<p> Alkaptonuria (AKU), a rare hereditary disorder, was the first disease to be interpreted as an inborn error of metabolism. Thedeficiency causes homogentisic aciduria, ochronosis, and arthritis. AKU patients are deficient for homogentisate 1,2 dioxygenase (<db_xref db="EC" dbkey="1.13.11.5"/>), the enzyme that mediates the conversion of homogentisate to maleylacetoacetate; a step in the catabolism of both tyrosine and phenylalanine. <reaction> Homogentisate + O(2) = 4-maleylacetoacetate. </reaction> </p>