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  Albinism

Albinism:

It involves a spectrum of clinical syndromes characterized through hypomelanosis that is reduced synthesis of melanin pigment in skin and eyes. The defect is in the enzyme tyrosinase that is accountable for the biosynthesis of the pigment and therefore the individual appears bleached. In this circumstance, melanin is not synthesized in the melanocytes and influences the skin, hair, sclera and choroids etc. Melanin is synthesized from tyrosine as displayed in the below figure.

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                                                 Diagram: Synthesis of melanin from tyrosine

Usually, two types of albinism are observed. That is

1. Oculo-cutaneous albinism:
In this type of albinism, there is decreased pigmentation of skin and eyes,

2. Ocular albinism:
In this type of albinism, there is decreased pigmentation of just only eyes and not the skin.

Alkaptonuria:

This is an uncommon inborn fault of metabolism of phenylalanine and tyrosine.  Approximate occurrence is 2-5 per million live births.

Cause:

The disease is characterised through the deficiency of homogentisate oxidase that catalyses the translation of tyrosine to acetyl coA and acetate. Within this reaction sequence homogentisic acid (homogentisate) is an intermediate that is oxidised through the enzyme homogentisate oxidase. This effects in the accumulation of homogentisic acid as displayed in the below figure.

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                                                  Figure: Role of homogentisate oxidase in tyrosine catabolism

Symptoms:

Homogentisic acid builds up in the tissues and blood and as well appears in urine. Very much striking clinical manifestation is incidence of dark urine on standing in air. Homogentisic acid such as several derivatives of tyrosine is readily oxidized to black pigments. These pigments are known as alkaptons. In an exposed air Urine slowly turns black from top to bottom.

In the cases of long standing, deposition of homogentisic acid derivatives in cartilages of ears and another exposed places leading to generalized pigmentation of connective tissues and deposition in joints leading to arthritis, a condition is known as ochronosis. This is because of the of oxidation homogentisate through polyphenol oxidase, creating benzoquinone acetate that polymerizes and binds to connective tissue macromolecules.

 

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