Immunological identification and determination of xanthine oxidase in cells and tissues

Insight into oxidative stress in varicocele-associated male infertility: part 1

Varicocele is recognized as the leading cause of male infertility because it can impair spermatogenesis through several distinct pathophysiological mechanisms. Current evidence supports oxidative stress as a key element in the pathophysiology of varicocele-related infertility, although these mechanisms have not yet been fully described. Measurement of the reactive oxygen species and other markers of oxidative stress, including the levels of the antioxidant enzymes catalase and superoxide dismutase, can provide valuable information on the extent of oxidative stress and might guide therapeutic management strategies. The testis can respond to varicocele-associated cell stressors, such as heat stress, ischaemia or production of vasodilators (for example, nitric oxide) at the expense of the generation of excessive reactive oxygen species. These responses have their own implications in exacerbating the underlying oxidative stress and on the subsequent infertility.

NADPH oxidase plays a central role in cone cell death in retinitis pigmentosa

Retinitis pigmentosa (RP) is a collection of diseases in which rod photoreceptors die from a variety of mutations. After rods die, the level of tissue oxygen in the outer retina becomes elevated and there is progressive oxidative damage to cones that ultimately triggers apoptosis. In this study, we investigated the hypothesis that NADPH oxidase (Nox) and/or xanthine oxidase serve as critical intermediaries between increased tissue oxygen and the generation of excessive reactive oxygen species that cause oxidative damage to cones. Apocynin, a blocker of Nox, but not allopurinol, a blocker of xanthine oxidase, markedly reduced the superoxide radicals visualized by hydroethidine in the outer retina in the retinal degeneration-1 (rd1(+/+)) model of RP. Compared to rd1(+/+) mice treated with vehicle, those treated with apocynin, but not those treated with allopurinol, had significantly less oxidative damage in the retina measured by ELISA for carbonyl adducts. Apocynin-treated, but not allopurinol-treated, rd1(+/+) mice had preservation of cone cell density, increased mRNA levels for m- and s-cone opsin, and increased mean photopic b-wave amplitude. In Q344ter mice, a model of dominant RP in which mutant rhodopsin is expressed, apocynin treatment preserved photopic electroretinogram b-wave amplitude compared to vehicle-treated controls. These data indicate that Nox, but not xanthine oxidase, plays a critical role in generation of the oxidative stress that leads to cone cell death in RP and inhibition of Nox provides a new treatment strategy.

Xanthine oxidoreductase and cardiovascular disease: molecular mechanisms and pathophysiological implications

There is substantial evidence that oxidative stress participates in the pathophysiology of cardiovascular disease. Biochemical, molecular and pharmacological studies further implicate xanthine oxidoreductase (XOR) as a source of reactive oxygen species in the cardiovascular system. XOR is a member of the molybdoenzyme family and is best known for its catalytic role in purine degradation, metabolizing hypoxanthine and xanthine to uric acid with concomitant generation of superoxide. Gene expression of XOR is regulated by oxygen tension, cytokines and glucocorticoids. XOR requires molybdopterin, iron-sulphur centres, and FAD as cofactors and has two interconvertible forms, xanthine oxidase and xanthine dehydrogenase, which transfer electrons from xanthine to oxygen and NAD(+), respectively, yielding superoxide, hydrogen peroxide and NADH. Additionally, XOR can generate superoxide via NADH oxidase activity and can produce nitric oxide via nitrate and nitrite reductase activities. While a role for XOR beyond purine metabolism was first suggested in ischaemia-reperfusion injury, there is growing awareness that it also participates in endothelial dysfunction, hypertension and heart failure. Importantly, the XOR inhibitors allopurinol and oxypurinol attenuate dysfunction caused by XOR in these disease states. Attention to the broader range of XOR bioactivity in the cardiovascular system has prompted initiation of several randomised clinical outcome trials, particularly for congestive heart failure. Here we review XOR gene structure and regulation, protein structure, enzymology, tissue distribution and pathophysiological role in cardiovascular disease with an emphasis on heart failure.

Ultrastructural localization of xanthine oxidoreductase activity in isolated rat liver cells

Xanthine oxidoreductase (XOR) can exist in a dehydrogenase form (XD) and an oxidase form (XO). The D-form uses NAD as cofactor and the O-form uses oxygen as second substrate and produces oxygen radicals. Both enzymes have a high affinity for hypoxanthine and xanthine as substrate and produce uric acid, a potent antioxidant. In the present study, XOR activity was demonstrated with the ferricyanide method in permeabilized isolated rat liver cells at the electron microscopical level. Moreover, ultrastructural localization of XO activity in these cells was studied with the cerium salt method. Activity of both XOR and XO was found in matrix and core of peroxisomes of rat liver parenchymal cells. Only XOR activity was present as well in the cytoplasm of rat liver parenchymal cells. In Kupffer cells and sinusoidal endothelial cells, XOR activity was demonstrated in vesicles and occasionally on granular endoplasmic reticulum. XO activity was not found in Kupffer cells and sinusoidal endothelial cells. The presence of uric acid oxidase activity in matrix and core of peroxisomes as was found previously suggests further breakdown of purines to allantoin in peroxisomes. It is suggested that the major function of XOR activity in the cytoplasm of rat liver parenchymal cells and in sinusoidal cells is not the production of oxygen radicals, but rather the production of uric acid which can act as a potent antioxidant.

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.

Mouse mammary gland xanthine oxidoreductase: purification, characterization, and regulation

Xanthine oxidoreductase (XOR) has been purified from lactating mouse mammary tissue and its properties and developmental expression have been characterized. XOR was purified 80-fold in two steps using benzamidine-Sepharose affinity chromatography. The purified enzyme had a specific activity of 5.7 U/mg and an activity to flavin ratio of 192. SDS-polyacrylamide gel electrophoresis showed that it was composed of a single (150 kDa) band and N-terminal sequence analysis verified that it was intact mouse XOR. Isoelectric focusing showed that purified XOR was composed of three catalytically active, electrophoretic variants with pI values of 7.55, 7.65, and 7.70. The majority of the XOR activity in both pregnant and lactating mammary glands was shown to exist as NAD+-dependent dehydrogenase (XD form), while the enzyme in freshly obtained mouse milk exits as O2-dependent oxidase (XO form). The activity and protein levels of XOR selectively increased in mammary tissue during pregnancy and lactation. The time course of these increases was biphasic and correlated with the functional maturation of the mammary gland. These results indicate that XOR may have novel, mammary gland-specific functions, in addition to its role in purine metabolism.

Endothelial cells potentiate oxidant-mediated Kupffer cell phagocytic killing

Phagocytosis and killing of circulating organisms by Kupffer cells (KCs) are discrete, important components of host defense. However, the killing mechanism(s) are not fully understood, and the potential role of adjacent nonparenchymal cells such as hepatic endothelial cells has not been defined. Rat KCs -/+ an hepatic endothelial cell enriched cellular fraction (HECEF) were incubated with Candida parapsilosis and assayed for phagocytosis and phagocytic killing by validated fluorochromatic vital staining. The role of reactive oxygen metabolites in KC phagocytic functions was examined by inhibition with superoxide dismutase and/or catalase. Diphenyleneiodonium and allopurinol were used to examine the potential roles of NADPH oxidase and xanthine oxidase, respectively, in generating these toxic oxidants. Coculture with HECEF increased KC phagocytic activity (from 75% to 88%) and candidacidal activity (from 20% to 31%). Superoxide dismutase, catalase, diphenyleneiodonium, or allopurinol caused inhibition of candidacidal activity, but did not affect phagocytosis, and did not block the potentiation of phagocytosis or of killing caused by coculture with HECEF. Reactive oxygen intermediates generated by both NADPH oxidase and xanthine oxidase-dependent pathways are important in KC killing of Candida parapsilosis. In vitro, KC phagocytosis and killing are potentiated (via a non-oxidant-mediated mechanism) by coculture with a preparation of hepatic non-parenchymal cells composed primarily of endothelial cells.

Role of reactive oxygen metabolites in murine peritoneal macrophage phagocytosis and phagocytic killing

This study was designed to quantify the role of reactive oxygen metabolites (ROMs) in two distinct components of murine peritoneal macrophage activity, phagocytosis and killing, and to discriminate quantitatively the degree to which each component is dependent on NADPH oxidase and/or xanthine oxidase. A fluorochromatic vital staining technique was modified to simultaneously quantify phagocytosis and microbicidal activity of macrophages incubated with Candida parapsilosis targets. To determine the role of ROMs, macrophages were preincubated with free radical scavengers [superoxide dismutase (SOD) and/or catalase] or with selective inhibitors of xanthine oxidase (XO, e.g., allopurinol) or NADPH oxidase [diphenyleneiodonium (DPI)]. Phagocytosis was not affected by treatment of macrophages with SOD, catalase, allopurinol, or DPI. Candidacidal activity, however, was inhibited by SOD, allopurinol, or DPI. The inhibitory effects of DPI and allopurinol were additive. Histochemical and biochemical assays demonstrated substantial quantities of XO in murine peritoneal macrophages. The findings suggest that the generation of ROMs by XO- and NADPH oxidase-dependent pathways are each important for phagocytic killing by murine peritoneal macrophages.

Carboxy-terminal cytoplasmic domain of mouse butyrophilin specifically associates with a 150-kDa protein of mammary epithelial cells and milk fat globule membrane

A cDNA encoding mouse butyrophilin was obtained by reverse transcriptase-coupled polymerase chain reaction (RT-PCR) using poly (A)+ RNA from lactating mouse mammary gland as a template and screening a cDNA library with the RT-PCR-amplified fragment as a probe. DNA sequencing and computer analysis revealed that it has a rather long 3'-untranslated sequence and that the carboxy-terminal cytoplasmic domain was well conserved between mouse and bovine butyrophilins. To elucidate the biological function of butyrophilin, the cytoplasmic region expressed as fusion protein with glutathione S-transferase (GST) was purified and incubated with the cell lysate of mouse mammary epithelial cell lines, COMMA-ID and HC11. A 150-kDa protein was shown to specifically associate with the cytoplasmic domain and the protein increased in amount when the cells were treated with basal medium supplemented with lactogenic hormones such as prolactin, insulin and glucocorticoid. N-terminal amino acid sequencing indicated that the protein is xanthine dehydrogenase/oxidase which has been cloned from mouse liver. Further, the cytoplasmic domain also bound xanthine dehydrogenase/oxidase from bovine milk fat globule membrane. These results suggest that butyrophilin might be physiologically associated with xanthine dehydrogenase/oxidase and might function in a complex form in milk fat secretion.

A re-evaluation of the tissue distribution and physiology of xanthine oxidoreductase

Xanthine oxidoreductase is an enzyme which has the unusual property that it can exist in a dehydrogenase form which uses NAD+ and an oxidase form which uses oxygen as electron acceptor. Both forms have a high affinity for hypoxanthine and xanthine as substrates. In addition, conversion of one form to the other may occur under different conditions. The exact function of the enzyme is still unknown but it seems to play a role in purine catabolism, detoxification of xenobiotics and antioxidant capacity by producing urate. The oxidase form produces reactive oxygen species and, therefore, the enzyme is thought to be involved in various pathological processes such as tissue injury due to ischaemia followed by reperfusion, but its role is still a matter of debate. The present review summarizes information that has become available about the enzyme. Interpretations of contradictory findings are presented in order to reduce confusion that still exists with respect to the role of this enzyme in physiology and pathology.

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.

High levels of xanthine oxidoreductase in rat endothelial, epithelial and connective tissue cells

The localization of xanthine oxidoreductase activity was investigated in unfixed cryostat sections of various rat tissues by an enzyme histochemical method which specifically demonstrates both the dehydrogenase and oxidase forms of xanthine oxidoreductase. High activity was found in epithelial cells from skin, vagina, uterus, penis, liver, oral and nasal cavities, tongue, esophagus, fore-stomach and small intestine. In addition activity was demonstrated in sinusoidal cells of liver and adrenal cortex, endothelial cells in various organs and connective tissue fibroblasts. Xanthine oxidoreductase produces urate which is a scavenger of oxygen-derived radicals. Because the enzyme is found in epithelial and endothelial cells which are subject to relatively high oxidant stress, it is postulated that in these cells xanthine oxidoreductase is involved in the antioxidant enzyme defense system. In addition, a possible role for the enzyme in proliferation and differentiation processes is discussed.

Responses of vascular endothelial oxidant metabolism to lipopolysaccharide and tumor necrosis factor-alpha

Quantification of intracellular and extracellular levels and production rates of reactive oxygen species is crucial to understanding their contribution to tissue pathophysiology. We measured basal rates of oxidant production and the activity of xanthine oxidase, proposed to be a key source of O2- and H2O2, in endothelial cells. Then we examined the influence of tumor necrosis factor-alpha and lipopolysaccharide on endothelial cell oxidant metabolism, in response to the proposal that these inflammatory mediators initiate vascular injury in part by stimulating endothelial xanthine oxidase-mediated production of O2- and H2O2. We determined a basal intracellular H2O2 concentration of 32.8 +/- 10.7 pM in cultured bovine aortic endothelial cells by kinetic analysis of aminotriazole-mediated inactivation of endogenous catalase. Catalase activity was 5.72 +/- 1.61 U/mg cell protein and glutathione peroxidase activity was much lower, 8.13 +/- 3.79 mU/mg protein. Only 0.48 +/- 0.18% of total glucose metabolism occurred via the pentose phosphate pathway. The rate of extracellular H2O2 release was 75 +/- 12 pmol.min-1.mg cell protein-1. Intracellular xanthine dehydrogenase/oxidase activity determined by pterin oxidation was 2.32 +/- 0.75 microU/mg with 47.1 +/- 11.7% in the oxidase form. Intracellular purine levels of 1.19 +/- 1.04 nmol hypoxanthine/mg protein, 0.13 +/- 0.17 nmol xanthine/mg protein, and undetectable uric acid were consistent with a low activity of xanthine dehydrogenase/oxidase. Exposure of endothelial cells to 1000 U/ml tumor necrosis factor (TNF) or 1 microgram/ml lipopolysaccharide (LPS) for 1-12 h did not alter basal endothelial cell oxidant production or xanthine dehydrogenase/oxidase activity. These results do not support a casual role for H2O2 in the direct endothelial toxicity of TNF and LPS.

Free radical ablation for the prevention of post-ischemic renal failure following renal transplantation

The toxic metabolites of oxygen, including those which are free radicals, have been found to constitute a fundamental common pathway of tissue injury in a wide variety of disease processes, including injury in many organs resulting from post-ischemic reperfusion. Research efforts designed to prevent or ameliorate tissue injury have therefore centered on the pharmacologic inhibition of free radical-mediated mechanisms. This approach has particular application to post-ischemic renal failure seen in renal transplantation, after a well-defined period of graft ischemia, followed by reperfusion.

cDNA cloning of a mouse mammary epithelial cell surface protein reveals the existence of epidermal growth factor-like domains linked to factor VIII-like sequences

A 2.1-kilobase cDNA coding for a surface protein of mammary epithelial cells has been isolated from a mouse mammary gland lambda gt11 cDNA library. Sequence analysis of this cDNA reveals an open reading frame of 1389 base pairs that defines a protein with a molecular mass of 51.5 kDa. Structural analysis of the predicted sequence identifies two putative functional domains of the protein: (i) an N-terminal cysteine-rich region that is similar to epidermal growth factor-like domains of Drosophila Notch-1 protein and (ii) a large segment of the sequence that exhibited 54.5% identity with C-terminal domains of human coagulation factors VIII and V. These similarities in structure are used to predict the possible functions of the protein and its means of interaction with the cell surface. mRNA expression was detectable in mammary tissue from nonpregnant animals but was maximal in the lactating gland. In cultured cells, mRNA levels also correlated with the degree of cellular differentiation.

Xanthine oxidoreductase activity in perfused hearts of various species, including humans

Oxygen free radicals generated by xanthine oxidase have been implicated in cardiac damage. The activity of xanthine oxidase/reductase in adult rat heart is considerable. Its assay gives controversial results for other species, for example, rabbits and humans. Therefore, we perfused isolated hearts of various species, including explanted human hearts, to measure the conversion of exogenous hypoxanthine to xanthine and urate. We assayed these purines with high-performance liquid chromatography. The apparent xanthine oxidoreductase activities, calculated as release of xanthine plus 2x urate, were (milliunits per gram wet weight, mean +/- SEM) mice 33 +/- 3 (n = 5), rats 28.5 +/- 1.4 (n = 9), guinea pigs 14.4 +/- 1.0 (n = 5), rabbits 0.59 +/- 0.09 (n = 5), pigs less than 0.1 (n = 6), humans 0.31 +/- 0.04 (n = 7), and cows 3.7 +/- 0.8 (n = 4). In rabbit heart the conversion of hypoxanthine to xanthine was slow, and that of xanthine to urate was even slower. On the other hand, guinea pig and human heart released little xanthine, indicating that xanthine breakdown exceeds its formation. We conclude that isolated perfused mouse, rat, guinea pig, and also bovine hearts show considerable xanthine oxidoreductase activity, contrasting rabbit, porcine, and diseased human hearts.

Modulation of fibroblast proliferation by oxygen free radicals

The major unexplained phenomenon in fibrotic conditions is an increase in replicating fibroblasts. In this report we present evidence that oxygen free radicals can both stimulate and inhibit proliferation of cultured human fibroblasts, and that fibroblasts themselves release superoxide (O2.-) free radicals. Fibroblasts released O2.- in concentrations which stimulated proliferation, a finding confirmed by a dose-dependent inhibition of proliferation by free radical scavengers. Oxygen free radicals released by a host of agents may thus provide a very fast, specific and sensitive trigger for fibroblast proliferation. Prolonged stimulation may result in fibrosis, and agents which inhibit free radical release may have a role in the prevention of fibrosis.

Histochemical localization and possible antibacterial role of xanthine oxidase in the bovine mammary gland

Xanthine oxidase (XO) was demonstrated to be present in the teat canal and secretory tissue of the bovine mammary gland by histochemical techniques. Homogenates of these tissues were able to replace XO in an antibacterial assay with Streptococcus uberis. The action of XO on its substrate hypoxanthine was shown to provide an essential component for anti-streptococcal activity mediated by lactoperoxidase. A mechanism is proposed whereby the interaction of XO, lactoperoxidase and thiocyanate may provide antibacterial activity in the teat canal.

Lesch-Nyhan syndrome and its pathogenesis: purine concentrations in plasma and urine with metabolite profiles in CSF

Purine metabolism in the Lesch-Nyhan syndrome has been re-examined in 10 patients. Hypoxanthine and xanthine concentrations in plasma and CSF and urinary excretion have been studied, on and off allopurinol treatment, using high performance liquid chromatographic methods. Accumulation of the substrate, hypoxanthine, of the missing hypoxanthine guanine phosphoribosyltransferase (HPRT) enzyme, is more marked in urine and in CSF than in plasma. The greater increase in CSF is consistent with the most metabolically active tissue, brain, showing the most marked functional changes. The function of HPRT seems to be the recycling of hypoxanthine which is released from tissues in increasing quantities as energy use, ATP 'turnover', in the tissue increases. The existing screening method for HPRT deficiency, the ratio of the urinary concentration of urate to that of creatinine, shows overlap between the values in severe HPRT deficiency and in controls; this overlap is not found with a urinary hypoxanthine/creatinine molar concentration ratio.

Effect of allopurinol on ischemia and reperfusion-induced cerebral injury in spontaneously hypertensive rats

In spontaneously hypertensive rats, we studied the participation of xanthine oxidase-linked free radical in ischemia and reperfusion-induced cerebral injury, using allopurinol, a xanthine oxidase inhibitor. The loss of righting reflex was noted in some animals after a 4 hour occlusion of bilateral common carotid arteries and 19 of 25 animals died within 72 hours after reperfusion. One hour after reperfusion, the cerebral water content increased significantly, with an increase in sodium content and a decrease in potassium content. In 7 animals treated with oral administrations of allopurinol (200 mg/kg) 24 hours and 1 hour before occlusion, no death was found either during occlusion or after reperfusion, and the loss of righting reflex was noted in only one animal 24-72 hours following reperfusion. The increase in cerebral water content and accompanied changes in electrolyte contents were clearly prevented by allopurinol. These results suggest the possibility that the production of xanthine oxidase-linked free radical participates in cerebral injury due to ischemia and reperfusion in spontaneously hypertensive rats.

Purification and tissue-specific expression of casein kinase from the lactating guinea-pig mammary gland

A serine-specific casein kinase, an integral membrane protein of the lactating guinea-pig mammary gland, has been purified from a Golgi-enriched membrane fraction, using a combination of sucrose gradient centrifugation and chromatography on ATP-agarose. The enzyme comprises a polypeptide of estimated Mr 74 000 as judged by sodium dodecyl sulphate/polyacrylamide gel electrophoresis, compared with a monomer Mr of 50 000 as determined by sucrose gradient centrifugation in the presence of 500 mM NaCl and 0.1% Triton X-100. Kinetic studies show that the purified enzyme exhibits kinetic constants distinctly different from the rabbit reticulocyte casein kinases I and II, whilst polyclonal antisera raised against the mammary gland enzyme did not cross-react with soluble liver or reticulocyte protein kinase activities. Immunoblotting and immunocytochemical analyses demonstrate the mammary gland enzyme's apparently unique location in lactating mammary gland tissue. Comparative studies with polyclonal antisera raised against bovine galactosyltransferase, show that casein kinase and galactosyltransferase have a similar intracellular localisation in the lactating mammary gland as judged by immunocytochemistry at the light level, but that casein kinase was unique to mammary gland whereas galactosyltransferase could be found in other tissues. The results extended our earlier observations which suggest a Golgi location for casein kinase, and demonstrate that future studies using this enzyme may well prove advantageous for the study of intracellular mechanisms involved in the biogenesis of organelles, in this instance the Golgi apparatus.

High concentrations of antibodies to xanthine oxidase in human and animal sera. Molecular characterization

The widespread occurrence of antibodies (IgG) specific to xanthine oxidase in both normal (nonimmune) human and animal sera, and in antisera raised against a diversity of unrelated antigens is described. A study of sera from 81 humans revealed that xanthine oxidase-specific IgG represents a high proportion (1-8%) of total IgG. No obvious correlation to pathological events or symptoms of disease could be found. These xanthine oxidase-specific antibodies could be isolated by immunoaffinity chromatography on purified human or bovine xanthine oxidase and showed specific binding to the enzyme polypeptide of Mr 155,000 in immunoblotting experiments. By immunofluorescence microscopy they displayed the same cell type-specific reaction as experimentally induced antibodies, i.e., the staining of lactating mammary gland epithelium and capillary endothelium. The naturally occurring xanthine oxidase-specific antibodies consisted of polyclonal IgG of various subclasses. F(ab')2 preparations gave immune-reactions identical to those of IgG. The human xanthine oxidase-specific IgG cross-reacted with the bovine enzyme and both human and animal antibodies partially inhibited its activity. The xanthine oxidase activity of human milk lipid globules and supernatant fractions from various human tissues was extremely low when compared with that of the bovine antigen. The enzyme protein, however, was effectively precipitated from these sources by both the human and bovine antibodies. We suggest that the exceptionally high concentrations of antibodies against one protein, xanthine oxidase, are due to self-immunization to the xanthine oxidase antigen present in endothelial cells of capillaries. We do not exclude, however, nutritional contributions of bovine milk antigen to the appearance of xanthine oxidase antibodies in human sera. The possible biological functions of this immunological reaction are discussed.

Characterization of a monoclonal antibody to bovine xanthine oxidase

The isolation of a hybridoma cell line, C-41, secreting monoclonal antibody to bovine xanthine oxidase (EC 1.2.3.2), is described. The specificity of this antibody was determined by solid-phase immunoassay, immunoblotting procedures, affinity chromatography, immunoelectrophoresis and precipitation techniques. The results are compared with those obtained in similar specificity studies on a previously described monoclonal antibody secreted by hybridoma cell line A-94 [Mather, Nace, Johnson & Goldsby (1980) Biochem. J. 188, 925-928]. This latter antibody appears to bind to xanthine oxidase only when the enzyme is immobilized on a solid support such as a plastic plate or nitrocellulose paper. Potential problems in the determination of the specificity of monoclonal antibodies, especially towards membrane proteins of unknown biological activity, are discussed.

Protein synthesis in lactating guinea-pig mammary tissue perfused in vitro. II. Biogenesis of milk-fat-globule membrane proteins

Guinea-pig mammary tissue was perfused in vitro, radiolabelled with [35S]methionine and intracellular protein precursors of the milk-fat-globule membrane (FGM) recovered by immunoabsorption techniques. Labelled xanthine oxidase was solely detected in post-microsomal supernatants and butyrophilin in carbonate-washed membranes. A major glycoprotein (Gp 55), was initially present in a membrane-bound form, but after longer perfusion times a fraction of this protein was recovered in the post-microsomal supernatant. These results are discussed with reference to formation of the apically-derived FGM.