The hypothesis that is proposed is that tyrosinase, an enzyme widely found within the human body is implicated in the ochronosis that occurs in alkaptonuria; an autosomal recessive condition first used by Archibald Garrod to describe the theory of “Inborn Errors of Metabolism.” The disease results from the absence of a single enzyme in the liver that breaks down homogentisic acid; this molecule becomes systemically elevated in sufferers. The condition is characterised by a clinical triad of symptoms; homogentisic aciduria from birth, ochronosis (darkening) of collagenous tissues (from ∼30years of age) and ochronotic osteoarthropathy in weight bearing joints due to long term ochronosis in them (from ∼40years of age). Tyrosinase, a polyphenol oxidase has been shown in many species to contribute to the darkening of tissues in many organisms; including humans in the production of melanin. Tyrosinase under the right conditions shows alterations in its substrate specificity and may contribute to the darkening seen in AKU where it moves away from polymerising tyrosine but also homogentisic acid, the causative molecule in alkaptonuria, that is present in excess.