Question

Melanuria (black urine) is caused by abnormal catabolism of which of the following amino acid?
A) Alanine
B) Tyrosine
C) Proline
D) Tryptophan

Answer 1

The person suffering from the disease alkaptonuria has black coloured urine. This is because in alkaptonuria, the homogentisic acid is accumulated inside the body and when it comes in contact with air, it forms alkapton which is black in colour.

Complete answer:
Alkaptonuria is a genetic disorder that is caused due to mutation in the homogentisic dioxygenase gene (HGD). This gene codes for the enzyme homogentisate 1,2- dioxygenase. This enzyme helps in the processing (or breakdown) of amino acids phenylalanine and tyrosine. The mutation in the HGD gene causes the accumulation of homogentisic acid in the body. The oxidation and polymerization of homogentisic acid results in the formation of a pigment called alkapton.

The alkapton is deposited in the connective tissues, nose, ears, bones and cartilages. This condition is called ochronosis. This results in discolouration of bones and cartilage and makes them brittle. With time, when the person reaches 20-30 years of age, early signs of osteoarthritis appear. However, the disorder is not much harmful as most of the homogentisic acid is excreted through urine. Hence, the infants often remain asymptomatic even if they are suffering from the disease.

The disease can be easily diagnosed by a traditional method which is a change in colour of urine to black-brown colour. Other methods include various chemical tests such as Benedict’s test, ferric chloride test etc. There is no specific treatment of the disease as it is not dangerous. However, the person is recommended to consume a protein diet having low phenylalanine concentration.

Phenylketonuria is a metabolic disorder in which phenylalanine is accumulated in the body. It is a rare disorder that results in mental retardation.

Hyperprolinemia is a metabolic disorder that is caused by the accumulation of proline in the body. The enzyme proline oxidase does not work properly and hence, cannot catalyse the proline amino acid.

Hypertryptophanemia is a metabolic disorder that is caused due to the accumulation of amino acid tryptophan in the blood. It is a rare autosomal recessive disorder that results in musculoskeletal effects such as joint pain.

Thus the answer to the above question is option (B) Tyrosine.

Note: The Lusitanus first described about the disease alkaptonuria in 1649. It is an example of an inborn error of metabolism which is inherited to children from their parents who are carriers or affected by the disease. However, the disease occurs in the person who is homozygous for the mutated allele.