Homogentisate oxidase, deficiency

Answer A. Adults with alcaptonuria show a high prevalence of ochronotic arthritis due to deficiency of homogentisate oxidase. 

Alkaptonuria is an autosomal recessive, benign disorder with a normal life expectancy. It is caused by a deficiency of homogentisate oxidase (Fig. 47.2). Homogentisate accumulates, is excreted in the urine and is gradually oxidised to a black pigment when exposed to air. It is usually detected when the nappies (or diapers) show black staining. 

Correct answer = B. Alkaptonuria is a rare metabolic disease involving a deficiency in homogentisic acid oxidase, and the subsequent accumulation of homogentisic acid in the urine, which turns dark upon standing. The elevation of methylmalonate (due to methylmalonyl CoA mutase deficiency), phenylpyruvate (due to phenylalanine hydroxlyase deficiency), a-ketoisovalerate (due to branched-chain a-ketoacid dehydrogenase deficiency), and homocystine (due to cystathionine synthase deficiency) are inconsistent with a healthy child with darkening of the urine. 

Alkaptonuria is a rare inborn error of metabolism caused by a lack of the enzyme homogentisic acid oxidase. The deficiency causes a failure to oxidize homogentisate, which is excreted in the urine in abundance. Polymerization of the metabolite, especially under alkaline conditions in the presence of oxygen, leads to the production of a black polymer. This makes the urine black if it is exposed to air for a few hours and in vivo, cartilage and other connective tissues become pigmented and take on an orange color (hence the name ochronosis for this state of the tissues the ocher color is readily seen postmortem). In later years the patients develop severe arthritis. 

A rare inborn error of metabolism in which there is a deficiency of the enzyme homogentisic acid oxidase. It is inherited as an autosomal recessive. 

The oxidative breakdown of the amino acid tyrosine contains a stage in which homogentisic acid is converted to maley-lacetoacetic acid, a reaction catalysed by homogentisic acid oxidase. A deficiency of this enzyme results in the accumulation... 

An organic acid found in the urine of patients with alkaptonuria, a hereditary condition in which there is a deficiency of homo-gentisic acid oxidase, one of the enzymes of tyrosine metabolism. As a result, homogentisic acid accumulates in the blood, tissues and urine. Urines containing homogentisic acid turn dark on standing due to oxidation and polymerization of the compound to the black pigment alkapton. 

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