Ochronosis is

The metabolic defect in alkaptonuria is a defective homogentisate oxidase (EC 1.13.11.5), the enzyme that catalyzes reaction 3 of. The urine darkens on exposure to air due to oxidation of excreted homogentisate. Late in the disease, there is ahritis and connective tissue pigmentation (ochronosis) due to oxidation of homogentisate to benzoquinone acetate, which polymerizes and binds to connective tissue.Reference: Harper biochemistry, 30th edition, page no 304