Hepatic oxalate production: the role of hydroxypyruvate

Fructose increases risk for kidney stones: potential role in metabolic syndrome and heat stress

Background

Fructose intake, mainly as table sugar or high fructose corn syrup, has increased in recent decades and is associated with increased risk for kidney stones. We hypothesized that fructose intake alters serum and urinary components involved in stone formation.

Methods

We analyzed a previously published randomized controlled study that included 33 healthy male adults (40–65 years of age) who ingested 200 g of fructose (supplied in a 2-L volume of 10% fructose in water) daily for 2 weeks. Participants were evaluated at the Unit of Nephrology of the Mateo Orfila Hospital in Menorca. Changes in serum levels of magnesium, calcium, uric acid, phosphorus, vitamin D, and intact PTH levels were evaluated. Urine magnesium, calcium, uric acid, phosphorus, citrate, oxalate, sodium, potassium, as well as urinary pH, were measured.

Results

Ingestion of fructose was associated with an increased serum level of uric acid (p < 0.001), a decrease in serum ionized calcium (p = 0.003) with a mild increase in PTH (p < 0.05) and a drop in urinary pH (p = 0.02), an increase in urine oxalate (p = 0.016) and decrease in urinary magnesium (p = 0.003).

Conclusions

Fructose appears to increase urinary stone formation in part via effects on urate metabolism and urinary pH, and also via effects on oxalate. Fructose may be a contributing factor for the development of kidney stones in subjects with metabolic syndrome and those suffering from heat stress.

Trial registration

ClinicalTrials.gov NCT00639756 March 20, 2008.

Potential cytotoxic effect of hydroxypyruvate produced from D-serine by astroglial D-amino acid oxidase

D-amino acid oxidase (DAO) is a flavoenzyme that exists in the kidney, liver and brain of mammals. This enzyme catalyzes the oxidation of D-amino acids to the corresponding α-keto acid, hydrogen peroxide and ammonia. Recently D-serine, one of the substrates of DAO, has been found in the mammalian brain, and shown to be a co-agonist of the N-methyl-D-aspartate (NMDA) receptor in glutamate neurotransmission. In this study, we investigated the metabolism of extracellular D-serine and the effects of D-serine metabolites to study the pathophysiological role of DAO. Treatment with a high dose of D-serine induced the cell death in dose-dependent manner in DAO-expressing cells. Moreover, overexpression of DAO in astroglial cells induced the enhanced cytotoxicity. The treatment with 1 mM beta-hydroxypyruvate (HPA), uniquely produced from the D-serine metabolism by DAO activity, also induced cell death, comprising apoptosis, in the astroglial cell, but not in the other cells derived from liver and kidney. Taken together, we consider that high dose of extracellular D-serine induced cell death by the production of not only hydrogen peroxide but also HPA as a result of DAO catalytic activity in astroglial cell. Furthermore, this cytotoxicity of HPA is observed uniquely in astroglial cells expressing DAO.

Effect of soda consumption on urinary stone risk parameters

BACKGROUND AND PURPOSE

Fluid consumption has been demonstrated to influence kidney stone formation. Studies have shown that consumption of cola may be a risk factor for stone disease, while fluids containing citric acid may attenuate stone activity. Diet was not always controlled in these investigations, however. We undertook a study to determine the impact of three different fluids on urinary stone risk factors.

SUBJECTS AND METHODS

Six healthy nonstone-forming adults were placed on a standardized metabolic diet and consumed three different types of fluid during three 5-day periods. There was a 2-day washout between each sequence. The three fluids administered during these periods were Le Bleu water, caffeine-free Diet Coke, and Fresca (citrate containing). These two soda preparations were chosen to prevent the known increase in calcium excretion promoted by carbohydrates and caffeine. Twenty-four hour urine specimens were collected on days 4 and 5 of each sequence. The following urinary parameters were measured: Volume, calcium, oxalate, creatinine, uric acid, citrate, sodium, magnesium, phosphorus, sulfate, urea nitrogen, pH, and supersaturation indices. A paired t test was used for statistical analysis.

RESULTS

Urinary volumes were significantly higher and supersaturation of calcium oxalate significantly lower compared with a self-selected dietary regimen. A decrease in uric acid was also seen in the Fresca cohort. There were no statistically significant differences for any of the urinary parameters.

CONCLUSION

There is no increased risk or benefit to consuming Fresca or caffeine-free Diet Coke compared with Le Bleu bottled water with respect to stone formation.

Determinants of 24-hour urinary oxalate excretion

BACKGROUND AND OBJECTIVES

Higher levels of urinary oxalate substantially increase the risk of calcium oxalate kidney stones. However, the determinants of urinary oxalate excretion are unclear. The objective was to examine the impact of dietary factors, age, body size, diabetes, and urinary factors on 24-h urinary oxalate.

DESIGN, SETTING, PARTICIPANTS, AND MEASUREMENTS

We conducted a cross-sectional study of 3348 stone forming and non-stone-forming participants in the Health Professionals Follow-up Study (men), the Nurses' Health Study (older women), and the Nurses' Health Study II (younger women).

RESULTS

Median urinary oxalate was 39 mg/d in men, 27 mg/d in older women, and 26 mg/d in younger women. Participants in the highest quartile of dietary oxalate excreted 1.7 mg/d more urinary oxalate than participants in the lowest quartile (P trend 0.001). The relation between dietary and urinary oxalate was similar in individuals with and without nephrolithiasis. Participants consuming 1000 mg/d or more of vitamin C excreted 6.8 mg/d more urinary oxalate than participants consuming <90 mg/d (P trend < 0.001). Body mass index, total fructose intake, and 24-h urinary potassium, magnesium, and phosphorus levels also were positively associated with urinary oxalate. Calcium intake and age were inversely associated with urinary oxalate. After adjustment for body size, participants with diabetes excreted 2.0 mg/d more urinary oxalate than those without diabetes (P < 0.01).

CONCLUSIONS

The impact of dietary oxalate on urinary oxalate appears to be small. Further investigation of factors influencing urinary oxalate may lead to new approaches to prevent calcium kidney stones.

Fructose consumption and the risk of kidney stones

Fructose consumption has markedly increased over the past decades. This intake may increase the urinary excretion of calcium, oxalate, uric acid, and other factors associated with kidney stone risk. We prospectively examined the relationship between fructose intake and incident kidney stones in the Nurses' Health Study I (NHS I) (93,730 older women), the Nurses' Health Study II (NHS II) (101,824 younger women), and the Health Professionals Follow-up Study (45,984 men). Food frequency questionnaires were used to assess free fructose and sucrose intake every 4 years. Total-fructose intake was calculated as free fructose plus half the intake of sucrose, and expressed as percentage of total energy. Cox proportional hazard regressions were adjusted for age, body mass index (BMI), thiazide use, caloric intake, and other dietary factors. We documented 4902 incident kidney stones during a combined 48 years of follow-up. The multivariate relative risks of kidney stones significantly increased for participants in the highest compared to the lowest quintile of total-fructose intake for all three study groups. Free-fructose intake was also associated with increased risk. Non-fructose carbohydrates were not associated with increased risk in any cohort. Our study suggests that fructose intake is independently associated with an increased risk of incident kidney stones.

Glycolate and glyoxylate metabolism in HepG2 cells

Oxalate synthesis in human hepatocytes is not well defined despite the clinical significance of its overproduction in diseases such as the primary hyperoxalurias. To further define these steps, the metabolism to oxalate of the oxalate precursors glycolate and glyoxylate and the possible pathways involved were examined in HepG2 cells. These cells were found to contain oxalate, glyoxylate, and glycolate as intracellular metabolites and to excrete oxalate and glycolate into the medium. Glycolate was taken up more effectively by cells than glyoxylate, but glyoxylate was more efficiently converted to oxalate. Oxalate was formed from exogenous glycolate only when cells were exposed to high concentrations. Peroxisomes in HepG2 cells, in contrast to those in human hepatocytes, were not involved in glycolate metabolism. Incubations with purified lactate dehydrogenase suggested that this enzyme was responsible for the metabolism of glycolate to oxalate in HepG2 cells. The formation of14C-labeled glycine from14C-labeled glycolate was observed only when cell membranes were permeabilized with Triton X-100. These results imply that peroxisome permeability to glycolate is restricted in these cells. Mitochondria, which produce glyoxylate from hydroxyproline metabolism, contained both alanine:glyoxylate aminotransferase (AGT)2 and glyoxylate reductase activities, which can convert glyoxylate to glycine and glycolate, respectively. Expression of AGT2 mRNA in HepG2 cells was confirmed by RT-PCR. These results indicate that HepG2 cells will be useful in clarifying the nonperoxisomal metabolism associated with oxalate synthesis in human hepatocytes.

Glyoxylate synthesis, and its modulation and influence on oxalate synthesis

PURPOSE

We define the major pathways of hepatic oxalate synthesis in humans, examine the association with other metabolic pathways and identify ways that oxalate synthesis may be modified. In addition, we suggest what is required for further progress in this area.

MATERIALS AND METHODS

We consolidated relevant data primarily from recently published literature, considered new pharmacological approaches to decrease oxalate synthesis, and formulated an overview of the regulation and modification of oxalate synthesis pathways.

RESULTS

Experiments with animals, including humans, animal cells and in vitro preparations of cellular components, support the existence of a major metabolic pathway linking the amino acids serine, glycine and alanine. Oxalate synthesis is a minor, secondary reaction of a cascade of reactions termed the glyoxylate pathway, which has a prominent role in gluconeogenesis and ureagenesis. The enzymatic steps and effectors which regulate glyoxylate and oxalate synthesis are not well characterized. Pharmacological approaches can reduce oxalate synthesis by diminishing the glyoxylate pool and possibly modifying enzymatic reactions leading to glyoxylate synthesis.

CONCLUSIONS

The individual steps associated with glyoxylate and oxalate synthesis can be identified. The glyoxylate pathway has a significant functional role in intermediary liver metabolism but the way it is regulated is uncertain. Oxalate synthesis can be modified by drugs, indicating that primary and idiopathic hyperoxaluria may respond to pharmacological intervention.

Idiopathic calcium oxalate urolithiasis and endogenous oxalate production

Despite the great effort that has gone into investigating urolithiasis, this condition still persists as one of the major ailments of the urinary tract. Calcium oxalate urolithiasis is the most common form, accounting for some 60 to 80% of total stones. This review examines the elements (i.e., urine volume and pH and urinary excretion of calcium, oxalate, citrate, urate, magnesium, pyrophosphate, and glycosaminoglycans) that give rise to idiopathic calcium oxalate urolithiasis. Treatment strategies for idiopathic calcium oxalate urolithiasis, including lithotripsy, also are discussed. Urinary oxalate excretion is a major risk factor for calcium oxalate urolithiasis, with 85 to 95% of the urinary load derived endogenously. The factors controlling endogenous oxalate production are reviewed, including pathways for the diversion of glyoxylate from oxalate production. The use of beta-aminothiols and other substances to reduce endogenous oxalate production in subjects with idiopathic calcium oxalate urolithiasis is also discussed. A review of current methodologies for the determination of urinary oxalate is also included.