Analysis of Purine Metabolism to Elucidate the Pathogenesis of Acute Kidney Injury in Renal Hypouricemia

Renal hypouricemia is a disease caused by the dysfunction of renal urate transporters. This disease is known to cause exercise-induced acute kidney injury, but its mechanism has not yet been established. To analyze the mechanism by which hypouricemia causes renal failure, we conducted a semi-ischemic forearm exercise stress test to mimic exercise conditions in five healthy subjects, six patients with renal hypouricemia, and one patient with xanthinuria and analyzed the changes in purine metabolites. The results showed that the subjects with renal hypouricemia had significantly lower blood hypoxanthine levels and increased urinary hypoxanthine excretion after exercise than healthy subjects. Oxidative stress markers did not differ between healthy subjects and hypouricemic subjects before and after exercise, and no effect of uric acid as a radical scavenger was observed. As hypoxanthine is a precursor for adenosine triphosphate (ATP) production via the salvage pathway, loss of hypoxanthine after exercise in patients with renal hypouricemia may cause ATP loss in the renal tubules and consequent tissue damage.

Modified forearm ischemic test in hypouricemic patients

Renal hypouricemia sometimes leads to exercise-induced acute kidney injury (EIAKI) of unknown pathogenesis. In order to elucidate the various pathological conditions associated with hypouricemia, we analyzed the effects of low uric acid level on energy metabolism. We have modified semi-ischemic forearm exercise test and performed this test in one Japanese healthy volunteer, three patients with hereditary renal hypouricemia and one patient with hereditary xanthinuria of Czech origin. Forearm exercise was performed by squeezing a hand dynamometer with the sphygmomanometer cuff pressure kept at the mean arterial pressure. Venous blood was drawn five times (before exercise, 3, 10, 30, 45 minutes after the start of exercise) in each tests. The mean plasma lactate concentration increased from a baseline of 1.3 (range 0.7-1.8 mmol/L) to 4.0 (range 2.0-5.5 mmol/L) at 3 minutes after the start of exercise. The plasma hypoxanthine concentrations were quite low before exercise (0-2.9 μmol/L), but increased markedly to a range of 13.6-28.8 μmol/L after 10 minute forearm ischemia. Our protocol allowed us to conclude that the load was sufficient for observing metabolic changes in temporally hypoxia and in following recovery phase. The test was well tolerated and safe, we did not observe any adverse reactions including EIAKI.

Hereditary xanthinuria is not so rare disorder of purine metabolism

Hereditary xanthinuria (type I) is caused by an inherited deficiency of the xanthine oxidorectase (XDH/XO), and is characterized by very low concentration of uric acid in blood and urine and high concentration of urinary xanthine, leading to urolithiasis. Type II results from a combined deficiency of XDH/XO and aldehyde oxidase. Patients present with hematuria, renal colic, urolithiasis or even acute renal failure. Clinical symptoms are the same for both types. In a third type, clinically distinct, sulfite oxidase activity is missing as well as XDH/XO and aldehyde oxidase. The prevalence is not known, but about 150 cases have been described so far. Hypouricemia is sometimes overlooked, that´s why we have set up the diagnostic flowchart. This consists of a) evaluation of uric acid concentrations in serum and urine with exclusion of primary renal hypouricemia, b) estimation of urinary xanthine, c) allopurinol loading test, which enables to distinguish type I and II; and finally assay of xanthine oxidoreductase activity in plasma with molecular genetic analysis. Following this diagnostic procedure we were able to find first patients with hereditary xanthinuria in our Czech population. We have detected nine cases, which is one of the largest group worldwide. Four patients were asymptomatic. All had profound hypouricemia, which was the first sign and led to referral to our department. Urinary concentrations of xanthine were in the range of 170-598 mmol/mol creatinine (normal < 30 mmol/mol creatinine). Hereditary xanthinuria is still unrecognized disorder and subjects with unexplained hypouricemia need detailed purine metabolic investigation.

Purine disorders with hypouricemia

Hypouricemia is defined as a serum urate levels less than 2 mg/dL (119 µmol/L). Primary hypouricemia is caused by disorders of purine metabolism and transport. This laboratory finding is sometimes overlooked and, following two genetic defects, should be considered in differential diagnosis of unexplained hypouricemia. Hereditary xanthinuria is autosomal recessive and due to mutations in xanthine oxidase, leading to over-production of xanthine and minimal production of urate. Patients have very low serum urate levels and suffer from elevated levels of xanthine in the urine, leading to xanthine stones, haematuria, and sometimes occult chronic kidney failure. Hypouricemia is the key to diagnosis. Hereditary renal hypouricemia is a new genetic defect of renal transport of uric acid. Two types were distinguished: a) renal hypouricemia type 1, caused by the defects in the SLC22A12 gene coding the human urate transporter 1 (hURAT1) and b) renal hypouricemia type 2, caused by the defects in the SLC2A9 gene, which encodes GLUT9 transporter. This disorder predisposes patients to exercise-induced acute renal failure and/or nephrolithiasis. Diagnosis is based on two markers: hypouricemia (<119 µmol/L) and increased fractional excretion of uric acid (>10%). Over one hundred cases were identified in Japan and and this number is unique worldwide. Several patients were described in Macedonia. We were able to detect four Czech families with hereditary xanthinuria and eight cases of hereditary renal hypouricemia. In conclusion, hereditary xanthinuria and hereditary renal hypouricemia are still unrecognized conditions. Patients with unexplained hypouricemia need detailed purine metabolic investigations.

Genetic disorders resulting in hyper- or hypouricemia

Serum uric acid concentrations are governed by the balance of urate production and excretion. Besides well-known secondary causes of hyperuricemia, such as myeloproliferative diseases, decreased renal function, and excessive dietary purine intake, there are a number of genetic disorders that result in hyper- or hypouricemia. Renal impairment in these disorders may be associated with the development of chronic kidney disease, acute kidney injury, or urate nephrolithiasis. These conditions are frequently misdiagnosed, not because the diagnosis is complicated and difficult to ascertain, but rather because of a lack of awareness of the particular condition. The first important step in the diagnosis is obtaining a detailed family history, with evaluation of serum and urinary urate concentrations. This review will aid physicians in identifying these inherited kidney disorders associated with hyperuricemia and hypouricemia. Identification of these conditions will help to explain the pathogenesis of different types of gout, and may extend insights into the urate transport and chronic kidney disease.

Acute kidney injury in two children caused by renal hypouricaemia type 2

BACKGROUND

Renal hypouricaemia is a heterogeneous inherited disorder characterized by impaired tubular uric acid transport with severe complications, such as acute kidney injury and nephrolithiasis. Type 1 is caused by a loss-of-function mutation in the SLC22A12 gene (OMIM #220150), while type 2 is caused by defects in the SLC2A9 gene (OMIM #612076).

CASE-DIAGNOSIS/TREATMENT

The cases of two children, a 12- and a 14-year-old boy with acute kidney injury (proband 1: urea 9.4 mmol/l, creatinine 226 μmol/l; proband 2: urea 11.7 mmol/l, creatinine 202 μmol/l) are described. Both are offspring of nonconsanguineous couples in the UK. The concentrations of serum uric acid were consistently below the normal range (0.03 and 0.04 mmol/l) and expressed as an increase in the fractional excretion of uric acid (46 and 93 %).

CONCLUSIONS

A sequencing analysis of the coding region of uric acid transporters SLC22A12 and SLC2A9 was performed. Analysis of genomic DNA revealed two unpublished missense transitions, p.G216R and p.N333S in the SLC2A9 gene. No sequence variants in SLC22A12 were found. Our findings suggest that homozygous and/or compound heterozygous loss-of-function mutations p.G216R and p.N333S cause renal hypouricaemia via loss of uric acid absorption and do lead to acute kidney injury.

Diagnostic tests for primary renal hypouricemia

Primary renal hypouricemia is a genetic disorder characterized by defective renal uric acid (UA) reabsorption with complications such as nephrolithiasis and exercise-induced acute renal failure. The known causes are: defects in the SLC22A12 gene, encoding the human urate transporter 1 (hURAT1), and also impairment of voltage urate transporter (URATv1), encoded by SLC2A9 (GLUT9) gene. Diagnosis is based on hypouricemia (<119 μmol/L) and increased fractional excretion of UA (>10%). To date, the cases with mutations in hURAT1 gene have been reported in East Asia only. More than 100 Japanese patients have been described. Hypouricemia is sometimes overlooked; therefore, we have set up the flowchart for this disorder. The patients were selected for molecular analysis from 620 Czech hypouricemic patients. Secondary causes of hyperuricosuric hypouricemia were excluded. The estimations of (1) serum UA, (2) excretion fraction of UA, and (3) analysis of hURAT1 and URATv1 genes follow. Three transitions and one deletion (four times) in SLC22A12 gene and one nucleotide insertion in SLC2A9 gene in seven Czech patients were found. Three patients had acute renal failure and urate nephrolithiasis. In addition, five nonsynonymous sequence variants and three nonsynonymous sequence variants in SLC2A9 gene were found in two UK patients suffering from acute renal failure. Our finding of the defects in SLC22A12 and SLC2A9 genes gives further evidence of the causative genes of primary renal hypouricemia and supports their important role in regulation of serum urate levels in humans.

Novel mutations in xanthine dehydrogenase/oxidase cause severe hypouricemia: biochemical and molecular genetic analysis in two Czech families with xanthinuria type I

BACKGROUND

The article describes the clinical, biochemical, enzymological and molecular genetics findings in two patients from two families with xanthinuria type I.

METHODS

Biochemical analysis using high performance liquid chromatography, allopurinol loading test and analysis of xanthine oxidase activity in plasma and of uromodulin excretion in urine were performed. Sequencing analysis of the xanthine dehydrogenase gene and the haplotype and statistical analyses of consanguinity were performed.

RESULTS

Probands showed extremely low concentrations of uric acid, on seven occasions under the limit of detection. The concentration of uric acid in 38-year-old female was 15 μmol/L in serum and 0.04 mmol/L in urine. Excretion of xanthine in urine was 170 mmol/mol creatinine. The concentration of uric acid in 25-year-old male was 0.03 mmol/L in urine. Excretion of xanthine in urine was 141 mmol/mol creatinine. The allopurinol loading test confirmed xanthinuria type I. The xanthine oxidase activities in patients were 0 and 0.4 pmol/h/mL of plasma. We found three nonsense changes: p.P214QfsX4 and unpublished p.R825X and p.R881X.

CONCLUSIONS

We found two nonconsanguineous compound heterozygotes with xanthinuria type I caused by three nonsense changes. The methods used did not confirm consanguinity in the probands, thus there might be an unconfirmed biological relationship or mutational hotspot.

Novel homozygous insertion in SLC2A9 gene caused renal hypouricemia

Renal hypouricemia is a heterogeneous inherited disorder characterized by impaired uric acid handling in the renal tubules. Patients are usually asymptomatic; however, some may experience urolithiasis and/or acute kidney injury. Most of the described patients (compound heterozygous and/or homozygous) are Japanese with mutations in the SLC22A12 gene (OMIM #220150). Four patients with renal hypouricemia caused by heterozygous defects and two families with homozygous mutations in the SLC2A9 gene have been recently described (OMIM #612076). We describe the clinical history, biochemical and molecular genetics findings of a Czech family with renal hypouricemia. The concentration of serum uric acid in the proband (16-year-old Czech girl with unrelated parents) was 0.17 ± 0.05 mg/dl and expressed as an increase in the fractional excretion of uric acid (194 ± 99%). The sequencing analysis of the coding region of uric acid transporters SLC22A12, SLC2A9, SLC17A3, ABCC4 and ABCG2, was performed. Analysis of genomic DNA revealed novel one nucleotide homozygote insertion in exon 3 in the SLC2A9 gene in proband and her brother resulting in a truncated protein (p.Ile118HisfsX27). No sequence variants in other candidate uric acid transporter were found. Homozygous loss-of-function mutations cause massive renal hypouricemia via total loss of uric acid absorption; however, they do not necessarily lead to nephrolithiasis and acute kidney injury. In contrast to previously reported heterozygous patients with renal hypouricemia type 2, we did not find even slight hypouricemia and found no decrease in the FE-UA of the heterozygous parents of the reported siblings.

Unusual presentation of Kelley-Seegmiller syndrome

Female carriers of hypoxanthine-guanine phosphoribosyltransferase (HPRT) deficiency have somatic cell mosaicism of HPRT activity and are healthy. We report a 50-year-old woman without gout or nephrolithiasis. She was never on allopurinol. Normal serum uric acid concentrations, increased plasma hypoxanthine, and xanthine were found. HPRT activity in erythrocytes was surprisingly low: at 8.6 nmol h(-1) mg (-1) haemoglobin. Mutation analysis revealed a heterozygous HPRT gene mutation, c.215A > G (p.Tyr72Cys). Assessment of X-inactivation ratio has shown that > 75% of the active X-chromosome bears the mutant allele and could explain these unusual, previously undescribed findings.

Polymorphisms of the TPMT gene in the Czech healthy population and patients with inflammatory bowel disease

Genetic variation in thiopurine S-methyltransferase (TPMT) is a major factors for wide variation in the metabolism and safety of thiopurine drugs. We investigated the frequency of functional gene polymorphisms in 396 patients with inflammatory bowel disease and 300 healthy subjects. Frequencies of functionally deficient alleles TPMT*2, TPMT*3A, TPMT*3B, and TPMT*3B in the patient group were 0.1%, 4.3%, 0.1%, and 0.4%, respectively, and were similar to those of healthy subjects in the Czech population. Our results provide necessary information for pharmacoeconomic studies in the Czech Republic.

An unusual cause of renal amyloidosis secondary to gout: the first description of familial occurrence

BACKGROUND

AA amyloidosis caused by the chronic inflammation accompanying gouty arthritis is extremely rare and familial occurrence has not been described so far.

CASE REPORT

We present the case of two brothers (47 and 44 years old) with 7- and 10-year history of hyperuricaemia and chronic tophaceous gout with polyarticular involvement. The enzymatic assay performed in their erythrocytes proved the partial hypoxanthine-guanine phosphoribosyl transferase deficiency (Kelley-Seegmiller syndrome), the genetic defect of purine metabolism. Later on they developed proteinuria and chronic renal insufficiency /CRI/. Renal biopsy disclosed the combination of AA amyloidosis and gouty nephropathy in both the cases. Despite the standard treatment the older brother progressed to chronic renal failure. On the contrary, the younger one being longterm treated with oral colchicin have stabilized CRI.

CONCLUSIONS

Only several cases of AA renal amyloidosis until recently, secondary to gout have been reported. Our case represents the first report of familial occurrence of this extremely rare disease.

Analysis of excretion fraction of uric acid

Excretion fraction of uric acid (EFUA), is one of the most important hallmarks for diagnosis of familial juvenile hyperuricemic nephropathy (FJHN) and hereditary renal hypouricemia. EFUA was measured in 20 patients with FJHN. However, low excretion fraction (<6%) was found also in healthy FJHN family members and healthy controls (ref. ranges EFUA: men 6-12%, women 6-20%). Similar finding of low EFUA was reported recently. Distribution of EFUA was further studied in 2,416 healthy controls, which were selected from 6,000 samples and divided according to age. In conclusion, finding of low EFUA in family members is a risk factor for renal damage and indication for purine metabolic investigations with subsequent molecular biology analysis. As EFUA could be found also in healthy controls--it should be interpreted with care and other features of FJHN (such as hyperuricemia, progressive renal disease in family) should be taken to account.

Purine and pyrimidine metabolism: a firm basis for a transformed society

Purines and pyrimidines form the backbone of DNA and RNA. Hence, modification of purine and pyrimidine metabolism can have serious effects on normal functioning of a subject. These aspects formed the main topics for an International and a European Series of meetings, dedicated to the metabolism in man. In order to streamline the organization of these meetings the European Society was transformed to an International society: the Purine and Pyrimidine Society (www.ppsociety.org). This special issue of Nucleosides, Nucleotides, and, Nucleic, Acids highlights the last European meeting in Prague, focusing on inborn errors, cardiac diseases, inflammatory diseases, rheumatology, haematology, cancer, virology, genetic polymorphism, specific methodology, and, of course, metabolism. The meeting in Chicago in 2007 will be the first meeting of the Purine and Pyrimidine Society.

[Diagnostic aspects of familial juvenile hyperuriceamic nephropathy]

BACKGROUND; Familial juvenile hyperuricemic nephropathy (FJHN) is a genetic disorder with the autosomal dominant mode of hereditability; characterized with hyperuricemia, gout and progressive renal disease. Characterization of the disease together with clinical and biochemical findings in patients of Czech population is described.

METHODS AND RESULTS

The bloodlines of three Czech families with FJHN were set up on the basis of their family history. The specimens of blood and urine were taken from 57 family members for biochemical investigations and isolations of genomic DNA. Blood and urinary concentrations of the uric acid and creatinine together with values of excretion fraction of uric acid and Kaufman's index were determined. Based on these results diagnosis of FJHN was established or confirmed in 19 patients. One additional patient was diagnosed on the results of linkage analysis.

CONCLUSIONS

FJHN is a disorder sharing non-specific clinical and biochemical signs with the group of familial renal disorders. The effective diagnosis is difficult due to the heterogeneity of the disorder and limited availability of molecular genetic analysis. Detailed purine metabolic investigation together with precise family history is thus necessary and very important in family members with hyperuricemia and/or gout (particularly in childhood or young women) as well as in patients with familial renal disease.

Familial juvenile hyperuricaemic nephropathy (FJHN): linkage analysis in 15 families, physical and transcriptional characterisation of the FJHN critical region on chromosome 16p11.2 and the analysis of seven candidate genes

Familial juvenile hyperuricaemic nephropathy (FJHN) is an autosomal dominant renal disease characterised by juvenile onset of hyperuricaemia, gouty arthritis, and progressive renal failure at an early age. Recent studies in four kindreds showed linkage of a gene for FJHN to the same genomic interval on chromosome 16p11.2, where the gene for the phenotypically similar medullary cystic disease type 2 (MCKD2) has been localised. In this study we performed linkage analysis in additional 15 FJHN families. Linkage of FJHN to 16p11.2 was confirmed in six families, which suggests that, in a large proportion of FJHN kindreds, the disease is likely to be caused by a gene or genes located outside of 16p11.2. Haplotype analysis of the new and previously analysed families provided two non-overlapping critical regions on 16p11.2-FJHN1, delimited by markers D16S499-D16S3036 and FJHN2, delimited by markers D16S412-D16S3116. Considering MCKD2 to be a distinct molecular entity, the analysis suggests that as many as three kidney disease genes may be located in close proximity on 16p11.2. From genomic databases we compiled integrated physical and transcription maps of whole critical genomic region in which 45 known genes and 129 predicted loci have been localised. We selected, analysed and found no pathogenic mutations in seven candidate genes. The linkage and haplotype analysis reported here demonstrates the genetic heterogeneity of FJHN. The report of integrated physical and mostly in-silico predicted transcription maps of the FJHN critical region provides a basis for precise experimental annotation of the current transcript map, which is essential for final identification of the FJHN gene(s).

Human adenylosuccinate lyase (ADSL), cloning and characterization of full-length cDNA and its isoform, gene structure and molecular basis for ADSL deficiency in six patients

Adenylosuccinate lyase (ADSL) is a bifunctional enzyme acting in de novo purine synthesis and purine nucleotide recycling. ADSL deficiency is a selectively neuronopathic disorder with psychomotor retardation and epilepsy as leading traits. Both dephosphorylated enzyme substrates, succinylaminoimidazole-carboxamide riboside (SAICAr) and succinyladenosine (S-Ado), accumulate in the cerebrospinal fluid (CSF) of affected individuals with S-Ado/SAICAr concentration ratios proportional to the phenotype severity. We studied the disorder at various levels in a group of six patients with ADSL deficiency. We identified the complete ADSL cDNA and its alternatively spliced isoform resulting from exon 12 skipping. Both mRNA isoforms were expressed in all the tissues studied with the non-spliced form 10-fold more abundant. Both cDNAs were expressed in Escherichia coli and functionally characterized at the protein level. The results showed only the unspliced ADSL to be active. The gene consists of 13 exons spanning 23 kb. The promotor region shows typical features of the housekeeping gene. Eight mutations were identified in a group of six patients. The expression studies of the mutant proteins carried out in an attempt to study genotype-phenotype correlation showed that the level of residual enzyme activity correlates with the severity of the clinical phenotype. All the mutant enzymes studied in vitro displayed a proportional decrease in activity against both of their substrates. However, this was not concordant with strikingly different concentration ratios in the CSF of individual patients. This suggests either different in vivo enzyme activities against each of the substrates and/or their different turnover across the CSF-blood barrier, which may be decisive in determining disease severity.

Familial juvenile hyperuricemic nephropathy: localization of the gene on chromosome 16p11.2-and evidence for genetic heterogeneity

Familial juvenile hyperuricemic nephropathy (FJHN), is an autosomal dominant renal disease characterized by juvenile onset of hyperuricemia, gouty arthritis, and progressive renal failure at an early age. Using a genomewide linkage analysis in three Czech affected families, we have identified, on chromosome 16p11.2, a locus for FJHN and have found evidence for genetic heterogeneity and reduced penetrance of the disease. The maximum two-point LOD score calculated with allowance for heterogeneity (HLOD) was 4.70, obtained at recombination fraction 0, with marker D16S3036; multipoint linkage analysis yielded a maximum HLOD score of 4.76 at the same location. Haplotype analysis defined a 10-cM candidate region between flanking markers D16S501 and D16S3113, exhibiting crossover events with the disease locus. The candidate interval contains several genes expressed in the kidney, two of which-uromodulin and NADP-regulated thyroid-hormone-binding protein-represent promising candidates for further analysis.

Combined adenine phosphoribosyltransferase and N-acetylgalactosamine-6-sulfate sulfatase deficiency

We describe a Czech patient with combined adenine phosphoribosyltransferase (APRT) deficiency (2,8-dihydroxyadenine urolithiasis) and N-acetylgalactosamine-6-sulfate sulfatase (GALNS) deficiency (mucopolysaccharidosis Type IVA, Morquio disease A). Adenine and its extremely insoluble derivative, 2,8-dihydroxyadenine, were identified in the urine, and APRT deficiency was confirmed in erythrocytes. There was excessive excretion of keratan sulfate in the urine, and GALNS deficiency was confirmed in leukocytes. GALNS and APRT are both located on chromosome 16q24.3, suggesting that the patient had a deletion involving both genes. PCR amplification of genomic DNA indicated that a novel junction was created by the fusion of sequences distal to GALNS exon 2 and proximal to APRT exon 3, and that the size of the deleted region was approximately 100 kb. The deletion breakpoints were localized within GALNS intron 2 and APRT intron 2. Several other genes, including the alpha subunit of cytochrome B (CYBA), which is deleted or mutated in the autosomal form of chronic granulomatous disease, are located in the 16q24.3 region, but PCR amplification showed that this gene was present in the proband. A patient with hemizygosity for GALNS deficiency and APRT deficiency has been reported from Japan recently. These findings indicate that: (i) APRT is located telomeric to GALNS; (ii) GALNS and APRT are transcribed in the same orientation (centromeric to telomeric); and (iii) combined APRT/GALNS deficiency may be more common than hitherto realized.

Identification and determination of succinyladenosine in human cerebrospinal fluid

Succinyladenosine (S-Ado) is a biochemical marker of adenylosuccinase deficiency--the genetic defect of purine de novo synthesis. S-Ado has been previously reported as normally undetectable in cerebrospinal fluid (CSF) of children not suffering from this defect. In present study, we employed solid-phase extraction and thin-layer chromatography for isolation of a compound with spectral and chromatographic characteristics identical to S-Ado from human CSF. The high-performance liquid chromatography-negative-ion electrospray ionization mass spectrometry analysis confirmed that the isolated compound is S-Ado. We established the reference values of S-Ado in CSF of children (1.1+/-0.4 micromol/l; mean +/- S.D; n = 26) by means of reversed-phase HPLC method on a C18 column with UV detection.

[Familial juvenile gouty nephropathy]

The authors present the description of a family comprising father (his mother had died middle-aged from renal failure) and his two children aged 15 and 17 years who developed is young age (already in the second decade) gouty arthritis and primary interstitial nephritis. Based on the laboratory finding of hyperuricaemia with disproportionately low urate excretion and excretion of excessive uric acid formation, an enzyme defect and other renal disease the authors diagnosed familial gouty juvenile nephropathy. This diagnosis was confirmed also by histological examination of renal biopsy in the youngest member of the family. It is a disease which belongs into the group of hereditary types of nephritis. In the literature worldwide some nine families were described, in the Czech Republic it is the first description of this condition.

The allopurinol loading test for identification of carriers for ornithine carbamoyl transferase deficiency: studies in a healthy control population and females at risk

The increase in orotidine excretion following a 300 mg allopurinol dose has been used for carrier detection in ornithine carbamoyl transferase (OCT) deficiency. This test was evaluated, using three collection periods, in 23 healthy women, 4 obligate heterozygotes and 32 other women at risk of being carriers of OCT deficiency. Four methods for the analysis of orotidine and orotic acid were compared. Using the most reproducible method, the excretion of orotic acid in controls was found to be consistently higher than that of orotidine in all three periods. The distribution of both orotic acid and orotidine excretion of controls was skewed so that standard deviations (S.D.) were calculated after logarithmic transformation. All four obligate heterozygotes showed orotic acid and orotidine excretion in excess of 3 S.D. above the control mean and a further 7 women had one or more excretion values in excess of 3 S.D., while 16 gave a value of less than 2 S.D. for both metabolites. We conclude that the predictive value of the test is good, that both orotic acid and orotidine should be measured to reduce the risk of misclassification and that values greater than 2 S.D. for both in one or more periods should be used as the cut-off point to identify carriers.