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This is a rare inborn error of metabolism of phenylalanine and tyrosine.
Estimated incident is 2-5 per million live births.
The disease is characterised by the deficiency of homogentisate oxidase which catalyses the conversion of tyrosine to acetyl coA and acetate. In this reaction sequence homogentisic acid (homogentisate) is an intermediate which is oxidised by the enzyme homogentisate oxidase.This results in the accumulation of homogentisic acid as shown in the figure 7.4.
Homogentisic acid accumulates in the tissues and blood and also appears in urine. Most striking clinical manifestation is occurrence of dark urine on standing in air. Homogentisic acid like many derivatives of tyrosine is readily oxidized to black pigments. These pigments are called as alkaptons. Urine in an exposed air slowly turns black from top to bottom.
In long standing cases, deposition of homogentisic acid derivatives in cartilages of ears and other exposed places leading to generalized pigmentation of connective tissues and deposition in joints leading to arthritis, a condition is called ochronosis. This is due to the of oxidation homogentisate by polyphenol oxidase, forming benzoquinone acetate, which polymerizes and binds to connective tissue macromolecules.
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