Melanins, production defects

Albinism refers to a group of conditions in which a defect in tyrosine metabolism results in a deficiency in the production of melanin. These defects result in the partial or full absence of pigment from the skin, hair, and eyes. Albinism appears in different forms, and it may be inherited by one of several modes autosomal recessive, autosomal dominant, or X-linked. Complete albinism (also called tyrosinase-negative oculocutaneous albinism) results from a defi ciency of tyrosinase activity, causing a total absence of pigment from the hair, eyes, and skin (Figure 20.20). It is the most severe form of the condition. Affected people may appear to have white hair, skin, and iris color, and they may have vision defects. They also have photophobia (sunlight is painful to their eyes), they sun burn easily, and do not tan. 

Malignant melanocytes present defective melanosomes and tend to exhibit up-regulated melanogenesis. Melanogenuria (in the form of dark urine) is observed in some patients with widespread disease. End-product pigments, enzymes and melanin precursors or intermediates of the melanogenesis have therefore been measured in urine and blood from melanoma patients for more than 30 years. 

Albinism is an example of a genetic defect with serious consequences. The enzyme tyrosinase is deficient. Consequently, melanin, a black pigment found in skin, hair, and eyes, is not produced. It is formed from tyrosine in several cell types, for example, the melanocytes in skin. In such cells, tyrosinase converts tyrosine to DOPA and DOPA to dopaquinone. A large number of molecules of the latter product, which is highly reactive, condense to form melanin. Because of the lack of pigment, affected individuals (called albinos) are extremely sensitive to sunlight. In addition... 

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