Alkaptonuria is an inborn error of metabolism that presents most notably with the occurrence of black urine when the urine is exposed to air. For this reason, it is also called the “Black Urine” disease. It is an autosomal recessive disorder and is therefore inherited genetically. Alkaptonuria is very rare, with a presentation of about 1:250,000(ref1). The causality behind Alkaptonuria is a deficiency in homogentisic acid 1,2 dioxygenase, an enzyme that is found in the liver and plays a key role in the catabolism of amino acids such as phenylalanine and tyrosine. Due to the lack of homogentisic acid 1,2 dioxygenase, the affected individual will experience a build-up of homogentisic acid in the body, namely in fibrous and cartilaginous tissue(ref1). Homogentisic acid is part of the catabolic chain from phenylalanine and tyrosine to fumarate and acetoacetic acid. Alkaptonuria received its title from the early name for homogentisic acid, alkapton.
Alkaptonuria is an autosomal recessive disorder that is genetically inherited in a Mendelian fashion within a family. It arises when individuals inherit two defective HGD genes from their parents. The gene HGD is located on the long arm of chromosome 3 and encodes homogentisic acid 1,2 dioxygenase (ref2). Although it is first and foremost a genetically linked condition, Alkaptonuria has also been observed in a patient who, following a liver transplant began to show symptoms of the disease. This confirmed that the location of homogentisic acid production is, in fact, the liver (ref3).
Signs and Diagnostics:
The alternate name for Alkaptonuria is the “black urine” disease, which is informative in the nature of the presentation of this disease. When exposed to air, the urine of the affected will turn to a dark brown-black color. In newborns, the urine discoloration can be observed by the parents and is an immediate cause for concern. Fresh urine, however, does not undergo...