The effects of (L)-2-oxothiazolidine-4-carboxylate on urinary oxalate excretion

Future treatments for hyperoxaluria

PURPOSE OF REVIEW

The review of potential therapies in the treatment of hyperoxaluria is timely, given the current excitement with clinical trials and the mounting evidence of the importance of oxalate in both kidney stone and chronic kidney disease.

RECENT FINDINGS

Given the significant contribution of both endogenous and dietary oxalate to urinary oxalate excretions, it is not surprising therapeutic targets are being studied in both pathways. This article covers the existing data on endogenous and dietary oxalate and the current targets in these pathways.

SUMMARY

In the near future, there will likely be therapies targeting both endogenous and dietary oxalate, especially in subsets of kidney stone formers.

Transport via SLC5A8 (SMCT1) is obligatory for 2-oxothiazolidine-4-carboxylate to enhance glutathione production in retinal pigment epithelial cells

PURPOSE

To evaluate the role of SLC5A8 in the transport of 2-oxothiazolidine-4-carboxylate (OTC) and to determine whether OTC augments glutathione production in RPE cells, thereby providing protection against oxidative stress.

METHODS

SLC5A8-mediated transport of OTC was monitored in Xenopus laevis oocytes by electrophysiological means. Saturation kinetics, Na(+)-activation kinetics, and inhibition by ibuprofen were analyzed by monitoring OTC-induced currents as a measure of transport activity. Oxidative stress was induced in ARPE-19 cells and primary RPE cells isolated from wild type and Slc5a8(-/-) mouse retinas using H(2)O(2), and the effects of OTC on cell death and intracellular glutathione concentration were examined.

RESULTS

Heterologous expression of human SLC5A8 in X. laevis oocytes induced Na(+)-dependent inward currents in the presence of OTC under voltage-clamp conditions. The transport of OTC via SLC5A8 was saturable, with a K(t) of 104 ± 3 μM. The Na(+)-activation kinetics was sigmoidal with a Hill coefficient of 1.9 ± 0.1, suggesting involvement of two Na(+) in the activation process. Ibuprofen, a blocker of SLC5A8, inhibited SLC5A8-mediated OTC transport; the concentration necessary for half-maximal inhibition was 17 ± 1 μM. OTC increased glutathione levels and protected ARPE-19 and primary RPE cells isolated from wild type mouse retinas from H(2)O(2)-induced cell death. These effects were abolished in primary RPE isolated from Slc5a8(-/-) mouse retinas.

CONCLUSIONS

OTC is a transportable substrate for SLC5A8. OTC augments glutathione production in RPE cells, thereby protecting them from oxidative damage. Transport via SLC5A8 is obligatory for this process.

Pyridoxamine lowers kidney crystals in experimental hyperoxaluria: A potential therapy for primary hyperoxaluria

BACKGROUND

Primary hyperoxaluria is a rare genetic disorder of glyoxylate metabolism that results in overproduction of oxalate. The disease is characterized by severe calcium oxalate nephrolithiasis and nephrocalcinosis, resulting in end-stage renal disease (ESRD) early in life. Most patients eventually require dialysis and kidney transplantation, usually in combination with the replacement of the liver. Reduction of urinary oxalate levels can efficiently decrease calcium oxalate depositions; yet, no treatment is available that targets oxalate biosynthesis. In previous in vitro studies, we demonstrated that pyridoxamine can trap reactive carbonyl compounds, including intermediates of oxalate biosynthesis.

METHODS

The effect of PM on urinary oxalate excretion and kidney crystal formation was determined using the ethylene glycol rat model of hyperoxaluria. Animals were given 0.75% to 0.8% ethylene glycol in drinking water to establish and maintain hyperoxaluria. After 2 weeks, pyridoxamine treatment (180 mg/day/kg body weight) started and continued for an additional 2 weeks. Urinary creatinine, glycolate, oxalate, and calcium were measured along with the microscopic analysis of kidney tissues for the presence of calcium oxalate crystals.

RESULTS

Pyridoxamine treatment resulted in significantly lower (by approximately 50%) levels of urinary glycolate and oxalate excretion compared to untreated hyperoxaluric animals. This was accompanied by a significant reduction in calcium oxalate crystal formation in papillary and medullary areas of the kidney.

CONCLUSION

These results, coupled with favorable toxicity profiles of pyridoxamine in humans, show promise for therapeutic use of pyridoxamine in primary hyperoxaluria and other kidney stone diseases.

(L)-2-oxothiazolidine-4-carboxylate in the treatment of primary hyperoxaluria type 1

OBJECTIVE

To evaluate the short-term efficacy of (l)-2-oxothiaolidine-4-carboxylate (OTZ, which reduces urinary oxalate excretion in normal subjects) in the treatment of primary hyperoxaluria type 1 (PH1) in a phase II study.

PATIENTS AND METHODS

Two patients with PH1 received intravenous infusions of OTZ (100 mg/kg body weight for 2 h) given every 8 h for four doses. One patient also received a placebo treatment. Urine samples (24-h collections) were obtained before and during OTZ treatment and assayed for oxalate, citrate, creatinine, sulphate and pH. Daily blood samples were assayed for plasma oxalate and serum creatinine.

RESULTS

Urinary oxalate excretion was unaffected by OTZ treatment. Plasma oxalate declined in both individuals with OTZ treatment, but the effect was small. Plasma cysteine was normal in one patient, rising from a mean (sd) of 36 (3.7) micromol/L before treatment to a peak of 141 micromol/L after OTZ, but was not detected in samples from the other patient. The ratio of oxalate to creatinine clearances was high in both patients, with mean values of 3.1 and 3.8.

CONCLUSIONS

Treatment with OTZ did not lead to clinically significant changes in urinary oxalate excretion. The high clearance of oxalate in these patients suggests a substantial renal secretion of oxalate.

Microbial relatives of the seed storage proteins of higher plants: conservation of structure and diversification of function during evolution of the cupin superfamily

This review summarizes the recent discovery of the cupin superfamily (from the Latin term "cupa," a small barrel) of functionally diverse proteins that initially were limited to several higher plant proteins such as seed storage proteins, germin (an oxalate oxidase), germin-like proteins, and auxin-binding protein. Knowledge of the three-dimensional structure of two vicilins, seed proteins with a characteristic beta-barrel core, led to the identification of a small number of conserved residues and thence to the discovery of several microbial proteins which share these key amino acids. In particular, there is a highly conserved pattern of two histidine-containing motifs with a varied intermotif spacing. This cupin signature is found as a central component of many microbial proteins including certain types of phosphomannose isomerase, polyketide synthase, epimerase, and dioxygenase. In addition, the signature has been identified within the N-terminal effector domain in a subgroup of bacterial AraC transcription factors. As well as these single-domain cupins, this survey has identified other classes of two-domain bicupins including bacterial gentisate 1, 2-dioxygenases and 1-hydroxy-2-naphthoate dioxygenases, fungal oxalate decarboxylases, and legume sucrose-binding proteins. Cupin evolution is discussed from the perspective of the structure-function relationships, using data from the genomes of several prokaryotes, especially Bacillus subtilis. Many of these functions involve aspects of sugar metabolism and cell wall synthesis and are concerned with responses to abiotic stress such as heat, desiccation, or starvation. Particular emphasis is also given to the oxalate-degrading enzymes from microbes, their biological significance, and their value in a range of medical and other applications.

Modulation of endogenous glutathione availability

Glutathione is quantitatively the most important antioxidant and scavenger. In addition it has a number of important functions in amino acid transport across membranes, in protein synthesis and degradation, in gene regulation and in cellular redox regulation. It becomes more and more evident that depletion of glutathione is associated with states of severe diseases. From this perspective, the possibility of manipulating the availability of glutathione becomes a very attractive form of treatment. In this review, new insights into pathophysiology and the regulation of glutathione metabolism, in addition to effects of precursors to and stimulants of glutathione synthesis, are covered. It is very likely that glutathione precursors will soon be an important pharmacological tool for treatment in a number of diseased states.

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.

The effect of (L)-cysteine and (L)-2-oxothiazolidine-4-carboxylic acid (OTZ) on urinary oxalate excretion: studies using a hyperoxaluric rat model

PURPOSE

To determine the efficacy of (L)-cysteine and (L)-2-oxothiazolidine-4-carboxylic acid (OTZ) in reducing urinary oxalate excretion under hyperoxaluric conditions and to determine whether by inclusion of glycolate in a standard diet, cysteine:glyoxylate adduct can be detected in hyperoxaluric rats given either compound.

MATERIALS AND METHODS

Hyperoxaluria (200% above basal) was induced 2 days prior to commencement of the studies and maintained throughout. After a 3 days baseline, animals were randomly allocated to a control or treatment group. Standard diet containing either (L)-cysteine or OTZ was then fed to the treatment groups for 5 days while standard diet alone was fed to the control groups. Urinary oxalate excretion was subsequently monitored and average daily rates were then compared with basal values. Plasma and urine were analyzed for adduct.

RESULTS

Both (L)-cysteine and OTZ significantly reduced urinary oxalate excretion relative to the basal hyperoxaluric level (28.6 +/- 1.5 micromol./day). While (L)-cysteine reduced oxalate excretion over the 5 day treatment period by only 7.82 +/- 1.39 micromol./day (27%), OTZ reduced it by 12.34 +/- 1.58 micromol./day (43%). Adduct could not be detected in plasma or urine in this study.

CONCLUSIONS

This study confirms that both (L)-cysteine and OTZ are effective in reducing urinary oxalate excretion under hyperoxaluric conditions, with OTZ being more effective than (L)-cysteine. These compounds were shown to be 3- to 4-fold more effective in reducing urinary oxalate excretion under hyperoxaluric conditions when compared with the results from previous studies under normooxaluric conditions.