Glucuronic acid

Glucuronic acid

Names
IUPAC name (2S,3S,4S,5R,6R)-3,4,5,6-​Tetrahydroxyoxane-2-carboxylic acid
Other names β-D-glucopyranuronic acid
Identifiers
CAS Number	6556-12-3
ChEBI	CHEBI:28860
ChemSpider	392615
DrugBank	DB03156
Jmol interactive 3D	Image
KEGG	C00191
MeSH	Glucuronic+acid
PubChem	441478
UNII	8A5D83Q4RW
InChI InChI=1S/C6H10O7/c7-1-2(8)4(5(10)11)13-6(12)3(1)9/h1-4,6-9,12H,(H,10,11)/t1-,2-,3+,4-,6-/m0/s1 Key: AEMOLEFTQBMNLQ-WAXACMCWSA-N InChI=1/C6H10O7/c7-1-2(8)4(5(10)11)13-6(12)3(1)9/h1-4,6-9,12H,(H,10,11)/t1-,2-,3+,4-,6-/m0/s1 Key: AEMOLEFTQBMNLQ-WAXACMCWBB
SMILES O=C(O)[C@H]1O[C@H](O)[C@H](O)[C@@H](O)[C@@H]1O
Properties
Chemical formula	C6H10O7
Molar mass	194.14 g·mol^−1
Melting point	159 to 161 °C (318 to 322 °F; 432 to 434 K)[1]
Related compounds
Related uronic acids	Alluronic acid, Altruronic acid, Arabinuronic acid, Fructuronic acid, Galacturonic acid, Guluronic acid, Iduronic acid, Lyxuronic acid, Mannuronic acid, Psicuronic acid, Riburonic acid, Ribuluronic acid, Sorburonic acid, Tagaturonic acid, Taluronic acid, Xyluluronic acid, Xyluronic acid
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
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Infobox references

Glucuronic acid (from Ancient Greek γλυκύς "sweet" + οὖρον "urine") is an uronic acid that was first isolated from urine (hence the name). It is found in many gums such as Gum arabic (ca. 18 %) and Xanthan, and is important for the metabolism of microorganisms, plants and animals.

Properties

The β-D methyl glycoside of glucuronic acid in the low energy ⁴C₁ conformation of D-glucose

Glucuronic acid is a sugar acid derived from glucose, with its sixth carbon atom oxidized to a carboxylic acid. In living beings, this primary oxidation occurs with UDP-α-D-glucose (UDPG), not with the free sugar.

Glucuronic acid, like its precursor glucose, can exist as a linear (carboxo-)aldohexose (< 1 %), or as a cyclic hemiacetal (furanose or pyranose). Aldohexoses such as D-glucose are capable of forming two furanose forms (α and β) and two pyranose forms (α and β). By the Fischer convention, glucuronic acid has two stereoisomers (enantiomers), D- and L-glucuronic acid, depending on its configuration at C-5. Most physiological sugars are of the D-configuration. Due to ring closure, cyclic sugars have another asymmetric carbon atom (C-1), resulting in two more stereoisomers, named anomers. Depending on the configuration at C-1, there are two anomers of glucuronic acid, α- and β-form. In β-D-glucuronic acid the C-1 hydroxy group is on the same side of the pyranose ring as the carboxyl group. In the free sugar acid, the β-form is prevalent (~ 64 %), whereas in the organism, the α-form UDP-α-D-glucuronic acid (UDPGA) predominates.

Carbohydrate stereoisomers, which differ in configuration at only one (other) asymmetric C-atom, are named epimers. For example, D-mannuronic (C-2), D-alluronic (C-3), D-galacturonic (C-4), and L-iduronic acid (C-5) are epimers of glucuronic acid.

The nonplanar pyranose rings can assume either chair (in 2 variants) or boat conformation. The preferred conformation depends on spatial interference or other interactions of the substituents. The pyranose form of D-glucose and its derivative D-glucuronic acid prefer the chair ⁴C₁.

Additional oxidation at C-1 to the carboxyl level yields the dicarboxylic glucaric acid. Glucuronolactone is the self-ester (lactone) of glucuronic acid.

Direct oxidation of an aldose affects the aldehyde group first. A laboratory synthesis of a uronic acid from an aldose requires protecting the aldehyde and hydroxy groups from oxidation, for example by conversion to cyclic acetals (e. g., acetonides).

Functions

Proteoglycans

Glucuronic acid is a common building block of proteoglycans and glycoglycerolipids:

• Heparin is an inhibitor of blood coagulation, and occurs in mast cells, lung and liver.
• Chondroitin sulfate is found in large quantities in cartilage, aorta, connective tissue, bone, and skin.
• Dermatan sulfate is a proteoglycan in skin, heart, and blood vessels.
• Keratan sulfate being found in the cornea, cartilage, and bone.
• Hyaluronic acid occurs in large quantities in connective tissues, skin, cartilage, and synovial fluid.
• Glycoglycerolipids of glucuronic or galacturonic acids form the cell walls of bacteria.

Glucuronidation

Glucuronidation of (para-phenyl-)aniline by formation of a β-glycosidic bond. In the body, aniline is first hydroxylated during first-phase metabolism.

UDP-α-D-glucuronic acid (UDPGA) is often involved in the phase II metabolism (conjugation) of lipophilic xeno- and endobiotics. These linkages involve glycosidic bonds with thiol, amine and hydroxy groups, or esterification with the carboxyl and hydroxyl groups. This linkage process is known as glucuronidation (or glucuronide conjugation). Glucuronidation occurs mainly in the liver, although the enzymes responsible for its catalysis, UDP-glucuronyltransferases (UDP-GT), have been found in all major body organs, e.g., intestine, kidneys, brain, adrenal gland, spleen, and thymus.^[2][3] Analogous reactions occur with other UDP-uronic acids (e. g., D-galacturonic acid).

Glycosides resulting from glucuronidation are named (β-D-)glucuronides, its salts and esters are named glucuronates. The human body uses glucuronidation to make alcohols, phenols, carboxylic acids, mercaptans, primary and secondary aliphatic amines, and carbamates more water-soluble, and, in this way, allows for their subsequent elimination from the body through urine or faeces (via bile from the liver). The carboxyl group is ionized at physiological pH, making the conjugated compound water-soluble. Compounds with molecular masses > 300 are too large for renal excretion and will be excreted with bile into the intestine. Neonates are deficient in this conjugating system, making them particularly vulnerable to drugs such as chloramphenicol, which is inactivated by the addition of glucuronic acid, resulting in gray baby syndrome. Bilirubin is excreted in the bile as bilirubin diglucuronide (80 %), bilirubin monoglucuronide (20 %), and unconjugated bilirubin (< 1 %). In the Crigler-Najjar syndrome and the Gilbert syndrome, UDPGT activity is reduced or nearly absent due to mutations, resulting in jaundice.

Glucuronides may be hydrolyzed by β-glucuronidase present in intestinal microflora to the respective aglycone, which may be reabsorbed from the intestine and translocated back to the liver with the blood. The resulting cycle is called enterohepatic circulation. Compounds that undergo enterohepatic circulation are only slowly excreted and usually have a longer half-life in the body.

Certain glucuronides are electrophilic and may function in toxication processes. Covalent binding of the aglycone portions of several carboxylic acid (ester) glucuronides is known to occur to nucleophilic sites on serum albumin via transacylation reactions, for example.^[4]

Phenols, quantitatively important P450-derived metabolites of aromatic hydrocarbons, are substrates for both UDP-GT and sulfotransferases. Glucuronides predominate with phenol or a phenol precursor (benzene) in mammals because sulfate formation is a high-affinity, low-capacity system (due to sulfate depletion), whereas glucuronidation is a low-affinity, high-capacity system.^[4]

Use

Determination of urinary steroids and of steroid conjugates in blood. Ethyl glucuronide and ethyl sulfate are excreted in urine as metabolites of ethanol and are used to monitor alcohol use or dependence.

Glucuronic acid and gluconic acid are fermentation products in Kombucha tea.^[5]

Glucuronic acid is a precursor of ascorbic acid (vitamin C). Ascorbate can be biosynthesized by higher plants, algae, yeast and most animals. An adult goat produces ~13 g of vitamin C per day. This ability is lacking in some mammals (including humans and guinea pigs) and also in insects, invertebrates and most fishes. These species require external ascorbate supply, because they lack the biosynthetic enzyme L-gulonolactone oxidase.^[6]

The glucuronide 4-methylumbelliferyl-β-D-glucuronide (MUG) is used to test for the presence of Escherichia coli. E. coli produces the enzyme β-glucuronidase, which hydrolyzes the MUG molecule to a fluorescent product that is detectable under ultraviolet light.

The glucuronide laetrile (L-mandelonitrile-β-D-glucuronide) has been touted as an alternative cancer treatment.

References

1. D-Glucuronic acid at Sigma-Aldrich
2. Ohno, Shuji; Nakajin, Shizuo (2008-10-06). "Determination of mRNA Expression of Human UDP-Glucuronosyltransferases and Application for Localization in Various Human Tissues by Real-Time Reverse Transcriptase-Polymerase Chain Reaction". Drug Metabolism and Disposition (American Society for Pharmacology and Experimental Therapeutics) 37 (1): 32–40. doi:10.1124/dmd.108.023598. Retrieved 2010-11-07.
3. Bock K, Köhle C (2005). "UDP-glucuronosyltransferase 1A6: structural, functional, and regulatory aspects". Methods enzymol. 400: 57–75. doi:10.1016/S0076-6879(05)00004-2. PMID 16399343.
4. Tanya C McCarthy; Christopher J Sinal (2005), "Biotransformation", Encyclopedia of Toxicology 1 (2nd ed.), Elsevier, pp. 299–312, ISBN 0-12-745354-7
5. Blanc, P.J. (February 1996). "Characterization of the tea fungus metabolites". Biotechnology Letters 18 (2): 139. doi:10.1007/BF00128667.
6. Gerhard Michal; Dietmar Schomburg (2012), Biochemical Pathways: An Atlas of Biochemistry and Molecular Biology (2nd ed.), Wiley, p. 145a, ISBN 978-0-470-14684-2