Thermodynamics and stoicheiometry of the binding of substrate analogues to uricase

Rationalization design, soluble expression and PEG modification of highly active recombinant human-porcine uricase mutant protein

Uric acid is the end product of purine metabolism in humans due to inactivation of the uricase determined by the mutated uricase gene. Uricase catalyzes the conversion of uric acid into water-soluble allantoin that is easily excreted by the kidneys. Hyperuricemia occurs when the serum concentration of uric acid exceeds its solubility (7 mg/dL). However, modifications to improve the uricase activity is under development for treating the hyperuricemia. Here we designed 7 types of human-porcine chimeric uricase by multiple sequence comparisons and targeted mutagenesis. An optimal human-porcine chimeric uricase mutant (uricase-10) with both high activity (6.33 U/mg) and high homology (91.45 %) was determined by enzyme activity measurement. The engineering uricase was further modified with PEGylation to improve the stability of recombinant protein drugs and reduce immunogenicity, uricase-10 could be more suitable for the treatment of gout and hyperuricemia theoretically.

Enhancement of Thermostability of Aspergillus flavus Urate Oxidase by Immobilization on the Ni-Based Magnetic Metal-Organic Framework

The improvement in the enzyme activity of Aspergillus flavus urate oxidase (Uox) was attained by immobilizing it on the surface of a Ni-based magnetic metal–organic framework (NimMOF) nanomaterial; physicochemical properties of NimMOF and its application as an enzyme stabilizing support were evaluated, which revealed a significant improvement in its stability upon immobilization on NimMOF (Uox@NimMOF). It was affirmed that while the free Uox enzyme lost almost all of its activity at ~40–45 °C, the immobilized Uox@NimMOF retained around 60% of its original activity, even retaining significant activity at 70 °C. The activation energy (Ea) of the enzyme was calculated to be ~58.81 kJ mol⁻¹ after stabilization, which is approximately half of the naked Uox enzyme. Furthermore, the external spectroscopy showed that the MOF nanomaterials can be coated by hydrophobic areas of the Uox enzyme, and the immobilized enzyme was active over a broad range of pH and temperatures, which bodes well for the thermal and long-term stability of the immobilized Uox on NimMOF.

"Resurrected" human-source urate oxidase with high uricolytic activity and stability

As evidences showed that UOX(Gene ID: 391051), a single pseudogene formed after multiple mutations during human evolution, could be transcribed to mature mRNA and translated to two short peptides, we hypothesized that urate oxidase with higher homology with deduced human urate oxidase (dHU) might have lower immunogenicity. In this work, we constructed a "resurrected" human-source urate oxidase (rHU19) based on dHU. It obtained better uricolytic activity (8.29 U/mg) and catalytic efficiency (3.32 s^-1 μM^-1) compared with wild porcine urate oxidase (wPU) and FDA-approved porcine-baboon chimera (PBC). Maintaining high homology with dHU (93.75 %), rHU19 could be more suitable for the treatment of gout and hyperuricemia theoretically.

Enzyme Replacement Therapy for Genetic Disorders Associated with Enzyme Deficiency

Mutations in human genes might lead to loss of functional proteins, causing diseases. Among these genetic disorders, a large class is associated with the deficiency in metabolic enzymes, resulting in both an increase in the concentration of substrates and a loss in the metabolites produced by the catalyzed reactions. The identification of therapeutic actions based on small molecules represents a challenge to medicinal chemists because the target is missing. Alternative approaches are biology-based, ranging from gene and stem cell therapy, CRISPR/Cas9 technology, distinct types of RNAs, and enzyme replacement therapy (ERT). This review will focus on the latter approach that since the 1990s has been successfully applied to cure many rare diseases, most of them being lysosomal storage diseases or metabolic diseases. So far, a dozen enzymes have been approved by FDA/EMA for lysosome storage disorders and only a few for metabolic diseases. Enzymes for replacement therapy are mainly produced in mammalian cells and some in plant cells and yeasts and are further processed to obtain active, highly bioavailable, less degradable products. Issues still under investigation for the increase in ERT efficacy are the optimization of enzymes interaction with cell membrane and internalization, the reduction in immunogenicity, and the overcoming of blood-brain barrier limitations when neuronal cells need to be targeted. Overall, ERT has demonstrated its efficacy and safety in the treatment of many genetic rare diseases, both saving newborn lives and improving patients' life quality, and represents a very successful example of targeted biologics.

Expression, localization and metabolic function of "resurrected" human urate oxidase in human hepatocytes

A high serum uric acid (SUA) concentration is associated with hyperuricemia (HUA) and gout. In order to obtain long-acting therapeutic effect, correction of purine metabolism at genetic level is advantageous. For this purpose, we expressed three "human-like" urate oxidases in human hepatocytes (HL-7702) by lentivirus-mediated transduction. Enzymatic assay revealed that the recombinant urate oxidases expressed in HL-7702 cells were functionally active. Electron microscopy study showed that the recombinant enzymes were localized to peroxisome and formed distinct crystalloid core structures as in other mammal cells. Although similar rate of uric acid degradation was observed for all recombinant urate oxidases, HL-7702-pLVX-UOX₈₃ cells and HL-7702-pLVX-UOX_214/217 cells retained more cell viability compared with HL-7702-pLVX-UOX_PBC at high uric acid level. This study provides a new direction for the treatment of gout and hyperuricemia.

Development of Therapeutic Chimeric Uricase by Exon Replacement/Restoration and Site-Directed Mutagenesis

The activity of urate oxidase was lost during hominoid evolution, resulting in high susceptibility to hyperuricemia and gout in humans. In order to develop a more “human-like” uricase for therapeutic use, exon replacement/restoration and site-directed mutagenesis were performed to obtain porcine–human uricase with higher homology to deduced human uricase (dHU) and increased uricolytic activity. In an exon replacement study, substitution of exon 6 in wild porcine uricase (wPU) gene with corresponding exon in dhu totally abolished its activity. Substitutions of exon 5, 3, and 1–2 led to 85%, 60%, and 45% loss of activity, respectively. However, replacement of exon 4 and 7–8 did not significantly change the enzyme activity. When exon 5, 6, and 3 in dhu were replaced by their counterparts in wpu, the resulting chimera H_1-2P₃H₄P_5-6H_7-8 was active, but only about 28% of wPU. Multiple sequence alignment and homology modeling predicted that mutations of E24D and E83G in H_1-2P₃H₄P_5-6H_7-8 were favorable for further increase of its activity. After site-directed mutagenesis, H_1-2P₃H₄P_5-6H_7-8 (E24D & E83G) with increased homology (91.45%) with dHU and higher activity and catalytic efficiency than the FDA-approved porcine–baboon chimera (PBC) was obtained. It showed optimum activity at pH 8.5 and 35 °C and was stable in a pH range of 6.5–11.0 and temperature range of 20–40 °C.

DNA shuffling of uricase gene leads to a more "human like" chimeric uricase with increased uricolytic activity

Urate oxidase (Uox) is the enzyme involved in purine metabolism. Pseudogenization of Uox gene is the underlying mechanism of hyperuricemia and gout in human. Although Uox from various microorganisms has been used in clinical practice for many years, its application is limited by potential immunogenicity. In order to develop a more "human like" uricase, DNA shuffling was used to create chimeric uricase with both improved enzymatic activity and increased homology with deduced human uricase (dHU) gene. By using wild porcine uricase (wPU) gene and dhu as parental genes, a diverse chimeric library was generated. After preliminary screening by a "homebrew" high throughput protocol, approximately 100 chimeras with relatively high enzymatic activity were obtained. By further activity comparison of the purified enzymes, chimera-62 with increase in both activity and homology with dHU compared with wPU was selected. Its Km and catalytic efficiency were determined as 9.43±0.04μM and 2.67s(-1)μM(-1) respectively. There were 33 amino acid substitutions in chimera-62 when compared with dHU and 5 substitutions when compared with wPU. By homology modeling and 3-D structure analysis, it was speculated that mutations G248S and L266F contributed to the increased activity of chimera-62 by increasing the stability of α-helix and surface polarity respectively.

Thermal inactivation of uricase (urate oxidase): mechanism and effects of additives

Uricase (Urc) is an oxidoreductase enzyme of both general and commercial interest, the former because of its lack of a cofactor and the latter because of its use in the treatment of hyperuricemic disorders. Results of fluorometry and circular dichroism (CD) spectroscopy indicate that the main phase of thermal Urc inactivation follows an irreversible two-state mechanism, with loss of ~20% of the helical structure, loss of the majority of the tertiary structure, and partial exposure of tryptophan residues to solution being approximately concurrent with activity loss. Results of size exclusion chromatography and 8-anilinonaphthalene-1-sulfonate binding studies confirm that this process results in the formation of aggregated molten globules. In addition to this process, CD studies indicate the presence of a rapid reversible denaturation phase that is not completely coupled to the main phase. Urc inactivation is inhibited by the presence of glycerol and trimethylamine oxide, stabilizers of hydrophobic interactions and backbone structure respectively, confirming that loss of hydrophobic bonding and loss of helical structure are key events in the loss of Urc activity. NaCl, however, destabilizes the enzyme at elevated temperature, emphasizing the importance of ionic interactions to Urc stability. A model is developed in which interfacial disruption, involving local loss of hydrophobic interactions, ionic bonds, and helical structure, leads to Urc inactivation and aggregation. Additional studies of Urc inactivation at a more ambient temperature indicate that the inactivation process followed under such conditions is different from that followed at higher temperatures, highlighting the limitations of high-temperature enzyme stability studies.

Impact of large aggregated uricases and PEG diol on accelerated blood clearance of PEGylated canine uricase

Background

Uricase has proven therapeutic value in treating hyperuricemia but sufficient reduction of its immunogenicity may be the largest obstacle to its chronic use. In this study, canine uricase was modified with 5 kDa mPEG-SPA and the impact of large aggregated uricases and cross-linked conjugates induced by difunctional PEG diol on immunogenicity was investigated.

Methods and Findings

Recombinant canine uricase was first expressed and purified to homogeneity. Source 15Q anion-exchange chromatography was used to separate tetrameric and aggregated uricase prior to pegylation, while DEAE anion-exchange chromatography was used to remove Di-acid PEG (precursor of PEG diol) from unfractionated 5 kDa mPEG-propionic acid. Tetrameric and aggregated uricases were separately modified with the purified mPEG-SPA. In addition, tetrameric uricases was modified with unfractionated mPEG-SPA, resulting in three types of 5 kDa mPEG-SPA modified uricase. The conjugate size was evaluated by dynamic light scattering and transmission electron microscope. The influence of differently PEGylated uricases on pharmacokinetics and immunogenicity were evaluated in vivo. The accelerated blood clearance (ABC) phenomenon previously identified for PEGylated liposomes occurred in rats injected with PEGylated uricase aggregates. Anti-PEG IgM antibodies, rather than neutralizing antibodies, were found to mediate the ABC.

Conclusions

The size of conjugates is important for triggering such phenomena and we speculate that 40–60 nm is the lower size limit that can trigger ABC. Removal of the uricase aggregates and the PEG diol contaminant and modifying with small PEG reagents enabled ABC to be successfully avoided and sufficient reduction in the immunogenicity of 5 kDa mPEG-modified tetrameric canine uricase.

Structure-based characterization of canine-human chimeric uricases and its evolutionary implications

Uricase was lost in hominoids during primate evolution, but the inactivation mechanism remains controversial. To investigate the inactivation process of hominoid uricase, chimeric constructions between canine and human uricase were employed to screen the target regions that may contain labile or inactivated mutations in deduced human uricase. Four chimeric uricases were constructed and showed different enzymatic characteristics. Homology modeling, rational site-directed mutagenesis and DNA alignment were used to analyze the changes. Arg119 is conserved in functional mammalian uricases and its side-chains are crucial in maintaining the stability of the β-barrel core. A single CGT (Arg) to CAT (His) mutation at codon 119 that is shared by the human and great ape clade greatly reduces this stability and could cause the loss of uricase activity. We speculate that this missense mutation occurred first and inactivated the uricase protein in humans and great apes and that later the known nonsense mutation at codon 33 occurred and silenced the uricase gene. A single GTC (Val) to GCC (Ala) mutation at codon 296 in canine uricase is regarded as deleterious structural mutation, but such kinds of deleterious mutations have been widely accumulated in extant mammalian uricases. We speculate that a reduction in uricase activity has been an evolutionary tendency in mammals. Moreover, from structure-activity analysis of helix 2 in ancestral primate uricase, we suggest that before the inactivation of hominoid uricase, deleterious structural evolutionary changes had occurred in ancestral primates. The loss of hominoid uricase should be caused by progressive multistep mutations rather than a single mutation event.

Classification of difference between inhibition constants of an inhibitor to facilitate identifying the inhibition type

To identify the common inhibition types, the putative decision system is unsatisfactory. In a new decision system, Michaelis-Menten constants and maximal reaction rates were plotted versus inhibitor concentrations for deriving K(ik) and K(iv) as the inhibition constants, respectively; their difference was quantified as the ratio of the larger one to the smaller one. Such ratios below 2.0 suggested uncompetitive inhibitors, over 5.0 suggested noncompetitive or competitive inhibitors, and from 2.0 to 5.0 suggested mixed inhibitors. By the new decision system, (i) the simulation recovery of uncompetitive inhibitors under CVs of 2% or 5% was improved by four times, but that of competitive or noncompetitive inhibitors was improved slightly; (ii) the recovery of L-phenylalanine as an uncompetitive inhibitor of intestinal alkaline phosphatase reached 38%, while the putative decision system lost all; the recovery of xanthine as a competitive inhibitor of uricase was improved slightly. Therefore, the new decision system was better.

Strengthening intersubunit hydrogen bonds for enhanced stability of recombinant urate oxidase from Aspergillus flavus: molecular simulations and experimental validation

The aim of this study was to obtain molecular insight into the deactivation of recombinant urate oxidase (uricase, UOX, EC 1.7.3.3) (rUOX) from Aspergillus flavus. The enzyme is a tunnel-shaped homotetramer and has important clinical applications. By means of molecular dynamics simulations, multidimensional structural characterization and enzyme activity assays, we concluded that the thermal deactivation of UOX at neutral pH was associated with the loss of intersubunit hydrogen (H) bonds. This mechanism could also explain the deactivation of dilute aqueous UOX. Thermal deactivation of aqueous UOX due to dissociation of its subunits was ruled out. Displacement of H(2)O from the surface of UOX by less polar solvents such as methanol and dimethyl sulfoxide (DMSO) was proposed as an approach for strengthening intersubunit H bonds and consequently UOX stability. The effectiveness of this method was validated by both in silico and in vitro experiments. The results mentioned above provide insights for improving the stability of UOX and extending its applications. They may also be helpful for understanding the properties of other multimeric proteins.

Study of pH and temperature-induced transitions in urate oxidase (Uox-EC1.7.3.3) by microcalorimetry (DSC), size exclusion chromatography (SEC) and enzymatic activity experiments

Purified recombinant urate oxidase (urate oxygen oxidoreductase EC 1.7.3.3. re-Uox) has been studied by means of differential scanning calorimetry (DSC) in correlation with enzymatic activity measurements and size exclusion chromatography. Differential scanning calorimetry curves versus pH show two endothermal effects in the pH range 6-10. The first endotherm reveals a maximum stability between pH 7.25 and pH 9.5 corresponding to a temperature of transition T(m1) of 49.0 degrees C and an enthalpy of transition of 326 kJ mol(-1). This value dramatically decreases below pH 7.25. The behavior of the second endotherm is more complex but the temperature of transition T(m2) is constant between pH 9 and 7.25 and a maximum for the corresponding enthalpy is obtained near pH 8 with DeltaH(2)=272 kJ mol(-1). An optimal pH of 8.0 for the stability of the enzymatic activity at elevated temperature was also found which was in good agreement with calorimetric results. Reversibility of the first endotherm is obtained from 20 to 51.5 degrees C. The calorimetric result is correlated to enzymatic activity, purity by size exclusion chromatography (SEC) and protein concentration measurements. In contrast, for the second endotherm, after heating up to 68.9 degrees C, no reversibility was found. Interaction with structural analogues of urate has been studied by DSC. 8-Azahyooxanthine has only a small effect and caffeine has no effect at all. With 8-azaxanthine, a rapid increase of the T(m1) function of the concentration is obtained. At high concentration T(m1) reached the T(m2) value which remained unaffected.

Cloning, sequence analysis and expression in Escherichia coli of the gene encoding a uricase from the yeast-like symbiont of the brown planthopper, Nilaparvata lugens

A urate oxidase (uricase; EC 1.7.3.3) gene of the yeast-like fungal endosymbiont of the brown planthopper, Nilaparvata lugens, was cloned, and sequenced together with its flanking regions. The gene comprised a open reading frame of 987 bp, that was split into two parts by a single 96 bp intron. The encoded uricase was 296 amino acids with 62% sequence identity with that of Aspergillus flavus. The molecular weight deduced was 32,882, and the predicted isoelectric point was 6.06. The symbiont's uricase conserved all the known consensus motifs, except the C-terminal PTS-1, Ser-basic-Leu. The leucine at the third position of PTS-1 was replaced by serine in the C-terminus of the symbiont's uricase. The symbiont's uricase gene was successfully expressed in Escherichia coli, and the product, tagged with histidine residues, was purified. The symbiont's uricase, thus produced, was as active as those from plants and animals, but less active than those from other fungi.

Development of a two-site immunoassay of recombinant urate oxidase (SR 29142) and its use for determination of pharmacokinetic parameters in rats and baboons

Two monoclonal antibodies (Mabs), 12C7 and 11G11, both directed against recombinant urate oxidase (SR 29142), were selected for their epitope specificity to develop a two-site immunoassay of urate oxidase in plasma. A quantitative recovery of urate oxidase in plasma was obtained at all the concentrations tested, and the limit of quantification was found to be 0.5 ng/mL. Intra-and interassay coefficients of variation ranged from 1.2 to 6.7% and from 3.5 to 10.8%, respectively. The specificity of the two antibodies was studied in Western-blot experiments. This assay was used successfully to determine urate oxidase pharmacokinetic parameters after intravenous injection in rats and baboons. In these two species, urate oxidase pharmacokinetics was characterized by a low clearance and a low volume of distribution without gender difference.

High-level production of a peroxisomal enzyme: Aspergillus flavus uricase accumulates intracellularly and is active in Saccharomyces cerevisiae

Strains of Saccharomyces cerevisiae producing Aspergillus flavus uricase (Uox) have been constructed. An artificial promoter which combined the upstream and downstream sequences of the GAL7 and ADH2 promoters, respectively, was found to be efficient in directing the synthesis of uaZ mRNAs encoding Uox. A good proportionality between the copy number of the uaZ expression cassette and the level of Uox production was found in the range of 1-10 copies. Transformants accumulated active and soluble Uox to a level exceeding 13% of total protein, as deduced from enzymatic assays. This relative level could be improved two- to threefold by using a recipient strain in which the wild-type GAL4 gene had been deleted and which expressed a GAL4 construct placed under the control of the ADH2 promoter.

Transformation of Aspergillus flavus: construction of urate oxidase-deficient mutants by gene disruption

A transformation procedure based on the complementation of a genetic defect was developed using a nitrate reductase-deficient mutant of Aspergillus flavus. The initial transformation efficiency was improved 40-fold by combining factors in a planned experimental program. Although low, this transformation rate was sufficient to obtain transformants in which the urate oxidase-encoding gene (uaZ) was disrupted in a gene replacement experiment. These new uaZ- strains were unable to utilize uric acid as the unique nitrogen source and could be reversed directly to the wild-type phenotype in second order transformation experiments using a urate oxidase-expressing vector.

Purification and molecular properties of urate oxidase from Chlamydomonas reinhardtii

Urate oxidase (urate: oxygen oxidoreductase, EC 1.7.3.3) from the unicellular green alga Chlamydomonas reinhardtii has been purified to electrophoretic and immunological homogeneity by a procedure which includes as main steps ammonium sulfate fractionation, gel filtration, ion exchange and xanthine-agarose affinity chromatography. The native enzyme has a relative molecular mass (Mr) of 124,000 and consists of four identical or similar-sized subunits of Mr 31,000 each. The enzyme has a Stokes's radius of 3.87 nm, a sedimentation coefficient of 6.8 S and an f/f0 of 1.23, and exhibits its maximal absorption at 276 nm. Optimum pH was 8.5 and maximum activity was shown at 40 degrees C, with an activation energy of 53 kJ.mol-1 and a Q10 of 1.96. Absorption spectrum of native reduced enzyme showed two transient maxima at 392 and 570 nm, very similar to those of metal-urate complexes, which disappeared in the presence of cyanide. Inhibition by cyanide and neocuproin, but not by salicylhydroxamic acid, strongly suggests that copper is the metal involved in enzymatic urate oxidation. By using a sensitive photokinetic method for copper determination, a content of 4 mol of copper per mol of enzyme has been found.

Liver uricase in Camelus dromedarius: purification and properties

1. Uricase (urate: oxygen oxidoreductase, EC 1.7.3.3) was purified 750-fold from the liver of Camelus dromedarius. 2. The enzyme is a tetramer with a Mr of 100,000, displays high specificity for uric acid with a Km of 12 microM and is inhibited by a selected number of purine derivatives carrying oxygen at the C2 position. 3. The effect of pH and the inhibition by thiol compounds and chelating agents on the enzyme activity is reported. 4. Some lines of evidence suggesting the possibility of interaction of camel liver uricase with oligonucleotides are presented.

Non-classical inhibition of uricase by cyanide

The interactions of Aspergillus flavus uricase with the substrates O2 and urate and the inhibitors xanthine, cyanide, periodate and hydroxylamine were investigated. Under equilibrium conditions O2 does not bind directly to the enzyme, and the absence of O2 had no effect on either the binding stoicheiometry or binding constant for xanthine, as measured by equilibrium dialysis and microcalorimetry. Cyanide, periodate and hydroxylamine inhibit uricase in a non-classical manner. A decrease in initial velocity to a steady-state inhibited velocity can be observed on a time scale of minutes. The time-dependence, which is unaltered by prior incubation with the inhibitors, is consistent with a first-order transition. Rate constants for induction of inhibition are linearly dependent on inhibitor concentration, but independent of urate and O2 concentrations. Radioactively labelled urate forms a stable but reversible complex with uricase in the presence of cyanide and O2. These results were used to deduce the nature of enzyme-bound intermediates and thus for the proposal of a novel mechanism for cyanide inhibition.