REACTIONS OF THE CARBOHYDRATES 67 



CHO CHO COOH 



I I I 



H C OH H C OH H C OH 



I I I 



HO C H HO C H HO C H 



H C OH H C OH H C OH 



H C OH H C OH H C OH 



I I I 



CH 2 OH COOH COOH 



d-glucose. d-glucuronic acid. d-saccharic acid. 



Glucuronic acid is therefore at the same time an acid and an alde- 

 hyde. On boiling its solution or on prolonged standing it is trans- 

 formed into a crystalline lactone which is represented by the formula: 



CHO 

 H c OH 



Glucuronic acid yields the pentose reactions with orcin or phloro- 

 glucin and hydrochloric acid (see p. 62) and also the following reaction: 



Naphtho-resorcinol Reaction. A small amount of naphtho-resorcinol 

 is dissolved in concentrated hydrochloric acid and to this reagent is 

 added an equal volume of a solution of glucuronic acid. A violet-blue 

 coloration results which is soluble in ether. This reaction is not specific 

 for glucuronic acid, being given by many ketose and aldehyde acids. 

 It is, however, useful for the purpose of distinguishing between glu- 

 curonic acid and the pentoses. 



Glucuronic acid does not occur in the free condition in animal tissues, 

 nor has it as yet been identified in plants. In the form of ester-like 

 compounds, however, it is found in many plants, notably in Scutellaria, 

 and esters of glucuronic acid are found in^many parts of the body, 

 in the blood, the liver and in urine. The normal forms in which it is 

 found in urine are Phenyl-, Indoxyl- and Skatoxyl-glucuronic acids. 

 Indoxyl and skatoxyl are highly toxic products of intestinal putrefac- 

 tion; the compounds which they form with glucuronic acid are, however, 

 harmless. 



Under ordinary conditions, glucose is readily oxidized in the body to 

 carbon dioxide and water, passing through intermediate stages of which 

 lactic acid is one. But in the presence of some toxic agents it appears 

 that the oxidation of glucose is arrested at the formation of glucuronic 

 acid, which combines with the toxic substance, the compound being 



