Increased protein intake on controlled oxalate diets does not increase urinary oxalate excretion

A genome-scale gain-of-function CRISPR screen in CD8 T cells identifies proline metabolism as a means to enhance CAR-T therapy

Chimeric antigen receptor (CAR)-T cell-based immunotherapy for cancer and immunological diseases has made great strides, but it still faces multiple hurdles. Finding the right molecular targets to engineer T cells toward a desired function has broad implications for the armamentarium of T cell-centered therapies. Here, we developed a dead-guide RNA (dgRNA)-based CRISPR activation screen in primary CD8+ T cells and identified gain-of-function (GOF) targets for CAR-T engineering. Targeted knockin or overexpression of a lead target, PRODH2, enhanced CAR-T-based killing and in vivo efficacy in multiple cancer models. Transcriptomics and metabolomics in CAR-T cells revealed that augmenting PRODH2 expression reshaped broad and distinct gene expression and metabolic programs. Mitochondrial, metabolic, and immunological analyses showed that PRODH2 engineering enhances the metabolic and immune functions of CAR-T cells against cancer. Together, these findings provide a system for identification of GOF immune boosters and demonstrate PRODH2 as a target to enhance CAR-T efficacy.

The Relationship between Modern Fad Diets and Kidney Stone Disease: A Systematic Review of Literature

OBJECTIVES

Kidney stone disease (KSD) has a strong association with diet metabolic syndrome. This review aims at exploring the lithogenic risk posed by the current most popular diets. Our approach was to search for the effect of each diet type on the major urinary risk factors, to try to draw conclusions regarding the association of a specific diet type and KSD.

METHODS

This systematic review searched for the available literature exploring the association between the existing popular fad diets and KSD. Articles in English, French and Spanish were included, without restriction of the search period with the final search done in August 2021.

RESULTS

Total number of studies and studies for each diet type was as follows: 22 articles for the low carbohydrate diet, 20 articles for high protein diets, 26 articles for vegetarian and vegan diets. There exists a substantial variability in different low carbohydrate and high protein diets, and considerable overlap between modern popular fad diets. High carbohydrate intake might increase urine uric acid, calcium and oxalate levels. High protein diets increase urine calcium and uric acid and lower urine pH and citrate. Consumption of fruits and vegetables increases the urinary volume and urinary citrate. In vegan diets, sufficient daily calcium intake is important to avoid possible secondary hyperoxaluria.

CONCLUSIONS

Few studies evaluated the direct relationship between modern fad diets and KSD. In general, the reduction of carbohydrate in the diet, and counterbalancing protein rich diets with sufficient intake of fruits and vegetables, seem to play a protective role against KSD formation. Maintaining sufficient calcium intake in vegan and vegetarian diets is important. Additional research is needed to directly evaluate the link between KSD and each diet type.

Determination of reference interval (RI) of spot urinary oxalate to creatinine ratio in children of pakistani origin under six years of age: A cross-sectional study

Background

The gold standard screening method of hyperoxaluria in children is using 24-hour urine collection. Urine collection may be cumbersome and challenging for children. Reference intervals (RI) of oxalate for the Pakistani population are not readily available. Therefore we aimed to determine the oxalate to creatinine ratio (Ox: Cr) for Pakistani children <6 years of age.

Materials and Methods

A cross-sectional study was conducted at Aga Khan University from June 2018 to October 2019. Random urine samples from apparently healthy children < 6 years were collected and stored at -30°C until analysis after adding 6M HCl. Oxalate was measured on Micro lab 300 using a kit based on oxalate oxidase principle, while creatinine was measured by kinetic Jaffe reaction. Data was analyzed by EP evaluator and SPSS 23. Ox: Cr ratio was calculated and reported with 90% confidence interval (CI) and interquartile range (IQR).

Results

The mean age of study subjects (n=120) was 29 ±22.3 months with an M: F ratio of 1:1. Children of various ethnicities were included from all over Karachi. The majority of the subjects were Urdu speaking (37.5%). Median Ox: Cr was 0.13(0.10). No significant difference was noted in the median Ox: Cr ratio between various ethnicities (p>0.05). It was significantly different in group I to V which was 0.25 (IQR: 0.06), 0.19 (IQR: 0.11), 0.15 (IQR: 0.04), 0.11 (IQR: 0.06) and 0.08 (IQR: 0.04) respectively (pvalue <0.001).

Conclusion

The established RIs of Ox: Cr ratio was 0.05-0.34 (90% CI). Ox: Cr ratio showed a declining trend with age. Large scale reference interval studies are encouraged, taking diet and age into consideration.

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Reference intervals (RI) of oxalate for the Pakistani population are not readily available.

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Established RIs of Ox: Cr ratio was 0.05–0.34 (90% CI) in children under 6 years.

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Ox: Cr ratio showed a declining trend with age.

Contribution of Dietary Oxalate and Oxalate Precursors to Urinary Oxalate Excretion

Kidney stone disease is increasing in prevalence, and the most common stone composition is calcium oxalate. Dietary oxalate intake and endogenous production of oxalate are important in the pathophysiology of calcium oxalate stone disease. The impact of dietary oxalate intake on urinary oxalate excretion and kidney stone disease risk has been assessed through large cohort studies as well as smaller studies with dietary control. Net gastrointestinal oxalate absorption influences urinary oxalate excretion. Oxalate-degrading bacteria in the gut microbiome, especially Oxalobacter formigenes, may mitigate stone risk through reducing net oxalate absorption. Ascorbic acid (vitamin C) is the main dietary precursor for endogenous production of oxalate with several other compounds playing a lesser role. Renal handling of oxalate and, potentially, renal synthesis of oxalate may contribute to stone formation. In this review, we discuss dietary oxalate and precursors of oxalate, their pertinent physiology in humans, and what is known about their role in kidney stone disease.

Genetic Regulation of Liver Metabolites and Transcripts Linking to Biochemical-Clinical Parameters

Given the central metabolic role of the liver, hepatic metabolites and transcripts reflect the organismal physiological state. Biochemical-clinical plasma biomarkers, hepatic metabolites, transcripts, and single nucleotide polymorphism (SNP) genotypes of some 300 pigs were integrated by weighted correlation networks and genome-wide association analyses. Network-based approaches of transcriptomic and metabolomics data revealed linked of transcripts and metabolites of the pentose phosphate pathway (PPP). This finding was evidenced by using a NADP/NADPH assay and HDAC4 and G6PD transcript quantification with the latter coding for first limiting enzyme of this pathway and by RNAi knockdown experiments of HDAC4. Other transcripts including ARG2 and SLC22A7 showed link to amino acids and biomarkers. The amino acid metabolites were linked with transcripts of immune or acute phase response signaling, whereas the carbohydrate metabolites were highly enrich in cholesterol biosynthesis transcripts. Genome-wide association analyses revealed 180 metabolic quantitative trait loci (mQTL) (p < 10^-4). Trans-4-hydroxy-L-proline (p = 6 × 10^-9), being strongly correlated with plasma creatinine (CREA), showed strongest association with SNPs on chromosome 6 that had pleiotropic effects on PRODH2 expression as revealed by multivariate analysis. Consideration of shared marker association with biomarkers, metabolites, and transcripts revealed 144 SNPs associated with 44 metabolites and 69 transcripts that are correlated with each other, representing 176 mQTL and expression quantitative trait loci (eQTL). This is the first work to report genetic variants associated with liver metabolite and transcript levels as well as blood biochemical-clinical parameters in a healthy porcine model. The identified associations provide links between variation at the genome, transcriptome, and metabolome level molecules with clinically relevant phenotypes. This approach has the potential to detect novel biomarkers displaying individual variation and promoting predictive biology in medicine and animal breeding.

Dietary oxalate and kidney stone formation

Dietary oxalate is plant-derived and may be a component of vegetables, nuts, fruits, and grains. In normal individuals, approximately half of urinary oxalate is derived from the diet and half from endogenous synthesis. The amount of oxalate excreted in urine plays an important role in calcium oxalate stone formation. Large epidemiological cohort studies have demonstrated that urinary oxalate excretion is a continuous variable when indexed to stone risk. Thus, individuals with oxalate excretions >25 mg/day may benefit from a reduction of urinary oxalate output. The 24-h urine assessment may miss periods of transient surges in urinary oxalate excretion, which may promote stone growth and is a limitation of this analysis. In this review we describe the impact of dietary oxalate and its contribution to stone growth. To limit calcium oxalate stone growth, we advocate that patients maintain appropriate hydration, avoid oxalate-rich foods, and consume an adequate amount of calcium.

Diet and stone formation: a brief review of the literature

PURPOSE OF REVIEW

Urolithiasis is a very common condition, which can be accompanied by serious complications. Diagnosis, treatment, and prevention are of great cost for national health systems. Many studies have been published about the role of diet in both stone formation and prevention. The aim of this review is to summarize the most recent developments that correlate diet to lithiasis.

RECENT FINDINGS

Recurrent stone formers should undergo metabolic evaluation and stone chemical analysis. Current evidence propose difference approaches based on the metabolic disorder that is diagnosed. Diet could have a detrimental role in the prevention of recurrences. Prevention advises include increased fluid uptake, vegetables and fruit intake but decreased sugar, salt, and meat consumption.

SUMMARY

The analysis of the food contents and their role to lithogenesis prevention are of great importance. Modifying diet to prevent stones could help many people who suffer from lithiasis to avoid recurrence and the consequences. Furthermore, the cost for diagnosis and treatment could be significantly reduced. Therefore, the field of dietary factors in lithogenesis should be further investigated.

The influence of metabolic syndrome and its components on the development of nephrolithiasis

The prevalence of kidney stone disease is increasing, afflicting 7%–11% of the United States population. Multiple systemic conditions, including obesity and diabetes, are also on the rise. Further, the literature has demonstrated a strong association between metabolic syndrome, its components, and kidney stone disease. In this article, we aim to review the associations of metabolic syndrome and nephrolithiasis, discussing the pathophysiology, urinary parameters, and clinical presentations. With this knowledge, urologists will have a more comprehensive understanding of this complex population of metabolic stone formers enabling improved patient management and treatment of stone disease.

Whey protein and albumin effects upon urinary risk factors for stone formation

Protein supplements are consumed for an expected increase in muscle mass and improved exercise performance, but as their impact on lithogenic parameters are unknown, we aimed to evaluate the effects of Whey protein (WP) and Albumin upon the risk factors for nephrolithiasis. WP or Albumin supplements (one scoop/day) were administered for 3 days to 18 healthy volunteers, with 1-week washout period between them. Serum and 24-h urine samples were collected at baseline and after completing each intervention. All participants were asked to replicate their baseline diet during the subsequent urine collection. After WP or albumin, mean protein equivalent of nitrogen appearance (PNA) was significantly higher (p < 0.001), as the result of the consumption of each of the supplements, but mean urinary calcium, phosphorus, sodium, potassium, uric acid, citrate, oxalate, magnesium, creatinine, pH, and urinary saturation indices did not differ from baseline. However, individual increases higher than 50% in urinary calcium were observed in 39% of the individuals and variable decreases in urinary pH in 44 and 67% of them, respectively, after WP or Albumin. Increases higher than 50% in urinary sodium occurred in one-third of them after Albumin. A short-term consumption of WP or albumin by healthy subjects, under controlled diet, did not significantly change the mean lithogenic parameters. Nevertheless, the wide individual variation and relevant increases/decreases observed for urinary calcium, sodium, and pH suggest the need of a closer surveillance of these parameters and adequacy of diet in case of supplementation by stone formers.

Urinary oxalate to creatinine ratios in healthy Turkish schoolchildren

Aim: we aimed to establish reference values for urinary oxalate to creatinine ratios in healthy children aged 6–15 years and to investigate the relationship between their nutritional habits and oxalate excretion.

Materials and methods: Random urine specimens from 953 healthy children aged 6–15 years were obtained and analyzed for oxalate and creatinine. Additionally, a 24-h dietary recall form was prepared and given to them. The ingredient composition of the diet was calculated. The children were divided into three groups according to age: Group I (69 years, n = 353), Group II (10–12 years, n = 335), and Group III (13–15 years, n = 265).

Results: The 95th percentile of the oxalate to creatinine ratio for subjects aged 6–9, 10–12, and 13–15 years were 0.048, 0.042, and 0.042 mg/mg, respectively. The oxalate to creatinine ratio was significantly higher in Group 1 than in Group 2 and Group 3. Urinary oxalate excretion was positively correlated with increased protein intake and negatively correlated with age. A significant positive correlation was determined between urinary oxalate excretion and the proline, serine, protein, and glycine content of diet. Dietary proline intake showed a positive correlation with the urine oxalate to creatinine ratio and was found to be an independent predictor for urinary oxalate.

Conclusions: These data lend support to the idea that every country should have its own normal reference values to determine the underlying metabolic risk factor for kidney stone disease since regional variation in the dietary intake of proteins and other nutrients can affect normal urinary excretion of oxalate.

Dietary Protein and Potassium, Diet-Dependent Net Acid Load, and Risk of Incident Kidney Stones

BACKGROUND AND OBJECTIVES

Protein and potassium intake and the resulting diet-dependent net acid load may affect kidney stone formation. It is not known whether protein type or net acid load is associated with risk of kidney stones.

DESIGN, SETTING, PARTICIPANTS, & MEASUREMENTS

We prospectively examined intakes of protein (dairy, nondairy animal, and vegetable), potassium, and animal protein-to-potassium ratio (an estimate of net acid load) and risk of incident kidney stones in the Health Professionals Follow-Up Study (n=42,919), the Nurses' Health Study I (n=60,128), and the Nurses' Health Study II (n=90,629). Multivariable models were adjusted for age, body mass index, diet, and other factors. We also analyzed cross-sectional associations with 24-hour urine (n=6129).

RESULTS

During 3,108,264 person-years of follow-up, there were 6308 incident kidney stones. Dairy protein was associated with lower risk in the Nurses' Health Study II (hazard ratio for highest versus lowest quintile, 0.84; 95% confidence interval, 0.73 to 0.96; P value for trend <0.01). The hazard ratios for nondairy animal protein were 1.15 (95% confidence interval, 0.97 to 1.36; P value for trend =0.04) in the Health Professionals Follow-Up Study and 1.20 (95% confidence interval, 0.99 to 1.46; P value for trend =0.06) in the Nurses' Health Study I. Potassium intake was associated with lower risk in all three cohorts (hazard ratios from 0.44 [95% confidence interval, 0.36 to 0.53] to 0.67 [95% confidence interval, 0.57 to 0.78]; P values for trend <0.001). Animal protein-to-potassium ratio was associated with higher risk (P value for trend =0.004), even after adjustment for animal protein and potassium. Higher dietary potassium was associated with higher urine citrate, pH, and volume (P values for trend <0.002).

CONCLUSIONS

Kidney stone risk may vary by protein type. Diets high in potassium or with a relative abundance of potassium compared with animal protein could represent a means of stone prevention.

Dietary recommendations and treatment of patients with recurrent idiopathic calcium stone disease

This review describes the various dietary regimens that have been used to advise patients on how to prevent the recurrence of their calcium-containing kidney stones. The conclusion is that although there is some general advice that may be useful to many patients, it is more efficacious to screen each patient individually to identify his/her main urinary, metabolic, nutritional, environmental, and lifestyle risk factors for stone-formation and then tailor specific advice for that particular patient based on the findings from these investigations. If the patient can be motivated to adhere strictly to this conservative approach to the prophylactic management of their stone problem over a long time period, then it is possible to prevent them from forming further stones. This approach to stone management is considerably less expensive than any of the procedures currently available for stone removal or disintegration. In the UK, for each new stone episode prevented by this conservative approach to prophylaxis it is calculated to save the Health Authority concerned around £2000 for every patient treated successfully. In the long term, this accumulates to a major saving within each hospital budget if most stone patients can be prevented from forming further stones and when the savings are totalled up country-wide saves the National Exchequer considerable sums in unclaimed Sick Pay and industry a significant number of manpower days which would otherwise be lost from work. It is also of immense relief and benefit to the patients not to have to suffer the discomfort and inconvenience of further stone episodes.

Proline dehydrogenase 2 (PRODH2) is a hydroxyproline dehydrogenase (HYPDH) and molecular target for treating primary hyperoxaluria

The primary hyperoxalurias (PH), types 1-3, are disorders of glyoxylate metabolism that result in increased oxalate production and calcium oxalate stone formation. The breakdown of trans-4-hydroxy-L-proline (Hyp) from endogenous and dietary sources of collagen makes a significant contribution to the cellular glyoxylate pool. Proline dehydrogenase 2 (PRODH2), historically known as hydroxyproline oxidase, is the first step in the hydroxyproline catabolic pathway and represents a drug target to reduce the glyoxylate and oxalate burden of PH patients. This study is the first report of the expression, purification, and biochemical characterization of human PRODH2. Evaluation of a panel of N-terminal and C-terminal truncation variants indicated that residues 157-515 contain the catalytic core with one FAD molecule. The 12-fold higher k(cat)/K(m) value of 0.93 M⁻¹·s⁻¹ for Hyp over Pro demonstrates the preference for Hyp as substrate. Moreover, an anaerobic titration determined a K(d) value of 125 μM for Hyp, a value ~1600-fold lower than the K(m) value. A survey of ubiquinone analogues revealed that menadione, duroquinone, and CoQ₁ reacted more efficiently than oxygen as the terminal electron acceptor during catalysis. Taken together, these data and the slow reactivity with sodium sulfite support that PRODH2 functions as a dehydrogenase and most likely utilizes CoQ₁₀ as the terminal electron acceptor in vivo. Thus, we propose that the name of PRODH2 be changed to hydroxyproline dehydrogenase (HYPDH). Three Hyp analogues were also identified to inhibit the activity of HYPDH, representing the first steps toward the development of a novel approach to treat all forms of PH.

Relevance of dietary protein concentration and quality as risk factors for the formation of calcium oxalate stones in cats

The role of dietary protein for the development of feline calcium oxalate (CaOx) uroliths has not been conclusively clarified. The present study evaluated the effects of a varying dietary protein concentration and quality on critical indices for the formation of CaOx uroliths. Three diets with a high protein quality (10–11 % greaves meal/diet) and a varying crude protein (CP) concentration (35, 44 and 57 % in DM) were compared. Additionally, the 57 % CP diet was compared with a fourth diet that had a similar CP concentration (55 % in DM), but a lower protein quality (34 % greaves meal/diet). The Ca and oxalate (Ox) concentrations were similar in all diets. A group of eight cats received the same diet at the same time. Each feeding period was divided into a 21 d adaptation period and a 7 d sampling period to collect urine. There were increases in urinary volume, urinary Ca concentrations, renal Ca and Ox excretion and urinary relative supersaturation (RSS) with CaOx with increasing dietary protein concentrations. Urinary pH ranged between 6·34 and 6·66 among all groups, with no unidirectional effect of dietary protein. Lower renal Ca excretion was observed when feeding the diet with the lower protein quality, however, the underlying mechanism needs further evaluation. In conclusion, although the observed higher urinary volume is beneficial, the increase in urinary Ca concentrations, renal Ca and Ox excretion and urinary RSS CaOx associated with a high-protein diet may be critical for the development of CaOx uroliths in cats.

Internet program for facilitating dietary modifications limiting kidney stone risk

INTRODUCTION

Certain dietary modifications limit the risk of stone recurrence. Compliance is an important component of dietary therapy for stone prevention, and self-efficacy is an important ingredient of compliance. We developed an internet program to facilitate dietary compliance for stone prevention and performed a pilot study to assess its effectiveness.

MATERIALS AND METHODS

The internet program provides information regarding dietary modifications including increased fluid consumption, limited animal protein, sodium, and oxalate intake, and adequate calcium consumption. Participants record their daily food and fluid intake and receive immediate feedback as to whether they were compliant or not. Five adult calcium stone formers collected three 24 hour urine specimens on self-selected diets, three 24 hour urine specimens while on a stone preventive metabolic diet, and three 24 hour urine specimens after utilizing the internet program for 1 month. Urinary stone risk parameters were measured, and data were analyzed using repeated measures ANOVA and Student's t test.

RESULTS

All participants recorded their meals and snacks for each day and found the program easy to navigate. The mean time in hours from food consumption to log in was 35.25 +/- 70.8 hours. There were no statistically significant differences in stone risk factors between the controlled and internet dietary phases. Oxalate excretion was significantly higher during the self-selected dietary intake (p = 0.03).

CONCLUSIONS

This pilot study demonstrates that subjects appear to be compliant with utilization of an interactive internet program for stone prevention with dietary modifications. In addition, improvement in certain stone risk parameters occurred.

Optimum nutrition for kidney stone disease

We summarize the data regarding the associations of individual dietary components with kidney stones and the effects on 24-hour urinary profiles. The therapeutic recommendations for stone prevention that result from these studies are applied where possible to stones of specific composition. Idiopathic calcium oxalate stone-formers are advised to reduce ingestion of animal protein, oxalate, and sodium while maintaining intake of 800 to 1200 mg of calcium and increasing consumption of citrate and potassium. There are few data regarding dietary therapy of calcium phosphate stones. Whether the inhibitory effect of citrate sufficiently counteracts increasing urine pH to justify more intake of potassium and citrate is not clear. Reduction of sodium intake to decrease urinary calcium excretion would also be expected to decrease calcium phosphate stone recurrence. Conversely, the most important urine variable in the causation of uric acid stones is low urine pH, linked to insulin resistance as a component of obesity and the metabolic syndrome. The mainstay of therapy is weight loss and urinary alkalinization provided by a more vegetarian diet. Reduction in animal protein intake will reduce purine ingestion and uric acid excretion. For cystine stones, restriction of animal protein is associated with reduction in intake of the cystine precursor methionine as well as cystine. Reduction of urine sodium results in less urine cystine. Ingestion of vegetables high in organic anion content, such as citrate and malate, should be associated with higher urine pH and fewer stones because the amino acid cystine is soluble in more alkaline urine. Because of their infectious origin, diet has no definitive role for struvite stones except for avoiding urinary alkalinization, which may worsen their development.

Response to dietary oxalate after bariatric surgery

BACKGROUND AND OBJECTIVES

Bariatric surgery (BS) may be associated with increased oxalate excretion and a higher risk of nephrolithiasis. This study aimed to investigate urinary abnormalities and responses to an acute oxalate load as an indirect assessment of the intestinal absorption of oxalate in this population.

DESIGN, SETTING, PARTICIPANTS, & MEASUREMENTS

Twenty-four-hour urine specimens were collected from 61 patients a median of 48 months after BS (post-BS) as well as from 30 morbidly obese (MO) participants; dietary information was obtained through 24-hour food recalls. An oral oxalate load test (OLT), consisting of 2-hour urine samples after overnight fasting and 2, 4, and 6 hours after consuming 375 mg of oxalate (spinach juice), was performed on 21 MO and 22 post-BS patients 12 months after BS. Ten post-BS patients also underwent OLT before surgery (pre-BS).

RESULTS

There was a higher percentage of low urinary volume (<1.5 L/d) in post-BS versus MO (P<0.001). Hypocitraturia and hyperoxaluria (P=0.13 and P=0.36, respectively) were more frequent in BS versus MO patients. The OLT showed intragroup (P<0.001 for all periods versus baseline) and intergroup differences (P<0.001 for post-BS versus MO; P=0.03 for post-BS versus pre-BS). The total mean increment in oxaluria after 6 hours of load, expressed as area under the curve, was higher in both post-BS versus MO and in post-BS versus pre-BS participants (P<0.001 for both).

CONCLUSIONS

The mean oxaluric response to an oxalate load is markedly elevated in post-bariatric surgery patients, suggesting that increased intestinal absorption of dietary oxalate is a predisposing mechanism for enteric hyperoxaluria.

Primary hyperoxaluria type III--a model for studying perturbations in glyoxylate metabolism

Perturbations in glyoxylate metabolism lead to the accumulation of oxalate and give rise to primary hyperoxalurias, recessive disorders characterized by kidney stone disease. Loss-of-function mutations in HOGA1 (formerly DHDPSL) are responsible for primary hyperoxaluria type III. HOGA1 is a mitochondrial 4-hydroxy-2-oxoglutarate aldolase catalyzing the fourth step in the hydroxyproline pathway. We investigated hydroxyproline metabolites in the urine of patients with primary hyperoxaluria type III using gas chromatography-mass spectroscopy. Significant increases in concentrations of 4-hydroxy-2-oxoglutarate and its precursor and derivative 4-hydroxyglutamate and 2,4-dihydroxyglutarate, respectively, were found in all patients as compared to carriers of the corresponding mutations or healthy controls. Despite a functional block in the conversion of hydroxyproline to glyoxylate--the immediate precursor of oxalate--the production of oxalate increases. To explain this apparent contradiction, we propose a model of glyoxylate compartmentalization in which cellular glyoxylate is normally prevented from contact with the cytosol where it can be oxidized to oxalate. We propose that HOGA1 deficiency results in the accumulation of 4-hydroxy-2-oxoglutarate in the mitochondria and its transport into the cytosol where it is converted to glyoxylate by a different cytosolic aldolase. In human hepatocyte cell lines, we detected a cytosolic 4-hydroxy-2-oxoglutarate aldolase activity not due to HOGA1. These studies provide a diagnostic tool for primary hyperoxaluria type III and shed light on glyoxylate metabolism and the pathogenesis of primary hyperoxalurias.

Metabolism of [13C5]hydroxyproline in vitro and in vivo: implications for primary hyperoxaluria

Hydroxyproline (Hyp) metabolism is a key source of glyoxylate production in the body and may be a major contributor to excessive oxalate production in the primary hyperoxalurias where glyoxylate metabolism is impaired. Important gaps in our knowledge include identification of the tissues with the capacity to degrade Hyp and the development of model systems to study this metabolism and how to suppress it. The expression of mRNA for enzymes in the pathway was examined in 15 different human tissues. Expression of the complete pathway was identified in liver, kidney, pancreas, and small intestine. HepG2 cells also expressed these mRNAs and enzymes and were shown to metabolize Hyp in the culture medium to glycolate, glycine, and oxalate. [(18)O]- and [(13)C(5)]Hyp were synthesized and evaluated for their use with in vitro and in vivo models. [(18)O]Hyp was not suitable because of an apparent tautomerism of [(18)O]glyoxylate between enol and hydrated forms, which resulted in a loss of isotope. [(13)C(5)]Hyp, however, was metabolized to [(13)C(2)]glycolate, [(13)C(2)]glycine, and [(13)C(2)]oxalate in vitro in HepG2 cells and in vivo in mice infused with [(13)C(5)]Hyp. These model systems should be valuable tools for exploring various aspects of Hyp metabolism and will be useful in determining whether blocking Hyp catabolism is an effective therapy in the treatment of primary hyperoxaluria.

Metabolism of primed, constant infusions of [1,2-¹³C₂] glycine and [1-¹³C₁] phenylalanine to urinary oxalate

Experiments in humans and rodents using oral doses of glycine and phenylalanine have suggested that the metabolism of these amino acids contributes to urinary oxalate excretion. To better define this contribution, we have examined the primed, constant infusion of [1-(13)C(1)] phenylalanine and [1,2-(13)C(2)] glycine in the postabsorptive state in healthy adults. Subjects were infused for 5 hours, hourly urines were collected, and blood was drawn every 30 minutes. Ion chromatography/mass spectrometry was used to measure [(13)C] enrichment in urinary oxalate, glycolate, and hippurate; and the enrichment of (13)C-amino acids in plasma samples was measured by gas chromatography/mass spectrometry. Following infusion with either 6 μmol/(kg h) [1-(13)C(1)] phenylalanine or 6 μmol/(kg h) [1,2-(13)C(2)] glycine, no isotopic glycolate or oxalate was detected in urine. Based on the limits of detection of our ion chromatography/mass spectroscopy method, these data indicate that less than 0.7% of the urinary oxalate could be derived from phenylalanine catabolism and less than 5% from glycine catabolism. Infusions with high levels of [1,2-(13)C(2)] glycine, 60 μmol/(kg h), increased mean plasma glycine by 29% and the whole-body flux of glycine by 72%. Under these conditions, glycine contributed 16.0% ± 1.6% and 16.6% ± 3.2% to urinary oxalate and glycolate excretion, respectively. Experiments using cultured hepatoma cells demonstrated that only at supraphysiological levels (>1 mmol/L) did glycine and phenylalanine metabolism increase oxalate synthesis. These data suggest that glycine and phenylalanine metabolism make only minor contributions to oxalate synthesis and urinary oxalate excretion.

Influence of nutrition on feline calcium oxalate urolithiasis with emphasis on endogenous oxalate synthesis

The prevalence of calcium oxalate (CaOx) uroliths detected in cats with lower urinary tract disease has shown a sharp increase over the last decades with a concomitant reciprocal decrease in the occurrence of struvite (magnesium ammonium phosphate) uroliths. CaOx stone-preventative diets are available nowadays, but seem to be marginally effective, as CaOx urolith recurrence occurs in patients fed these diets. In order to improve the preventative measures against CaOx urolithiasis, it is important to understand its aetiopathogenesis. The main research focus in CaOx formation in cats has been on the role of Ca, whereas little research effort has been directed towards the role and origin of urinary oxalates. As in man, the exogenous origin of urinary oxalates in cats is thought to be of minor importance, although the precise contribution of dietary oxalates remains unclear. The generally accepted dietary risk factors for CaOx urolithiasis in cats are discussed and a model for the biosynthetic pathways of oxalate in feline liver is provided. Alanine:glyoxylate aminotransferase 1 (AGT1) in endogenous oxalate metabolism is a liver-specific enzyme targeted in the mitochondria in cats, and allows for efficient conversion of glyoxylate to glycine when fed a carnivorous diet. The low peroxisomal activity of AGT1 in cat liver is compatible with the view that felids utilised a low-carbohydrate diet throughout evolution. Future research should focus on understanding de novo biosynthesis of oxalate in cats and their adaptation(s) in oxalate metabolism, and on dietary oxalate intake and absorption by cats.