylamide gel gradient electrophoresis and displayed the characteristic absorption spectra of highly purified enzymes.

Bioreductive metabolism of mitomycin C in EMT6 mouse mammary tumor cells: cytotoxic and non-cytotoxic pathways, leading to different types of DNA adducts. The effect of dicumarol

The six DNA adducts formed in EMT6 mouse mammary tumor cells upon treatment with mitomycin C (MC) fall into two groups: (1) four guanine adducts of MC and (2) two guanine adducts derived from 2,7-diaminomitosene (2,7-DAM), the major reductive metabolite of MC. The two groups of adducts were proposed to originate from two pathways arising from reductive activation of MC: (a) direct alkylation of DNA and (b) formation of 2,7-DAM, which then alkylates DNA. The aim of this study was to test the validity of this proposal and to evaluate the significance of alkylation of DNA by 2,7-DAM. Treatment of the cells with 2,7-DAM itself yielded the same 2,7-DAM-guanine adducts as treatment with MC; however, 2,7-DAM was approximately 100-fold less cytotoxic than MC. The uptake and efflux of 2,7-DAM by EMT6 cells was comparable to that of MC, but 2,7-DAM alkylated DNA with higher efficiency than MC. These results validate the two proposed pathways and show that formation of 2,7-DAM-DNA adducts in MC-treated cells represents a relatively non-toxic pathway of reductive metabolism of MC. A selective stimulatory effect of dicumarol (DIC) on 2,7-DAM-DNA adduct formation in EMT6 cells treated with MC was also investigated. DIC had no effect on alkylation by MC in cell-free systems, nor did it have significant effects on adduct formation or cell survival for cells treated with 2,7-DAM. It is proposed that in the cell DIC stimulates a reductase enzyme located at subcellular sites where the activated MC species has no direct access to DNA and therefore is diverted into the non-cytotoxic pathway, which leads to the formation of 2,7-DAM and its adducts.

Comparison of oxygen radical generation from the reductive activation of doxorubicin, streptonigrin, and menadione by xanthine oxidase and xanthine dehydrogenase

Investigations into the enzymes responsible for the reductive activation of antineoplastic agents are of particular interest with regard to the use of these agents in the treatment of solid tumors. Xanthine oxidase (EC 1.1.3.22; XO) and xanthine dehydrogenase (EC 1. 1.1.204; XDH) are two enzymes capable of the reductive activation of antineoplastic agents. Previously, XDH, the enzymatic precursor of XO, was not extensively studied because of difficulties in its isolation. Research in the reductive activation of antineoplastic agents by XDH has increased with the discovery of a rapid and high-yield purification procedure for XDH. In the present investigation, the potential for drug activation of doxorubicin (DOX), streptonigrin (STN), and menadione (MD) by XO and XDH was assessed through oxygen consumption studies. These studies were conducted at pH 7.4 and pH 6.0 to reflect physiological and the acidic pH of solid tumors, respectively. Apparent kinetic constants were determined for DOX, STN, and MD activation by XO and XDH at both pH levels. Higher oxygen consumption was observed for XDH drug activation in comparison to XO drug activation at equivalent enzyme activity for both pH levels. Drug-induced oxygen consumption was affected by pH. Hence, drug activation for DOX, STN, and MD was dependent upon the form of the xanthine-converting enzyme and the pH.

Reductive activation of doxorubicin by xanthine dehydrogenase from EMT6 mouse mammary carcinoma tumors

The role of enzymes in the reductive activation of various chemotherapeutic agents is an area of considerable interest in studies to better understand the selective toxicities of these agents. Xanthine dehydrogenase (XDH) is an enzyme capable of reductive activation of chemotherapeutic agents. Previously, this enzyme has not been extensively studied because of difficulties in its isolation. We recently isolated this enzyme from EMT6 mouse mammary carcinoma cells and showed that this enzyme is capable of activating mitomycin C. In this study, we examined whether XDH could activate the clinically important antineoplastic agent, doxorubicin. Drug activation was determined under aerobic and hypoxic conditions and at various pHs in order to simulate the different environments found in solid tumors. The results of these studies show that XDH reacts with doxorubicin via a two-electron reduction. This reduction is different from the modified and more extensively studied form of the enzyme, xanthine oxidase (XO), which reacts with doxorubicin via a one-electron reduction. Under hypoxic conditions, the formation of large quantities of 7-deoxydoxorubicin aglycone, a deactivation product of doxorubicin metabolism, may serve to moderate doxorubicin's antineoplastic activity. Under aerobic conditions, however, XDH activation led to a greater rate of formation of oxygen radicals than XO thereby possibly potentiating doxorubicin's cytotoxicity to aerobic tumor cells. Kinetic constants were determined for doxorubicin activation by XDH.

Hypouricemic and uricosuric actions of AA-193 in a hyperuricemic rat model

In normal rats, consecutive administrations of AA-193 for 7 days maintained the dose-dependent uricosuric activity without significant changes of the plasma urate level. In clearance studies, AA-193 produced an increase in the fractional excretion of urate (FEua) namely an inhibition of the net urate reabsorption in the nephron, which was probably dependent on the plasma concentration of the agent. During in vitro studies, 1 mmol/L AA-193 had no effect on liver uricase activity and 0.2 mmol/L AA-193 did not inhibit xanthine dehydrogenase activity. Therefore, it is unlikely that AA-193 at physiologic doses has a significant effect on either the production or degradation of urate. To assess the hypouricemic effect of AA-193 derived from its uricosuric effect, we used uricase-inhibited rats produced by oxonate feeding. In the hyperuricemic rat model, consecutive administrations of AA-193 for 7 days increased urate excretion and decreased the plasma urate level. We conclude that AA-193 has a hypouricemic effect caused by increases in urate excretion in hyperuricemic rats.

The role of xanthine oxidase in oxidative damage caused by cytokines in cultured mouse hepatocytes

In the present study we have examined the potential role of xanthine oxidase (XO) in the intracellular oxidative stress induced by combinations of recombinant murine TNF alpha (rMuTNF alpha) and murine interferon-gamma (IFN gamma) in cultured mouse hepatocytes. IFN gamma alone and the combination of rMuTNF alpha and IFN gamma increased XO activity after a 4 hr exposure period. rMuTNF alpha alone increased XO activity only after 24 hr. At the later time point, the increased XO activity was accounted for by decreased XDH activity. However, the apparent conversion of XDH to XO cannot account for the early effects of rMuTNF alpha on hepatocyte function, particularly the onset of an oxidative stress (as indicated by efflux of GSSG from the hepatocytes). This effect is observed after two hours, and it is temporally the earliest sign of alteration of cellular function caused by rMuTNF alpha. Increased XO activity was not observed until 4 hr after treatment with rMuTNF alpha/IFN gamma. In addition, inhibition of XO activity with allopurinol did not ameliorate GSSG efflux from hepatocytes treated with the cytokines. However, the ATP depletion caused by the combination of rMuTNF alpha and IFN gamma and the cytotoxicity observed with the combined cytokines in cells pre-treated with BCNU (to inhibit glutathione reductase) was inhibited by allopurinol. These results show that the onset of oxidative stress in cultured mouse hepatocytes is not due to conversion of XDH to XO. However, events which follow the efflux of GSSG, such as ATP depletion and cytotoxicity in cells with impaired anti-oxidant defenses, may be partially due to increased XO activity, especially in cells treated with both rMuTNF alpha and IFN tau.

Purification and immunohistochemical tissue localization of human xanthine oxidase

Xanthine oxidase was purified 1600-fold from human liver cytosol. The purified enzyme was shown as a single band of 300 kDa on polyacrylamide gel electrophoresis and 150 kDa on SDS-PAGE. Using this purified enzyme, polyclonal antibody against xanthine oxidase was raised in a rabbit. On Ouchterlony's double immunodiffusion method, the raised antibody and the human liver cytosol made a precipitation line stained by activity stain and protein stain, respectively. With the raised anti-xanthine oxidase sera, the immunohistochemical localization of xanthine oxidase in human tissues was examined. Immunostaining of frozen hepatic tissue section showed that the cytoplasm of hepatocytes and endothelial lining cells were stained. In a number of other tissues, the xanthine oxidase antigen was detected only in the endothelial lining cells from heart, kidney, brain, aorta, lung and mesentery, except for the duodenal mucosa cells. A possible role for xanthine oxidase in the endothelial cells from various human tissues in the pathogenesis of reperfusion injury was suggested.

Purification and substrate inactivation of xanthine dehydrogenase from Chlamydomonas reinhardtii

Xanthine dehydrogenase (XDH) from the unicellular green alga Chlamydomonas reinhardtii has been purified to electrophoretic homogeneity by a procedure which includes several conventional steps (gel filtration, anion exchange chromatography and preparative gel electrophoresis). The purified protein exhibited a specific activity of 5.7 units/mg protein (turnover number = 1.9 .10(3) min-1) and a remarkable instability at room temperature. Spectral properties were identical to those reported for other xanthine-oxidizing enzymes with absorption maxima in the 420-450 nm region and a shoulder at 556 nm characteristic of molybdoflavoproteins containing iron-sulfur centers. Chlamydomonas XDH was irreversibly inactivated upon incubation of enzyme with its physiological electron donors xanthine and hypoxanthine, in the absence of NAD+, its physiological electron acceptor. As deduced from spectral changes in the 400-500 nm region, xanthine addition provoked enzyme reduction which was followed by inactivation. This irreversible inactivation also took place either under anaerobic conditions or whenever oxygen or any of its derivatives were excluded. Adenine, 8-azaxanthine and acetaldehyde which could act as reducing substrates of XDH were also able to inactivate it upon incubation. The same inactivating effect was observed with NADH and NADPH, electron donors for the diaphorase activity associated with xanthine dehydrogenase. In addition, partial activities of XDH were differently affected by xanthine incubation. We conclude that xanthine dehydrogenase inactivation by substrate is due to an irreversible process affecting mainly molybdenum center and that sequential and uninterrupted electron flow from xanthine to NAD+ is essential to maintain the enzyme in its active form.

Molecular cloning of a cDNA coding for mouse liver xanthine dehydrogenase. Regulation of its transcript by interferons in vivo

The cDNA coding for xanthine dehydrogenase (XD) is isolated from mouse liver mRNA by cross-hybridization with a DNA fragment of the Drosophila melanogaster homologue. Two lambda bacteriophage overlapping clones represent the copy of a 4538-nucleotide-residue-long transcript with an open reading frame of 4005 nucleotide residues, coding for a putative polypeptide of 1335 amino acid residues. Comparison of the deduced amino acid sequence of the mouse XD with those of the Drosophila and the rat homologues shows a high conservation of this protein (55% identity between mouse and Drosophila, and 94% identity between mouse and rat). RNA blotting analysis demonstrates that interferon-alpha (IFN-alpha) and its inducers, i.e. poly(I).poly(C), bacterial lipopolysaccharide (LPS) and tilorone (2,7-bis-[2-(diethylamino)ethoxy]fluoren-9-one), increase the expression of XD mRNA in liver. Poly(I).poly(C) also induces XD mRNA in several other tissues in vivo. Protein synthesis de novo is not required for the elevation of XD mRNA after IFN-alpha treatment, since cycloheximide does not block the induction. The elevation of XD mRNA concentration is relatively fast and precedes the induction of both XD and xanthine oxidase (XO) enzymic activities.

Purification and characterization of mouse liver xanthine oxidase

Xanthine oxidase (EC 1.1.3.22) is purified to homogeneity from mouse liver after induction with bacterial lipopolysaccharide. The enzyme has an apparent molecular weight of 300,000 in its native state and it is suggested to be constituted of two identical subunits of Mr 150,000 each. The isoelectric point is 6.7 and the apparent Km value for xanthine is 3.4 microM. The amino acid composition of mouse xanthine oxidase is quite similar to that of Drosophila xanthine dehydrogenase.

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.