Glucuronic Acid

GLUCURONIC ACID, COOH.(CH.OH)₄-CHO, a product of glucose oxidation, is a non-crystallizing syrupy substance that dissolves in water and alcohol. Upon boiling an aqueous solution of glucuronic acid, it partially converts into its anhydride, glucurone, C₆H₈O₆. Glucuronic acid rotates the plane of polarization to the right, gives reduction reactions similar to glucose, but does not ferment. Upon distillation with hydrochloric acid, glucuronic acid yields furfural, carbonic acid, and water; the formation of furfural under these conditions is the basis for the quantitative determination of glucuronic acid by the Tollens and Lefevre method and accounts for the ability of glucuronic acid and its conjugated compounds to produce the same colors when heated with HCl and orcin or phloroglucinol as pentoses do. The naphthoresorcinol reaction is used to distinguish it from pentoses: a blue-violet coloration upon heating with naphthoresorcinol and HCl. To detect glucuronic acid and its conjugated compounds, one can also use the formation of its sparingly soluble compound with para-bromophenylhydrazine. Glucuronic acid has not been found in a free state in animals and plants; it occurs in compounds with other substances, for example, in chondroitinsulfuric and mucoitinsulfuric acids, and especially in the form of so-called conjugated glucuronic acids, which belong to the β-glycosides. In small quantities, phenol-glucuronic acid, HOOC-CH-CH(OH)-CH(OH)-CH(OH)-CH.O.C₆H₅, as well as, in negligible quantities, indoxyl- and skatoxyl-glucuronic acids are normal constituents of urine (0.5–0.6 g of conjugated glucuronic acids are excreted in urine per day). Conjugated glucuronic acids are also found in the blood, bile, liver, and excrement, and also occur in plants.

In increased quantities, conjugated glucuronic acids appear in the urine upon the intake of various medicinal substances (e.g., chloral hydrate, camphor, naphthol, borneol, turpentine, morphine, and many others), as well as during increased putrefaction processes in the intestine, during severe respiratory disorders, in diabetes mellitus, and other pathological states. The synthesis of conjugated glucuronic acids from toxic substances of endogenous or exogenous origin serves to detoxify these poisonous substances. In cases of poisoning by phenols introduced into the body from the outside, the daily excretion of conjugated glucuronic acids increases colossally, reaching even 7 g. The liver is considered the main site of formation of conjugated glucuronic acids. Experiments clarifying the origin of conjugated glucuronic acids in the body are somewhat contradictory; however, it must still be acknowledged that glucose is the main starting material for the formation of glucuronic acid. The possibility of the formation of a portion of glucuronic acid from the body's proteins and fats is not excluded. Conjugated glucuronic acids are optically active and, with rare exceptions, rotate the plane of polarization to the left. Conjugated glucuronic acids possess reducing ability either directly or after hydrolysis, breaking down to form free glucuronic acid. These properties of conjugated glucuronic acids have practical significance in the determination of grape sugar in urine using reduction reactions and in the quantitative determination of glucose using a polarization apparatus. The synthesis of conjugated glucuronic acids, insofar as it occurs mainly in the liver, can be used for the functional examination of this organ. By giving the subject under examination camphor or menthol, one can judge the synthetic function of the liver by the amount of conjugated glucuronic acids excreted in the urine.