Melanin ( Ã, ( listen ) ; from the Greek: ????? weld , "black, dark") is a broad term for a group of natural pigments found in most of the organisms. Melanin is produced by the oxidation of amino acid tyrosine, followed by polymerization. The melanin pigment is produced in a special group of cells known as melanocytes.
There are three basic types of melanin: eumelanin, pheomelanin, and neuromelanin. The most common type is eumelanin, where there are two types - chocolate eumelanin and black eumelanin. Pheomelanin is a cysteine ​​derivative containing a polybenzothiazine part which is largely responsible for red hair color, among other pigmentations. Neuromelanin is found in the brain, although its function remains unclear.
In human skin, melanogenesis is initiated by exposure to UV radiation, causing the skin to become dark. Melanin is an effective light absorber; the pigment is able to dissipate more than 99.9% of the absorbed UV radiation. Due to these properties, melanin supposedly protects skin cells from UVB radiation damage, reducing the risk of cancer, and it is considered that exposure to UV radiation is associated with an increased risk of malignant melanoma, melanocyte cancer (melanin cells). Research shows a lower incidence of skin cancer in individuals with more dense melanin, the darker skin tone. However; the relationship between skin pigmentation and photoprotection is still under investigation.
Video Melanin
Man
In humans, melanin is the main determinant of skin color. It is also found in hair, the underlying pigmented tissue of the iris, and the stria vascularis of the inner ear. In the brain, tissues with melanin include the medulla and pigment-bearing neurons in the brainstem area, such as the coeruleus locus and substansia nigra. It also occurs in the reticular zone of the adrenal gland.
Melanin in the skin is produced by melanocytes, which are found in the basal layer of the epidermis. Although, in general, humans have similar melanocyte concentrations in their skin, melanocytes in some individuals and ethnic groups produce varying amounts of melanin. Some humans have little or no melanin synthesis in their bodies, a condition known as albinism.
Since melanin is a smaller component molecular aggregate, there are many different types of melanin with different proportions and bonding patterns of this component molecule. Both pheomelanin and eumelanin are found in human skin and hair, but eumelanin is the most abundant melanin in humans, as well as the form most likely to lack albinism.
Eumelanin
Eumelanin polymers have long been considered to contain many 5.6-dihydroxyindola (DHI) crosslinking polymers and 5,6-dihydroxyindol-2-carboxylic (DHICA).
Pheomelanin
Pheomelanins (or phaeomelanins) give a pink to red, depending on concentration. Pheomelanins are mainly concentrated in the lips, nipples, penis glands, and vagina. When a small amount of brown eumelanin in the hair, which would otherwise cause blond hair, mixed with red pheomelanin, the result is a strawberry blonde. Pheomelanin is also present in the skin, and red hairs consequently often have a more pink color on their skin as well.
In chemical terms, pheomelanins differ from eumelanins in oligomeric structures incorporating units of benzothiazine and benzothiazole produced instead of DHI and DHICA, when the L-cysteine ​​amino acid is present.
Trichochromes
Trichochromes (formerly called trichosiderins) are pigments produced from the same metabolic pathways as eumelanin and pheomelanins, but unlike those molecules, they have low molecular weights. They occur in some red human hair.
Neuromelanin
Neuromelanin (NM) is an insoluble dark polymer pigment produced in a specific population of catecholamergic neurons in the brain. Humans have the largest amount of NM, which is present in less amounts in other primates, and is not present in many other species. However, biological functions are still unknown, although NM humans have been shown to efficiently bind transition metals such as iron, as well as other potentially toxic molecules. Therefore, it may play an important role in apoptosis and associated Parkinson's disease.
Maps Melanin
Other organisms
Melanin has a very diverse role and function in various organisms. A melanin form makes the ink used by many cephalopods (see cephalopoda ink) as a defense mechanism against predators. Melanin also protects microorganisms, such as bacteria and fungi, against pressures that involve cellular damage such as UV radiation from the sun and reactive oxygen species. Melanin also protects against damage from high temperatures, chemical pressures (such as heavy metals and oxidizing agents), and biochemical threats (such as host defense against microbial invasion). Therefore, in many microbial pathogens (eg, in Cryptococcus neoformans, a fungus) melanins appear to play an important role in virulence and pathogenicity by protecting microbes against the host immune response. In invertebrates, a major aspect of the innate immune defense system against pathogen attack involves melanin. Within minutes of infection, microbes are formulated in melanin (melanization), and generation of free radical byproducts during capsule formation is considered helpful in killing them. Some types of fungi, called radiotropic fungi, seem to be able to use melanin as photosynthetic pigments that allow them to capture gamma rays and harness this energy for growth.
Darker bird feathers owe color to melanin and are less easily degraded by bacteria than non-pigmented or carotenoid-containing pigments. Feathers containing melanin are also 39% more resistant to abrasion than those not because the melanin grains help fill the space between the keratin strands that make up the fur.
Melanin is also important in mammalian pigmentation. The mammalian mantle pattern is determined by the agouti gene that regulates the distribution of melanin. Gene mechanisms have been extensively studied in mice to provide insight into the diversity of mammalian mantle patterns.
Melanin in arthropods has been observed to be deposited in layers to produce Bragg reflectors from alternating refractive indices. When the scale of this pattern corresponds to the wavelength of visible light, structural staining appears: giving a number of colorful species.
Arachnids are one of the few groups in which melanin has not been easily detected, although the researchers found data showing that spiders do produce melanin.
Several species of moths, including the Wood Tiger Moth, turn resources into melanin to improve their thermoregulation. Since the Wood Tiger moth has populations above a large number of latitudes, it has been observed that more and more northern populations lowered higher melanization rates. In both male yellow and white male phenotypes moth Tiger Wood, individuals with more melanin have a high ability to trap heat but increased predation rates due to weaker and less effective aposematic signals.
Plants
The melanin produced by plants is sometimes referred to as 'catechol melanins' because they can produce catechol in alkaline fusion. This is often seen in enzymatic browning of fruits such as bananas. Chestnut melanin shell can be used as an antioxidant and dye. Biosynthesis involves oxidation of indole-5,6-quinone by tyrosinase type of polyphenol oxidase from tyrosine and catecholamines leading to the formation of melanin catechol. Although many of these plants contain compounds that inhibit the production of melanin.
Biosynthetic path
The first step of the biosynthetic pathway for eumelanin and pheomelanin is catalyzed by tyrosinase.
- Tyrosine -> DOPA -> dopaquinone
Dopaquinone dapat bergabung dengan sistein dengan dua jalur menuju benzothiazine dan pheomelanins
- Dopaquinone cysteine ​​-> 5-S-cysteinyldopa -> benzothiazine intermediate -> pheomelanin
- Dopaquinone cysteine ​​-> 2-S-cysteinyldopa -> benzothiazine intermediate -> pheomelanin
Also, dopaquinone can be converted into leucodopachrome and follow two more paths to eumelanins
- Dopaquinone -> leucodopachrome -> dopachrome -> 5,6-dihydroxyindole-2-carboxylic acid -> quinone -> eumelanin
- Dopaquinone -> leucodopachrome -> dopachrome -> 5,6-dihydroxyindole -> quinone -> eumelanin
Detailed metabolic paths can be found in the KEGG database (see External links).
Microscopic sightings
Melanin is brown, can not be refractile, and smooth with individual grains having a diameter of less than 800 nanometers. It distinguishes melanin from the common, larger, thick, and refractory pigment, and ranges in color from green to yellow or red-brown. In high-pigmented lesions, melanin-dense aggregates may obscure histological details. A dilute solution of potassium permanganate is an effective melanin bleach.
Genetic disorders and disease status
There are about nine types of oculocutaneous albinism, most of which are autosomal recessive disorders. Certain ethnicities have a higher incidence of various forms. For example, the most common type, called oculocutaneous albinism type 2 (OCA2), is very common among people of black African descent. It is an autosomal recessive disorder characterized by a congenital reduction or absence of melanin pigment in the skin, hair, and eyes. The estimated frequency of OCA2 among African-Americans is 1 in 10,000, which contrasts with frequency 1 in 36,000 white Americans. In some African countries, the frequency of this disturbance is even higher, ranging from 1 in 2,000 to 1 in 5,000. Another form of Albinism, "oculocutaneous yellow albinism", seems to be more common among the Amish, mainly of Swiss and German descent. People with IB variants of this disorder generally have white hair and skin at birth, but quickly develop normal skin pigmentation in infancy.
Ocular albinism not only affects eye pigmentation but also visual acuity. People with albinism are usually a bad test, in the 20/60 to 20/400 range. In addition, two forms of albinism, with about 1 in 2700 most common among people of Puerto Rican origin, are linked to mortality beyond melanoma-related deaths.
The relationship between albinism and deafness is well known, though poorly understood. In his 1859 treatise On The Origin of Species, Charles Darwin observed that "a cat that is wholly white and has a generally deaf blue eye". In humans, hypopigmentation and deafness occur together in the rare Waardenburg syndrome, especially observed among the Hopi in North America. The incidence of albinism in the Hopi Indian has been estimated to be about 1 in 200 individuals. It is interesting to note that the same pattern of albinism and deafness has been found in other mammals, including dogs and rodents. However, the lack of melanin per se does not appear to be directly responsible for deafness associated with hypopigmentation, since most individuals lacking the enzymes needed to synthesize melanin have normal hearing functions. Conversely, the absence of melanocytes in the stria vascularis of the inner ear results in cochlear damage, although why this is, is not fully understood.
In Parkinson's disease, a disorder that affects neuromotor function, there is a decrease in neuromelanin in the substantia nigra and coeruleus locus as a consequence of the specific droping of dopaminergic and noradrenergic pigmented neurons. This results in reduced synthesis of dopamine and norepinephrine. While no correlation between race and neuromelanin levels in nigra substansia has been reported, a significantly lower incidence of Parkinson's in blacks than whites has "asked some people to suggest that skin melanin may somehow serve to protect neuromelanin in the substance. " nigra from external toxins. "
In addition to melanin deficiency, molecular weight of the melanin polymer may decrease with various factors such as oxidative stress, exposure to light, disturbances in relation to the melanosomal matrix protein, pH change, or local metal ion concentration. Decreased molecular weight or decreased polymerization rate ocular melanin has been proposed to convert the normal antioxidant polymer to pro-oxidant. In a pro-oxidant state, melanin has been suggested to be involved in the cause and development of macular degeneration and melanoma. Rasagiline, an important monotherapy drug in Parkinson's disease, has melanin-binding properties, and melanoma-reducing properties.
Higher levels of eumelanin can also be a disadvantage, however, beyond the higher disposition of vitamin D deficiency. Dark skin is a tricky factor in the removal of laser-wine stained ports. Effective in treating whites, in general, lasers are less successful in removing port wine stains in Asians or African descendants. Higher melanin concentrations in dark-skinned individuals simply spread and absorb laser radiation, inhibiting light absorption by targeted tissue. In the same way, melanin can complicate the laser treatment of other skin conditions in people with darker skin.
Spots and moles are formed where there is local concentration of melanin in the skin. They are strongly associated with pale skin.
Nicotine has an affinity for tissues containing melanin because of its predecessor function in melanin synthesis or irreversible melanin binding. It has been suggested to underlie the increased nicotine dependence and lower smoking cessation rates in darker pigmented individuals.
Human adaptation
Physiology
Melanocytes incorporate melanin granules into special cellular vesicles called melanosomes. These are then transferred to keratinocyte cells from the human epidermis. The melanosomes in each receiving cell accumulate above the cell nucleus, where they protect nuclear DNA from mutations caused by ionizing radiation from the sun's ultraviolet rays. In general, people whose ancestors lived for a long time in regions of the world near the equator had a larger amount of eumelanin on their skin. This makes their skin brown or black and protects them from exposure to high sunlight, which more often causes melanoma in lighter-skinned people.
Not all pigmentation effects are favorable. Pigmentation increases heat load in hot climates, and blacks absorb 30% more heat from sunlight than whites, although this factor can be offset by more sweating. In a cold climate blacks need more heat loss by radiation. Pigmentation also inhibits the synthesis of vitamin D, so in areas of poor children malnutrition is more susceptible to rickets than white children. Since pigmentation does not seem to be entirely favorable for life in the tropics, other hypotheses about biological significance have been advanced, for example secondary phenomena caused by adaptations to parasites and tropical diseases.
The origins of evolution
Early humans evolved to have dark skin color about 1.2 million years ago, as an adaptation to the loss of body hair that enhances the effects of UV radiation. Prior to the development of hairless hair, early humans had fair skin beneath their fur, similar to those found in other primates. Recent scientific evidence shows that modern humans anatomically evolved in Africa between 200,000 and 100,000 years, and then filled the whole world through a migration between 80,000 and 50,000 years ago, in some areas of intercross with certain early human species (Neanderthal, Denisova, and maybe other). It seems that the first modern humans had relatively large amounts of eumelanin-producing melanocytes, resulting in darker skin similar to today's indigenous Africans. Because some of these native people migrate and settle in Asia and Europe, selective pressures for eumelanin production decline in climates where the radiation from the sun is less intense. This ultimately results in the current human skin color range. Of the two common gene variants known to be associated with pale human skin, Mc1r does not seem to have a positive selection, while SLC24A5 has undergone a positive selection.
Effects
Like those who migrate north, those who migrate with light skin toward the equator adapt to much stronger solar radiation. Most people's skin gets dark when exposed to UV rays, giving them more protection when needed. This is the physiological goal of sun tanning. Dark-skinned people, who produce more eumelanin that protects the skin, have greater protection against sunburn and melanoma development, potentially lethal skin cancer forms, as well as other health issues associated with strong exposure to solar radiation, including specific photodegradation of vitamins such as riboflavin, carotenoids, tocopherol, and folate.
Melanin in the eye, in iris and choroid, helps to protect them from visible ultraviolet and high light; people with gray, blue, and green eyes are more at risk of eye problems associated with sunlight. Furthermore, the ocular lens becomes yellow with age, providing additional protection. However, the lens also becomes more rigid as it gets older, losing most of its accommodation - the ability to change shape to focus from near to far - possible losses due to cross-linking of proteins caused by UV exposure.
Recent studies have shown that melanin can serve a protective role in addition to photoprotection. Melanin is capable of effectively chelating metal ions through carboxylates and phenolic hydroxyl groups, in many cases much more efficient than the strong chelating ligand ethylenediaminetetraacetate (EDTA). Thus, it may serve to absorb toxic metal ions, protecting the rest of the cell. This hypothesis is supported by the fact that the loss of neuromelanin observed in Parkinson's disease is accompanied by elevated levels of iron in the brain.
Physical properties and technology applications
The evidence exists to support a crosslinked heteropolymer that is covalently bonded with a melanoprotein scaffold matrix. It has been proposed that the ability of melanin to act as an antioxidant is directly proportional to the degree of polymerization or molecular weight. The optimal conditions for effective polymerization of melanin monomers can lead to the formation of low-molecular-weight melanin, a pro-oxidant that has been implicated in the cause and development of macular degeneration and melanoma. Signaling pathways that enhance melanization regulation in retinal pigment epithelium (RPE) may also be involved in downregulation rod outer segment fagocytosis by RPE. This phenomenon is partly due to foveal sparing in macular degeneration.
See also
- Albino
- Albinism in biology
- Griscelli syndrome, syndrome characterized by hypopigmentation
- Human skin color
- Melanin Theory
- Melanism
- Melanogenesis, melanin production
- The risks and benefits of sun exposure
- Vitamin D
- Skin whitening
- Ferulic acid
References
External links
- Melanin absorption spectrum
- Tyrosine metabolism at KEGG
- Melanogenesis in KEGG
Source of the article : Wikipedia
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