Urinalysis - The Carter Center

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4. 12 Determination of Melanin
Introduction
Melanin is pigment derived from tyrosine, which is normally present in
hair, skin and in the choroid layer of the eye.
There are two recognized metabolic pathways for the conversion of
tyrosine to melanin:
1. The eumelanin pathway, which polymerize to brown or black
pigments.
2. The pheomelanin pathway – which polymerize to yellow or red
pigments.
Melanomas with pigments are normally transferred from melanocytes to
skin and mucus membrane cells.
In patients with tumors arising from the melanin producing cells, the
melanomas, the melanin may be excreted in the urine in large amount,
and its presence is indicative of metastasis of the tumor to the liver or
other organ.20% of patients with disseminated malignant melanoma
excrete a black urine due to melanin, or its precursor, the colorless
melanogen in the urine. The urine becomes black upon standing
(oxidation), where the chromogen /melanogen is changed into the
pigment called melanin which is a physical method to detect melanin.
Clinical Significance
Melanin occurs in metabolic tambours especially with metastasis of liver.
Chemical Tests for Melanin
There are two types of chemical test for melanin in urine depending up
on either by
a. Utilization of oxidizing agent (e.g. FeCl 3 )
b Utilization of reducing action of melanogen
c. Oxidation by atmospheric air