Human urate oxidase gene: cloning and partial sequence analysis reveal a stop codon within the fifth exon

The landscape of pathophysiology guided therapeutic strategies for gout treatment

INTRODUCTION

Gout is a common autoinflammatory disease caused by hyperuricemia with acute and/or chronic inflammation as well as tissue damage. Currently, urate-lowering therapy (ULT) and anti-inflammatory therapy are used as first-line strategies for gout treatment. However, traditional drugs for gout treatment exhibit some unexpected side effects and are not suitable for certain patients due to their comorbidity with other chronic disease.

AREAS COVERED

In this review, we described the pathophysiology of hyperuricemia and monosodium urate (MSU) crystal induced inflammatory response during gout development in depth and comprehensively summarized the advances in the investigation of promising ULT drugs as well as anti-inflammatory drugs that might be safer and more effective for gout treatment.

EXPERT OPINION

New drugs that are developed based on these molecular mechanisms exhibited great efficacy on reduction of disease burden both in vitro and in vivo, implying their potential for clinical application. Moreover, hyperthermia also showed regulation effect on MSU crystals formation and the signaling pathways involved in inflammation.

Functional Characterization of Clinically-Relevant Rare Variants in ABCG2 Identified in a Gout and Hyperuricemia Cohort

ATP-binding cassette subfamily G member 2 (ABCG2) is a physiologically important urate transporter. Accumulating evidence demonstrates that congenital dysfunction of ABCG2 is an important genetic risk factor in gout and hyperuricemia; recent studies suggest the clinical significance of both common and rare variants of ABCG2. However, the effects of rare variants of ABCG2 on the risk of such diseases are not fully understood. Here, using a cohort of 250 Czech individuals of European descent (68 primary hyperuricemia patients and 182 primary gout patients), we examined exonic non-synonymous variants of ABCG2. Based on the results of direct sequencing and database information, we experimentally characterized nine rare variants of ABCG2: R147W (rs372192400), T153M (rs753759474), F373C (rs752626614), T421A (rs199854112), T434M (rs769734146), S476P (not annotated), S572R (rs200894058), D620N (rs34783571), and a three-base deletion K360del (rs750972998). Functional analyses of these rare variants revealed a deficiency in the plasma membrane localization of R147W and S572R, lower levels of cellular proteins of T153M and F373C, and null urate uptake function of T434M and S476P. Accordingly, we newly identified six rare variants of ABCG2 that showed lower or null function. Our findings contribute to deepening the understanding of ABCG2-related gout/hyperuricemia risk and the biochemical characteristics of the ABCG2 protein.

Pharmacological urate-lowering approaches in chronic kidney disease

Chronic kidney disease (CKD) has become a global public health issue and uric acid (UA) remains a major risk factor of CKD. As the main organ for the elimination of UA, kidney owned a group of urate transporters in tubular epithelium. Kidney disease hampered the UA excretion, and the accumulation of serum UA in return harmed the renal function. Commercially, there are three kinds of agents targeting at urate-lowering, xanthine oxidoreductase inhibitor which prevents the production of UA, uricosuric which increases the concentration of UA in urine thus decreasing serum UA level, and uricase which converts UA to allantoin resulting in the dramatic decrement of serum UA. Of note, in patients with CKD, administration of above-mentioned agents, alone or combined, needs special attention. New evidence is emerging for the efficacy of several urate-lowering drugs for the treatment of hyperuricemia in patients with CKD. Besides, loads of novel and promising drug candidates and phytochemicals are in the different phases of research and development. As of today, there is insufficient evidence to recommend the widespread use of UA-lowering therapy to prevent or slow down the progression of CKD. The review summarized the evidence and perspectives about the treatment of hyperuricemia with CKD for medicinal chemist and nephrologist.

ABCG2 dysfunction increases serum uric acid by decreased intestinal urate excretion

ATP-binding cassette transporter G2 (ABCG2), also known as breast cancer resistance protein (BCRP), is identified as a high-capacity urate exporter and its dysfunction has an association with serum uric acid (SUA) levels and gout/hyperuricemia risk. However, pathophysiologically important pathway(s) responsible for the ABCG2-mediated urate excretion were unknown. In this study, we investigated how ABCG2 dysfunction affected the urate excretion pathways. First, we revealed that mouse Abcg2 mediates urate transport using the membrane vesicle system. The export process by mouse Abcg2 was ATP-dependent and not saturable under the physiological concentration of urate. Then, we characterized the excretion of urate into urine, bile, and intestinal lumen using in vivo mouse model. SUA of Abcg2-knockout mice was significantly higher than that of control mice. Under this condition, the renal urate excretion was increased in Abcg2-knockout mice, whereas the urate excretion from the intestine was decreased to less than a half. Biliary urate excretion showed no significant difference regardless of Abcg2 genotype. From these results, we estimated the relative contribution of each pathway to total urate excretion; in wild-type mice, the renal excretion pathway contributes approximately two-thirds, the intestinal excretion pathway contributes one-third of the total urate excretion, and the urate excretion into bile is minor. Decreased intestinal excretion could account for the increased SUA of Abcg2-knockout mice. Thus, ABCG2 is suggested to have an important role in extra-renal urate excretion, especially in intestinal excretion. Accordingly, increased SUA in patients with ABCG2 dysfunction could be explained by the decreased excretion of urate from the intestine.

The human peroxisome in health and disease: the story of an oddity becoming a vital organelle

Since the first report by Rhodin in 1954, our knowledge on mammalian microbodies/peroxisomes has known several periods. An initial two decades period (1954-1973) has contributed to the biochemical individualisation of peroxisomes as a new class of subcellular organelles (de Duve, 1965). The corresponding research period failed to define a clear role of mammalian peroxisomes in vital functions and intermediary metabolism, explaining why feeling that peroxisomes might be in the human cell oddities has prevailed during several decades. The period standing from 1973 to nowadays has progressively removed this cell oddity view of peroxisomes by highlighting vital function and metabolic role of peroxisomes in health and disease along with genetic and metabolic regulation of peroxisomal protein content, organelle envelope formation and protein signal targeting mechanisms. Research on peroxisomes and their response to various drugs and metabolites, dietary and physiological conditions has also played a key role in the discovery of peroxisome proliferator activated receptors (PPARs) belonging to the nuclear hormone receptor superfamily and for which impact in science and medicine goes now by far beyond that of the peroxisomes.

Characterization of purine catabolic pathway genes in coelacanths

Coelacanths are a critically valuable species to explore the gene changes that took place in the transition from aquatic to terrestrial life. One interesting and biologically relevant feature of the genus Latimeria is ureotelism. However not all urea is excreted from the body; in fact high concentrations are retained in plasma and seem to be involved in osmoregulation. The purine catabolic pathway, which leads to urea production in Latimeria, has progressively lost some steps, reflecting an enzyme loss during diversification of terrestrial species. We report the results of analyses of the liver and testis transcriptomes of the Indonesian coelacanth Latimeria menadoensis and of the genome of Latimeria chalumnae, which has recently been fully sequenced in the framework of the coelacanth genome project. We describe five genes, uricase, 5-hydroxyisourate hydrolase, parahox neighbor B, allantoinase, and allantoicase, each coding for one of the five enzymes involved in urate degradation to urea, and report the identification of a putative second form of 5-hydroxyisourate hydrolase that is characteristic of the genus Latimeria. The present data also highlight the activity of the complete purine pathway in the coelacanth liver and suggest its involvement in the maintenance of high plasma urea concentrations.

Decreased extra-renal urate excretion is a common cause of hyperuricemia

ABCG2, also known as BCRP, is a high-capacity urate exporter, the dysfunction of which raises gout/hyperuricemia risk. Generally, hyperuricemia has been classified into urate 'overproduction type' and/or 'underexcretion type' based solely on renal urate excretion, without considering an extra-renal pathway. Here we show that decreased extra-renal urate excretion caused by ABCG2 dysfunction is a common mechanism of hyperuricemia. Clinical parameters, including urinary urate excretion, are examined in 644 male outpatients with hyperuricemia. Paradoxically, ABCG2 export dysfunction significantly increases urinary urate excretion and risk ratio of urate overproduction. Abcg2-knockout mice show increased serum uric acid levels and renal urate excretion, and decreased intestinal urate excretion. Together with high ABCG2 expression in extra-renal tissues, our data suggest that the 'overproduction type' in the current concept of hyperuricemia be renamed 'renal overload type', which consists of two subtypes-'extra-renal urate underexcretion' and genuine 'urate overproduction'-providing a new concept valuable for the treatment of hyperuricemia and gout.

A distinct metabolic signature predicts development of fasting plasma glucose

Background

High blood glucose and diabetes are amongst the conditions causing the greatest losses in years of healthy life worldwide. Therefore, numerous studies aim to identify reliable risk markers for development of impaired glucose metabolism and type 2 diabetes. However, the molecular basis of impaired glucose metabolism is so far insufficiently understood. The development of so called 'omics' approaches in the recent years promises to identify molecular markers and to further understand the molecular basis of impaired glucose metabolism and type 2 diabetes. Although univariate statistical approaches are often applied, we demonstrate here that the application of multivariate statistical approaches is highly recommended to fully capture the complexity of data gained using high-throughput methods.

Methods

We took blood plasma samples from 172 subjects who participated in the prospective Metabolic Syndrome Berlin Potsdam follow-up study (MESY-BEPO Follow-up). We analysed these samples using Gas Chromatography coupled with Mass Spectrometry (GC-MS), and measured 286 metabolites. Furthermore, fasting glucose levels were measured using standard methods at baseline, and after an average of six years. We did correlation analysis and built linear regression models as well as Random Forest regression models to identify metabolites that predict the development of fasting glucose in our cohort.

Results

We found a metabolic pattern consisting of nine metabolites that predicted fasting glucose development with an accuracy of 0.47 in tenfold cross-validation using Random Forest regression. We also showed that adding established risk markers did not improve the model accuracy. However, external validation is eventually desirable. Although not all metabolites belonging to the final pattern are identified yet, the pattern directs attention to amino acid metabolism, energy metabolism and redox homeostasis.

Conclusions

We demonstrate that metabolites identified using a high-throughput method (GC-MS) perform well in predicting the development of fasting plasma glucose over several years. Notably, not single, but a complex pattern of metabolites propels the prediction and therefore reflects the complexity of the underlying molecular mechanisms. This result could only be captured by application of multivariate statistical approaches. Therefore, we highly recommend the usage of statistical methods that seize the complexity of the information given by high-throughput methods.

Genetics and experimental models of crystal-induced arthritis. Lessons learned from mice and men: is it crystal clear?

PURPOSE OF REVIEW

We examine the major genes in mice and humans involved in the pathogenesis of monosodium urate, calcium pyrophosphate dihydrate and hydroxyapatite crystal-induced arthritis.

RECENT FINDINGS

Several genetic causes of renal disease associated with hyperuricemia and gout provide insight into genes involved in renal urate handling. Mutations or polymorphisms in exons 4 and 5 and intron 4 of urate transporter 1 may be independent genetic markers of hyperuricemia and gout. Genetic analysis supports the role of ANKH mutations in calcium pyrophosphate dihydrate-induced arthritis. ANKH gain-of-function mutations were confirmed by functional studies; however, the crystals formed in ATD5 cells were basic calcium phosphate, not calcium pyrophosphate dihydrate, underlying the significance of chondrocyte differentiation state and the factors regulating normal and pathological mineralization. Animal models have implicated a general model of crystal-induced inflammation involving innate immunity through the NALP3 (Natch domain, leucine-rich repeat, and PYD-containing protein 3) inflammasome signaling through the interleukin-1 receptor and its signaling protein myeloid differentiation primary response protein 88.

SUMMARY

Genetic analysis has elucidated genes responsible for crystal formation and animal models have unveiled mechanisms in the development of crystal-induced arthritis. Future studies will hasten understanding of the pathology of crystal-induced arthritis and provide new therapies.

Amphioxus allantoicase: Molecular cloning, expression and enzymatic activity

Allantoicase, one of the purine metabolism enzymes, is progressively truncated during the chordate evolution, yet it is unknown when its activity became phylogenetically extinct. In this study, a cDNA encoding allantoicase was isolated from the gut cDNA library of amphioxus Branchiostoma belcheri tsingtauense. It is 2441 bp long, and contains an open reading frame encoding a protein of 392 amino acid residues. RT-PCR analysis showed that amphioxus allantoicase was strongly expressed in the hepatic caecum, and weakly expressed in other tissues including hind-gut, gill, muscle, notochord, testis and ovary. The parallel experiment was performed measuring the allantoicase activity in the same tissues revealed that its activity was high in the hepatic caecum, but low or undetectable in other tissues examined. These suggest that allantoicase remains in action in the primitive chordate amphioxus.

Selective pressure on the allantoicase gene during vertebrate evolution

During vertebrate evolution, the uric acid degradation pathway has been modified and several enzymes have been lost. Consequently, the end product of purine catabolism varies from species to species. In the past few years, we have focused our attention on vertebrate allantoicase (an uricolytic pathway enzyme), whose activity is present in certain fish and amphibians only, but whose mRNA we detected also in mammals. As allantoicase activity disappeared in amniotes, we wonder why these sequences not only remain present in the mammalian genome, but are still transcribed. To elucidate this issue, we have cloned and analyzed comparable cDNA sequences of different organisms from ascidians to mammals. The analysis of the nonsynonymous-synonymous substitution rate that we performed on the coding region comprising exons 3 to 8 by means of maximum likelihood suggested that a certain amount of purifying selection is acting on the allantoicase sequences. Some implications of the preservation of an apparently unnecessary gene in higher vertebrates are discussed.

Genomic organization and chromosome localization of the murine and human allantoicase gene

Allantoicase is one of the enzymes involved in uricolysis. The enzymes of this catabolic pathway (i.e. allantoinase, allantoicase, ureidoglycolate lyase and urease) were lost during vertebrate evolution and the causes for this loss are still unclear. In mammals, as well as in birds and reptiles, the activity of allantoicase is absent; notwithstanding, we recently cloned human and mouse cDNA sequences with high similarity with previously characterized allantoicases. In the present paper, we report the genomic organization of the allantoicase gene in mouse and in man. Both genes are constituted by 11 exons that appear to be very conserved; introns are more variable in length while maintain the same phase but for intron 4. We have also detected a second transcript of the human allantoicase gene in which exon 1 is absent. Moreover, the mouse gene maps in chromosome 12 at 13.0 cM from the centromere.

Property comparison of recombinant amphibian and mammalian allantoicases

Allantoicase is an enzyme involved in uric acid degradation. Although it is commonly accepted that allantoicase is lost in mammals, birds and reptiles, we have recently identified its transcripts in mice and humans. The mouse mRNA seems capable of encoding a functional allantoicase, therefore we expressed the Xenopus and mouse allantoicases (MAlc and XAlc, respectively) in Escherichia coli and characterized the recombinant enzymes. The two recombinant allantoicases show a similar temperature and pH stability but, although XAlc and MAlc share a 54% amino acid identity, they differ in sensitivity to bivalent cations, in substrate affinity and in the level of expression in tissues (as revealed by means of Western blot analysis). We propose that the loss of allantoicase activity in mouse is due to a low substrate affinity and to a reduced expression level of the enzyme.

The tree squirrel HP-25 gene is a pseudogene

The gene for the hibernation-specific protein HP-25 is expressed in the liver in hibernating species of the squirrel family (chipmunk and ground squirrel), but not in a nonhibernating species (tree squirrel). To investigate why the HP-25 gene is not expressed in the tree squirrel, we isolated the tree squirrel HP-25 gene and compared its gene structure and promoter activity with that of the chipmunk. The tree squirrel HP-25 gene is composed of three exons, and the gene structures are conserved between the tree squirrel and chipmunk. However, the tree squirrel HP-25 gene has an insertional mutation of 13 nucleotides in exon 2 that disrupts the ORF. In the chipmunk HP-25 gene, the 80-bp 5' flanking sequence is sufficient for the liver-specific promoter activity, and HNF-4, which binds to the sequence from nucleotides -67 to -51, is involved in its transcriptional regulation. In contrast, the corresponding tree squirrel 5' flanking sequence had almost no promoter activity in HepG2 cells, and HNF-4 did not bind to the corresponding region of the tree squirrel HP-25 gene. Furthermore, a tree squirrel-type G to A mutation at -57 in the chipmunk HP-25 gene promoter context abolished its binding to and transactivation by HNF-4. Thus, the point mutation in the HNF-4-binding site is likely to be involved in the lack of HP-25 gene expression in the tree squirrel.

Molecular cloning of mouse allantoicase cDNA

The uric acid degradation pathway is progressively lost during vertebrate evolution. In mammals, the end product of this catabolic pathway is allantoin and, therefore, no allantoicase should be present in mouse tissues. Surprisingly, we have found an expressed sequence tag (EST) from mouse testis with high similarity to allantoicase. To characterize this transcript, we have completely sequenced the corresponding EST clone insert and found a 1495 bp long cDNA coding for a 414 amino acid long protein. Identities of mouse versus microorganism allantoicases range from 25 to 30%. Identity reaches 54% when compared to Xenopus allantoicase. Among the tested tissues, only testis possesses the allantoicase transcript. Although no deleterious mutations were found in the coding region, no allantoicase activity could be detected in mouse testis.

Cell biology of peroxisomes and their characteristics in aquatic organisms

The general characteristics of peroxisomes in different organisms, including aquatic organisms such as fish, crustaceans, and mollusks, are reviewed, with special emphasis on different aspects of the organelle biogenesis and mechanistic aspects of peroxisome proliferation. Peroxisome proliferation and peroxisomal enzyme inductions elicited by xenobiotics or physiological conditions have become useful tools to study the mechanisms of peroxisome biogenesis. During peroxisome proliferation, the induction of peroxisomal proteins is heterogeneous, enzymes that show increased activity being involved in different aspects of lipid homeostasis. The process of peroxisome biogenesis is coordinately triggered by a whole array of structurally dissimilar compounds known as peroxisome proliferators, and investigating the effect of some of these compounds that commonly appear as pollutants in the environment on the peroxisomes of aquatic animals inhabiting marine and estuarine habitats seems interesting. It is also important to determine whether peroxisome proliferation in these animals is a phenomenon that might occur under normal physiological or season-related conditions and plays a metabolic or functional role. This would help set the basis for understanding the process of peroxisome biogenesis in aquatic animals.

Human allantoicase gene: cDNA cloning, genomic organization and chromosome localization

Uric-acid-degrading enzymes (uricase, allantoinase, allantoicase, ureidoglycolate lyase and urease) were lost during vertebrate evolution and the causes for this loss are still unclear. We have recently cloned the first vertebrate allantoicase cDNA from the amphibian Xenopus laevis. Surprisingly, we have found some mammalian expressed sequence tags (ESTs) that show high similarity with Xenopus allantoicase cDNA. From a human fetal spleen cDNA library and adult kidney EST clone, we have obtained a 1790 nucleotide long cDNA. The 3' end of this sequence reveals a substantial high identity with the corresponding portion of Xenopus allantoicase cDNA. In contrast, at the 5' end the human sequence diverges from that of Xenopus; since no continuous open reading frame can be found in this region, the hypothetical human protein appears truncated at its N-terminus. We proposed that such a transcript could be due to an incorrect splicing mechanism that introduces an intron portion at the 5' end of human cDNA. Allantoicase cDNA is expressed in adult testis, prostate, kidney and fetal spleen. By comparison with available genomic sequences deposited in database, we have determined that the human allantoicase gene consists of five exons and spans 8kb. We have also mapped the gene in chromosome 2.

cDNA cloning and analysis of tissue-specific expression of mouse peroxisomal straight-chain acyl-CoA oxidase

Straight-chain acyl-CoA oxidase is the first and rate limiting enzyme in the peroxisomal beta-oxidation pathway catalysing the desaturation of acyl-CoAs to 2-trans-enoyl-CoAs, thereby producing H2O2. To study peroxisomal beta-oxidation we cloned and characterized the cDNA of mouse peroxisomal acyl-CoA oxidase. It consists of 3778 bp, including a 1983-bp ORF encoding a polypeptide of 661 amino-acid residues. Like the rat and human homologue the C-terminus contains an SKL motif, an import signal present in several peroxisomal matrix proteins. Sequence analysis revealed high amino-acid homology with rat (96%) and human (87%) acyl-CoA oxidase in addition to minor homology ( approximately 40%) with other related proteins, such as rabbit trihydroxy-cholestanoyl-CoA oxidase, human branched chain acyl-CoA oxidase and rat trihydroxycoprostanoyl-CoA oxidase. Acyl-CoA oxidase mRNA and protein expression were most abundant in liver followed by kidney, brain and adipose tissue. During mouse brain development acyl-CoA oxidase mRNA expression was highest during the suckling period indicating that peroxisomal beta-oxidation is most critical during this developmental stage. Comparing tissue mRNA levels of peroxisome proliferator-activated receptor alpha and acyl-CoA oxidase, we noticed a constant relationship in all tissues investigated, except heart and adipose tissue in which much more, and respectively, much less, peroxisome proliferator-activated receptor alpha mRNA in proportion to acyl-CoA oxidase mRNA was found. Our data show that acyl-CoA oxidase is an evolutionary highly conserved enzyme with a distinct pattern of expression and indicate an important role in lipid metabolism.

Large-scale purification and further characterization of rat pristanoyl-CoA oxidase

The elution of pristanoyl-CoA oxidase from butyl-Sepharose required unusually high concentrations of ethylene glycol, enabling the large-scale purification of this oxidase in a single chromatographic step. The enzyme, the native molecular mass of which was estimated previously at 415 kDa by gel filtration (Van Veldhoven, P.P., Vanhove, G., Vanhoutte, F., Dacremont, G., Eyssen, H. J. & Mannaerts, G. P. (1991) J. Biol. Chem. 266, 24676-24683), migrated as a 513-kDa protein during native gel electrophoresis. It showed a typical flavoprotein spectrum and probably binds 4 mol FAD/mol enzyme. Its amino acid composition was different from those of other known acyl-CoA oxidases. Screening of different rat tissues, either for enzyme activity or by immunoblotting, revealed the highest level of pristanoyl-CoA oxidase in liver, followed by kidney, intestinal mucosa, spleen and lung. The oxidase activities, measured with 2-methylpalmitoyl-CoA as the substrate, in livers from other vertebrates including man were low compared to rat. This was also confirmed by immunoblotting which provided a clear signal only in rat liver, possibly indicating that pristanoyl-CoA oxidase might be a rat-specific oxidase.

Isolation of the human peroxisomal acyl-CoA oxidase gene: organization, promoter analysis, and chromosomal localization

Peroxisomal acyl-CoA oxidase (ACOX; EC 1.3.3.6) is the first enzyme of the fatty acid beta-oxidation pathway, which catalyzes the desaturation of acyl-CoAs to 2-trans-enoyl-CoAs, and it donates electrons directly to molecular oxygen, thereby producing H2O2. The discovery of carcinogenic peroxisome proliferators, which markedly increase the levels of this H2O2-producing ACOX in rat and mouse liver, generated interest in peroxisomal beta-oxidation system genes. The present study deals with the structural organization of human ACOX gene. This gene spans approximately 33 kb and consists of 14 exons and 13 introns. Primer-extension analysis revealed three principal cap sites, which were mapped at 50, 52, and 53 nt upstream of the initiator methionine codon. The 5' flanking region of the ACOX gene was sequenced up to 500 bp upstream of the cap sites. This promoter region is G + C-rich and contains three copies of the "GC box" hexanucleotides. Multiple GC boxes are a characteristic feature of the rat ACOX and bifunctional protein genes of the beta-oxidation system. A + T-rich TATA-boxlike sequences, TTTATTT and TTATT, have also been identified in this human ACOX gene, but typical CCAAT motifs are absent. This ACOX gene has been mapped to chromosome 17q25 by in situ hybridization, using a biotinlabeled probe.

Uric acid degrading enzymes, urate oxidase and allantoinase, are associated with different subcellular organelles in frog liver and kidney

On the basis of differential and density gradient centrifugation studies, the site of the uric acid degrading enzymes, urate oxidase and allantoinase, in amphibia was previously assigned to the hepatic peroxisomes. Using specific antibodies against frog urate oxidase and allantoinase, we have undertaken an immunocytochemical study of the localization of these two proteins in frog liver and kidney, and demonstrate that whereas urate oxidase is present in peroxisomes, allantoinase is localized in mitochondria. Urate oxidase and allantoinase were detected by immunoblot analysis in both frog liver and kidney. The subcellular localization of these two enzymes was ascertained by Protein A-gold immunocytochemical staining of Lowicryl K4M-embedded tissue. Peroxisomes in frog liver parenchymal cells and kidney proximal tubular epithelium contained a semi-dense subcrystalloid core, which was found to be the exclusive site of urate oxidase localization. Allantoinase was detected within mitochondria, but not in peroxisomes of hepatocytes or proximal tubular epithelium. No allantoinase was detected in the mitochondria of nonhepatic parenchymal cells in liver and of the cells lining the distal convoluted tubules of the kidney. These results demonstrate that, unlike rat kidney peroxisomes which lack urate oxidase, peroxisomes of frog kidney contain this enzyme. Contrary to previous assumptions, these studies also clearly establish that urate oxidase and allantoinase, the first two enzymes involved in uric acid degradation, are localized in different subcellular organelles in frog liver and kidney.

Genetic improvements of an industrial strain of Aspergillus flavus for urate oxidase production

Urate oxidase, an enzyme used in human therapy, is currently produced industrially by a strain of Aspergillus flavus. Two strategies of strain improvement were tested in order to obtain higher yields of urate oxidase. The first one, based on a classical mutation-selection protocol, led to the isolation of a mutant strain that overproduced uricase two-fold as compared to the industrial strain. The second one consisted in the construction of transformed strains that had integrated multiple copies of a urate oxidase-expression vector. A twenty-fold improvement in urate oxidase was obtained by this method.

Metabolic pathways in mammalian peroxisomes

This article summarizes our current knowledge of the metabolic pathways present in mammalian peroxisomes. Emphasis is placed on those aspects that are not covered by other articles in this issue: peroxisomal enzyme content and topology; the peroxisomal beta-oxidation system; substrates of peroxisomal beta-oxidation such as very-long-chain fatty acids, branched fatty acids, dicarboxylic fatty acids, prostaglandins and xenobiotics; the role of peroxisomes in the metabolism of purines, polyamines, amino acids, glyoxylate and reactive oxygen products such as hydrogen peroxide, superoxide anions and epoxides.

Rat urate oxidase produced by recombinant baculovirus expression: formation of peroxisome crystalloid core-like structures

Urate oxidase (EC 1.7.3.3), which catalyzes the oxidation of uric acid to allantoin, is present in most mammals but absent in humans and hominoid primates. In rats and most other mammals that catabolize uric acid to allantoin, this enzyme is localized within the crystalloid cores of peroxisomes present in liver parenchymal cells. To determine whether urate oxidase forms these crystalloid cores or whether core-forming protein(s) exist in association with urate oxidase, a baculovirus expression vector system was used to overproduce the full-length rat urate oxidase in Spodoptera frugiperda cells. Urate oxidase was expressed to a level of approximately 30% of the total protein in this system. Immunoblot analysis demonstrated that the baculovirus-generated protein had electrophoretic and immunologic properties similar to those of urate oxidase expressed in rat liver. Immunofluorescence and electron microscopic examination revealed that the overexpressed recombinant urate oxidase is present in both the cytoplasm and the nucleus of infected insect cells as numerous 1- to 3-microns discrete particles. These insoluble protein aggregates, which were positively stained for urate oxidase by protein A-gold immunocytochemical approach, did not appear to be delimited by a single membrane. They revealed a crystalloid structure reminiscent of rat peroxisomal core consisting of bundles of tubules with an inner diameter of approximately 50 A. The recombinant urate oxidase particles, isolated by a single-step procedure, were composed entirely of 35-kDa urate oxidase subunit. These studies indicate that rat urate oxidase is capable of forming insoluble crystalloid core-like structures.

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.

Molecular evolution of the urate oxidase-encoding gene in hominoid primates: nonsense mutations

Nucleotide sequences of portions of second and fifth exons of urate oxidase encoding gene (UOX) of chimpanzee, gorilla, orangutan, rhesus monkey and squirrel monkey obtained following amplification by polymerase chain reaction have been compared with corresponding sequences of human, baboon and rat UOX. Two or more nonsense mutations are found in the coding regions of this UOX gene thus far analyzed in human, chimpanzee, gorilla and orangutan, but not in the baboon, rhesus monkey and squirrel monkey. Of these nonsense mutations, the stop codon at amino acid position 33 is constant in the human and the three great apes suggesting that this may be the original mutation responsible for the inactivation of the UOX gene during hominoid evolution.

Structural analysis of the gene encoding rat uricase

The uricase gene was isolated from rat genomic DNA libraries. The gene spans 40 kb and consists of eight exons. All the exon-intron junctional sequences conform to the canonical GT/AG rule. The restriction map of the isolated clones and Southern blot analysis revealed that the enzyme is encoded by a single-copy gene. Analysis of the transcription initiation site of rat uricase mRNA indicated the differential use of consecutive nucleotides; the principal repeat is located 55 nucleotides upstream from the first methionine codon. Nucleotide sequence analysis of the 5'-flanking region showed the presence of a TATA (ATAAAA) sequence at nucleotides 30 to 25 and of a CAAT (GGTCAAT) sequence at nucleotides 63 to 57 upstream of the principal transcription initiation site. The 5'-flanking region contains another possible regulatory sequence (TGTCGACA) homologous to the cAMP-regulatory element. The palindromic sequence is located 158 to 151 nucleotides upstream from the transcription initiation site, surrounded by a direct repeat (TCAGCAA).

Rat urate oxidase: cloning and structural analysis of the gene and 5'-flanking region

The structural gene (UOX) encoding rat urate oxidase (UOX) spans at least 23 kb and is composed of eight exons and seven introns. All of the exon-intron splice junction sequences conformed to the GT/AG consensus established for eukaryotic genes. The transcription start point (tsp) was determined using S1-type nuclease protection riboprobe, and assigned to an adenine 54 nucleotides (nt) upstream of the ATG start codon. A 456-bp 5'-terminal fragment, starting at the ATG codon, carries a putative TATA (ATAAAA) sequence at -32, and two putative 'CAAT box' sequences at -62 and -71 bp upstream from the tsp. No sequence resembling 'GC' box hexanucleotides (GGGCGG or CCGCCC) was found. The structural features of the 5'-flanking region of the UOX gene are distinct from the 5'-flanking sequences of peroxisomal beta-oxidation system genes which contain one or more 'GC' box elements but lack TATA- and CAAT-like features [Osumi et al., J. Biol. Chem. 262 (1987) 8138-8143; Ishii et al., J. Biol. Chem. 262 (1987) 8144-8150]. The 5'-flanking region of the UOX gene reveals a sequence, TTAGTAATT at nt -276 from the tsp, which appears to be complementary to the underlined part of the liver-specific LF-B1/HNF-1 consensus sequence, GTTAATNATTAAC (where N = A, C, T, G or no nt).