Article: Alkaptonuria

Alkaptonuria (black urine disease, alcaptonuria or ochronosis) is a rare inherited genetic disorder of tyrosine metabolism. This is an autosomal recessive trait that is caused by a defect in the enzyme homogentisic acid oxidase (EC 1.13.11.5). The enzyme normally breaks down a toxic tyrosine byproduct, homogentisic acid (also called alkapton), which is harmful to bones and cartilage and is excreted in urine.

Symptoms

A distinctive characteristic of alkaptonuria is that urine or ear wax exposed to air turns reddish or inky black, depending on what one has eaten, after several hours because of the build-up of homogentisic acid. In adulthood, persons suffering from alkaptonuria develop progressive arthritis (especially of the spine), due to its degenerative effects on bones and cartilage.

Diagnosis

Presumptive diagnosis can be made by adding sodium or potassium hydroxide to urine and observing the formation of a dark brown to black pigment on the surface layer of urine within 30 minutes to 1 hour. Diagnosis can be confirmed by demonstrating the presence of homogentisic acid in the urine. This may be done by paper chromatography and thin-layer chromatography (Seegmiller, 1998).

Treatment

Prevention is not possible and the treatment is aimed at ameliorating symptoms.

See also

• Ochronosis
• Tyrosinemia