oxalate

The structure of the oxalate anion

Oxalate or ethanedioate is the divalent anion with formula C₂O₄²⁻ or (COO)₂²⁻, resulting from oxalic acid by loss of two protons. Either name is often used for a salt containing this anion (such as disodium oxalate, (Na⁺)₂C₂O₄²⁻), or an ester of oxalic acid (such as dimethyl oxalate, (CH₃)₂C₂O₄).

Removal of a single proton from oxalic acid results in the monovalent hydrogenoxalate anion HC₂O₄⁻. A salt with this anion is sometimes called an acid oxalate, monobasic oxalate, or hydrogen oxalate.

Occurrence in nature

Oxalate, sometimes called oxalic acid, occurs widely in nature, most notably fat hen (lamb's quarters), sorrel, and Oxalis species. The root and/or leaves of rhubarb and buckwheat are listed as being high in oxalic acid.^[1] It arises biosynthetically via the incomplete oxidation of carbohydrates.

Other edible plants that contain significant concentrations of oxalic acid include—in decreasing order—star fruit (carambola), black pepper, parsley, poppy seed, amaranth, spinach, chard, beets, cocoa, chocolate, most nuts, most berries, New Zealand Spinach (Tetragonia tetragonioides) and beans.

The gritty “mouth feel” one experiences when drinking milk with a rhubarb dessert is caused by precipitation of calcium oxalate. Thus, even dilute amounts of oxalic acid can readily "crack" the casein found in various dairy products.

Leaves of the tea plant (Camellia sinensis) contain among the greatest measured concentrations of oxalic acid relative to other plants. However the infusion beverage typically contains only low to moderate amounts of oxalic acid per serving, due to the small mass of leaves used for brewing.

Physiological effects

The affinity of divalent metal ions is sometimes reflected in their tendency to form insoluble precipitates. Thus in the body, oxalic acid also combines with metals ions such as Ca²⁺, Fe²⁺, and Mg²⁺ to deposit crystals of the corresponding oxalates, which irritate the gut and kidneys. Because it binds vital nutrients such as calcium, long-term consumption of foods high in oxalic acid can be problematic. Healthy individuals can safely consume such foods in moderation, but those with kidney disorders, gout, rheumatoid arthritis, or certain forms of chronic vulvar pain (vulvodynia) are typically advised to avoid foods high in oxalic acid or oxalates. The calcium oxalate crystals or precipitate (better known as kidney stones) obstruct the kidney tubules. An estimated 80% of kidney stones are formed from calcium oxalate.^[3]

Conversely, calcium supplements taken along with foods high in oxalic acid can cause calcium oxalate to precipitate out in the gut and drastically reduce the levels of oxalate absorbed by the body (by 97% in some cases.)^[4][5]

Oxalic acid can also be produced by the metabolism of ethylene glycol ("antifreeze"), glyoxylic acid or ascorbic acid (vitamin C).^{[dubious – discuss]}

Some Aspergillus species produce oxalic acid, which reacts with blood or tissue calcium to precipitate calcium oxalate.^[6] There is some preliminary evidence that the administration of probiotics can affect oxalic acid excretion rates^[7] (and presumably oxalic acid levels as well.)

Methods to reduce the oxalate content in food are of current interest.^[8]

Examples

• sodium oxalate - Na₂C₂O₄
• calcium oxalate - CaC₂O₄, a major component of kidney stones
• dimethyl oxalate - (CH₃)₂C₂O₄
• phenyl oxalate ester - (C₆H₅)₂C₂O₄
• potassium ferrioxalate - K₃[Fe(C₂O₄)₃], an iron complex with oxalate ligands

As a ligand

Oxalate, the conjugate base of oxalic acid, is an excellent ligand for metal ions. It usually binds as a bidentate ligand forming a 5-membered MO₂C₂ ring. An illustrative complex is potassium ferrioxalate, K₃[Fe(C₂O₄)₃]. The drug Oxaliplatin exhibits improved water solubility relative to older platinum-based drugs, avoiding the dose-limiting side-effect of nephrotoxicity. Oxalic acid and oxalates can be oxidized by permanganate in an autocatalytic reaction. One of the main applications of oxalic acid is a rust-removal, which arises because oxalate forms water soluble derivatives with the ferric ion.

Safety

Although unusual, consumption of oxalates (for example, the grazing of animals on oxalate-containing plants such as greasewood or human consumption of Sorrel) may result in kidney disease or even death due to oxalate poisoning. The presence of Oxalobacter formigenes in the gut flora can prevent this.

References

1. ^ Streitweiser, Andrew Jr.; Heathcock, Clayton H.: Introduction to Organic Chemistry, Macmillan 1976, p 737
2. ^ Resnick, Martin I.; Pak, Charles Y. C. (1990). Urolithiasis, A Medical and Surgical Reference. W.B. Saunders Company. pp. 158. ISBN 0721624391. 
3. ^ Coe FL, Evan A, Worcester E. (2005). "Kidney stone disease". J Clin Invest. 115 (10): 2598–608. doi:10.1172/JCI26662. PMID 16200192. 
4. ^ Morozumi M, Hossain RZ, Yamakawa KI, Hokama S, Nishijima S, Oshiro Y, Uchida A, Sugaya K, Ogawa Y (2006). "Gastrointestinal oxalic acid absorption in calcium-treated rats". Urol Res 34: 168. doi:10.1007/s00240-006-0035-7. PMID 16444511. 
5. ^ Hossain RZ, Ogawa Y, Morozumi M, Hokama S, Sugaya K. "Milk and calcium prevent gastrointestinal absorption and urinary excretion of oxalate in rats". Front Biosci.. PMID 12700095. 
6. ^ Pabuccuoglu U. (2005). "Aspects of oxalosis associated with aspergillosis in pathology specimens". Pathol Res Pract. 201 (5): 363–8. doi:10.1016/j.prp.2005.03.005. PMID 16047945. 
7. ^ Lieske JC, Goldfarb DS, De Simone C, Regnier C. (2005). "Use of a probiotic to decrease enteric hyperoxaluria". Kidney Int. 68 (3): 1244–9. doi:10.1111/j.1523-1755.2005.00520.x. PMID 16105057. 
8. ^ Betsche, T.; Fretzdorff, B. (2005). "Biodegradation of oxalic acid from spinach using cereal radicles". J Agric Food Chem. 53 (25): 9751–8. doi:10.1021/jf051091s. PMID 16332126.