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

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.

Loss of urate oxidase activity in hominoids and its evolutionary implications

We have determined and compared the promoter, coding, and intronic sequences of the urate oxidase (Uox) gene of various primate species. Although we confirm the previous observation that the inactivation of the gene in the clade of the human and the great apes results from a single CGA to TGA nonsense mutation in exon 2, we find that the inactivation in the gibbon lineage results from an independent nonsense mutation at a different CGA codon in exon 2 or from either one-base deletion in exon 3 or one-base insertion in exon 5, contrary to the previous claim that the cause is a 13-bp deletion in exon 2. We also find that compared with other organisms, the primate functional Uox gene is exceptional in terms of usage of CGA codons which are prone to TGA nonsense mutations. Nevertheless, we demonstrate rather strong selective constraint against nonsynonymous sites of the functional Uox gene and argue that this observation is consistent with the fact that the Uox gene is unique in the genome and evolutionarily conserved not only among animals but also among eukaryotes. Another finding that there are a few substitutions in the cis-acting element or CAAT-box (or both) of primate functional Uox genes may explain the lowered transcriptional activity. We suggest that although the inactivation of the hominoid Uox gene was caused by independent nonsense or frameshift mutations, the gene has taken a two-step deterioration process, first in the promoter and second in the coding region during primate evolution. It is also argued that the high concentration of uric acid in the blood of humans and nonhuman primates has developed molecular coevolution with the xanthine oxidoreductase in purine metabolism. However, it remains to be answered whether loss of Uox activity in hominoids is related to protection from oxidative damage and the prolonged life span.

Human gene mutation in pathology and evolution

Mutations in human gene pathology and evolution represent two sides of the same coin in that the same mechanisms that have frequently been implicated in disease-associated mutagenesis appear also to have been involved in potentiating evolutionary change. Indeed, the mutational spectra of germline mutations responsible for inherited disease, somatic mutations underlying tumorigenesis, polymorphisms (either neutral or functionally significant) and differences between orthologous gene sequences exhibit remarkable similarities, implying that they may have causal mechanisms in common. Since these different categories of mutation share multiple unifying characteristics, they should no longer be viewed as distinct entities but rather as portions of a continuum of genetic change that links population genetics and molecular medicine with molecular evolution.

Rasburicase: a potent uricolytic agent

Rasburicase (Fasurtec, Elitek, Sanofi-Synthelabo) is a new recombinant urate oxidase developed for the prevention and treatment of hyperuricaemia. It has a half-life of 17-21 h and produces rapid (within 4 h) and pronounced reductions in plasma uric acid concentrations. To date, rasburicase has been tested in four clinical trials conducted in patients with cancer (primarily haematological malignancies). It successfully alleviated hyperuricaemia in 98% of patients and prevented this complication in 99.6% of those who were at risk. In a stratified, randomised trial, rasburicase was more effective than allopurinol. Most patients had improved or stabilised renal function during rasburicase treatment, despite ongoing chemotherapy-induced tumour lysis. Except for occasional instances of haemolytic anaemia and methemoglobinaemia in patients with glucose-6-phosphate dehydrogenase deficiency, rasburicase was well-tolerated, with a low frequency of mild adverse events. Rasburicase is a safe and effective agent in the prevention or treatment of hyperuricaemia in cancer patients.

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.

Urate oxidase in the prophylaxis or treatment of hyperuricemia: the United States experience

Nonrecombinant urate oxidase (Uricozyme, Sanofi-Synthélabo, Inc, Paris, France) is a highly effective uricolytic agent, but its use is associated with hypersensitivity reaction manifested mainly by bronchospasm in approximately 5% of patients. Recently, several multi-institutional studies have evaluated the efficacy and safety of a recombinant urate oxidase (rasburicase). In a phase I/II study, all 131 patients with newly diagnosed acute lymphoblastic leukemia (ALL) or stage III/IV non-Hodgkin's lymphoma (NHL) experienced rapidly decreased plasma uric acid concentrations after receiving recombinant urate oxidase. Serum creatinine levels also decreased significantly. Toxicity was negligible, and none of the patients required dialysis. In a phase III trial, children with newly diagnosed ALL or stage III/IV NHL were stratified and randomized to receive recombinant urate oxidase or allopurinol. Results showed that the 27 patients who received recombinant urate oxidase had a significantly lower plasma uric acid concentration and a more rapid decline in serum creatinine level than did the 25 who took allopurinol. One patient in the recombinant urate oxidase group had hemolysis of unknown cause, and one in the allopurinol group required hemofiltration for hyperphosphatemia. To further assess the safety profile of recombinant urate oxidase, the data on 245 patients (173 children and 72 adults) who received this agent in a compassionate-use program were reviewed retrospectively. The drug produced dramatic decreases in uric acid concentrations in all patients. Nine patients (four children and five adults) had mild adverse reactions that were drug-related or of unknown etiology. These data suggest that recombinant urate oxidase is safe and effective in the prophylaxis and treatment of hyperuricemia associated with malignancy or chemotherapy.

The central nervous system inflammatory response to neurotropic virus infection is peroxynitrite dependent

We have recently demonstrated that increased blood-CNS barrier permeability and CNS inflammation in a conventional mouse model of experimental allergic encephalomyelitis are dependent upon the production of peroxynitrite (ONOO(-)), a product of the free radicals NO* and superoxide (O2*(-)). To determine whether this is a reflection of the physiological contribution of ONOO(-) to an immune response against a neurotropic pathogen, we have assessed the effects on adult rats acutely infected with Borna disease virus (BDV) of administration of uric acid (UA), an inhibitor of select chemical reactions associated with ONOO(-). The pathogenesis of acute Borna disease in immunocompetent adult rats results from the immune response to the neurotropic BDV, rather than the direct effects of BDV infection of neurons. An important stage in the BDV-specific neuroimmune response is the invasion of inflammatory cells into the CNS. UA treatment inhibited the onset of clinical disease, and prevented the elevated blood-brain barrier permeability as well as CNS inflammation seen in control-treated BDV-infected rats. The replication and spread of BDV in the CNS were unchanged by the administration of UA, and only minimal effects on the immune response to BDV Ags were observed. These results indicate that the CNS inflammatory response to neurotropic virus infection is likely to be dependent upon the activity of ONOO(-) or its products on the blood-brain barrier.

Systemic uric acid administration increases serum antioxidant capacity in healthy volunteers

Oxidative stress plays an important role in the development of atherosclerosis and contributes to tissue damage that occurs as a consequence, particularly in myocardial infarction and acute stroke. Antioxidant properties of uric acid have long been recognized and, as a result of its comparatively high serum concentrations, it is the most abundant scavenger of free radicals in humans. Elevation of serum uric acid concentration occurs as a physiologic response to increased oxidative stress-for example, during acute exercise-thus providing a counter-regulatory increase in antioxidant defenses. In view of its antioxidant properties, uric acid may have potentially important and beneficial effects within the cardiovascular system. We wished to investigate whether administration of uric acid was feasible and if it could have an impact on antioxidant function in vivo. We have, therefore, performed a randomized, placebo-controlled double-blind study of the effects of systemic administration of uric acid, 1,000 mg, in healthy volunteers, compared with vitamin C, 1,000 mg. We observed a significant increase in serum free-radical scavenging capacity from baseline during uric acid and vitamin C infusion, using two methodologically distinct antioxidant assays. The effect of uric acid was substantially greater than that of vitamin C.

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.

A randomized comparison between rasburicase and allopurinol in children with lymphoma or leukemia at high risk for tumor lysis

Standard therapy in the United States for malignancy-associated hyperuricemia consists of hydration, alkalinization, and allopurinol. Urate oxidase catalyzes the enzymatic oxidation of uric acid to a 5 times increased urine soluble product, allantoin. Rasburicase is a new recombinant form of urate oxidase available for clinical evaluation. This multicenter randomized trial compared allopurinol to rasburicase in pediatric patients with leukemia or lymphoma at high risk for tumor lysis. Patients received the assigned uric acid-lowering agent for 5 to 7 days during induction chemotherapy. The primary efficacy end point was to compare the area under the serial plasma uric acid concentration curves during the first 96 hours of therapy (AUC(0-96)). Fifty-two patients were randomized at 6 sites. In an intent-to-treat analysis, the mean uric acid AUC(0-96) was 128 +/- 70 mg/dL.hour for the rasburicase group and 329 +/- 129 mg/dL.hour for the allopurinol group (P <.0001). The rasburicase versus allopurinol group experienced a 2.6-fold (95% CI: 2.0-3.4) less exposure to uric acid. Four hours after the first dose, patients randomized to rasburicase compared to allopurinol achieved an 86% versus 12% reduction (P <.0001) of initial plasma uric acid levels. No antirasburicase antibodies were detected at day 14. This randomized study demonstrated more rapid control and lower levels of plasma uric acid in patients at high risk for tumor lysis who received rasburicase compared to allopurinol. For pediatric patients with advanced stage lymphoma or high tumor burden leukemia, rasburicase is a safe and effective alternative to allopurinol during initial chemotherapy.

Recombinant urate oxidase for the prophylaxis or treatment of hyperuricemia in patients With leukemia or lymphoma

PURPOSE

To improve the control of hyperuricemia in patients with leukemia or lymphoma, we tested a newly developed uricolytic agent, recombinant urate oxidase (SR29142; Rasburicase; Sanofi-Synthelabo, Inc, Paris, France), which catalyzes the oxidation of uric acid to allantoin, a highly water-soluble metabolite readily excreted by the kidneys.

PATIENTS AND METHODS

We administered Rasburicase intravenously, at 0.15 or 0.20 mg/kg, for 5 to 7 consecutive days to 131 children, adolescents, and young adults with newly diagnosed leukemia or lymphoma, who either presented with abnormally high plasma uric acid concentrations or had large tumor cell burdens. Blood levels of uric acid, creatinine, phosphorus, and potassium were measured daily. The pharmacokinetics of Rasburicase, the urinary excretion rate of allantoin, and antibodies to Rasburicase were also studied.

RESULTS

At either dosage, the recombinant enzyme produced a rapid and sharp decrease in plasma uric acid concentrations in all patients. The median level decreased by 4 hours after treatment, from 9.7 to 1 mg/dL (P =.0001), in the 65 patients who presented with hyperuricemia, and from 4.3 to 0.5 mg/dL (P =.0001) in the remaining 66 patients. Despite cytoreductive chemotherapy, plasma uric acid concentrations remained low throughout the treatment (daily median level, 0.5 mg/dL). The urinary excretion rate of allantoin increased during Rasburicase treatment, peaking on day 3. Serum phosphorus concentrations did not change significantly during the first 3 days of treatment, decreased significantly by day 4 in patients presenting with hyperuricemia (P =.0003), and fell within the normal range in all patients by 48 hours after treatment. Serum creatinine levels decreased significantly after 1 day of treatment in patients with or without hyperuricemia at diagnosis (P =.0003 and P =.02, respectively) and returned to normal range in all patients by day 6 of treatment. Toxicity was negligible, and none of the patients required dialysis. The mean plasma half-lives of the agent were 16.0 +/- 6.3 (SD) hours and 21.1 +/- 12.0 hours, respectively, in patients treated at dosages of 0.15 or 0.20 mg/kg. Seventeen of the 121 assessable patients developed antibodies to the enzyme.

CONCLUSION

Rasburicase is safe and highly effective for the prophylaxis or treatment of hyperuricemia in patients with leukemia or lymphoma.

The role of uric acid in protection against peroxynitrite-mediated pathology

Peroxynitrite, the product of the free radicals nitric oxide and superoxide, has been implicated in the pathogenesis of inflammatory CNS disorders. Uric acid, an effective scavenger of peroxynitrite, is a purine metabolite present at high levels in the serum of hominoids relative to lower-order animals due to the functional deletion of urate oxidase. Raising the normally low levels of uric acid in mice is therapeutic for experimental allergic encephalomyelitis, an animal model of multiple sclerosis. This therapeutic activity of uric acid is associated with the inhibition of peroxynitrite-induced tissue damage, blood-CNS barrier permeability changes, and CNS inflammation. Based on these findings we have concluded that peroxynitrite has an important role in promoting enhanced vascular permeability and inflammatory cell extravasation. We hypothesize that higher uric acid levels in hominoids evolved to protect against this process.

Protection of myelin basic protein immunized mice from free-radical mediated inflammatory cell invasion of the central nervous system by the natural peroxynitrite scavenger uric acid

Peroxynitrite (ONOO(-)), the product of nitric oxide (NO(radical)) and superoxide (O(2)(-radical)), is believed to be a major contributor to immunotoxicity when produced by activated cells expressing inducible nitric oxide synthase (iNOS). Uric acid (UA) is a natural scavenger of ONOO(-) that is present at high levels in the sera of humans and other higher order primates relative to most lower mammals. We have previously shown that UA treatment is therapeutic in experimental allergic encephalomyelitis (EAE), a rodent model of multiple sclerosis (MS). In this study we have examined the effect of UA therapy on the dynamics of the appearance of iNOS-positive cells in central nervous system (CNS) tissue of mice subjected to the stimuli that cause EAE. The results indicate that UA prevents activated monocytes from entering CNS tissue where they may contribute to the pathogenesis of MS and other CNS diseases.

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.

Xenopus allantoicase: molecular cloning, enzymatic activity and developmental expression

Allantoicase is one of the enzymes of the purine degradation pathway and, interestingly, it appears to be lost, together with uricase and allantoinase, during mammalian evolution. Only allantoicases from the ascomycetes S. pombe, S. cerevisiae, and N. crassa have already been cloned, although the activity has been reported also in fishes and amphibians. By screening a cDNA expression library of Xenopus liver, we have cloned a 1491-bp-length cDNA coding for a 389 amino acid protein that shows an high similarity with the enzyme allantoicase. We have found that allantoicase mRNA is abundantly expressed in kidney and liver, but at much lower level is also present in brain, testis, intestine, and lung. We have detected enzymatic activity in crude extract from kidney, liver, and lung; we have also determined kinetic parameters (K(m) = 8.44 mM, V(max) = 6. 94 micromol min(-1) per mg protein) in kidney. During embryo development, we have detected allantoicase transcript and activity starting from 1 and 5 days after fertilization, respectively.

Uric acid, a peroxynitrite scavenger, inhibits CNS inflammation, blood-CNS barrier permeability changes, and tissue damage in a mouse model of multiple sclerosis

Peroxynitrite (ONOO(-)), a toxic product of the free radicals nitric oxide and superoxide, has been implicated in the pathogenesis of CNS inflammatory diseases, including multiple sclerosis and its animal correlate experimental autoimmune encephalomyelitis (EAE). In this study we have assessed the mode of action of uric acid (UA), a purine metabolite and ONOO(-) scavenger, in the treatment of EAE. We show that if administered to mice before the onset of clinical EAE, UA interferes with the invasion of inflammatory cells into the CNS and prevents development of the disease. In mice with active EAE, exogenously administered UA penetrates the already compromised blood-CNS barrier, blocks ONOO(-)-mediated tyrosine nitration and apoptotic cell death in areas of inflammation in spinal cord tissues and promotes recovery of the animals. Moreover, UA treatment suppresses the enhanced blood-CNS barrier permeability characteristic of EAE. We postulate that UA acts at two levels in EAE: 1) by protecting the integrity of the blood-CNS barrier from ONOO(-)-induced permeability changes such that cell invasion and the resulting pathology is minimized; and 2) through a compromised blood-CNS barrier, by scavenging the ONOO(-) directly responsible for CNS tissue damage and death.

Hydrogen peroxide generation in peroxisome proliferator-induced oncogenesis

Peroxisome proliferators are a structurally diverse group of non-genotoxic chemicals that induce predictable pleiotropic responses including the development of liver tumors in rats and mice. These chemicals interact variably with peroxisome proliferator-activated receptors (PPARs), which are members of the nuclear receptor superfamily. Evidence derived from mice with PPARalpha gene disruption indicates that of the three PPAR isoforms (alpha, beta/delta and gamma), the isoform PPARalpha is essential for the pleiotropic responses induced by peroxisome proliferators. Peroxisome proliferator-induced activation of PPARalpha leads to profound transcriptional activation of genes encoding for the classical peroxisomal beta-oxidation system and cytochrome P450 CYP 4A isoforms, CYP4A1 and CYP4A3, among others. Livers with peroxisome proliferation manifest substantial increases in the expression of H(2)O(2)-generating peroxisomal fatty acyl-CoA oxidase, the first enzyme of the classical peroxisomal fatty acid beta-oxidation system, and of microsomal cytochrome P450 4A1 and 4A3 genes. Disproportionate increases in H(2)O(2)-generating enzymes and H(2)O(2)-degrading enzyme catalase and reductions in glutathione peroxidase activity by peroxisome proliferators, lead to increased oxidative stress in liver cells. Sustained oxidative stress resulting from chronic increases in H(2)O(2)-generating enzymes manifests as massive accumulation of lipofuscin in hepatocytes, and increased levels of 8-hydroxydeoxyguanosine adducts in liver DNA; this supports the hypothesis that oxidative stress plays a critical role in the development of liver tumors induced by these non-genotoxic chemical carcinogens. Evidence also indicates that cells stably overexpressing H(2)O(2)-generating fatty acyl-CoA oxidase or urate oxidase, when exposed to appropriate substrate(s), reveal features of neoplastic conversion including growth in soft agar and formation of tumors in nude mice. Mice with disrupted fatty acyl-CoA oxidase gene (AOX(-/-) mice), which encodes the first enzyme of the PPARalpha regulated peroxisomal beta-oxidation system, exhibit profound spontaneous peroxisome proliferation, including development of liver tumors, indicative of sustained activation of PPARalpha by the unmetabolized substrates of acyl-CoA oxidase. With the exception of fatty acyl-CoA oxidase, all PPARalpha responsive genes including CYP4A1 and CYP4A3 are up-regulated in the livers of these AOX(-/-) mice. Thus, the substrates of acyl-CoA oxidase serve as endogenous ligands for this receptor leading to a receptor-enzyme cross-talk, because acyl-CoA oxidase gene is transcriptionally regulated by PPARalpha. Peroxisome proliferators induce only a transient increase in liver cell proliferation and this may serve as an additional contributory factor, rather than play a primary role in liver tumor development. Thus, sustained activation of PPARalpha by either synthetic or natural ligands leads to reproducible pleiotropic responses culminating in the development of liver tumors. This phenomenon of peroxisome proliferation provides fascinating challenges in exploring the molecular mechanisms of cell specific transcription, and in identifying the PPARalpha responsive target genes, as well as events involved in their regulation. Genetically altered animals and cell lines should enable investigations on the role of H(2)O(2)-producing enzymes in neoplastic conversion.

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.

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.