Subunit structure of bovine milk xanthine oxidase. Effect of limited cleavage by proteolytic enzymes on activity and structure

The Future Challenge of Reactive Oxygen Species (ROS) in Hypertension: From Bench to Bed Side

Reactive oxygen species (ROS) act as signaling molecules that control physiological processes, including cell adaptation to stress. Redox signaling via ROS has quite recently become the focus of much attention in numerous pathological contexts, including neurodegenerative diseases, kidney and cardiovascular disease. Imbalance in ROS formation and degradation has also been implicated in essential hypertension. Essential hypertension is characterized by multiple genetic and environmental factors which do not completely explain its associated risk factors. Thereby, even if advances in therapy have led to a significant reduction in hypertension-associated complications, to interfere with the unbalance of redox signals might represent an additional therapeutic challenge. The decrease of nitric oxide (NO) levels, the antioxidant activity commonly found in preclinical models of hypertension and the ability of antioxidant approaches to reduce ROS levels have spurred clinicians to investigate the contribution of ROS in humans. Indeed, particular effort has recently been devoted to understanding how redox signaling may contribute to vascular pathobiology in human hypertension. However, although biomarkers of oxidative stress have been found to positively correlate with blood pressure in preclinical model of hypertension, human data are less convincing. We herein provide an overview of the most relevant mechanisms via which oxidative stress might contribute to the pathophysiology of essential hypertension. Moreover, alternative approaches, which are directed towards improving antioxidant machinery and/or interfering with ROS production, are also discussed.

Structural and conformational analysis of the oxidase to dehydrogenase conversion of xanthine oxidoreductase

Xanthine oxidoreductase (XOR) is a 300-kDa homodimer that can exist as an NAD+-dependent dehydrogenase (XD) or as an O2-dependent oxidase (XO) depending on the oxidation state of its cysteine thiols. Both XD and XO undergo limited cleavage by chymotrypsin and trypsin. Trypsin selectively cleaved both enzyme forms at Lys184, while chymotrypsin cleaved XD primarily at Met181 but cleaved XO at Met181 and at Phe560. Chymotrypsin, but not trypsin, cleavage also prevented the reductive conversion of XO to XD; thus the region surrounding Phe560 appears to be important in the interconversion of the two forms. Size exclusion chromatography showed that disulfide bond formation reduced the hydrodynamic volume of the enzyme, and two-dimensional gel electrophoresis of chymotrypsin-digested XO showed significant, disulfide bond-mediated, conformational heterogeneity in the N-terminal third of the enzyme but no evidence of disulfide bonds between the N-terminal and C-terminal regions or between XOR subunits. These results indicate that intrasubunit disulfide bond formation leads to a global conformational change in XOR that results in the exposure of the region surrounding Phe560. Conformational changes within this region in turn appear to play a critical role in the interconversion between the XD and XO forms of the enzyme.

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.

Cysteines involved in the interconversion between dehydrogenase and oxidase forms of bovine xanthine oxidoreductase

Mammalian xanthine oxidoreductase exists intracellularly in its dehydrogenase form. However, outside of this reducing milieu the enzyme quickly transforms into an oxidase form. Interconversion can be controlled by sulfhydryl reactive reagents, suggesting that disulfide bridging is linked to this phenomenon. The present work identified cysteines involved in the interconversion process. Purified enzyme was subjected to mild reduction with 1,4-dithioerythriol to regain dehydrogenase activity, and the accessible cysteines were labeled with specific radioactive alkylation reagents, iodoacetic acid. This partial alkylation stabilizes the dehydrogenase form, presumable by hindering formation of disulfide bond(s). Six of 38 cysteines were found to be labeled (residues 169, 170, 535, 992, 1317, and 1325). The significance of this labeling of bovine xanthine oxidoreductase is discussed in relation to structural knowledge about the enzyme, and especially by comparison with the AA sequences of avian and invertebrate enzymes, which do not undergo conversion. Cysteines 535 and 992 are the most likely marked residues to be involved in the interconversion, whereas the other cysteines are located too far from the cofactorbinding areas in xanthine oxidoreductase.

A review and proposed nomenclature for major proteins of the milk-fat globule membrane

The characteristics and possible functions of the most abundant proteins associated with the bovine milk-fat globule membrane are reviewed. Under the auspices of the Milk Protein Nomenclature Committee of the ADSA, a revised nomenclature for the major membrane proteins is proposed and discussed in relation to earlier schemes. We recommend that proteins be assigned specific names as they are identified by molecular cloning and sequencing techniques. The practice of identifying proteins according to their Mr, electrophoretic mobility, or staining characteristics should be discontinued, except for uncharacterized proteins. The properties and amino acid sequences of the following proteins are discussed in detail: MUC1, xanthine dehydrogenase/oxidase, CD36, butyrophilin, adipophilin, periodic acid Schiff 6/7 (PAS 6/7), and fatty acid binding protein. In addition, a compilation of less abundant proteins associated with the bovine milk-fat globule membrane is presented.

Irreversible conversion of xanthine dehydrogenase into xanthine oxidase by a mitochondrial protease

Irreversible conversion of xanthine dehydrogenase (XDH) to its oxygen free radical producing oxidase (XO) form occurs through an uncharacterized proteolytic process, which was studied in human liver. Upon incubation of fresh unfrozen liver cytosol, XDH remained intact. When recombinant human XDH was coincubated with subcellular fractions of human liver, the mitochondrial intermembrane space was shown to contain a heat-labile activity that converted XDH irreversibly to XO. This activity is resistant to inhibitors of all major groups of proteases. We postulate that this novel type of proteolytic enzyme is released into the cytosol upon mitochondrial damage.

The trypanocidal Cape buffalo serum protein is xanthine oxidase

Plasma and serum from Cape buffalo (Syncerus caffer) kill bloodstream stages of all species of African trypanosomes in vitro. The trypanocidal serum component was isolated by sequential chromatography on hydroxylapatite, protein A-G, Mono Q, and Superose 12. The purified trypanocidal protein had a molecular mass of 150 kDa, and activity correlated with the presence of a 146-kDa polypeptide detected upon reducing sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Amino acid sequences of three peptide fragments of the 146-kDa reduced polypeptide, ligand affinity and immunoaffinity chromatography of the native protein, and sensitivity to pharmacological inhibitors, identified the trypanocidal material as xanthine oxidase (EC 1.1.3.22). Trypanocidal activity resulted in the inhibition of trypanosome glycolysis and was due to H2O2 produced during catabolism of extracellular xanthine and hypoxanthine by the purine catabolic enzyme.

Xanthine dehydrogenase/xanthine oxidase and oxidative stress

Xanthine dehydrogenase (XDH) and xanthine oxidase (XOD) are single-gene products that exist in separate but interconvertible forms. XOD utilizes hypoxanthine or xanthine as a substrate and O2 as a cofactor to produce superoxide (·O2 (-)) and uric acid. XDH acts on these same substrates but utilizes NAD as a cofactor to produce NADH instead of ·O2 (-) and uric acid. XOD has been proposed as a source of oxygen radicals in polymorphonuclear, endothelial, epithelial, and connective tissue cells. However, several questions remain about the physiological significance and functions of XOD on aging and oxidative stress. XOD is reported to play an important role in cellular oxidative status, detoxification of aldehydes, oxidative injury in ischemia-reperfusion, and neutrophil mediation. For example, XOD may serve as a messenger or mediator in the activation of neutrophil, T cell, cytokines, or transcription in defense mechanisms rather than as a free radical generator of tissue damage. Emerging evidence on the synergistic interactions of ·O2 (-), a toxic product of XOD and nitric oxide, may be another illustration of XOD involvement in tissue injury and cytotoxicity in an emergent condition such as ischemia or inflammation.

Purification and characterization of a prokaryotic xanthine dehydrogenase from Comamonas acidovorans

Xanthine dehydrogenase (XDH) is induced in Comamonas acidovorans cells incubated in a limited medium with hypoxanthine as the only carbon and nitrogen source. The enzyme has been purified to homogeneity using standard techniques and characterized. It contains two subunits with M(r) values of 90 and 60 kDa. Gel filtration studies show the enzyme to have an alpha 2 beta 2 native structure. No precursor form of the enzyme is observed on Western blot analysis of cell extracts obtained at various stages of enzyme induction. Metal analysis of the purified enzyme shows 1.1 Mo, 4.0 Fe, and 3.6 phosphorus atoms per alpha beta protomer. Cofactor analysis shows the enzyme to contain a single molybdopterin mononucleotide and one FAD per alpha beta protomer. Electron spin resonance and circular dichroism spectral studies of the oxidized and reduced forms of the enzyme suggest the Fe centers to be two nonidentical [2Fe-2S] clusters. Electron spin resonance signals due to Mo(V) and neutral FAD radical are also observed in the reduced form of the enzyme. Purified enzyme preparations ranged from 70% to 100% functionality. The enzyme is irreversibly inactivated by CN- and is inhibited on incubation with allopurinol. With xanthine and NAD+ as substrates the enzyme has a specific activity of 50 units/mg, a kcat value of 120 s-1, an activity/flavin ratio of 1930, and respective Km values of 66 and 160 mM. Using 8-D-xanthine as substrate, a DV value of 1.8 is found with no change in Km. Thus, the Km and KD values of the enzyme for xanthine are equal. These data show Comamonas XDH to exhibit structural properties similar to bovine milk xanthine oxidase/dehydrogenase and to chicken liver xanthine dehydrogenase. Although the bacterial enzyme exhibits a 6-7-fold greater turnover rate than bovine or avian enzymes, the catalytic efficiencies (as measured by V/K) are similar for all three enzymes.

Purification of the bovine xanthine oxidoreductase from milk fat globule membranes and cloning of complementary deoxyribonucleic acid

The amino acid sequence of the bovine xanthine oxidoreductase was determined by cloning and sequencing cDNA clones encoding the enzyme. Partial amino acid sequence corresponding to 54% of the total sequence were also determined from purified bovine milk xanthine oxidoreductase, showing identity with the translated cDNA sequence. The cDNA of 4719 nucleotides included a 5' untranslated region of 96 nucleotides, an open reading frame encoding a xanthine oxidoreductase of 1332 amino acid residues, and a 3' untranslated region of 624 nucleotides including two polyadenylation signals and a poly (A) tail of 74 nucleotides. The identity between the amino acid sequence of the bovine xanthine oxidoreductase and xanthine oxidoreductase from mammalian species was 86 to 90%.

The structure of chicken liver xanthine dehydrogenase. cDNA cloning and the domain structure

The amino acid sequence of chicken liver xanthine dehydrogenase (EC 1.1.1.204) was determined by cDNA cloning and partial amino acid sequencing of the purified enzyme. The enzyme consisted of 1358 amino acids with calculated molecular mass of 149,633 Da. In order to compare the structure of the chicken and rat enzymes, limited proteolysis was performed with the purified chicken liver xanthine dehydrogenase. When the enzyme was digested with subtilisin, it was not converted from the NAD-dependent dehydrogenase type to the O2-dependent oxidase type, in contrast with the mammalian enzyme. However, the enzyme was cleaved mainly into three fragments in a manner similar to that for the rat enzyme at pH 8.2 (20, 37, and 84 kDa) and retaining a full complement of redox centers. The cleavage sites were identified by determination of amino-terminal sequences of the produced fragments. It was concluded that the 20-kDa fragment was amino-terminal, the 84-kDa fragment carboxyl-terminal, and the 37-kDa fragment an intermediate portion in the enzyme protein. On the other hand, when the enzyme was digested with the same protease at pH 10.5, the sample contained only the 20- and 84-kDa portions and lacked the 37-kDa portion. The resultant sample possessed xanthine dichlorophenol indophenol reductase activity, indicating that the molybdenum center remained intact. The absorption spectrum showed the sample was very similar to deflavo-enzyme. From these results and sequence analyses, the domain structure of the enzyme is discussed.

Immunohistochemical localization of xanthine oxidase in human cardiac and skeletal muscle

The generation of a monoclonal antibody specific to xanthine oxidase and its use in the distribution of the enzyme in human tissue is described. Xanthine oxidase was purified from human and bovine milk by a rapid method, allowing for minimal proteolytic degradation, and the purified enzyme preparations were used for the immunization of BALB/c mice as well as for the subsequent selection of hybridomas. The hybridoma clone X1-7, IgG (2a, kappa-light chain) was selected for further analysis and demonstrated to precipitate xanthine oxidase from human liver and skeletal muscle extracts. As determined by SDS-polyacrylamide gel electrophoresis of eluates from affinity chromatography, the X1-7 antibody bound to a main protein of 155 kDa, from human milk and skeletal muscle, and to proteins of 155, 143 and 95 kDa from human liver. Immunohistochemical studies, using two of the monoclonal antibodies with differing epitope specificity, revealed xanthine oxidase to be localized mainly in the vascular smooth muscle cells but also in a proportion of endothelial cells of capillaries and smaller vessels in both human cardiac and skeletal muscle. Immunoreactivity was additionally observed in human macrophages and mast cells. The results of the present study confirm previous reports of the presence of xanthine oxidase in capillary endothelial cells, but also demonstrates additional localization of the enzyme in vascular smooth muscle cells, macrophages and mast cells. The current findings verify that the distribution of xanthine oxidase in human tissue includes cardiac and skeletal muscle.

Studies on the induction and phosphorylation of xanthine dehydrogenase in cultured chick embryo hepatocytes

Chick embryo hepatocytes, cultured in a chemically defined medium, were used to investigate hormonal requirements for xanthine-dehydrogenase induction and to determine whether the enzyme is phosphorylated. Triiodothyronine is found to be required to induce the synthesis of active enzyme. Inclusion of sodium tungstate in the medium resulted in the complete loss of enzyme activity but no decrease of immunochemically detectable levels of enzyme. Immunoprecipitated xanthine dehydrogenase from cell extracts migrates with enzyme purified from adult chicken liver on SDS/PAGE. Both the native 150-kDa subunit and the 130-kDa form of the enzyme is observed. N-terminal sequence analysis of the 150-kDa subunit shows the following; Ala-Pro-Pro-Glu-Thr-Gly-Asp-Glu-Leu-Val-Phe-Phe-Val-Asn-Gly-Lys-Lys-Val- Val which is similar to the published N-terminal sequences of rat, mouse and insect xanthine dehydrogenases. Autoradiography of denaturing gels of xanthine dehydrogenase isolated from 32P(i)-labeled hepatocytes demonstrates that the 150-kDa and the 130-kDa forms of the enzyme are phosphorylated. Chemical phosphate analysis of acid-precipitated, electrophoretically pure chicken liver xanthine dehydrogenase also shows the presence of covalently bound phosphate. Phosphoamino acid analysis of both 32P-labeled forms of the enzyme demonstrates the presence of phosphoserine. Thus, chicken liver xanthine dehydrogenase contains a phosphoserine residue as found previously in bovine milk xanthine oxidase [Davis, M. D., Edmondson, D. E. & Müller, F. (1984) Eur. J. Biochem. 145, 237-250].

Xanthine dehydrogenase and 2-furoyl-coenzyme A dehydrogenase from Pseudomonas putida Fu1: two molybdenum-containing dehydrogenases of novel structural composition

The constitutive xanthine dehydrogenase and the inducible 2-furoyl-coenzyme A (CoA) dehydrogenase could be labeled with [185W]tungstate. This labeling was used as a reporter to purify both labile proteins. The radioactivity cochromatographed predominantly with the residual enzymatic activity of both enzymes during the first purification steps. Both radioactive proteins were separated and purified to homogeneity. Antibodies raised against the larger protein also exhibited cross-reactivity toward the second smaller protein and removed xanthine dehydrogenase and 2-furoyl-CoA dehydrogenase activity up to 80 and 60% from the supernatant of cell extracts, respectively. With use of cell extract, Western immunoblots showed only two bands which correlated exactly with the activity stains for both enzymes after native polyacrylamide gel electrophoresis. Molybdate was absolutely required for incorporation of 185W, formation of cross-reacting material, and enzymatic activity. The latter parameters showed a perfect correlation. This evidence proves that the radioactive proteins were actually xanthine dehydrogenase and 2-furoyl-CoA dehydrogenase. The apparent molecular weight of the native xanthine dehydrogenase was about 300,000, and that of 2-furoyl-CoA dehydrogenase was 150,000. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis of both enzymes revealed two protein bands corresponding to molecular weights of 55,000 and 25,000. The xanthine dehydrogenase contained at least 1.6 mol of molybdenum, 0.9 ml of cytochrome b, 5.8 mol of iron, and 2.4 mol of labile sulfur per mol of enzyme. The composition of the 2-furoyl-CoA dehydrogenase seemed to be similar, although the stoichiometry was not determined. The oxidation of furfuryl alcohol to furfural and further to 2-furoic acid by Pseudomonas putida Fu1 was catalyzed by two different dehydrogenases.

Preparation of antibodies against xanthine oxidase from human milk

1. Human xanthine oxidase [XO; EC 1.2.3.2.] was isolated by a non-proteolytic method from fresh human milk. Final purification of the protein was achieved by hydroxyapatite chromatography. Most (less than 95%) of the enzyme was released in the 0.40 M phosphate fraction at pH 6.8. 2. The specific activity of this preparation was found to be 0.047 microM min-1 mg-1 with xanthine as substrate. 3. Sodium dodecyl sulfate (SDS)-polyacrylamide gel electrophoresis (PAGE) separated two subunits, each with a mol. wt approximately 122 kDa. 4. On non-denaturing acrylamide gels both of these subunits exhibited oxidase-like activity with xanthine as substrate in the presence of nitroblue tetrazolium and molecular oxygen. 5. Immunoconjugates of XO were prepared by the keyhole limpet hemocyanin (KLH)- and glutaraldehyde-crosslinking techniques. 6. Polyclonal antibodies to XO were raised by i.m. injection of these conjugates into female New Zealand rabbits. 7. Western blot analysis using the semi-dry technique was employed to confirm the specificity of the antibody.

Characteristics of purified cows' milk xanthine oxidase and its submolecular characteristics

Xanthine oxidase (EC 1.2.3.2) was purified from fresh cows' milk by differential centrifugation and hydroxylapatite chromatography in the absence of reducing agents and proteases. The purified isolate possessed an absorbance at 280 nm:absorbance at 450 nm ratio of 4.84; an absorbance (1 cm at 280 nm 1%) of 11.9; an activity:absorbance at 450 nm of 141, a specific activity of 3.59 units/mg; and detectable dehydrogenase activity. The enzyme preparation was obtained in a reversible oxidase form that could be partially converted to xanthine dehydrogenase in the presence of 10mM dithiothreitol or 1% mercaptoethanol. Amino acid analyses revealed that the enzyme was hydrophobic in nature and that lysine constituted its N-terminal residue. The protein contained 22 disulfide and 38 sulfhydryl groups, four of which were detectable in the undenatured protein complex. Discontinuous PAGE in the presence of selected dissociation agents did not result in further resolution. Sodium dodecyl sulfate-PAGE of the purified enzyme revealed a sharp zone with a molecular weight of 151,000 +/- 4000 (i.e., monomer). The purified enzyme exhibited oxidase activity in the presence of 6 M urea and following limited proteolysis by trypsin, chymotrypsin, plasmin, pancreatin, pepsin, and papain. Proteolyzed xanthine oxidase migrated as a single zone in polyacrylamide gels in the presence and absence of dissociating agents such as 1% mercaptoethanol and 6 M urea. Restricted digestion of xanthine oxidase by proteases was indicated by the presence of three major zones with molecular weights ranging from 85,000 to 100,000, 30,000 to 35,000, and 18,000 to 20,000 commonly observed in SDS gels. Amino acid profiles of the principal peptidyl fragments of trypsin-cleaved xanthine oxidase indicated their hydrophobic nature and lysine as the N-terminal residue for all fragments.

The redox centers of xanthine oxidase are on independent structural domains of the enzyme

Xanthine oxidase employs four electron transport sites (flavin adenine dinucleotide (FAD), molybdenum, and two FeS centers) in catalyzing a variety of redox reactions. To determine whether the redox sites reside in independent domains of the enzyme, the temperature of heat inactivation of each site's catalytic activity was determined, except that no attempt was made to distinguish between the two FeS sites. In the oxidase form of xanthine oxidase, the order of thermal stabilities was Mo greater than FAD greater than FeS, while after conversion to its dehydrogenase form the above ranking was Mo greater than FeS greater than FAD. The small but reproducible difference in heat inactivation temperatures among the redox sites demonstrated that the sites are located in separate domains of the enzyme. To confirm the above segregation of redox centers, the temperature of heat-induced release of each redox cofactor from its site on the enzyme was examined. These temperatures were found to be different for each redox cofactor and agreed closely with the heat inactivation temperatures measured above. The data thus demonstrate that both heat inactivation and cofactor release derive from thermal unfolding of independent domains. Using a technique termed "thermal digestion analysis," the FAD domain was located in a approximately equal to 42,000-Da tryptic fragment, while the FeS and Mo domains were isolated in a trypsin-resistant 92,000-Da fragment. We conclude that xanthine oxidase is constructed in modular fashion with the redox sites located in independent structural domains.

Xanthine oxidase from human liver: purification and characterization

Xanthine oxidase [EC 1.2.3.2] was purified 2000-fold from human liver. The last step of the procedure involved affinity chromatography. The resulting preparation showed two closely migrating bands of enzyme activity after gel electrophoresis under nondenaturing conditions. No other proteins were detected on these gels. The average particle mass of the enzyme was 300 kDa as determined by size-exclusion chromatography. This together with results of gel electrophoresis under denaturing conditions suggested that the native enzyme was composed of two subunits of approximately 150 kDa each. The electrophoretic patterns also indicated that a portion of these subunits had undergone partial proteolysis. The substrate specificity of the purified human enzyme was studied using an assay in which phenazine ethosulfate coupled the transfer of electrons from the reduced enzyme to cytochrome c. Hypoxanthine, 2-hydroxypurine, xanthine, 2-aminopurine, and adenine were among the most efficient purine substrates studied. Most purine nucleosides tested were oxidized at detectable rates, but with relatively high Km values. The 2'-deoxyribonucleosides were more efficient substrates than were the corresponding ribonucleosides or arabinonucleosides. In a direct comparison with xanthine oxidase from bovine milk, the human enzyme showed a similar specificity toward purine substrates. However, considerable differences between the bovine and human enzymes were observed with nucleoside substrates. With xanthine as the substrate for the human enzyme, 20% of the total electron flow was univalently transferred to oxygen to produce superoxide radicals.

Guinea pig liver aldehyde oxidase as a sulfoxide reductase: its purification and characterization

Guinea pig aldehyde oxidase was purified about 120-fold at a yield of 26% from liver cytosol by sequential column chromatography using DEAE-cellulose, FMN-Sepharose 4B, and Sephacryl S-300. The purified enzyme showed many similarities with the rabbit liver aldehyde oxidase reported by other workers with respect to its absolute spectra, molecular weight, and cofactor compositions of molybdenum, FAD, and nonheme iron. This enzyme efficiently utilized 2-hydroxypyrimidine and benzaldehyde as electron donors while N1-methylnicotinamide was 40 times less effective than 2-hydroxypyrimidine. Diphenyl sulfoxide was reduced anaerobically to diphenyl sulfide in the presence of electron donors. This activity was highly susceptible to SKF 525-A as well as the known inhibitors for aldehyde oxidase such as menadione, estradiol, and potassium cyanide. This enzyme also reduced dibenzyl sulfoxide, phenothiazine sulfoxide, D-biotin methyl ester d-sulfoxide, and quinoline N-oxide, but not L-methionine sulfoxide, dimethyl sulfoxide, D-biotin methyl ester l-sulfoxide, and D-biotin d- and l-sulfoxides, as well as diphenyl sulfone. These results indicate that aldehyde oxidase in guinea pig liver functions as a sulfoxide reductase with selective substrate specificity under anaerobic conditions.

Identification and characterization of the principal proteins of the fat-globule membrane from guinea-pig milk

The milk-fat-globule membrane (MFGM) was isolated from guinea-pig milk and the membrane-associated proteins and glycoproteins characterized by electrophoretic techniques. Major components of the membrane included PAS-I, a sialoglycoprotein of Mr greater than or equal to 200000, the redox enzyme xanthine oxidase and the glycoprotein, butyrophilin. Membrane preparations also contained two other glycoproteins, GP-80 and GP-55, of Mr 80000 and 55000, respectively. Comparison of guinea-pig xanthine oxidase and butyrophilin with proteins from bovine MFGM by peptide mapping procedures, showed that the two proteins in both species were similar, but not identical. GP-55 may also be related to glycoproteins of Mr 45000 and 48000 in the bovine membrane. The integral and peripheral components of guinea-pig MFGM were identified by treating membrane preparations with sodium carbonate solutions at high pH and by partitioning the membrane proteins in solutions of Triton X-114. By these criteria xanthine oxidase and GP-55 appeared to be peripheral components and GP-80 an integral protein of the membrane. PAS-I and butyrophilin displayed hydrophilic properties in Triton X-114 solutions, but could not be removed from membrane preparations with sodium carbonate. Possible reasons for these ambiguous data are discussed. The observed similarity between several of the proteins of guinea-pig and bovine MFGM implies that these proteins may have specific functions related to milk secretion in mammary tissue, e.g. in the budding of milk-fat globules or the exocytosis of milk protein and lactose at the apical surface.

Purification and properties of a novel ferricyanide-linked xanthine dehydrogenase from Pseudomonas putida 40

The isolation of a xanthine dehydrogenase from Pseudomonas putida 40 which utilizes ferricyanide as an electron acceptor at high efficiency is presented. The new activity is separate from the NAD+ and oxygen-utilizing activities of the same organism but displays a broad pattern for reducing substrates typical of those of previously studied xanthine-oxidizing enzymes. Unlike the previously studied enzymes, the new enzyme appears to lack flavin but possess heme and is resistant to cyanide treatment. However, sensitivity of the purified enzyme to methanol and the selective elimination of the activity when tungstate is added to certain growth media suggest a role for molybdenum. The enzyme is subject to a selective proteolytic action during processing which is not accompanied by denaturation or loss of activity and which is minimized by the continuous exposure of the activity to EDTA and phenylmethylsulfonyl fluoride. Electrophoresis of the denatured enzyme in the presence of sodium dodecyl sulfate suggests that the enzyme is constructed of subunits with a molecular weight of approximately 72,000. Electrophoresis under native conditions of a purified enzyme previously exposed to magnesium ion reveals a series of major and minor activity bands which display some selectivity toward both electron donors and acceptors. An analysis of the effect of gel concentration on this pattern suggests that the enzyme forms a series of charge and size isomers with a pair of trimeric forms predominating. Comparison of the rate of sedimentation of the enzyme in sucrose gradients with its elution profile from standardized Sepharose 6B columns suggests a molecular weight of 255,000 for the major form of the native enzyme.

Identification and partial characterization of the xanthine oxidase transitions of the milk fat globule membrane

Differential scanning calorimetry of bovine milk fat globule membranes (MFGM) yields five to eight transitions, depending on the conditions employed during isolation and assay of the membranes. Transitions A, B, and C were shown in a previous publication to derive from lipid melting, while transition D was found to stem from the unfolding of a structural protein termed butyrophilin [K. C. Appell, T. W. Kennan, and P. S. Low (1982) Biochim. Biophys. Acta 690, 243-250]. In this report we present evidence that the E1, E2, and F endotherms derive from the major MFGM protein, xanthine oxidase. Support for this contention derives from (i) thermal gel analysis; (ii) thermal inactivation analysis; (iii) comparison of the calorimetric properties of endotherms I, II, and III of purified xanthine oxidase with transitions E1, E2, and F of MFGM; (iv) comparison of the properties of a peculiar exotherm in scans of both the purified enzyme and MFGM; and (v) examination of the effects of specific ligands, reducing agents, and pH on both the xanthine oxidase and MFGM transition. The existence of three independent endotherms (I, II, and III) in purified xanthine oxidase demonstrates that the enzyme is composed of multiple independent domains. The interconversion of transitions I (E1) and II (E2) with a change in the redox conditions of the medium implies that these two transitions may be manifestations of the interconvertible dehydrogenase and oxidase forms of the enzyme, respectively. The relative independence of the I/II transitions from transition III further shows that only slight interaction between the major domains of xanthine oxidase exists.

Purification and properties of purine hydroxylase II from Aspergillus nidulans

Purine hydroxylase II from Aspergillus nidulans has been purified to near homogeneity. The enzyme has a pI of 5.7, a molecular weight of 300,000, and two subunits with molecular weight of 153,000 each. The enzyme contains 2 FAD, 2 molybdenum atoms, and 4 (2 Fe-2S) iron-sulfur centers per molecule and exhibits broad specificity for reducing and oxidizing substrates. Among the more notable characteristics are the ability to oxidize hypoxanthine and nicotinic acid but not xanthine and virtually complete inactivity with oxygen. Moreover, while the enzyme is inactivated by borate and methanol, it is very resistant to cyanide and arsenite and it not inactivated by allopurinol. At infinite concentrations of reducing and oxidizing substrates, the Km for hypoxanthine was 119 microM, for nicotinic acid was 136 microM, and for NAD+ was 525 microM.

[Xanthine oxidase in homogenized cow's milk and Oster's hypothesis: a review]

Oster has postulated that the enzyme xanthine oxidase in homogenized cow's milk is the cause of myocardial infarction and angina pectoris. This enzyme may be absorbed by ingestion, especially of the small particles of the fat globules, and then carried by lymph streams to the arterial vascular system, where it is deposited into the myocardium. Then it destroys the aldehydes liberated from the cell membrane-based plasmalogens. This results in the intimal damage to the cell membranes of the arterial intima and the myocardium and ultimately in the development of typical atherosclerotic lesions in the arteries. The presented review is a critical approach to this hypothesis. The following factors are discussed: - the influence of conditions prevailing in the intestine and the stomach on the activity of the xanthine oxidase in milk, - the possibility of this enzyme being absorbed in the intestine, - the formation of antibodies against absorbed xanthine oxidase and - the behaviour of xanthine oxidase administered intravenously. Compared with present knowledge, this theory gives little evidence only.

Detection of xanthine oxidase and immunologically related proteins in fractions from bovine mammary tissue and milk after electrophoresis in polyacrylamide gels containing sodium dodecyl sulphate

A solid-phase immunoassay was used to detect xanthine oxidase in fractions from bovine mammary glands after electrophoresis in polyacrylamide gels containing sodium dodecyl sulphate. Under these conditions the major proportion of xanthine oxidase in either mammary tissue or mild could be recovered as a protein of mol.wt. 150 000. In mammary tissue approx. 80% of the enzyme was in a soluble form and the remainder was accounted for in either 'mitochondrial' or microsomal fractions after tissue homogenization and fractionation. Affinity chromatography of either detergent-solubilized microsomal membranes or postmicrosomal supernatants on immobilized antibody to xanthine oxidase yielded a single protein that cross-reacted with antibody to the enzyme. In milk presumptive degradation products of the enzyme were detected in minor quantities with mol.wts. of 43 000 in the whey fraction and 90 000 in fat-globule membrane. Only the undegraded enzyme was present in the skim-milk membrane fraction. Xanthine oxidase is therefore synthesized and secreted as a protein with a monomeric mol.wt. of 150 000 and is not subjected to extensive proteolytic degradation during the storage of milk in mammary alveoli. The significance of the results is discussed in relation to the overall protein composition of the membranes of milk-fat globules and skim milk.