[19] Uric acid: Functions and determination

Host-Derived Cytotoxic Agents in Chronic Inflammation and Disease Progression

At inflammatory sites, cytotoxic agents are released and generated from invading immune cells and damaged tissue cells. The further fate of the inflammation highly depends on the presence of antagonizing principles that are able to inactivate these host-derived cytotoxic agents. As long as the affected tissues are well equipped with ready-to-use protective mechanisms, no damage by cytotoxic agents occurs and resolution of inflammation is initiated. However, long-lasting and severe immune responses can be associated with the decline, exhaustion, or inactivation of selected antagonizing principles. Hence, cytotoxic agents are only partially inactivated and contribute to damage of yet-unperturbed cells. Consequently, a chronic inflammatory process results. In this vicious circle of permanent cell destruction, not only novel cytotoxic elements but also novel alarmins and antigens are liberated from affected cells. In severe cases, very low protection leads to organ failure, sepsis, and septic shock. In this review, the major classes of host-derived cytotoxic agents (reactive species, oxidized heme proteins and free heme, transition metal ions, serine proteases, matrix metalloproteases, and pro-inflammatory peptides), their corresponding protective principles, and resulting implications on the pathogenesis of diseases are highlighted.

The Dual Role of Myeloperoxidase in Immune Response

The heme protein myeloperoxidase (MPO) is a major constituent of neutrophils. As a key mediator of the innate immune system, neutrophils are rapidly recruited to inflammatory sites, where they recognize, phagocytose, and inactivate foreign microorganisms. In the newly formed phagosomes, MPO is involved in the creation and maintenance of an alkaline milieu, which is optimal in combatting microbes. Myeloperoxidase is also a key component in neutrophil extracellular traps. These helpful properties are contrasted by the release of MPO and other neutrophil constituents from necrotic cells or as a result of frustrated phagocytosis. Although MPO is inactivated by the plasma protein ceruloplasmin, it can interact with negatively charged components of serum and the extracellular matrix. In cardiovascular diseases and many other disease scenarios, active MPO and MPO-modified targets are present in atherosclerotic lesions and other disease-specific locations. This implies an involvement of neutrophils, MPO, and other neutrophil products in pathogenesis mechanisms. This review critically reflects on the beneficial and harmful functions of MPO against the background of immune response.

Oxidative stress biomarkers in four Bloom syndrome (BS) patients and in their parents suggest in vivo redox abnormalities in BS phenotype

OBJECTIVE

To evaluate an association of Bloom syndrome (BS) phenotype with an in vivo prooxidant state.

METHODS

The following endpoints were measured in 4 BS patients, their 6 parents, and 78 controls: a) leukocyte and urinary 8-hydroxy-2'-deoxyguanosine (8-OHdG); b) blood glutathione (GSSG and GSH), c) plasma levels of some plasma antioxidants (uric acid, UA, ascorbic acid, AA, alpha- and gamma-tocopherol), and of glyoxal (Glx) and methylglyoxal (MGlx).

RESULTS

Leukocyte 8-OHdG levels were significantly increased in the 4 BS patients vs. 40 controls (p=0.04), while the urinary 8-OHdG levels were non-significantly increased in BS patients. Glutathione disulfide levels and GSSG/GSH ratio were significantly decreased in BS patients vs. 44 controls (p=0.02). The plasma levels of UA in BS patients were significantly increased vs. 24 controls (p=0.005). No significant alterations were found in the in the plasma levels of Glx, MGlx, AA, and tocopherol. No changes in the tested parameters were found in the BS heterozygotes.

CONCLUSION

This report shows a significant increase in oxidative DNA damage in leukocytes and in plasma UA levels from 4 BS patients. Should these data be confirmed in more extensive BS patient groups, an involvement of oxidative stress in the clinical BS phenotype might be suggested.

Inhibition by uric acid of free radicals that damage biological molecules

To clarify the antioxidative role of uric acid, its ability to scavenge carbon-centered and peroxyl radicals and its inhibitory effect on lipid peroxidation induced by various model systems were examined. Uric acid efficiently scavenged carbon-centered and peroxyl radicals derived from the hydrophilic free radical generator 2,2'-azobis-(2-amidinopropane)-dihydrochloride (AAPH). All damage to biological molecules, including protein, DNA and lipids induced by AAPH, was strongly prevented by uric acid. In contrast, alpha-tocopherol had little effect on damage to biological molecules. Lipid peroxidation by the lipophilic free radical generator 2,2'-azobis(2,4-dimethylvaleronitrile) (AMVN) was little inhibited by uric acid, but not by alpha-tocopherol. Copper-induced lipid peroxidation was inhibited by uric acid and alpha-tocopherol. NADPH- and ADP-Fe(3+)-dependent microsomal lipid peroxidation was efficiently inhibited by alpha-tocopherol, but not by uric acid. Uric acid seems to scavenge free radicals in hydrophilic conditions to inhibit lipid peroxidation on the lipid-aqueous boundary, and the antioxidation is only little in lipophilic conditions.

Autoantibody against oxidized low-density lipoproteins may be enhanced by cigarette smoking

A total of 59 healthy male subjects (32 smokers and 27 nonsmokers) who had no reported systemic disease and did not take alcohol and vitamin supplementation were included. The levels of autoantibody to oxidized low-density lipoproteins (ox-LDL) in smokers and age-matched nonsmokers were compared. The plasma levels of antioxidants that can affect the formation of ox-LDL were also measured, and correlation analyses between anti ox-LDL IgG and plasma antioxidants, controlling for age and body mass index (BMI), were performed. Plasma alpha-tocopherol and uric acid concentrations of nonsmokers (2.78+/-1.09 microg/mg total lipid and 6.96+/-1.69 mg/dl, respectively) were significantly higher than those of smokers (1.68+/-0.48 microg/mg total lipid and 6.15+/-1.14 mg/dl, respectively) (P<0.05). Although plasma ascorbate and retinol levels were not significantly different between smokers and nonsmokers, smokers older than 45 years old had significantly lower plasma ascorbate levels (0.32+/-0.17 mg/dl) than age-matched nonsmokers (0. 53+/-0.14 mg/dl) (P=0.036). Higher level of plasma anti ox-LDL IgG was noted in the group of smokers compared with nonsmokers (515+/-409 mU/ml vs. 407+/-268 mU/ml, respectively) under the statistic method of Chi-Square test (P=0.049). A significant negative correlation was found between plasma anti ox-LDL IgG and alpha-tocopherol in the combined population as well as in the smoker group (r=-0.26, p=0.047; r=-0.48, p=0.006; respectively). However, there was no correlation between plasma anti ox-LDL IgG and the levels of other antioxidants. These results suggest that reduced concentrations of alpha-tocopherol are associated with cigarette smoking. The significantly negative correlation between plasma anti ox-LDL IgG and alpha-tocopherol in the entire study population as well as in the smoker group suggests that plasma alpha-tocopherol may be partially effective if not totally at protecting LDL from oxidative damage caused by cigarette smoking and dietary supplementation with alpha-tocopherol may provide a protective effect against LDL oxidation, especially in smokers.

Vitamin E and heart disease: basic science to clinical intervention trials

A review is presented of studies on the effects of vitamin E on heart disease, studies encompassing basic science, animal studies, epidemiological and observational studies, and four intervention trials. The in vitro, cellular, and animal studies, which are impressive both in quantity and quality, leave no doubt that vitamin E, the most important fat-soluble antioxidant, protects animals against a variety of types of oxidative stress. The hypothesis that links vitamin E to the prevention of cardiovascular disease (CVD) postulates that the oxidation of unsaturated lipids in the low-density lipoprotein (LDL) particle initiates a complex sequence of events that leads to the development of atherosclerotic plaque. This hypothesis is supported by numerous studies in vitro, in animals, and in humans. There is some evidence that the ex vivo oxidizability of a subject's LDL is predictive of future heart events. This background in basic science and observational studies, coupled with the safety of vitamin E, led to the initiation of clinical intervention trials. The three trials that have been reported in detail are, on balance, supportive of the proposal that supplemental vitamin E can reduce the risk for heart disease, and the fourth trial, which has just been reported, showed small, but not statistically significant, benefits. Subgroup analyses of cohorts from the older three trials, as well as evidence from smaller trials, indicate that vitamin E provides protection against a number of medical conditions, including some that are indicative of atherosclerosis (such as intermittent claudication). Vitamin E supplementation also produces an improvement in the immune system and protection against diseases other than cardiovascular disease (such as prostate cancer). Vitamin E at the supplemental levels being used in the current trials, 100 to 800 IU/d, is safe, and there is little likelihood that increased risk will be found for those taking supplements. About one half of American cardiologists take supplemental vitamin E, about the same number as take aspirin. In fact, one study suggests that aspirin plus vitamin E is more effective than aspirin alone. There are a substantial number of trials involving vitamin E that are in progress. However, it is possible, or even likely, that each condition for which vitamin E provides benefit will have a unique dose-effect curve. Furthermore, different antioxidants appear to act synergistically, so supplementation with vitamin E might be more effective if combined with other micronutrients. It will be extremely difficult to do trials that adequately probe the dose-effect curve for vitamin E for each condition that it might affect, or to do studies of all the possible combinations of other micronutrients that might act with vitamin E to improve its effectiveness. Therefore, the scientific community must recognize that there never will be a time when the science is "complete." At some point, the weight of the scientific evidence must be judged adequate; although some may regard it as early to that judgement now, clearly we are very close. In view of the very low risk of reasonable supplementation with vitamin E, and the difficulty in obtaining more than about 30 IU/day from a balanced diet, some supplementation appears prudent now.

Ascorbate and urate are the strongest determinants of plasma antioxidative capacity and serum lipid resistance to oxidation in Finnish men

Copper-induced plasma lipoprotein oxidation resistance has usually been determined in separated low density lipoprotein (LDL) fractions, that do not contain water-soluble antioxidants present in blood plasma. The aim of this study was to find the main determinants of the measurements of copper-induced lipid oxidation resistance (lag time) in whole serum and plasma total peroxyl radical trapping capacity (TRAP) in a population sample of smoking (n = 25) or non-smoking (n = 26) middle aged men at high risk of cardiovascular diseases. Smokers had significantly lower plasma ascorbic acid values, but only slightly lower alpha-tocopherol, beta-carotene and serum urate values than non-smokers. Plasma ascorbic acid concentration explained 23.5% of the lag time variation (standardized regression coefficient beta = 0.48; P = 0.004) in smokers and 5.6% in non-smokers. Serum urate concentration was the strongest determinant of lag time in non-smokers (beta = 0.64, P < 0.001). In addition, serum albumin, lipid standardized alpha-tocopherol and serum high density lipoprotein (HDL) cholesterol entered the multivariate regression mode for lag time. For plasma TRAP, only urate and ascorbic acid entered the multivariate regression model. Lag times in serum and in isolated very low density lipoprotein (VLDL) and LDL fraction did not correlate, but the maximal rate of these reactions correlated significantly. These results confirm that lipid peroxidation resistance in serum or plasma are associated with ascorbic acid, urate, alpha-tocopherol, albumin and HDL concentrations. The measurement of lipid oxidation resistance in whole serum might be more physiological than in isolated lipoprotein fraction, as the effects of water-soluble antioxidants are not artificially removed.

Toxicity of oxygen from naturally occurring redox-active pro-oxidants

The survival of all aerobic life forms requires the ground-state of molecular oxygen, O2. However, the activation of O2 to reactive oxygen species (ROS) is responsible for universal toxicity. ROS are responsible in deleterious intracellular reactions associated with oxidative stress including membrane lipid peroxidation, and the oxidation of proteins and DNA. Redox-active allelochemicals such as quinones and phenolic compounds are involved in activating O2 to its deleterious forms including superoxide anion free radical, O2.-, hydrogen peroxide, H2O2, and hydroxyl radical, OH. Molecular oxygen is also activated in biologically relevant photosensitizing reactions to the singlet form, 1O2. The insect lifestyle exposes them to a broad diversity of pro-oxidant allelochemicals and, like mammalian species, they have developed an elaborate antioxidant system comprised of chemical antioxidants and a bank of anti-oxidant enzymes. We have found that an insect's antioxidant adaptation to a particular food correlates well with its risk of exposure to potential pro-oxidants.

Uric acid is reduced in the substantia nigra in Parkinson's disease: effect on dopamine oxidation

Postmortem caudate and substantia nigra tissue samples from human parkinsonian patients (PD) and age-matched controls (NC) were analyzed for uric acid (UA), dopamine (DA), and ascorbic acid (AA) by HPLC/UV/ED. Uric acid and DA levels were significantly lower in the substantia nigra of PD by 54% and 85%, respectively. In the caudate, DA levels were significantly lower while UA levels were nonsignificantly reduced (0.10 < p < 0.05). Ascorbic acid levels were not significantly different from the controls in either brain region. Conditions favorable for oxidative stress were evaluated by measuring the oxidation of DA in individual brain homogenates. The rate constant for DA oxidation in control caudate was 0.34 x 10(-2) min-1 and in parkinsonian caudate was 4.20 x 10(-2) min-1. In control and parkinsonian substantia nigra DA oxidation rate constants were 2.82 x 10(-2) min-1 and 4.57 x 10(-2) min-1, respectively. Addition of UA or catalase to parkinsonian homogenate decreased the rate of DA oxidation, while addition of uricase to control homogenate increased the rate of DA oxidation. The data support the hypothesis that UA is decreased in nigrostriatal dopamine neurons in parkinsonian patients which contributes to an environment susceptible to oxidative stress, particularly through dopamine oxidation reactions.

Interaction between allopurinol and copper: possible role in myocardial protection

Allopurinol, a potent inhibitor of xanthine oxidase, is known to effectively protect the heart against damage in patients undergoing cardiac bypass surgery. There is still an ambiguity concerning the presence of xanthine oxidase in the human heart. Thus, the mechanism underlying the protective effect of allopurinol is unclear. Transition metal ions, such as iron and copper, can participate in single-electron reactions and mediate the formation of oxygen-derived free radicals. In this study the interaction between allopurinol and Cu(II) was investigated. Spectrophotometric investigation shows that allopurinol (0-0.8 mM) form a 1:1 complex with Cu(II) ions (0-0.8 mM) with a specific absorbance peak at 364 nm. Also, the rate constant (k) for the copper-catalyzed aerobic oxidation of ascorbate was markedly decreased in the presence of allopurinol (from 0.068 min-1 to 0.014 min-1). Allopurinol substantially reduced the copper-mediated and ascorbate-driven DNA breakage. Spectrophotometric measurements did not indicate a specific interaction between iron ions and allopurinol. It is suggested that the beneficial effects of allopurinol during reperfusion of the heart could stem from its chelation of copper, yielding a complex with low redox activity.

Monitoring of oxidative free radical damage in vivo: analytical aspects

Free radical damage is an important factor in many pathological and toxicological processes. During the last decade a wide range of methods has been developed to determine free radical damage in various biological fluids and at various stages of development. This review offers an overview of the state of the art of monitoring free radical damage in vivo, with special emphasis on the analytical aspects of non-invasive methods.

Uric acid content of Drosophila decreases with aging

Free uric acid concentrations declined with aging in male Oregon R Drosophila melanogaster by 59% or more between 0 and 50 days of adult age. Free xanthine concentrations increased between 0 and 5 days of age and declined by 75% between 5 and 50 days of age. Xanthine oxidase activity was maximal for newly emerged flies and then declined rapidly reaching a minimum at 9 days of age. After 9 days of age xanthine concentrations may be the limiting factor for the production of uric acid by xanthine oxidase in aging fruit flies. Declining uric acid concentrations may represent a loss of antioxidant potential in aging Drosophila.

Mechanisms for regulating oxygen toxicity in phytophagous insects

The antioxidant enzymatic defense of insects for the regulation of oxygen toxicity was investigated. Insect species examined were lepidopterous larvae of the cabbage looper (Trichoplusia ni), southern armyworm (Spodoptera eridania), and black swallowtail (Papilio polyxenes). These phytophagous species are subject to both endogenous and exogenous sources of oxidative stress from toxic oxygen radicals, hydrogen peroxide (H2O2) and lipid peroxides (LOOH). In general, the constitutive levels of the enzymes superoxide dismutase (SOD), catalase (CAT), glutathione transferase (GT), and its peroxidase activity (GTpx), and glutathione reductase (GR), correlate well with natural feeding habits of these insects and their relative susceptibility to prooxidant plant allelochemicals, quercetin (a flavonoid), and xanthotoxin (a photoactive furanocoumarin). Induction of SOD activity which rapidly destroys superoxide radicals, appears to be the main response to dietary prooxidant exposure. A unique observation includes high constitutive activity of CAT and a broader subcellular distribution in all three insects than observed in most mammalian species. These attributes of CAT appear to be important in the prevention of excessive accumulation of cytotoxic H2O2. Unlike mammalian species, insects possess very low levels of a GPOX-like activity toward H2O2. Irrefutable proof that this activity is due to a selenium-dependent GPOX found in mammals, is lacking at this time. However, the activity of selenium-independent GTpx is unusually high in insects, suggesting that GTpx and not GPOX plays a prominent role in scavenging deleterious LOOHs. The GSSG generated from the GPOX and GTpx reactions may be reduced to GSH by GR activity. A key role of SOD in protecting insects from prooxidant toxicity was evident when its inhibition resulted in enhanced toxicity towards prooxidants. The role of antioxidant compounds in protecting these insects from toxic forms of oxygen has not been explored in depth. A major finding, however, is that these insects are lutein accumulators. Lutein is a dihydroxy (diol) derivative of beta-carotene, and it is a good quencher of activated forms of oxygen and free radicals. Levels of lutein are highest in P. polyxenes which specializes in feeding on prooxidant-containing plants.

Serum urate level and cancer

Histograms of the serum uric acid level of cancer patients (n = 501) and healthy controls (n = 15150) are compared. The statistical evaluation of the data yields significantly lower uric acid levels for cancer patients than for healthy persons (P less than 0.0001 for males, P less than 0.001 for females). These results support the theory that reactive species of oxygen (singlet oxygen and oxygen radicals) play an important role during carcinogenesis.

Photodynamic Production of Superoxide In Vitro by Altertoxins in the Presence of Reducing Agents

Superoxide production by the three 4,9-dihydroxyperylene-3,10-quinone fungal toxins, altertoxins I, II, and III, was stimulated on illumination with broad-spectrum light. As determined previously for cercosporin, superoxide production by illuminated altertoxins was increased by the addition of the reducing substances ergothioneine or urate; ascorbate also effectively increased superoxide production. Illuminated urate alone engendered some superoxide production.

How relevant is the reoxidation of ferrocytochrome c by hydrogen peroxide when determining superoxide anion production?

In a recent publication [(1987) FEBS Lett. 210, 195-198] the authors claim the use of cytochrome c to detect superoxide anion underestimates the real rate of superoxide anion formation on the basis that: (i) the rate of uric acid formation by xanthine oxidase is about 4-fold faster than the rate of cytochrome c reduction and (ii) hydrogen peroxide formed upon dismutation of the superoxide anion generated by xanthine oxidase is capable of reoxidizing ferrocytochrome c. That paper may have been misleading for readers not very familiar with the field of oxygen radicals, since both assumptions are, in fact, incorrect. In this report we demonstrate that the build up in concentration of H2O2 during most reactions in which superoxide anion is being produced is not enough to affect the rate of cytochrome c reduction. Our results suggest that the authors may have been misled by an artifact due to exposure of the samples containing H2O2 to UV light, which generates hydroxyl radicals by photolysis.

Role of hydrogen peroxide in the cytotoxicity of the xanthine/xanthine oxidase system

1. The survival of mammalian epithelial cells exposed in vitro to the xanthine/xanthine oxidase system in phosphate-buffered saline (PBS) or serum-containing medium (SCMEM) was investigated. 2. The cytotoxic effect observed depended on the composition of the medium in which the enzymic reaction was carried out; a surviving fraction of 5 x 10(-5) was found for cells exposed in PBS and 5.2 x 10(-1) for those in SCMEM. 3. The cytotoxic product(s) formed by the xanthine/xanthine oxidase system was relatively stable in PBS; survival of cells incubated after completion of the enzymic reaction was always less than that found for cells exposed during the reaction in the same system. 4. Superoxide dismutase or mannitol present during the enzymic reaction did not inhibit the cytotoxic effect. 5. NaN3 (a single-oxygen quencher and a catalase inhibitor) added to the system in SCMEM caused a reduction in survival to the level observed for cells exposed to the enzymic reaction in PBS. 6. Catalase completely protected cells, but no protection was observed when both catalase and NaN3 were present in the reaction mixture. 7. A similar cytotoxic effect was produced when cells were treated with H2O2 alone. 8. The rate of H2O2 decomposition in medium was accelerated by the presence of serum, but this was completely inhibited by NaN3. 9. It is concluded that H2O2 is the major cytotoxic product formed by the xanthine/xanthine oxidase system.

Measurement of allantoin and uric acid in human body fluids. A potential index of free-radical reactions in vivo?

Free-radical attack upon uric acid generates allantoin [Ames, Cathcart, Schwiers & Hochstein (1981) Proc. Natl. Acad. Sci. U.S.A. 78, 6858-6862]. Methods are described for the accurate measurement of uric acid and allantoin in human body fluids. The concentrations of uric acid and allantoin in human serum and synovial fluid are reported. It is suggested that measurement of changes in allantoin concentration may be a useful index of free-radical reactions taking place in vivo.

Release of iron from ferritin by xanthine oxidase. Role of the superoxide radical

Mobilization of iron from ferritin by xanthine oxidase was studied under aerobic and anaerobic conditions. Aerobic iron release amounted to approx. 3.7 nmol/ml in 10 min. This amount was decreased by approx. 30% under anaerobic conditions. Aerobic iron mobilization involved two mechanisms. About 70% was released by O2.- generated by xanthine oxidase. The rest was released by O2(.-)-independent mechanisms, which also accounted for the total iron release when O2 was absent. A possible transfer of reducing equivalents directly from xanthine oxidase to ferritin is discussed. The results imply that, in pathological conditions with increased formation of O2.-, iron may be released from ferritin. Furthermore, in hypoxic tissues xanthine oxidase can release iron from ferritin by an O2(.-)-independent process. Free iron is liable to catalyse the formation of the extremely reactive and damaging OH. radical.

Uric acid-iron ion complexes. A new aspect of the antioxidant functions of uric acid

In order to survive in an oxygen environment, aerobic organisms have developed numerous mechanisms to protect against oxygen radicals and singlet oxygen. One such mechanism, which appears to have attained particular significance during primate evolution, is the direct scavenging of oxygen radicals, singlet oxygen, oxo-haem oxidants and hydroperoxyl radicals by uric acid. In the present paper we demonstrate that another important 'antioxidant' property of uric acid is the ability to form stable co-ordination complexes with iron ions. Formation of urate-Fe3+ complexes dramatically inhibits Fe3+-catalysed ascorbate oxidation, as well as lipid peroxidation in liposomes and rat liver microsomal fraction. In contrast with antioxidant scavenger reactions, the inhibition of ascorbate oxidation and lipid peroxidation provided by urate's ability to bind iron ions does not involve urate oxidation. Association constants (Ka) for urate-iron ion complexes were determined by fluorescence-quenching techniques. The Ka for a 1:1 urate-Fe3+ complex was found to be 2.4 X 10(5), whereas the Ka for a 1:1 urate-Fe2+ complex was determined to be 1.9 X 10(4). Our experiments also revealed that urate can form a 2:1 complex with Fe3+ with an association constant for the second urate molecule (K'a) of approx. 4.5 X 10(5). From these data we estimate an overall stability constant (Ks approximately equal to Ka X K'a) for urate-Fe3+ complexes of approx. 1.1 X 10(11). Polarographic measurements revealed that (upon binding) urate decreases the reduction potential for the Fe2+/Fe3+ half-reaction from -0.77 V to -0.67 V. Thus urate slightly diminishes the oxidizing potential of Fe3+. The present results provide a mechanistic explanation for our previous report that urate protects ascorbate from oxidation in human blood. The almost saturating concentration of urate normally found in human plasma (up to 0.6 mM) represents 5-10 times the plasma ascorbate concentration, and is orders of magnitude higher than the 'free' iron ion concentration. These considerations point to the physiological significance of our findings.

Formation of hydroxyl radicals from NADH and NADPH in the presence of copper salts

The interaction of copper salts with NADH or NADPH in the presence of hydrogen peroxide at physiological pH is shown to produce hydroxyl radicals. The physiological significance of these results is discussed.

Conservation of vitamin C by uric acid in blood

Urate at physiological concentrations increased the stability of ascorbate in human serum approximately fivefold. These measurements were made by depleting human serum of urate with uricase. In model experiments (using phosphate buffer instead of serum), urate protected against iron-induced ascorbate oxidation and, to a lesser extent, pH-induced ascorbate oxidation. In both human serum and in phosphate buffer, urate exerted its protective effect without itself undergoing measurable oxidation as determined by spectrophotometric and high pressure liquid chromatography (HPLC) techniques. These experiments suggest an important physiological role for urate in preserving ascorbate in blood and other biological fluids.