Hyperoxalemia
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Oxalate (IUPAC: ethanedioate) is an anion with the formula C2O42−. This dianion is colorless. It occurs naturally, including in some foods. It forms a variety of salts, for example sodium oxalate (Na2C2O4), and several esters such as dimethyl oxalate (C2O4(CH3)2). It is a conjugate base of
oxalic acid Oxalic acid is an organic acid with the systematic name ethanedioic acid and formula . It is the simplest dicarboxylic acid. It is a white crystalline solid that forms a colorless solution in water. Its name comes from the fact that early inve ...
. At neutral pH in aqueous solution, oxalic acid converts completely to oxalate.


Relationship to oxalic acid

The dissociation of
proton A proton is a stable subatomic particle, symbol , H+, or 1H+ with a positive electric charge of +1 ''e'' elementary charge. Its mass is slightly less than that of a neutron and 1,836 times the mass of an electron (the proton–electron mass ...
s from oxalic acid proceeds in a stepwise manner; as for other polyprotic acids, loss of a single proton results in the monovalent hydrogenoxalate anion . A salt with this anion is sometimes called an acid oxalate, monobasic oxalate, or hydrogen oxalate. The equilibrium constant ( ''K''a) for loss of the first proton is (p''K''a = 1.27). The loss of the second proton, which yields the oxalate ion, has an equilibrium constant of (p''K''a = 4.28). These values imply, in solutions with neutral pH, no oxalic acid and only trace amounts of hydrogen oxalate exist. The literature is often unclear on the distinction between H2C2O4, , and , and the collection of species is referred to as oxalic acid.


Structure

The oxalate anion exists in a nonplanar conformation where the O–C–C–O dihedrals approach 90° with approximate D2d symmetry. When chelated to cations, oxalate adopts the planar, D2h conformation. However, in the structure of Cs2C2O4 the O–C–C–O dihedral angle is 81(1)°. Therefore, Cs2C2O4 is more closely approximated by a D2d symmetry structure because the two CO2 planes are staggered. Two structural forms of Rb2C2O4 have been identified by single-crystal X-ray diffraction: one contains a planar and the other a staggered oxalate. The barrier to rotation about this bond is calculated to be roughly 2–6 kcal/mol for the free dianion, . Such results are consistent with the interpretation that the central carbon–carbon bond is regarded as a single bond with minimal π interactions between the two units. This barrier to rotation about the C−C bond (which formally corresponds to the difference in energy between the planar and staggered forms) may be attributed to electrostatic interactions as unfavorable O−O repulsion is maximized in the planar form.


Occurrence in nature

Oxalate occurs in many plants, where it is synthesized by the incomplete oxidation of saccharides. Several plant foods such as the root and/or leaves of
spinach Spinach (''Spinacia oleracea'') is a leafy green flowering plant native to central and western Asia. It is of the order Caryophyllales, family Amaranthaceae, subfamily Chenopodioideae. Its leaves are a common edible vegetable consumed either f ...
,
rhubarb Rhubarb is the fleshy, edible stalks ( petioles) of species and hybrids (culinary rhubarb) of ''Rheum'' in the family Polygonaceae, which are cooked and used for food. The whole plant – a herbaceous perennial growing from short, thick rhizo ...
, and buckwheat are high in oxalic acid and can contribute to the formation of kidney stones in some individuals. Other oxalate-rich plants include fat hen ("lamb's quarters"), sorrel, and several '' Oxalis'' species. The root and/or leaves of rhubarb and buckwheat are high in oxalic acid. Other edible plants with significant concentrations of oxalate include, in decreasing order, star fruit ( carambola), black pepper,
parsley Parsley, or garden parsley (''Petroselinum crispum'') is a species of flowering plant in the family Apiaceae that is native to the central and eastern Mediterranean region (Sardinia, Lebanon, Israel, Cyprus, Turkey, southern Italy, Greece, Por ...
,
poppy seed Poppy seed is an oilseed obtained from the opium poppy (''Papaver somniferum''). The tiny, kidney-shaped seeds have been harvested from dried seed pods by various civilizations for thousands of years. It is still widely used in many countries, ...
,
amaranth ''Amaranthus'' is a cosmopolitan genus of annual or short-lived perennial plants collectively known as amaranths. Some amaranth species are cultivated as leaf vegetables, pseudocereals, and ornamental plants. Catkin-like cymes of densely pack ...
,
chard Chard or Swiss chard (; ''Beta vulgaris'' subsp. ''vulgaris'', Cicla Group and Flavescens Group) is a green leafy vegetable. In the cultivars of the Flavescens Group, the leaf stalks are large and often prepared separately from the leaf blade; ...
, beets,
cocoa Cocoa may refer to: Chocolate * Chocolate * ''Theobroma cacao'', the cocoa tree * Cocoa bean, seed of ''Theobroma cacao'' * Chocolate liquor, or cocoa liquor, pure, liquid chocolate extracted from the cocoa bean, including both cocoa butter and ...
, chocolate, most
nuts Nut often refers to: * Nut (fruit), fruit composed of a hard shell and a seed, or a collective noun for dry and edible fruits or seeds * Nut (hardware), fastener used with a bolt Nut or Nuts may also refer to: Arts, entertainment, and media Com ...
, most berries, fishtail palms, New Zealand spinach ('' Tetragonia tetragonioides''), and
bean A bean is the seed of several plants in the family Fabaceae, which are used as vegetables for human or animal food. They can be cooked in many different ways, including boiling, frying, and baking, and are used in many traditional dishes th ...
s. Leaves of the tea plant (''
Camellia sinensis ''Camellia sinensis'' is a species of evergreen shrub or small tree in the flowering plant family Theaceae. Its leaves and leaf buds are used to produce the popular beverage, tea. Common names include tea plant, tea shrub, and tea tree (not to ...
'') contain among the greatest measured concentrations of oxalic acid relative to other plants. However, the beverage derived by infusion in hot water typically contains only low to moderate amounts of oxalic acid due to the small mass of leaves used for brewing.


Physiological effects

Excess consumption has been linked to gout and kidney stones. Many metal ions form insoluble precipitates with oxalate, a prominent example being
calcium oxalate Calcium oxalate (in archaic terminology, oxalate of lime) is a calcium salt of oxalic acid with the chemical formula . It forms hydrates , where ''n'' varies from 1 to 3. Anhydrous and all hydrated forms are colorless or white. The monohydrate ...
, the primary constituent of the most common kind of kidney stones. The highly insoluble iron(II) oxalate appears to play a major role in gout, in the nucleation and growth of the otherwise extremely soluble sodium urate. This explains why gout usually appears after age 40, when ferritin levels in blood exceed 1 μg/L . Foods high in oxalate are often avoided by people at risk of gout. In studies with rats, calcium supplements given along with foods high in oxalic acid can cause calcium oxalate to precipitate in the gut and reduce the levels of oxalate absorbed by the body (by 97% in some cases). Some fungi of the genus '' Aspergillus'' produce oxalic acid.


As a ligand for metal ions

Oxalate also forms
coordination compound A coordination complex consists of a central atom or ion, which is usually metallic and is called the ''coordination centre'', and a surrounding array of bound molecules or ions, that are in turn known as ''ligands'' or complexing agents. Many ...
s where it is sometimes abbreviated as ox. It is commonly encountered as a
bidentate ligand In coordination chemistry, a ligand is an ion or molecule (functional group) that binds to a central metal atom to form a coordination complex. The bonding with the metal generally involves formal donation of one or more of the ligand's electr ...
, such as in potassium ferrioxalate. When the oxalate chelates to a single metal center, it always adopts the planar conformation. As a bidentate ligand, it forms a 5-membered MC2O2 ring. An illustrative complex is potassium ferrioxalate, K3 e(C2O4)3 The drug
oxaliplatin Oxaliplatin, sold under the brand name Eloxatin among others, is a cancer medication (platinum-based antineoplastic class) used to treat colorectal cancer. It is given by injection into a vein. Common side effects include numbness, feeling ti ...
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 rust-removal, which arises because oxalate forms water-soluble derivatives with the ferric ion.


Excess

An excess oxalate level in the blood is termed ''hyperoxalemia'', and high levels of oxalate in the urine is termed hyperoxaluria.


Acquired

Although unusual, consumption of oxalates (for example, the grazing of animals on oxalate-containing plants such as ''
Bassia hyssopifolia ''Bassia hyssopifolia'' is a species of flowering plant in the family Amaranthaceae, known by the common names five-horn smotherweed, five-hook bassia, and thorn orache.wood sorrel or, specifically in excessive quantities, black tea) may result in
kidney disease Kidney disease, or renal disease, technically referred to as nephropathy, is damage to or disease of a kidney. Nephritis is an inflammatory kidney disease and has several types according to the location of the inflammation. Inflammation can ...
or even death due to oxalate poisoning. '' The New England Journal of Medicine'' reported acute oxalate nephropathy "almost certainly due to excessive consumption of iced tea" in a 56-year-old man, who drank "sixteen 8-ounce glasses of iced tea daily" (roughly ). The authors of the paper hypothesized that acute oxalate nephropathy is an underdiagnosed cause of kidney failure and suggested thorough examination of patient dietary history in cases of unexplained kidney failure without proteinuria (an excess of protein in the urine) and with large amounts of calcium oxalate in urine sediment. ''
Oxalobacter formigenes ''Oxalobacter formigenes'' is a Gram negative oxalate-degrading anaerobic bacterium that was first isolated from the gastrointestinal tract of a sheep in 1985. To date, the bacterium has been found to colonizes the large intestines of numerous ...
'' in the
gut flora Gut microbiota, gut microbiome, or gut flora, are the microorganisms, including bacteria, archaea, fungi, and viruses that live in the digestive tracts of animals. The gastrointestinal metagenome is the aggregate of all the genomes of the gut mi ...
may help alleviate this.


Congenital

Primary hyperoxaluria is a rare, inherited condition, resulting in increased excretion of oxalate, with oxalate stones being common.


References


Further reading

* * *


External links


Oxalate.org
- Oxalate content of 750+ foods from university and government sources {{Oxalates Anticoagulants Carboxylate anions