Albinism Melanin

Melanins are a group of natural pigments foimd in many organisms. A lack of melanin results in albinism. Melanin is produced by the oxidation of the amino acid tyrosine, followed by pol5unerization. These compounds are shown in Figure 5.5. 

Tyrosine is a precursor of thyroid hormones as well as L-dopa. Both thyroxine and L-dopa are employed in clinical medicine thyroxine to treat hypothyroid patients and L-dopa to treat patients with Parkinsonism. L-dopa is also the precursor to the pigment of the skin known as melanin. The enzyme that catalyzes the transformation of tyrosine into L-dopa, tyrosine hydroxylase, also catalyzes the transformation of L-dopa into melanin. Albinism is a genetic disease in which a mutation in the gene encoding tyrosine hydroxylase results in an inactive enzyme. People with albinism have no pigment in their skin, hair, or retina. 

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. 

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

B. PKU is caused by a deficiency of phenylalanine hydroxylase, which converts phenylalanine to tyrosine. A defect in tyrosine degradation causes homogentisic acid to accumulate and produce dark pigments (alcaptonuria). A defect in the conversion of tyrosine to the skin pigment melanin causes albinism. 

Melanin is the normal pigment of the skin and mammalian hair. Carcinomatous growths in which abnormal melanin formation occurs are known as melanomas. A congenital metabolic defect in w hich skin pigmentation does not occur is known as albinism, and is inherited as a recessive Mende-lian character (cf. 40). Albinos occur in many species besides man (e.g., the pink-eyed white rabbit). As adrenaline formation is apparently unimpaired in albinos, the metabolic block presumably lies in the conversion of dihydroxyphenylalanine to melanin, as shown in diagram 6, rather than in the conversion of tyrosine to dihydroxyphenylalanine. However, the exact nature of the block has not been established. 

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. 

Fig. 48.4. The pathways of catecholamine and melanin biosynthesis. PLP = pyridoxal phosphate. BH4 = tetrahydrobiopterin. The shaded boxes indicate the enzymes, which, when defective, lead to albinism.

In albinism, either the copper-dependent tyrosine hydroxylase of melanocytes (which is distinct from the tyrosine hydroxylase found in the adrenal medulla) or other enzymes that convert tyrosine to melanins may be defective. Individuals with albinism suffer from a lack of pigment in the skin, hair, and eyes, and they are sensitive to sunlight. 

Albinism Absence of pigment (melanin) which protects against UV damage in skin and iris of eye skin cancer. 

In alkaptonuria, homogentisic acid (28) is excreted in excessive amounts indicating that in this metabolic disease the opening of the aromatic ring and complete oxidation to CO2 and H2O is blocked. In albinism, there is a deficiency of the enzyme systems responsible for the formation of melanin from tyrosine (24) (Figure 3.7). 

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. 

Albinism is a condition characterized by total or partial absence of melanin at its normal sites—skin, sclera, etc. Melanin is elaborated by the melanocyte, a special type of cell that originates from the neural crest in the embryo. The cells are found in the skin, in the uveal tract, and in the retina [89, 90]. 

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. 

An enzyme involved in the conversion of tyrosine to melanin. One form of albinism is due to a deficiency of tyrosinase in melanocytes. 

L-dopaquinone (36). Figure 4.14. The conversion of L-dopaqxiinone (36) to dopa-melanin is then assumed to be spontaneous. Albinism, which is characterised in mammals by a complete or partial lack of melanin pigments in the skin and hair, results from a deficiency in tyrosinase activity. It represents a further example of a disease which has resulted from an inborn error of metabolism . 

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