Advanced Techniques in Biology & Medicine
Open Access

Ochronosis

This form is also referred to as alkaptonuria (black urine disease). It is an autosomal recessive disease which is caused by inheriting a genetic defect in the enzyme homogentisate 1,2-dioxygenase (homogentisic acid oxidase or HGD). HGD has a role in the catabolism of the amino acids phenylalanine and tyrosine. When HGD is functionally deficient (as is the case in alkaptonuria), there is an accumulation of homogentisic acid (HGA), a toxic by-product of tyrosine metabolism, in the liver. This occurs because HGD uses HGA as a substrate to convert it into maleylacetoacetic acid. In HGD deficiency, some of the HGA is excreted in urine, and oxidized in the air into benzoquinone acetate (BQA). This compound then undergoes polymerization to yield a black colour.

This disease may first be noticed in infancy by observing that the urine is much darker than normal. Unfortunately, it is rare that the condition is diagnosed this early, because the urine turns dark only after being exposed to air for several hours.