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  1. Alkaptonuria (MIM 203500; gene symbol AKU) is a rare, hereditary, metabolic disease in which homogentisic acid, an intermediary product in the metabolism of phenylalanine and tyrosine, cannot be further metabolized. The metabolic defect causes a characteristic triad of homogentisic aciduria, ochronosis, and arthritis.

  2. The cause of the disease is a constitutional lack of the enzyme homogentisic acid oxidase. This enzyme normally exists primarily in the liver and kidney. It requires oxygen, ferrous iron, and sulfhydryl groups to open the ring of homogentisic acid.

  3. The condition is inherited as an autosomal recessive disease. No metabolic method for the detection of heterozygotes has been devised; mutation analysis for this purpose is feasible. The molecular basis of alkaptonuria has been demonstrated to be defects in the gene coding for homogentisic acid oxidase (symbol HGO). The AKU and HGO genes are the same and map to human chromosome 3q21-q23; the nucleotide sequence (GenBank U63008) is divided into 14 exons over 60 kb of genomic DNA.

  4. The relationships between the metabolic defect and the complications ochronosis and arthritis remain a challenging research problem of the future. Even though the lack of homogentisic acid oxidase is the ultimate cause of these complications, the mechanisms that bring them about are unknown.

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