Albinism Tyrosinase

In albinism, tyrosinase is defective and tyrosine cannot be converted to the skin pigment melanin. 

A copper-containing enzyme responsible for the conversion of tyrosine to the dark pigment, melanin. Skin color depends, in part, upon the concentration of melanin. A genetic absence of tyrosinase results in albinism. Tyrosinase is also the enzyme responsible for the browning of cut surfaces of certain fruits and vegetables as such, it is described by the generic term phenolase. 

Giebel, L. B., and Spritz, R.A. (1992). The molecular basis of type I (tyrosinase-deficient) human oculocutaneous albinism. Pigment Cell Res. Suppl. 2 101-106. 

King, R. A., Mentink, M. M., and Oetting, W. S. (1991). Non-random distribution of missense mutations within the human tyrosinase gene in type I (tyrosinase-related) oculocutaneous albinism. Mol. Biol. Med. 8 19-29. 

Copper is a component of many enzymes including amine oxidase, lysyl oxidase, ferroxidase, cytochrome oxidase, dopamine P-hydroxylase, superoxide dismutase and tyrosinase. This latter enzyme is present in melanocytes and is important in formation of melanin controlling the colour of skin, hair and eyes. Deficiency of tyrosinase in skin leads to albinism. Cu " ion plays an important role in collagen formation. 

As the melanin structure grows, it becomes more colored giving various shades of brown color to our skin. This brown coloration acts to help protect deeper skin elements from being damaged by the UV radiation. The absence of the enzyme tyrosinase that converts tyrosine to melanin can lead to albinism. 

The classic and most severe form of albinism, a complete lack of melanin in the hair and skin and of color in the iris, is due to complete deficiency of tyrosinase. 

Albinism <3 Melanin synthesis from tyrosine Tyrosine 3-monooxygenase (tyrosinase) Lack of pigmentation white hair, pink skin... 

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. 

Albinism The most common type, oculocutaneous albinism, results in white hair, pink skin, and an extreme photophobia owing to lack of pigment in the eye Tyrosinase of the melanocyte is absent... 

Tyrosine can also be acted on by the enzyme tyrosinase and converted into a dark pigment. This enzyme is quite interesting to study because it is vulnerable to a genetic mutation that makes it heat labile (i.e., it only works correctly in the cooler areas of the body). The consequence of this mutation is a lack of pigmentation in humans (albinism) and, conversely, the characteristic pattern of dark pigmentation at the ends of the nose,... 

The initial reaction in this pathway is catalyzed by tyrosinase, a copper enzyme. Its absence produces albinism, lack of all pigment. The structure of L-dopa is shown in Figure 16.9. 

Albinism is also caused by lack of the enzyme tyrosinase, resulting from a genetic defect. 

Enzyme deficiency diseases. A variety of metabolic diseases are caused by deficiencies or malfunctions of enzymes, due originally to gene mutation. Albinism, for example, may be caused by the absence of tyrosinase, an enzyme essential for the production of cellular pigments. The hereditary lack of phenylalanine hydroxylase results in the disease phenylketonuria (PKU) PKU is usually managed by dietary modifications, but intravenous... 

Lack of melanin production (hypomelanosis) gives rise to several hereditary disorders collectively known as albinism. Some forms result from deficiency of tyrosinase. The inheritance pattern of albinism varies with type. Affected individuals have increased susceptibility to various... 

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... 

Tyrosine also has an important role in the central nervous system and melanocyte and is the precursor of both melanins and catecholamines (epinephrine and norepinephrine). The conversion to these products takes place in the appropriate tissues, usually melanocyte, the central nervous system, or the adrenal gland. In each of these tissues, the enzyme tyrosinase catalyzes the conversion of tyrosine to dihydroxyphenylalanine (DOPA) by hydroxylating the ring adjacent to the parahydroxy group. This is a catechol ring. If this were an amine instead of an amino acid, it would be a catecholamine. The DOPA is a precursor of catecholamines in the adrenal gland and central nervous system. In melanocyte, the DOPA is converted to melanine. In the disease albinism, the tyrosinase in the... 

Catecholamine formation. The conversion of tyrosine to catecholamines and melanins is outlined above. The purple lettering under an enzyme indicates the resulting disease when that enzyme is missing. In the case of albinism, an isoenzyme of tyrosinase is missing in the melanocyte, but not in other tissues. 

Albinism conductance regulator Tyrosinase ethnic groups 0.02 schoolchildren in Zimbabwe [16,19,20]... 

Nakamura, E. et al., A novel mutation of the tyrosinase gene causing oculocutaneous albinism type 1 (OCAl), J. Dermatol. Sci., 28,102,2002. 

Tyrosinase EC 1.10.3.1 Skin (118) 4 Colorless Hydroxylation of tyrosine to dihydroxypheny1 alanine (dopa) Melanogenesis Absent in albinism Quantitative data on tyrosinase derived from mushroom... 

Albinism is a disorder of the synthesis or processing of the skin pigment melanin (Fig. 47.2). Oculocutaneous albinism type 1 (OCA type 1) is an autosomal recessive disorder of tyrosinase resulting in the complete absence of pigment from the hair, eyes and skin. The lack of melanin in the skin makes patients with OCA type 1 vulnerable to skin cancer. 

Oxidative conversion of dopa into M. is catalysed by the enzyme Tyrosinase (EC 1.14.18.1) (see). Absence of tyrosinase (usually an autosomal inherited defect in the ability to synthesize the enzyme) results in albinism most groups of mammals occasionally produce albino individuals, which completely lack any M. pigmentation of eyes, skin, hair, feathers, etc. Albinism may also result from 1. deficient melanin polymerization, 2. failure to synthesize the protein matrix of the melanin granule, 3. lack of tyrosine, and 4. presence of tyrosinase inhibitors. 

There are two forms of oculocutaneous albinism a tyrosinase negative due to a tyrosinase defect and a tyrosinase positive of unknown origin (probably caused by inavailability of tyrosine to the melanocyte). Both forms are inherited as autosomal recessive traits, but the genes for each type are found in different loci. Ocular albinism is inherited as an X-linked recessive trait. It is associated with vision defects, nystagmus, and head nodding. The metabolic defect is unknown. 

A condition in which there is an absence of the pigment melanin. A congenital deHciency of the enzyme tyrosinase, which converts tyrosine to dihydroxyphenylalanine, an intermediate in melanin formation, causes one form of albinism. It has a recessive mode of inheritance. 

---