Urate Binding Research Articles

Water-soluble hexakis-imidazolium cages: synthesis and selective binding of sodium urate

Selective binding of uric acid, the molecule that causes gout, by imidazolium-incorporated molecular cages in water.

The structure of Deinococcus radiodurans transcriptional regulator HucR retold with the urate bound

HucR is a MarR family protein of Deinococcus radiodurans, which binds tightly to the intergenic region of HucR and the uricase gene to inhibit their expression. Urate (or uric acid) antagonizes the repressor function of HucR by binding to HucR to impede its association with the cognate DNA. The previously reported crystal structure of HucR was without the bound urate showing significant structural homology to other MarR structures. In this paper, we report the crystal structure of HucR determined with the urate bound. However, despite the fact that the urate is found at a site well-known to harbor ligands in other MarR family proteins, the overall HucR structure indicates that no significant change in structure takes place with the urate bound. Structure analysis further suggests that the urate interaction in HucR is mediated by histidine/glutamate side chains and ordered water molecules stabilized by various residues. Such interaction is quite unique compared to other known structural interactions between urate and its binding proteins. Furthermore, structural comparison of the apo- and the urate bound forms allows us to hypothesize that the Trp20-mediated water network in the apo-form stabilizes the proper HucR fold for cognate DNA binding, and that urate binding, also via Trp20, and the consequent reorganization of water molecules in the binding pocket, likely disrupts the DNA binding configuration to result in the attenuated DNA binding.

Large-scale whole-exome sequencing association studies identify rare functional variants influencing serum urate levels

Elevated serum urate levels can cause gout, an excruciating disease with suboptimal treatment. Previous GWAS identified common variants with modest effects on serum urate. Here we report large-scale whole-exome sequencing association studies of serum urate and kidney function among ≤19,517 European ancestry and African-American individuals. We identify aggregate associations of low-frequency damaging variants in the urate transporters SLC22A12 (URAT1; p = 1.3 × 10−56) and SLC2A9 (p = 4.5 × 10−7). Gout risk in rare SLC22A12 variant carriers is halved (OR = 0.5, p = 4.9 × 10−3). Selected rare variants in SLC22A12 are validated in transport studies, confirming three as loss-of-function (R325W, R405C, and T467M) and illustrating the therapeutic potential of the new URAT1-blocker lesinurad. In SLC2A9, mapping of rare variants of large effects onto the predicted protein structure reveals new residues that may affect urate binding. These findings provide new insights into the genetic architecture of serum urate, and highlight molecular targets in SLC22A12 and SLC2A9 for lowering serum urate and preventing gout.

Human Mutations in SLC2A9 (Glut9) Affect Transport Capacity for Urate.

SLC2A9 or Glut9 is a voltage sensitive urate transporter, mainly expressed in the kidneys, the liver, and the intestine. Human Glut9 loss-of-function mutations were identified in familial hypouricemia, and several single nucleotide polymorphisms (SNPs) were associated with lower serum urate, further indicating that Glut9 is a major determinant of serum uric acid level. To get insights in Glut9 transport characteristics, we systematically analyzed the function of known human Glut9 mutants using 14C-urate uptake assay and two-electrode voltage clamp (TEVC) in the Xenopus laevis oocyte expression system. Surface expression was assessed by immunostaining and biotinylation. We found decreased urate transport by flux studies for most of the variants. No variant was permissive for glucose transport. We could further differentiate two behaviors among the mutants: those harboring poor overall and cell-surface expression leading to low activity and those fully expressed at the cell surface, but presenting decreased activity. We studied the latter by TEVC and observed, in depolarized conditions, decreased inward currents measured in presence of 400 μM urate, partially reversed in 1 mM urate. In addition, we showed that C210F displays lower transport ability. By contrast, N333S showed decreased urate transport activity and urate affinity, suggesting that it may belong to the urate binding pocket. Systematic analysis of Glut9 mutants confirms Glut9 as putative target for the treatment of hyperuricemia and brings new insights in Glut9 structure – function.

Identification of Key Residues for Urate Specific Transport in Human Glucose Transporter 9 (hSLC2A9)

Human glucose transporter 9 (hSLC2A9) is critical in human urate homeostasis, for which very small deviations can lead to chronic or acute metabolic disorders. Human SLC2A9 is unique in that it transports hexoses as well as the organic anion, urate. This ability is in contrast to other homologous sugar transporters such as glucose transporters 1 and 5 (SLC2A1 & SLC2A5) and the xylose transporter (XylE), despite the fact that these transporters have similar protein structures. Our in silico substrate docking study has revealed that urate and fructose bind within the same binding pocket in hSLC2A9, yet with distinct orientations, and allowed us to identify novel residues for urate binding. Our functional studies confirmed that N429 is a key residue for both urate binding and transport. We have shown that cysteine residues, C181, C301 and C459 in hSLC2A9 are also essential elements for mediating urate transport. Additional data from chimæric protein analysis illustrated that transmembrane helix 7 of hSLC2A9 is necessary for urate transport but not sufficient to allow urate transport to be induced in glucose transporter 5 (hSLC2A5). These data indicate that urate transport in hSLC2A9 involves several structural elements rather than just a unique substrate binding pocket.

Histidine switch controlling pH-dependent protein folding and DNA binding in a transcription factor at the core of synthetic network devices.

Therapeutic strategies have been reported that depend on synthetic network devices in which a urate-sensing transcriptional regulator detects pathological levels of urate and triggers production or release of urate oxidase. The transcription factor involved, HucR, is a member of the multiple antibiotic resistance (MarR) protein family. We show that protonation of stacked histidine residues at the pivot point of long helices that form the scaffold of the dimer interface leads to reversible formation of a molten globule state and significantly attenuated DNA binding at physiological temperatures. We also show that binding of urate to symmetrical sites in each protein lobe is communicated via the dimer interface. This is the first demonstration of regulation of a MarR family transcription factor by pH-dependent interconversion between a molten globule and a compact folded state. Our data further suggest that HucR may be utilized in synthetic devices that depend on detection of pH changes.

Systematic Characterization of SLC2A9 (Glut9) Variants Associated with Serum Uric Acid Levels

Glut9 is a voltage sensitive urate transporter, mainly expressed in the kidney, the liver and the intestines. It constitutes a major determinant of serum uric acid levels and variants have been associated with hyperuricemia and hypouricemia. In particular, several Glut9 loss-of-function mutations were identified as responsible for familial hypouricemia type II in humans and hyperuricemia in dogs. Moreover, several single nucleotide polymorphisms have been associated with lower serum urate. To get insights in Glut9 transport characteristics, we analyzed the expression and function of each Glut9 variant. GLUT9 variants were prepared by site-directed mutagenesis and expressed in Xenopus laevis oocytes. Fourty-eight hours later the function of each individual variant was studied using (i) 14C-urate uptake (n=30, 3 batches) in presence of 400µM urate and (ii) two-electrodes voltage clamp in presence of 400µM and 1mM urate (n=15, 3 batches). Surface expression was assessed by immunostaining and biotinylation (n=3). We found decreased urate transport by flux studies for most of the variants. We could further differentiate two different behaviors among the variants: those harboring poor surface expression, and, more interestingly, those keeping full cell surface expression, but harboring decreased function. We further studied the latter by electrophysiology and observed decreased inward currents measured in presence of 400 µM urate, an observation that could be partially reversed in presence of 1mM urate. These results show that most of loss-of-function Glut9 variants are non-functional due to poor surface expression. We identified three variants involved in urate binding. Systematic analysis of loss-function Glut9 mutants brings insight in Glut9 structure – function.

Ligand-binding pocket bridges DNA-binding and dimerization domains of the urate-responsive MarR homologue MftR from Burkholderia thailandensis.

Members of the multiple antibiotic resistance regulator (MarR) family often regulate gene activity by responding to a specific ligand. In the absence of ligand, most MarR proteins function as repressors, while ligand binding causes attenuated DNA binding and therefore increased gene expression. Previously, we have shown that urate is a ligand for MftR (major facilitator transport regulator), which is encoded by the soil bacterium Burkholderia thailandensis. We show here that both mftR and the divergently oriented gene mftP encoding a major facilitator transport protein are upregulated in the presence of urate. MftR binds two cognate sites in the mftR-mftP intergenic region with equivalent affinity and sensitivity to urate. Mutagenesis of four conserved residues previously reported to be involved in urate binding to Deinococcus radiodurans HucR and Rhizobium radiobacter PecS significantly reduced protein stability and DNA binding affinity but not ligand binding. These data suggest that residues equivalent to those implicated in ligand binding to HucR and PecS serve structural roles and that MftR relies on distinct residues for ligand binding. MftR exhibits a two-step melting transition suggesting independent unfolding of the dimerization and DNA-binding regions; urate binding or mutations in the predicted ligand-binding sites result in one-step unfolding transitions. We suggest that MftR binds the ligand in a cleft between the DNA-binding lobes and the dimer interface but that the mechanism of ligand-mediated attenuation of DNA binding differs from that proposed for other urate-responsive MarR homologues. Since DNA binding by MftR is attenuated at 37 °C, our data also suggest that MftR responds to both ligand and a thermal upshift by attenuated DNA binding and upregulation of the genes under its control.

Differential regulation of hexameric and dodecameric hemocyanin from A. leptodactylus

The oxygen binding properties of hemocyanins are regulated on a short time scale by effectors such as l-lactate, urate and protons, and on longer time scales by expression of the different types of subunits. For Astacus leptodactylus it was shown previously that acclimation to higher temperatures leads to increased levels of a 6-meric hemocyanin species, whereas at lower temperatures the 12-meric form prevails. Here we show that the temperature dependence of the two forms supports the idea, that the maintenance of high affinity towards oxygen is the driving force for the differential expression of these hemocyanins. Furthermore, the two different types of hemocyanin differ not only in the affinity to oxygen, but also with respect to their interaction with l-lactate: while the 12-meric form displays a normal shift in oxygen affinity upon the addition of l-lactate this allosteric regulation is absent in the 6-meric form. Exclusive binding of l-lactate to the 12-meric form was supported by isothermal titration calorimetry. These results indicate that l-lactate binds either at the interface between the two hexamers or at subunit α′ which is responsible for the formation of the 12-mers and is not present in the 6-meric form. Urate has a comparable effect on the oxygen affinity of 6-meric and 12-meric forms and also binds to a similar extent to the oxygenated state as determined by isothermal titration calorimetry. Thus, urate and l-lactate do not seem to share the same binding sites. Interestingly, urate binding sites with no allosteric effect seem to exist, which is unusual. This article is part of a Special Issue entitled: Oxygen Binding and Sensing Proteins.

Urate Is a Ligand for the Transcriptional Regulator PecS

PecS is a member of the MarR ( multiple antibiotic resistance regulator) family, which has been shown in Erwinia to regulate the expression of virulence genes. MarR homologs typically bind a small molecule ligand, resulting in attenuated DNA binding. For PecS, the natural ligand has not been identified. We have previously shown that urate is a ligand for the Deinococcus radiodurans-encoded MarR homolog HucR ( hypothetical uri case regulator) and identified residues responsible for ligand binding. We show here that all four residues involved in urate binding and propagation of conformational changes to DNA recognition helices are conserved in PecS homologs, suggesting that urate is the ligand for PecS. Consistent with this prediction, Agrobacterium tumefaciens PecS specifically binds urate, and urate attenuates DNA binding in vitro. PecS binds two operator sites in the intergenic region between the divergent pecS gene and pecM genes, one of which features two partially overlapping repeats to which PecS binds as a dimer on opposite faces of the duplex. Notably, urate dissociates PecS from cognate DNA, allowing transcription of both genes in vivo. Taken together, our data show that urate is a ligand for PecS and suggest that urate serves a novel function in signaling the colonization of a host plant.

Urate Responsive MarR Homolog from Agrobacterium tumefaciens

MarR family transcriptional regulators are extensively studied for their involvement in modulating a vast number of metabolic and response pathways in bacteria and archaea. The natural ligands for many of the ligand responsive homologs of this family are elusive due to their low affinity or broad spectrum of selectivity. We have previously identified urate to be the natural ligand for HucR, a MarR homolog in Deinococcus radiodurans, and identified amino acid residues involved in binding of urate and the mechanism of eliciting DNA binding antagonism. Sequence analysis shows that a subset of MarR homologs conserves these residues in an all‐or‐none fashion suggesting that they would accept urate as their ligand. Using Agrobacterium tumefaciens‐encoded PecS as a model we show that these homologs specifically interact with urate and exhibit attenuated DNA binding as a result. PecS binds with sub‐nanomolar affinity to the intergenic region between pecS and pecM genes as a dimer of dimers on the same face of DNA and addition of urate dissociates them from the cognate DNA while allowing the transcription of both genes. Urate is produced as a byproduct of oxidative burst in plant immunity against invading bacteria. PecS response to urate may result in an upregulation of defence mechanisms and secretion of beneficial antioxidants to protect pathogenic bacteria. This work is supported by NSF (MCB‐0744240).

Mechanistic implications of urate interaction with transcriptional regulator HucR from Deinococcus radiodurans

The hypothetical uricase regulator (HucR) is a ligand‐responsive MarR family transcriptional regulator from D. radiodurans that regulates uricase expression. Regulation of uric acid levels may contribute to the bacterium's extreme resistance to reactive oxygen species. Molecular docking was used to predict the urate binding site in HucR. A site was predicted above the DNA recognition helices, and mutation of residues predicted to contact uric acid, R80, D73, and W20, shows that all three residues are required for DNA binding antagonism in presence of urate. Although DNA binding is not antagonized by urate, the R80 mutant shows Trp fluorescence quenching at 338 nm on urate binding, indicating ligand binding to the predicted site. In contrast, mutation of D73 abolishes fluorescence quenching with increasing urate concentrations. Other purines with the exception of xanthine have no effect on DNA binding affinity of HucR and display no concentration‐dependent fluorescence quenching. Based on these observations we propose that urate‐dependent perturbation of DNA binding by HucR may be due to the binding of N3‐deprotonated urate changing the orientation of D73 which in turn affects the disposition of the recognition helix due to altered interaction with R106. Due to high DNA and ligand binding affinity, HucR serves as a model to study mechanistic implications of ligand binding to MarR type transcriptional regulators.

Two types of urate binding sites on hemocyanin from the crayfish Astacus leptodactylus: an ITC study

The oxygen binding behaviour of hemocyanins from Crustacea is regulated by small organic compounds such as urate and l-lactate. We investigated the binding characteristics of urate and the related compound caffeine to the 2×6-meric hemocyanin of A. leptodactylus under fully oxygenated conditions employing isothermal titration calorimetry (ITC). An analysis of urate and caffeine binding based on a model of n identical binding sites resulted in approximately four binding sites for caffeine and eight for urate. This result suggests that the binding process for these effectors is more complex than this most simple model. Therefore, we introduced a number of alternative models. Displacement experiments helped to select the appropriate model. Based on these experiments, at least two different types of binding sites for urate and caffeine exist on the 2×6-meric hemocyanin of A. leptodactylus. The two binding sites differ strongly in their specificity towards the two analogues. It can be hypothesized that two different subunit types (β and γ) are responsible for the two types of binding sites.

Binding of urate and caffeine to haemocyanin analysed by isothermal titration calorimetry.

Haemocyanin serves as an oxygen carrier in the haemolymph of decapod crustaceans. The oxygen-binding behaviour of the pigment is modulated by the two major anaerobic metabolites, l-lactate and urate. The binding of these two metabolites to haemocyanin has been investigated mainly indirectly by following the effector-induced changes in the oxygen-binding properties of the respiratory pigment. Only a few direct investigations of effector binding, employing ultracentrifugation techniques and equilibrium dialysis, have been carried out. No evidence for cooperative binding for either effector was detected using these methods. However, isothermal titration calorimetry (ITC) offers a useful tool to gain additional insight into the binding of effectors to these highly allosterically regulated macromolecules. By applying the ITC method to the fully oxygenated dodecameric haemocyanin of the lobster Homarus vulgaris, cooperativity in binding has been found for the urate analogue caffeine but not for urate itself: using urate and the urate analogue caffeine as ligands, two conformations of the oxygenated pigment were detected.

Binding of Urate and Caffeine to Hemocyanin of the Lobster Homarus vulgaris (E.) As Studied by Isothermal Titration Calorimetry

Hemocyanin serves as an oxygen carrier in the hemolymph of the European lobster Homarus vulgaris. The oxygen binding behavior of the pigment is modulated by metabolic effectors such as lactate and urate. Urate and caffeine binding to 12-meric hemocyanin (H. vulgaris) was studied using isothermal titration calorimetry (ITC). Binding isotherms were determined for fully oxygenated hemocyanin between pH 7.55 and 8.15. No pH dependence of the binding parameters could be found for either effector. Since the magnitude of the Bohr effect depends on the urate concentration, the absence of any pH dependence of urate and caffeine binding to oxygenated hemocyanin suggests two conformations of the pigment under deoxygenated conditions. Urate binds to two identical binding sites (n = 2) each with a microscopic binding constant K of 8500 M(-1) and an enthalpy change DeltaH degrees of -32.3 kcal mol(-1). Caffeine binds cooperatively to hemocyanin with two microscopic binding constants: K(1) = 14 100 M(-1) and K(2) = 40 400 M(-1). The corresponding enthalpy changes in binding are as follows: DeltaH degrees (1) = -23.3 kcal mol(-1) and DeltaH degrees (2) = -27.1 kcal mol(-1). The comparison of urate and caffeine binding to the oxygenated pigment indicates the existence of two protein conformations for oxygen-saturated hemocyanin. Since effector binding is not influenced by protons, four different conformations are required to create a convincing explanation for caffeine and urate binding curves. This was predicted earlier on the basis of the analysis of oxygen binding to lobster hemocyanin, employing the nesting model.

Kinetic mechanism and cofactor content of soybean root nodule urate oxidase.

The kinetic mechanism of urate oxidase isolated from soybean root nodules has been determined by initial velocity kinetic studies monitoring oxygen uptake, in order to avoid potential artifacts in the spectrophotometric assay which arise from absorbance due to unidentified products of the enzymatic reaction. Urate and O2 bind to the enzyme sequentially; xanthine is a competitive inhibitor versus urate and a noncompetitive inhibitor versus O2, which suggests that urate binds to the enzyme before O2. This kinetic mechanism was confirmed by an 18O isotope-trapping experiment, which demonstrated that O2 does not bind productively to the enzyme in the absence of urate. The pH dependence of V and (V/K)urate reveal the presence of an ionizable residue on the enzyme with a pK of approximately 6.2, which must be unprotonated for the catalytic reaction to occur. The (V/K)O2 profile is pH independent; these data are accomodated by a model in which a unimolecular step intervenes between the binding of urate and O2. The pKi profile for 9-methylurate, a competitive inhibitor versus urate, is pH independent, confirming that the protonation state of the ionizable residue is not important for binding. The pKi profile for xanthine defines a pK of 7.4, which demonstrates that the monoanion of xanthine binds to the enzyme; by analogy, the monoanion of urate is predicted to be the substrate. The four isomeric N-methylurates were examined as potential inhibitors of urate oxidase. Only 9-methylurate showed significant inhibition suggesting that ionization at N9 of urate is not required for binding; it is proposed that the N3-deprotonated urate monoanion is the species which binds to urate oxidase. The gene encoding urate oxidase was cloned from soybeans and expressed in Escherichia coli. The metal content of the recombinant enzyme was examined by inductively coupled argon plasma emission spectroscopy, and only trace quantities of copper were found. The molecular mass of the protein was determined by MALDI-TOF mass spectrometry and found to be 35,059.8 Da. The calculated molecular mass of urate oxidase is 35,052 Da; therefore, these data suggest that there is no covalently bound cofactor in urate oxidase.

Urate transport in Homarus americanus hepatopancreas: studies on membrane vesicles and R cells

[2-14C]urate uptake was studied in hepatopancreatic basolateral membrane vesicles and in R cell suspensions of the American lobster by Millipore filtration techniques. Unspecific binding of urate to the vesicular membrane was 25.5 +/- 3.0% of equilibrium. Vesicular uptake showed a diffusional and a saturable component (Km) 0.37 +/- 0.04 mM and maximal velocity (Vmax) 16.5 +/- 1.2 pmol urate.mg protein-1.s-1). [2-14C]urate uptake was significantly trans-stimulated by urate. Purine analogues, probenecid, p-aminohippuric acid, pyrazinoic, and oxonic acid cis-inhibited urate transport. Urate uptake was not affected by Na+ or K+ transmembrane gradients but stimulated by 1 mM 2-oxoglutarate at the cis-side in Na(+)-containing media. Cellular urate uptake was inhibited by pyrazinoic acid. Uptake was saturable (Km 0.53 +/- 0.11 mM and Vmax 3.7 +/- 0.4 pmol urate.mg protein-1.s-1) and Na(+)-independent. However, 2-oxoglutarate stimulated uptake in Na(+)-containing media. These results suggest that urate uptake across the basolateral membrane occurs via a specific, Na(+)-independent transport system that may operate in the exchange mode accepting 2-oxoglutarate as countertransported substrate. In vivo, urate uptake thereby would be a tertiary active system driven by a 2-oxoglutarate gradient established across the cell membrane by the operation of a Na(+)-2-oxoglutarate cotransport system.

Allosteric Modulation of Haemocyanin Oxygen-Affinity by L-Lactate and Urate in the Lobster Homarus Vulgaris:II. Characterization of Specific Effector Binding Sites

ABSTRACT The haemocyanin of Homarus vulgaris possesses specific binding sites for L-lactate and urate, two allosteric modulators of haemocyanin oxygen-affinity. The affinities for both ligands have been determined. The dissociation constants, KD, are 0.87±0.26mmoll−1 for L-lactate and 0.03±0.01mmoll−1 for urate at 15°C and pH 8.0. The affinity of the haemocyanin is about 40 times larger for urate than for L-lactate. The stoichiometry of the binding is two ligands per dodecamer in both cases. L-Lactate does not compete with urate for its binding site and vice versa, indicating that the ligand binding sites are independent of each other. The specificity of urate binding to haemocyanin was investigated in competition experiments with allantoin, caffeine and hypoxanthine. The purine derivatives caffeine and hypoxanthine reduce the binding of urate to haemocyanin, whereas allantoin has no effect. Thus, the purine ring system seems to be essential for the binding of urate to haemocyanin.

Induction of profound hypouricemia by a non-sedating thiobarbiturate

5-[N-phenylcarboxamido]-2-thiobarbituric acid (merbarone) is a non-sedating derivative of thiobarbituric acid originally developed for anticancer use. In the initial clinical study, a profound reduction in serum uric acid was observed. In 20 patients who received five daily doses of merbarone ranging from 100 to 750 mg/m 2, serum uric acid concentration was reduced from a mean pretreatment value of 5.7 ± 1.6 mg/dL to a mean lowest value of 1.3 ± 0.5 mg/dL. In most patients, the onset of the effect occurred with 24 hours and was maximal by 48 to 72 hours. Metabolic studies in two patients showed an increase in urinary uric acid excretion within 24 hours after initiation of drug treatment. A marked increase in fractional excretion of uric acid was sustained throughout the period of drug treatment. Urinary excretion of total oxypurines (xanthine and hypoxanthine) was increased twofold to threefold relative to baseline levels. Ultrafiltration studies showed that merbarone did not significantly displace binding of urate from albumin. When merbarone was incubated with xanthine oxidase in vitro, several reaction products were observed, including 2-oxo-2-desthio-merbarone and a compound with retention time similar to 4′-OH-merbarone. Both of these compounds have been described previously as metabolites of merbarone in human subjects. The parent drug and both metabolites were found to inhibit xanthine oxidase (K i = 41, 36, and 240 μmol/L, respectively). However, this inhibitory effect was substantially less potent than allopurinol (K i = 0.025 μmol/L). This study indicates that merbarone induces profound hypouricemia primarily by increasing uric acid excretion. Since urate binding to plasma protein is not affected, uricosuria is probably caused by a direct drug action on the renal tubule. Oxidation of merbarone by xanthine oxidase may be an important mechanism for drug metabolism in humans. Merbarone and both of its major metabolites inhibit xanthine oxidase in vitro. However, the magnitude of the increase in urinary oxypurine excretion is considerably less than that reported for allopurinol. Since merbarone does not cause adverse reactions at doses which induce marked hypouricemia, current studies are evaluating merbarone as treatment for hyperuricemia in patients who are intolerant of allopurinol.

Urate binding globulin, interactions with immunoglobulins.

The presence of an urate binding to serum proteins may influence the deposition in joints, and the renal excretion of urate. So several authors have studied these interactions with different results. By use of an affinity chromatography procedure we demonstrated the presence of at least two different serum proteins which recognize urate: albumin and α-globulin (1). Application of this technique led to isolation and purification of the urate binding globulin from human serum (2). This protein, which migrates as α2 globulin is devoid of enzymatic activity, shows an apparent molecular weight of 70000 + 4000 Daltons and it has been partially characterized as previously reported (3). In this paper we describe the presence of an urate binding globulin-IgG complex in human plasma or plasma fraction III according to Cohn.