Alkaptonuria - Symptoms, Causes and Treatment

By: Corwin Brown

Alkaptonuria is an inherited condition that causes urine to turn black when exposed to air. Ochronosis, a buildup of dark pigment in connective tissues such as cartilage and skin, is also characteristic of the disorder. This blue-black pigmentation usually appears after age 30. People with alkaptonuria typically develop arthritis, particularly in the spine and large joints, beginning in early adulthood. Other features of this condition can include heart problems, kidney stones, and prostate stones.

Symptoms
The main symptoms of alkaptonuria are due to the accumulation of homogentisic acid in tissues. In the joints this leads to cartilage damage, specifically in the spine and leading to low back pain at a young age in most cases, but also of the hip and shoulder. Joint replacement surgery (hip and schoulder) is often necessary at a relatively young age.

Urine in an infant's diaper may darken and can turn almost black after several hours. However, many persons with this condition may not know they have it until mid-adulthood, around age 40, when joint and other problems occur.

Urine in an infant's diaper may darken and can turn almost black after several hours. However, many persons with this condition may not know they have it until mid-adulthood, around age 40, when joint and other problems occur.

Causes
Alkaptonuria is caused by the deficiency of an enzyme known as homogentisic acid oxidase (HGAO). Normally, this enzyme performs a crucial step in a metabolic pathway by converting a chemical, homogentisic acid (HGA), into another form to meet the body's needs. As normal amounts of the HGAO enzyme are missing, HGA is not broken-down and accumulates in the body. Some is eliminated in the urine, and the rest is deposited in body tissues where it is harmful. The result is a blue-black discolouration of connective tissue including bone, cartilage and skin (otherwise known as ochronosis).

The gene defect makes the body unable to properly break down certain proteins (tyrosine and phenylalanine). As a result, a substance called homogentisic acid builds up in the skin and other body tissues. The acid leaves the body through the urine. The urine turns brownish-black when it mixes with air.

Inability to convert homogentisic acid to maleylacetoacetic acid results in accumulation of the former. Homogentisic acid is subsequently converted to benzoquinone acetic acid and spontaneously polymerized. Deposition of the polymer in association with cartilage is the initiating pathophysiologic cause of the arthritis.

Diets low in protein--especially in amino acids, phenylalanine (found in aspartame), and tyrosine--help reduce the levels of HGA, thereby lessening the amount of pigment deposited in body tissues. Symptoms of alkaptonuria (e.g., arthropathy, cardiovascular disease) are treated when possible. Unfortunately, the course of the disease remains unchanged, and no cure is available.

Treatment
No treatment modality has been unequivocally demonstrated to reduce the complications of alkaptonuria. Commonly recommended treatments include dietary restriction of phenylalanine and tyrosine and large doses of ascorbic acid (vitamin C). Dietary restriction may be effective in children, but benefits in adults have not been demonstrated.

Some patients benefit from high-dose vitamin C. This has been shown to decrease the build up of brown pigment in the cartilage and may slow the development of arthritis.

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