Indican

Indican is a colourless organic compound, soluble in water, naturally occurring in Indigofera plants. It is a precursor of indigo dye.

Chemical reactions
Indican is a glycoside. Common and significant reactions involving indican are as follows:
 * The hydrolysis of indican yields β-D-glucose and indoxyl.
 * The reaction of indoxyl (indican) with a mild oxidizing agent, such as atmospheric oxygen, yields blue indigo dye.

Biosynthesis
A reaction, similar to that used to produce indigo dye, is seen in the normal population, who excrete small amounts of the chemical in their urine. Normal urine reacting to hydrogen peroxide does at times produce a bluish tinge. Tryptophan is first converted to indole (which is excreted in faeces), then to indican by bacteria in the gut. Indican, being water-soluble, is then excreted through the urine. Following absorption from the gut, indole is converted to 3-hydroxy indole (indoxyl or indican) in the liver, where it is again then conjugated with sulfuric acid or glucuronic acid through normal xenobiotic metabolism pathways. It is then transported to the kidneys for excretion.

The enzyme "indoxyl esterase" has been found in humans and is involved in another pathway of chemical reactions involving indoxyl.

Pathology
Individuals affected by blue diaper syndrome exhibit a defect in tryptophan metabolism. Tryptophan is first converted to indole, then to indican by bacteria in the gut. Indican is then excreted into the urine and from there into the diaper where, upon exposure to air, it is oxidised by atmospheric oxygen to indigo blue dye.

Indican interferes with many commercial procedures for measuring total bilirubin which can be a problem for renal failure patients whose blood indican levels are raised. It can cause gastrointestinal symptoms in patients whose protein absorption is reduced, as in Hartnup's disease, allowing for greater bacterial decomposition of the tryptophan to indole and its conversion to indican.