The Intricacies of Renal Phosphate Reabsorption-An Overview.

To maintain an optimal body content of phosphorus throughout postnatal life, variable phosphate absorption from food must be finely matched with urinary excretion. This amazing feat is accomplished through synchronised phosphate transport by myriads of ciliated cells lining the renal proximal tubules. These respond in real time to changes in phosphate and composition of the renal filtrate and to hormonal instructions. How they do this has stimulated decades of research. New analytical techniques, coupled with incredible advances in computer technology, have opened new avenues for investigation at a sub-cellular level. There has been a surge of research into different aspects of the process. These have verified long-held beliefs and are also dramatically extending our vision of the intense, integrated, intracellular activity which mediates phosphate absorption. Already, some have indicated new approaches for pharmacological intervention to regulate phosphate in common conditions, including chronic renal failure and osteoporosis, as well as rare inherited biochemical disorders. It is a rapidly evolving field. The aim here is to provide an overview of our current knowledge, to show where it is leading, and where there are uncertainties. Hopefully, this will raise questions and stimulate new ideas for further research.

Evaluation of the economic burden of kidney stone disease in the UK: a retrospective cohort study with a mean follow-up of 19 years.

OBJECTIVE: To estimate the cost of kidney stone disease (KSD) in England. PATIENTS AND METHODS: We conducted a retrospective cohort study of patients with KSD, referred to a metabolic stone clinic between 1990 and 2007 using electronic records of patients with KSD in a tertiary referral centre, to determine cost using UK National Health Service (NHS) tariff, with subsequent extrapolation to the entire England population. Those with no documentation and <5 years follow-up were excluded. The outcome measure was calculation of cost (as per 2018 NHS tariff) presented as lower and higher estimates for: per episode; total within the cohort; and estimation of initial, 5-,10- and 15-year costs for the cohort and total population in England. Linear regression was used to examine for significant predictors of per episode and total cost. RESULTS: A total of 781 patients were included in the study after 1000 records were screened for inclusion, with a mean follow-up of 19 years. The mean (SD) overall costs per episode were between £1277 (1724) and £2887 (2492). Total initial costs for the cohort were between £950 842 and £2 336 442, rising to between £1.43 million and £3.02 million at 15 years of follow-up. Estimated cost in 2010 in England alone was between £190 million and £324 million. CONCLUSION: KSD is a costly disease, comparable to the combined cost of prostate and bladder cancer in UK.

Monocarboxylate transporter 1 deficiency and ketone utilization.

Ketoacidosis is a potentially lethal condition caused by the imbalance between hepatic production and extrahepatic utilization of ketone bodies. We performed exome sequencing in a patient with recurrent, severe ketoacidosis and identified a homozygous frameshift mutation in the gene encoding monocarboxylate transporter 1 (SLC16A1, also called MCT1). Genetic analysis in 96 patients suspected of having ketolytic defects yielded seven additional inactivating mutations in MCT1, both homozygous and heterozygous. Mutational status was found to be correlated with ketoacidosis severity, MCT1 protein levels, and transport capacity. Thus, MCT1 deficiency is a novel cause of profound ketoacidosis; the present work suggests that MCT1-mediated ketone-body transport is needed to maintain acid-base balance.

The acetaminophen metabolite N-acetyl-p-benzoquinone imine (NAPQI) inhibits glutathione synthetase in vitro; a clue to the mechanism of 5-oxoprolinuric acidosis?

1. Metabolic acidosis due to accumulation of l-5-oxoproline is a rare, poorly understood, disorder associated with acetaminophen treatment in malnourished patients with chronic morbidity. l-5-Oxoprolinuria signals abnormal functioning of the γ-glutamyl cycle, which recycles and synthesises glutathione. Inhibition of glutathione synthetase (GS) by N-acetyl-p-benzoquinone imine (NAPQI) could contribute to 5-oxoprolinuric acidosis in such patients. We investigated the interaction of NAPQI with GS in vitro. 2. Peptide mapping of co-incubated NAPQI and GS using mass spectrometry demonstrated binding of NAPQI with cysteine-422 of GS, which is known to be essential for GS activity. Computational docking shows that NAPQI is properly positioned for covalent bonding with cysteine-422 via Michael addition and hence supports adduct formation. 3. Co-incubation of 0.77 µM of GS with NAPQI (25-400 µM) decreased enzyme activity by 16-89%. Inhibition correlated strongly with the concentration of NAPQI and was irreversible. 4. NAPQI binds covalently to GS causing irreversible enzyme inhibition in vitro. This is an important novel biochemical observation. It is the first indication that NAPQI may inhibit glutathione synthesis, which is pivotal in NAPQI detoxification. Further studies are required to investigate its biological significance and its role in 5-oxoprolinuric acidosis.

Clinical pattern, mutations and in vitro residual activity in 33 patients with severe 5, 10 methylenetetrahydrofolate reductase (MTHFR) deficiency.

BACKGROUND: Severe methylenetetrahydrofolate reductase (MTHFR) deficiency is a rare inborn defect disturbing the remethylation of homocysteine to methionine (<200 reported cases). This retrospective study evaluates clinical, biochemical genetic and in vitro enzymatic data in a cohort of 33 patients. METHODS: Clinical, biochemical and treatment data was obtained from physicians by using a questionnaire. MTHFR activity was measured in primary fibroblasts; genomic DNA was extracted from cultured fibroblasts. RESULTS: Thirty-three patients (mean age at follow-up 11.4 years; four deceased; median age at first presentation 5 weeks; 17 females) were included. Patients with very low (<1.5%) mean control values of enzyme activity (n = 14) presented earlier and with a pattern of feeding problems, encephalopathy, muscular hypotonia, neurocognitive impairment, apnoea, hydrocephalus, microcephaly and epilepsy. Patients with higher (>1.7-34.8%) residual enzyme activity had mainly psychiatric symptoms, mental retardation, myelopathy, ataxia and spasticity. Treatment with various combinations of betaine, methionine, folate and cobalamin improved the biochemical and clinical phenotype. During the disease course, patients with very low enzyme activity showed a progression of feeding problems, neurological symptoms, mental retardation, and psychiatric disease while in patients with higher residual enzyme activity, myelopathy, ataxia and spasticity increased. All other symptoms remained stable or improved in both groups upon treatment as did brain imaging in some cases. No clear genotype-phenotype correlation was obvious. DISCUSSION: MTHFR deficiency is a severe disease primarily affecting the central nervous system. Age at presentation and clinical pattern are correlated with residual enzyme activity. Treatment alleviates biochemical abnormalities and clinical symptoms partially.

2-Pentanone production from hexanoic acid by Penicillium roqueforti from blue cheese: is this the pathway used in humans?

Production of 2-pentanone, a methylketone, is increased in fasting ketotic humans. Its origin is unknown. We hypothesised that it is formed via ß -oxidation of hexanoic acid by the peroxisomal pathway proposed for methylketone-producing fungi and yeasts. We used Penicillium roqueforti cultured on fat (margarine) to investigate 2-pentanone production. Headspace gas of incubates of the mould with a range of substrates was analysed using solid-phase microextraction with gas chromatography-mass spectrometry. Consistent with the proposed pathway, 2-pentanone was formed from hexanoic acid, hexanoyl-CoA, hexanoylcarnitine, and ethyl-3-oxohexanoic acid but not from ethylhexanoic, 2-ethylhexanoic, octanoic, or myristic acids, octanoylcarnitine, or pentane. However, the products from deuterated (D) hexanoic-D11 acid and hexanoic-2, 2-D2 acid were 9D- and 2D-2-pentanone, respectively, and not 8D- and 1D-2-pentanone as predicted. When incubated under (18)O2/(14)N2, there was only a very small enrichment of [(16)O2]- with [(18)O2]-containing 2-pentanone. These are new observations. They could be explained if hydrogen ions removed from hexanoyl-CoA by acyl-CoA oxidase at the commencement of ß -oxidation were cycled through hydrogen peroxide and reentered the pathway through hydration of hexenoyl-CoA. This would protect other proteins from oxidative damage. Formation of 2-pentanone through a ß -oxidation cycle similar to Penicillium roqueforti would be consistent with observations in humans.

When is an SNP not an SNP?

Genomic duplications are important sources of structural change and gene innovation. In humans, the most recent and highly identical sequences (>90% homology, >1 kb long) are known as segmental duplications (SDs). Single-nucleotide variants or single-nucleotide polymorphisms within SDs have not been systematically assessed due to limitations around mapping short-read sequencing data. Single-nucleotide variant rs62486260 was flagged in a study of familial renal stone disease but it was unclear whether it was real or an artifact resulting from the presence of a SD. We describe in silico and wet-lab approaches to investigate this, using segment-specific long-PCR assays, followed by short PCR for Sanger sequencing. Our conclusion was that rs62486260 is an artifact. Our approach can be generalized to deal with other such situations.

The method described includes a two-step procedure for determining whether an apparent single-nucleotide polymorphism may be an artifact resulting from the presence of a duplicated genomic region/pseudogene. Step one involves identifying sequence differences between the two duplicated regions and designing a long PCR assay to specifically amplify each region separately. Step 2 involves amplifying a short PCR product which flanks the single-nucleotide polymorphism of interest, from the long products generated in step 1.

Biochemistry and pathophysiology of the Transient Potential Receptor Vanilloid 6 (TRPV6) calcium channel.

TRPV6 is a Transient Receptor Potential Vanilloid (TRPV) cation channel with high selectivity for Ca2+ ions. First identified in 1999 in a search for the gene which mediates intestinal Ca2+ absorption, its far more extensive repertoire as a guardian of intracellular Ca2+ has since become apparent. Studies on TRPV6-deficient mice demonstrated additional important roles in placental Ca2+ transport, fetal bone development and male fertility. The first reports of inherited deficiency in newborn babies appeared in 2018, revealing its physiological importance in humans. There is currently strong evidence that TRPV6 also contributes to the pathogenesis of some common cancers. The recently reported association of TRPV6 deficiency with non-alcoholic chronic pancreatitis suggests a role in normal pancreatic function. Over time and with greater awareness of TRPV6, other disease-associations are likely to emerge. Powerful analytical tools have provided invaluable insights into the structure and operation of TRPV6. Its roles in Ca2+ signaling and carcinogenesis, and the use of channel inhibitors in cancer treatment are being intensively investigated. This review first briefly describes the biochemistry and physiology of the channel, and analytical methods used to investigate these. The focus subsequently shifts to the clinical disorders associated with abnormal expression and the underlying pathophysiology. The aims of this review are to increase awareness of this channel, and to draw together findings from a wide range of sources which may help to formulate new ideas for further studies.

Hsp40 chaperones promote degradation of the HERG potassium channel.

Loss of function mutations in the hERG (human ether-a-go-go related gene or KCNH2) potassium channel underlie the proarrhythmic cardiac long QT syndrome type 2. Most often this is a consequence of defective trafficking of hERG mutants to the cell surface, with channel retention and degradation at the endoplasmic reticulum. Here, we identify the Hsp40 type 1 chaperones DJA1 (DNAJA1/Hdj2) and DJA2 (DNAJA2) as key modulators of hERG degradation. Overexpression of the DJAs reduces hERG trafficking efficiency, an effect eliminated by the proteasomal inhibitor lactacystin or with DJA mutants lacking their J domains essential for Hsc70/Hsp70 activation. Both DJA1 and DJA2 cause a decrease in the amount of hERG complexed with Hsc70, indicating a preferential degradation of the complex. Similar effects were observed with the E3 ubiquitin ligase CHIP. Both the DJAs and CHIP reduce hERG stability and act differentially on folding intermediates of hERG and the disease-related trafficking mutant G601S. We propose a novel role for the DJA proteins in regulating degradation and suggest that they act at a critical point in secretory pathway quality control.

Exome sequencing identifies a disease variant of the mitochondrial ATP-Mg/Pi carrier SLC25A25 in two families with kidney stones.

BACKGROUND: Calcium kidney stones are common and recurrences are often not preventable by available empiric remedies. Their etiology is multifactorial and polygenic, and an increasing number of genes are implicated. Their identification will enable improved management. METHODS: DNA from three stone-formers in a Southampton family (UK) and two from an Italian family were analyzed independently by whole exome sequencing and selected variants were genotyped across all available members of both pedigrees. A disease variant of SLC25A25 (OMIM 608745), encoding the mitochondrial ATP-Mg/Pi carrier 3 (APC3) was identified, and analyzed structurally and functionally with respect to its calcium-regulated transport activity. RESULTS: All five patients had a heterozygous dominant SLC25A25 variant (rs140777921; GRCh37.p13: chr 9 130868670 G>C; p.Gln349His; Reference Sequence NM_001006641.3). Non-stone formers also carried the variant indicating incomplete penetrance. Modeling suggests that the variant lacks a conserved polar interaction, which may cause structural instability. Calcium-regulated ATP transport was reduced to ~20% of the wild type, showing a large reduction in function. CONCLUSION: The transporter is important in regulating mitochondrial ATP production. This rare variant may increase urine lithogenicity through impaired provision of ATP for solute transport processes in the kidney, and/or for purinergic signaling. Variants found in other genes may compound this abnormality.

Potential in vitro effects of carbon nanotubes on human aortic endothelial cells.

Respiratory exposure of mice to carbon nanotubes induces pulmonary toxicity and adverse cardiovascular effects associated with atherosclerosis. We hypothesize that the direct contact of carbon nanotubes with endothelial cells will result in dose-dependent effects related to altered cell function and cytotoxicity which may play a role in potential adverse pulmonary and cardiovascular outcomes. To test this hypothesis, we examined the effects of purified single- and multi-walled carbon nanotubes (SWCNT and MWCNT) on human aortic endothelial cells by evaluating actin filament integrity and VE-cadherin distribution by fluorescence microscopy, membrane permeability by measuring the lactate dehydrogenase (LDH) release, proliferation/viability by WST-1 assay, and overall functionality by tubule formation assay. Marked actin filament and VE-cadherin disruption, cytotoxicity, and reduced tubule formation occurred consistently at 24 h post-exposure to the highest concentrations [50-150 microg/10(6) cells (1.5-4.5 microg/ml)] for both SWCNT and MWCNT tested in our studies. These effects were not observed with carbon black exposure and carbon nanotube exposure in lower concentrations [1-10 microg/10(6) cells (0.04-0.4 microg/ml)] or in any tested concentrations at 3 h post-exposure. Overall, the results indicate that SWCNT and MWCNT exposure induce direct effects on endothelial cells in a dose-dependent manner.

Phosphaturia in kidney stone formers: Still an enigma.

Calcium kidney stones are common worldwide. Most are idiopathic and composed of calcium oxalate. Calcium phosphate is present in around 80% and may initiate stone formation. Stone production is multifactorial with a polygenic genetic contribution. Phosphaturia is found frequently among stone formers but until recently received scant attention. This review examines possible mechanisms for the phosphaturia and its relevance to stone formation from a wide angle. There is a striking lack of clinical data. Phosphaturia is associated, but not correlated, with hypercalciuria, increased 1,25 dihydroxy-vitamin D [1,25 (OH)2D], and sometimes evidence of disturbances in proximal renal tubular function. Phosphate reabsorption in the proximal renal tubules requires tightly regulated interaction of many proteins. Paracellular flow through intercellular tight junctions is the major route of phosphate absorption from the intestine and can be reduced therapeutically in hyperphosphatemic patients. In monogenic defects stones develop when phosphaturia is associated with hypercalciuria, generally explained by increased 1,25 (OH)2D production in response to hypophosphatemia. Calcification does not occur in disorders with increased FGF23 when phosphaturia occurs in isolation and 1,25 (OH)2D is suppressed. Candidate gene studies have identified mutations in the phosphate transporters, but in few individuals. One genome-wide study identified a polymorphism of the phosphate transporter gene SLC34A4 associated with stones. Others did not find mutations obviously linked to phosphate reabsorption. Future genetic studies should have a wide trawl and should focus initially on groups of patients with clearly defined phenotypes. The global data should be pooled.

Protein kinase Calpha activates c-Src and induces podosome formation via AFAP-110.

We report that the actin filament-associated protein AFAP-110 is required to mediate protein kinase Calpha (PKCalpha) activation of the nonreceptor tyrosine kinase c-Src and the subsequent formation of podosomes. Immunofluorescence analysis demonstrated that activation of PKCalpha by phorbol 12-myristate 13-acetate (PMA), or ectopic expression of constitutively activated PKCalpha, directs AFAP-110 to colocalize with and bind to the c-Src SH3 domain, resulting in activation of the tyrosine kinase. Activation of c-Src then directs the formation of podosomes, which contain cortactin, AFAP-110, actin, and c-Src. In a cell line (CaOV3) that has very little or no detectable AFAP-110, PMA treatment was unable to activate c-Src or effect podosome formation. Ectopic expression of AFAP-110 in CaOV3 cells rescued PKCalpha-mediated activation of c-Src and elevated tyrosine phosphorylation levels and subsequent formation of podosomes. Neither expression of activated PKCalpha nor treatment with PMA was able to induce these changes in CAOV3 cells expressing mutant forms of AFAP-110 that are unable to bind to, or colocalize with, c-Src. We hypothesize that one major function of AFAP-110 is to relay signals from PKCalpha that direct the activation of c-Src and the formation of podosomes.

Solid-phase microextraction: investigation of the metabolism of substances that may be abused by inhalation.

Purified liquefied petroleum gas (LPG), a mixture of butane, isobutane, and propane, is commonly abused by inhalation. Little is known about the mammalian metabolism of these substances. Metabolism of other hydrocarbons, including n-hexane and cyclohexane, has been studied in vitro using a range of liver preparations, with metabolites analyzed by static headspace techniques. Solid-phase microextraction (SPME) for sampling metabolites in the headspace of incubates of volatile compounds with activated rat liver microsomes is investigated. Cyclohexanol and cyclohexanone were formed from cyclohexane and 1-, 2-, and 3-hexanol and 2-hexanone from n-hexane as predicted. Secondary alcohols are found for the other compounds studied, except for propene and isobutane, together with 2-propanone and 2-butanone from propane and n-butane, respectively. Samples from three individuals who died following LPG abuse contained a range of putative n-butane metabolites: n-butanol, 2-butanol, 2,3-butanediol, 3-hydroxy-2-butanone, and 2,3-butanedione. To our knowledge, the last three compounds have not been proposed as metabolites of n-butane in man. These might be produced through similar metabolic pathways to those of n-hexane and n-heptane. The findings indicate the value of SPME for investigating the metabolism of volatile substances and for detecting and monitoring exposure to these compounds.

A novel pyrroline-5-carboxylic acid and acetoacetic acid adduct in hyperprolinaemia type II.

BACKGROUND: From investigations of a child with hyperprolinaemia type II, we demonstrated in vitro that pyridoxal phosphate forms a novel adduct with a proline metabolite, pyrroline-5-carboxylic acid, through Claisen condensation. Studies indicated that this was a previously unsuspected generic reaction of aldehydes and some ketones. We have subsequently found the acetoacetic acid adduct in both plasma and urine from the affected child. METHODS: Mixtures of acetoacetic acid and pyrroline-5-carboxylic acid were co-incubated at pH 7.4 and 37 degrees C, dried, or extracted and dried, derivatised and analysed by gas chromatography/mass spectrometry (GC/MS). Urine and plasma from the child were analysed. RESULTS: Fourteen new peaks were found in derivatised pyrroline-5-carboxylic acid/acetoacetic acid co-incubates. From accurate molecular mass data, the four largest peaks were probably diastereoisomers of tri-trimethylsilyl (tri-TMS) derivatives of alcohol adducts formed by Claisen condensation. Eight other peaks were mono- and di-trimethylsilyl derivatives of the adduct and a decarboxylated product. The adduct was demonstrated unequivocally in the child's acute urine and traces in plasma. CONCLUSIONS: Pyrroline-5-carboxylic acid forms an adduct with acetoacetic acid, which was present in urine of a sick child with hyperprolinaemia type II. Evidence suggests it formed in vivo. The biological significance of this novel reaction of aldehydes and ketones merits investigation.

Solid-phase extraction in clinical biochemistry.

In order to measure low concentrations of analytes in plasma and urine, it is often necessary to extract and concentrate them. With solid-phase extraction (SPE), this is achieved by partitioning the analytes between a solid and a liquid or headspace vapour. A wide range of high-quality materials is now available to do this, offering a variety of separation modes for different applications. These include partitioning using reversed-phase, normal-phase, ion-exchange, restricted-access and immunoaffinity sorbents or molecularly imprinted polymers and, increasingly, combinations of these processes. Solid-phase microextraction was introduced to analyse volatile and semi-volatile compounds. The range of sampling formats has expanded from simple packed syringes to cartridges, disks, SPE pipette tips and 96-well plates. These developments have facilitated automated off- and on-line sample processing. The basic principles of SPE and the recent innovations are reviewed here. This is a technological growth area. Some of the developments are finding application in clinical toxicology. However, they could also be of wider value in clinical chemistry--for example, for analyses of volatile and non-volatile metabolites, peptides, radioactive elements and trace metal speciation.

Ammonia metabolism and hyperammonemic disorders.

Human adults produce around 1000 mmol of ammonia daily. Some is reutilized in biosynthesis. The remainder is waste and neurotoxic. Eventually most is excreted in urine as urea, together with ammonia used as a buffer. In extrahepatic tissues, ammonia is incorporated into nontoxic glutamine and released into blood. Large amounts are metabolized by the kidneys and small intestine. In the intestine, this yields ammonia, which is sequestered in portal blood and transported to the liver for ureagenesis, and citrulline, which is converted to arginine by the kidneys. The amazing developments in NMR imaging and spectroscopy and molecular biology have confirmed concepts derived from early studies in animals and cell cultures. The processes involved are exquisitely tuned. When they are faulty, ammonia accumulates. Severe acute hyperammonemia causes a rapidly progressive, often fatal, encephalopathy with brain edema. Chronic milder hyperammonemia causes a neuropsychiatric illness. Survivors of severe neonatal hyperammonemia have structural brain damage. Proposed explanations for brain edema are an increase in astrocyte osmolality, generally attributed to glutamine accumulation, and cytotoxic oxidative/nitrosative damage. However, ammonia neurotoxicity is multifactorial, with disturbances also in neurotransmitters, energy production, anaplerosis, cerebral blood flow, potassium, and sodium. Around 90% of hyperammonemic patients have liver disease. Inherited defects are rare. They are being recognized increasingly in adults. Deficiencies of urea cycle enzymes, citrin, and pyruvate carboxylase demonstrate the roles of isolated pathways in ammonia metabolism. Phenylbutyrate is used routinely to treat inherited urea cycle disorders, and its use for hepatic encephalopathy is under investigation.

Male hypercalciuric stone formers with low renal calcium reabsorption.

AIMS: Hypercalciuria is a common poorly understood abnormality among stone formers. We aimed to identify hypercalciuric male stone formers with a primary defect in renal calcium reabsorption and to look for associated risk factors. METHODS: A retrospective cross-sectional database study of 623 male idiopathic calcium stone formers with normal plasma ultrafilterable calcium levels attending the Southampton stone clinic. Filtered calcium was estimated from plasma ultrafilterable calcium (60% of total plasma calcium) and 24 h creatinine clearance. Reabsorbed calcium was the difference between filtered and excreted calcium. RESULTS: 276 men had hypercalciuria (urine calcium >7.50 mmol/24 h); 347 had normocalciuria. Hypercalciuric men filtered more calcium than normocalciuric men: median values 247 and 227 mmol/24 h, but the ranges overlapped (175-371 and 153-316 mmol/24 h). However, across the entire filtration range, hypercalciuric men reabsorbed less of the filtered calcium. Among the hypercalciuric men, we noticed differences between those with high and low filtration. We therefore compared data for hypercalciuric men in the highest and lowest filtration quintiles (n=55). Men with high filtration were younger at their first stone episode and had significantly higher plasma ultrafilterable calcium and calcium reabsorption, urinary calcium, oxalate, urate and creatinine excretion and creatinine clearance. 35% with high filtration and 40% with low filtration had recurrent stones; 27% and 20%, respectively, had an affected first-degree relative. CONCLUSIONS: Hypercalciuric men reabsorbed proportionately less filtered calcium than normocalciuric men. Among hypercalciuric men, the risks for stones were higher in those with a high than a low filtered calcium load and presentation was earlier.

Characterisation of risk factors for stones in hyperuricosuric men attending a stone clinic.

Hyperuricosuria is common among stone formers, but its significance is uncertain. To progress our understanding and target treatment, we need to identify and characterise patients with uniform underlying pathology. We aimed to identify hyperuricosuric patients with a primary defect in renal urate reabsorption (renal hyperuricosuria) and to look for associated risk factors for stones. We undertook a retrospective cross-sectional database study of 666 male stone formers attending the Southampton stone clinic. We estimated filtered urate from plasma urate and 24-h creatinine clearance, and the net percentage reabsorbed. 153 men had hyperuricosuria (urine urate >4.80 mmol/24 h); 513 had normouricosuria. Hyperuricosuric men filtered more urate (median 68.1 and 52.5 mmol/24 h) but the ranges overlapped. Thirty hyperuricosuric men with filtered urate below the median for normouricosuria were selected as the renal hyperuricosuria group. Their normal plasma urate and high urate clearance substantiated this classification. In comparison with 60 normouricosuric stone formers matched for filtration, they had a higher incidence of hypercalciuria (67 versus 40%), but similar, high, frequencies of hyperoxaluria (25 and 11%) and phosphaturia (40 and 27%).There were no differences in age at first stone, incidence of stone recurrence or positive family history (20 and 25%). The findings demonstrate multiple risk factors for stones in this subgroup. In comparison, the 30 hyperuricosuric men with the highest filtration had a higher incidence of hyperoxaluria (58%) but fewer (7%) had a positive family history. Creatinine clearance was raised in 73%. An excessive protein intake might be a major correctable factor underlying these abnormalities.

Who makes uric acid stones and why--observations from a renal stones clinic.

AIMS: Excessively acidic urine is the dominant factor in uric acid stone formation. Recent evidence implicating insulin resistance has revived interest in its causation. We reviewed data on uric acid stone formers attending a general stones clinic to find out whether this supports and adds to current concepts. METHODS: A retrospective database study of 1504 stone formers investigated at the Southampton renal stones clinic from 1990 to March 2007. Uric acid stone formers and idiopathic calcium stone formers were compared using non-parametric tests. RESULTS: Fifty-nine patients (3.9%; 43 men) had uric acid stones. In men the commonest associated conditions were diabetes (20%), gout (20%) and an ileostomy (15%); in women, diabetes (33%), urinary infections (27%) and hyperparathyroidism (20%). Most patients with diabetes (85% of men, 75% of women), however, produced calcium stones. Risk factors did not differ significantly between calcium and uric acid stone formers with diabetes, gout or ileostomies. The median urine pH of men with idiopathic calcium stones was 6.20, idiopathic uric acid stones 5.47, diabetes 5.68, gout 6.05, diabetes and gout 5.20 and ileostomy 5.10. Plasma urate was higher with gout and idiopathic uric acid stones. Urate excretion was increased in gout. Oxalate excretion was lower with idiopathic uric acid stones (new finding). Urine volume decreased and oxalate concentration increased with ileostomy. CONCLUSIONS: Uric acid stones are increased in diabetes, but most patients with diabetes make calcium stones. Different mechanisms may explain low pH with diabetes, gout and idiopathic stones. Low oxalate excretion with idiopathic urate stones needs confirmation.