Content deleted Content added
No edit summary Tags: Mobile edit Mobile web edit |
→Pheomelanin: add that pheomelanins also now found in fish with citaiton |
||
| (14 intermediate revisions by 7 users not shown) | |||
Line 2:
{{Distinguish|Melamine|Melatonin}}
{{Use dmy dates|date=May 2022}}
{{cs1 config |display-authors=6 |name-list-style=vanc }}
{{Infobox material
| name = Melanin
Line 12 ⟶ 13:
[[File:Micrograph of keratinocytes, basal cells and melanocytes in the epidermis.jpg|thumb|Micrograph of the epidermis, with melanin labeled at left.]]
'''Melanin''' ({{IPAc-en|ˈ|m|ɛ|l|ə|n|ɪ|n|audio=melanin-pronunciation.ogg}}; {{etymology|grc|''{{wikt-lang|grc|μέλας}}'' ({{grc-transl|μέλας}})|black, dark}}) is a family of [[biomolecule]]s organized as [[oligomer]]s or [[polymer]]s, which among other functions provide the [[Biological pigment|pigments]] of many [[organism]]s.<ref name="Casadevall 2018 pp. 319–320">{{
There are five basic types of melanin: [[#Eumelanin|eumelanin]], [[#Pheomelanin|pheomelanin]], [[neuromelanin]], [[allomelanin]] and [[pyomelanin]].<ref name="
In the human skin, melanogenesis is initiated by exposure to [[UV radiation]], causing the skin to darken. Eumelanin is an effective absorbent of light; the pigment is able to dissipate over 99.9% of absorbed UV radiation.<ref name="Meredith">{{
== Melanin types ==
Line 22 ⟶ 23:
=== Eumelanin ===
[[File:Eumelanine.svg|thumb|Part of the structural formula of eumelanin. "(COOH)" can be COOH or H, or (more rarely) other [[substituent]]s. The arrow denotes where the polymer continues.]]
Eumelanin has two forms linked to [[5,6-dihydroxyindole]] (DHI) and [[DHICA|5,6-dihydroxyindole-2-carboxylic acid]] (DHICA). DHI-derived eumelanin is dark brown or black and insoluble, and DHICA -derived eumelanin which is lighter and soluble in alkali. Both eumelanins arise from the oxidation of tyrosine in specialized organelles called [[Melanosome|melanosomes]]. This reaction is catalyzed by the enzyme [[tyrosinase]]. The initial product, [[L-Dopaquinone|dopaquinone]] can transform into either 5,6-dihydroxyindole (DHI) or 5,6-dihydroxyindole-2-carboxylic acid (DHICA). DHI and DHICA are oxidized and then polymerize to form the two eumelanins.<ref name="Alaluf Heath Carter Atkins 2001 pp. 337–3472"
In natural conditions, DHI and DHICA often co-polymerize, resulting in a range of eumelanin polymers. These polymers contribute to the variety of melanin components in human skin and hair, ranging from light yellow/red pheomelanin to light brown DHICA-enriched eumelanin and dark brown or black DHI-enriched eumelanin. These final polymers differ in solubility and color.<ref name="Alaluf Heath Carter Atkins 2001 pp. 337–3472"
Analysis of highly pigmented ([[Fitzpatrick scale|Fitzpatrick type]] V and VI) skin finds that DHI-eumelanin comprises the largest portion, approximately 60–70%, followed by DHICA-eumelanin at 25–35%, and pheomelanin only 2–8%. Notably, while an enrichment of DHI-eumelanin occurs in during [[sun tanning]], it is accompanied by a decrease in DHICA-eumelanin and pheomelanin.<ref name="Alaluf Heath Carter Atkins 2001 pp. 337–3472">{{
=== Pheomelanin ===
[[File:Pheomelanine.svg|thumb|Part of the structural formula of pheomelanin. "(COOH)" can be COOH or H, or (more rarely) other [[substituent]]s. The arrows denote where the polymer continues.]]
Pheomelanins (or phaeomelanins) impart a range of yellowish to reddish colors.<ref>{{
Pheomelanin production is highly dependent on [[cysteine]] availability, which is transported into the melanosome, reacting with dopaquinone to form cys-dopa. Cys-dopa then undergoes several transformations before forming pheomelanin.<ref name="Alaluf Heath Carter Atkins 2001 pp. 337–3472"
Pheomelanins, unlike euemanins, are rare in lower organisms<ref name=":0" /> with claims they are an "evolutionary innovation in the tetrapod lineage"<ref name="eumelanin" /> but recent research finds them also in some fish.<ref name="Xuetal" />
=== Neuromelanin ===
{{main|Neuromelanin}}
Neuromelanin (NM) is an insoluble polymer pigment produced in specific populations of [[Catecholaminergic|catecholaminergic neurons]] in the brain. Humans have the largest amount of NM, which is present in lesser amounts in other primates, and totally absent in many other species.<ref>{{
=== Other forms of melanins ===
Up until the 1960s, melanin was classified into eumelanin and pheomelanin. However in 1955 a melanin associated with nerve cells was discovered, neuromelanin. In 1972 a water-soluble form, pyomelanin was discovered. In 1976, allomelanin, the fifth form of the melanins was found in nature.<ref name="
==== Peptidomelanin ====
[[File:A niger melanoliber.png|thumb|SEM micrograph of Aspergillus niger (strain: melanoliber) conodiophore possessing a large number of small conidospores (colorized). These spores release peptidomelanin into the surrounding medium during germination.]]
[[File:Peptidomelanin.jpg|thumb|The biochemical composition of peptidomelanin]]
Peptidomelanin is another water-soluble form of melanin.<ref>{{Cite journal |last1=Kolipakala |first1=Rakshita Sukruth |last2=Basu |first2=Suranjana |last3=Sarkar |first3=Senjuti |last4=Biju |first4=Beneta Merin |last5=Salazar |first5=Daniela |last6=Reddy |first6=Likhit |last7=Pradeep |first7=Pushya |last8=Yuvapriya |first8=Muniraj Krishnaveni |last9=Nath |first9=Shrijita |last10=Gall |first10=Riley |last11=Samprathi |first11=Anish Hemanth |last12=Balaji |first12=Harshitha |last13=Koundinya |first13=Eeshaan A. B. |last14=Shetye |first14=Aparna |last15=Nagarajan |first15=Deepesh |date=2024-08-07 |title=Fungal Peptidomelanin: A Novel Biopolymer for the Chelation of Heavy Metals |journal=ACS Omega |language=en |doi=10.1021/acsomega.4c03704 |issn=2470-1343 |pmc=11359623}}</ref> It was found to be secreted into the surrounding medium by germinating [[Aspergillus niger]] (strain: melanoliber) spores. Peptidomelanin is formed as a copolymer between L-DOPA eumelanin and short [[peptide]]s that form a 'corona', that are responsible for the substance's solubility. The peptide chains are linked to the L-DOPA core polymer via peptide bonds. This lead to a proposed biosynthetic process involving the hydroxylation of tyrosinylated peptides formed via proteases during sporogenesis, which are then incorporated autoxidatively into a growing L-DOPA core polymer.
==== Selenomelanin ====
It is possible to enrich melanin with [[selenium]] instead of [[sulphur]]. This selenium analogue of pheomelanin has been successfully synthesized through chemical and biosynthetic routes using selenocystine as a feedstock.<ref>{{
==== Trichochromes ====
Trichochromes (formerly called trichosiderins) are pigments produced from the same [[metabolic pathway]] as the eumelanins and pheomelanins, but unlike those molecules they have low molecular weight. They occur in some red human hair.<ref>{{
== Humans ==
[[File:Albinistic girl papua new guinea.jpg|thumb|[[Albinism in humans|Albinism]] occurs when melanocytes produce little melanin. This albino girl is from [[Papua New Guinea]].]]
In humans, melanin is the primary determinant of [[human skin color|skin color]]. It is also found in hair, the pigmented tissue underlying the [[iris (anatomy)|iris]] of the eye, and the [[stria vascularis of cochlear duct|stria vascularis]] of the [[inner ear]]. In the brain, tissues with melanin include the [[adrenal medulla|medulla]] and pigment-bearing neurons within areas of the [[brainstem]], such as the [[locus coeruleus]]. It also occurs in the [[zona reticularis]] of the [[adrenal gland]].<ref name=":0">{{Cite journal |last=Solano |first=F. |year=2014 |title=Melanins: Skin Pigments and Much More—Types, Structural Models, Biological Functions, and Formation Routes |journal=New Journal of Science |volume=2014 |pages=1–28 |doi=10.1155/2014/498276 |doi-access=free}}</ref>
The melanin in the skin is produced by [[melanocyte]]s, which are found in the [[Stratum germinativum|basal layer]] of the [[epidermis (skin)|epidermis]]. Although, in general, human beings possess a similar concentration of melanocytes in their skin, the melanocytes in some individuals and ethnic groups produce variable amounts of melanin. The ratio of eumelanin (74%) and pheomelanin (26%) in the epidermis is constant regardless of the degree of pigmentation.<ref name="i034">{{
Because melanin is an aggregate of smaller component molecules, there are many different types of melanin with different proportions and bonding patterns of these component molecules. 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 be deficient in albinism.<ref>{{Cite web |title=oculocutaneous albinism |url=https://ghr.nlm.nih.gov/condition/oculocutaneous-albinism |access-date=25 September 2017 |website=Genetics Home Reference |archive-date=17 July 2020 |archive-url=https://web.archive.org/web/20200717051343/https://ghr.nlm.nih.gov/condition/oculocutaneous-albinism |url-status=live }}</ref>
== Other organisms ==
Melanins have very diverse roles and functions in various organisms. A form of melanin makes up the ink used by many [[cephalopod]]s (see [[cephalopod ink]]) as a defense mechanism against predators. Melanins also protect microorganisms, such as bacteria and fungi, against stresses that involve cell damage such as [[UV radiation]] from the sun and [[reactive oxygen species]]. Melanin also protects against damage from high temperatures, chemical stresses (such as [[heavy metals]] and [[oxidizing agents]]), and biochemical threats (such as host defenses against invading microbes).<ref name="Hamilton & Gomez">{{
In [[fish]], melanin occurs not only in the skin but also in internal organs such as eyes. Most fish species use eumelanin,<ref name="FishChromatophores">{{
The darker [[feather]]s of birds owe their color to melanin and are less readily degraded by bacteria than unpigmented ones or those containing [[carotenoid]] pigments.<ref>{{Cite journal |last1=Gunderson |first1=Alex R. |last2=Frame |first2=Alicia M. |last3=Swaddle |first3=John P. |last4=Forsyth |first4=Mark H. |date=1 September 2008 |title=Resistance of melanized feathers to bacterial degradation: is it really so black and white? |journal=Journal of Avian Biology |volume=39 |issue=5 |pages=539–545 |doi=10.1111/j.0908-8857.2008.04413.x}}</ref> Feathers that contain melanin are also 39% more resistant to abrasion than those that do not because melanin granules help fill the space between the [[keratin]] strands that form feathers.<ref>{{Cite journal |last=Bonser |first=Richard H. C. |year=1995 |title=Melanin and the Abrasion Resistance of Feathers |url=https://sora.unm.edu/node/105022 |journal=Condor |volume=97 |issue=2 |pages=590–591 |doi=10.2307/1369048 |jstor=1369048 |access-date=25 September 2017 |archive-date=23 January 2020 |archive-url=https://web.archive.org/web/20200123211823/https://sora.unm.edu/node/105022 |url-status=live }}</ref><ref>{{Cite journal |last1=Galván |first1=Ismael |last2=Solano |first2=Francisco |date=8 April 2016 |title=Bird Integumentary Melanins: Biosynthesis, Forms, Function and Evolution |journal=International Journal of Molecular Sciences |volume=17 |issue=4 |page=520 |doi=10.3390/ijms17040520 |pmc=4848976 |pmid=27070583
Melanin is also important in [[mammal]]ian pigmentation.<ref>{{
Melanin in [[arthropods]] has been observed to be deposited in layers thus producing a [[Bragg reflector]] of alternating refractive index. When the scale of this pattern matches the wavelength of visible light, [[structural coloration]] arises: giving a number of species an [[iridescent]] color.<ref>{{Cite book |last=Neville |first=A. C. |url=https://books.google.com/books?id=VQHtCAAAQBAJ&pg=PA121 |title=Biology of the Arthropod Cuticle
[[Arachnid]]s are one of the few groups in which melanin has not been easily detected, though researchers found data suggesting spiders do in fact produce melanin.<ref name="HsiungBlackledge2015">{{
Some moth species, including the [[Parasemia plantaginis|wood tiger moth]], convert resources to melanin to enhance their thermoregulation. As the wood tiger moth has populations over a large range of latitudes, it has been observed that more northern populations showed higher rates of melanization. In both yellow and white male phenotypes of the wood tiger moth, individuals with more melanin had a heightened ability to trap heat but an increased predation rate due to a weaker and less effective [[Aposematism|aposematic]] signal.<ref>{{Cite journal |last1=Hegna |first1=Robert H. |last2=Nokelainen |first2=Ossi |last3=Hegna |first3=Jonathan R. |last4=Mappes |first4=Johanna |year=2013 |title=To quiver or to shiver: increased melanization benefits thermoregulation, but reduces warning signal efficacy in the wood tiger moth |journal=Proc. R. Soc. B |volume=280 |issue=1755 |pages=20122812 |doi=10.1098/rspb.2012.2812
Melanin protects ''[[Drosophila]]'' flies and [[laboratory mouse|mice]] against DNA damage from non-UV radiation.<ref name="Mosse-et-al-2001">{{
=== Plants ===
[[File:Indol-5,6-chinon.svg|thumb|Chemical structure of indole-5,6-quinone]]
Melanin produced by plants are sometimes referred to as 'catechol melanins' as they can yield [[catechol]] on alkali fusion. It is commonly seen in the [[enzymatic browning]] of fruits such as bananas. Chestnut shell melanin can be used as an antioxidant and coloring agent.<ref>{{
== Interpretation as a single monomer ==
It is now understood that melanins do not have a single structure or stoichiometry. {{citation needed|date=December 2023}} Nonetheless, chemical databases such as PubChem include structural and empirical formulae; typically ''3,8-Dimethyl-2,7-dihydrobenzo[1,2,3-''cd'':4,5,6-''c''′''d''′]diindole-4,5,9,10-tetrone''. This can be thought of as a single monomer that accounts for the measured elemental composition and some properties of melanin, but is unlikely to be found in nature.<ref name="Melanins: Skin Pigments and Much Mo">{{
{{chembox
Line 107 ⟶ 118:
== Biosynthetic pathways ==
The first step of the biosynthetic pathway for both eumelanins and pheomelanins is [[Enzyme catalysis|catalysed]] by [[tyrosinase]].<ref>{{Cite journal |
:[[Tyrosine]] → [[L-DOPA|DOPA]] → [[dopaquinone]]
Dopaquinone can combine with [[cysteine]] by two pathways to benzothiazines and pheomelanins
Line 130 ⟶ 141:
== Microscopic appearance ==
Melanin is brown, non-refractile, and finely granular with individual granules having a diameter of less than 800 nanometers. This differentiates melanin from common [[hemosiderin|blood breakdown pigments]], which are larger, chunky, and refractile, and range in color from green to yellow or red-brown. In heavily pigmented lesions, dense aggregates of melanin can obscure histologic detail. A dilute solution of [[potassium permanganate]] is an effective melanin bleach.<ref>{{Cite web |title=Melanin |url=https://pubchem.ncbi.nlm.nih.gov/compound/Melanin |
== Genetic disorders and disease states ==
There are approximately nine types of [[oculocutaneous albinism]], which is mostly an autosomal recessive disorder. Certain ethnicities have higher incidences of different forms. For example, the most common type, called oculocutaneous albinism type 2 (OCA2), is especially frequent among people of black African descent and white Europeans. People with OCA2 usually have fair skin, but are often not as pale as OCA1. They (OCA2 or OCA1? see comments in History) have pale blonde to golden, strawberry blonde, or even brown hair, and most commonly blue eyes. 98.7–100% of modern Europeans are carriers of the derived allele [[SLC24A5]], a known cause of nonsyndromic oculocutaneous albinism. 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 a frequency of 1 in 36,000 in white Americans.<ref>{{Cite web |title=Oculocutaneous Albinism<!-- Bot generated title --> |url=http://albinism.med.umn.edu/mmm.htm |url-status=dead |archive-url=https://web.archive.org/web/20081223001138/http://albinism.med.umn.edu/mmm.htm
Ocular albinism affects not only eye pigmentation but visual acuity, as well. People with albinism typically test poorly, within the 20/60 to 20/400 range. In addition, two forms of albinism, with approximately 1 in 2,700 most prevalent among people of Puerto Rican origin, are associated with mortality beyond melanoma-related deaths.
The connection between albinism and [[deafness]] is well known, though poorly understood. In his 1859 treatise ''[[On the Origin of Species]]'', [[Charles Darwin]] observed that "cats which are entirely white and have blue eyes are generally deaf".<ref>{{
In Parkinson's disease, a disorder that affects [[Motor neuron|neuromotor]] functioning, there is decreased neuromelanin in the substantia nigra and locus coeruleus as a consequence of specific dropping out of dopaminergic and noradrenergic pigmented neurons. This results in diminished [[dopamine]] and [[norepinephrine]] synthesis. While no correlation between race and the level of neuromelanin in the substantia nigra has been reported, the significantly lower incidence of Parkinson's in blacks than in whites has "prompt[ed] some to suggest that cutaneous melanin might somehow serve to protect the neuromelanin in substantia nigra from external toxins."<ref>{{Cite web |title=Lewy Body Disease |url=http://www.seniorpsychiatry.com/pages/articles/lewy.html |url-status=dead |archive-url=https://web.archive.org/web/20090721054743/http://www.seniorpsychiatry.com/pages/articles/lewy.html
In addition to melanin deficiency, the molecular weight of the melanin polymer may be decreased by various factors such as oxidative stress, exposure to light, perturbation in its association with melanosomal [[matrix protein]]s, changes in [[pH]], or in local concentrations of metal ions. A decreased molecular weight or a decrease in the degree of polymerization of '''ocular melanin''' has been proposed to turn the normally anti-oxidant polymer into a [[pro-oxidant]]. In its pro-oxidant state, melanin has been suggested to be involved in the causation and progression of [[macular degeneration]] and [[melanoma]].<ref>{{
Higher eumelanin levels also can be a disadvantage, however, beyond a higher disposition toward vitamin D deficiency. Dark skin is a complicating factor in the laser removal of [[port-wine stain]]s. Effective in treating white skin, in general, lasers are less successful in removing port-wine stains in people of Asian or African descent. Higher concentrations of melanin in darker-skinned individuals simply diffuse and absorb the laser radiation, inhibiting light absorption by the targeted tissue. In a similar manner, melanin can complicate laser treatment of other dermatological conditions in people with darker skin.
Line 147 ⟶ 158:
[[Freckles]] and [[mole (skin marking)|mole]]s are formed where there is a localized concentration of melanin in the skin. They are highly associated with pale skin.
[[Nicotine]] has an affinity for melanin-containing tissues because of its precursor function in melanin synthesis or its irreversible binding of melanin. This has been suggested to underlie the increased [[Nicotine#Dependence|nicotine dependence]] and lower [[smoking cessation]] rates in darker pigmented individuals.<ref>{{
== Human adaptations ==
===Physiology===
Melanocytes insert granules of melanin into specialized cellular [[Vesicle (biology)|vesicles]] called [[melanosome]]s. These are then transferred into the [[keratinocyte]] cells of the human [[Epidermis (skin)|epidermis]]. The melanosomes in each recipient cell accumulate atop the [[cell nucleus]], where they protect the nuclear [[DNA]] from mutations caused by the [[ionizing radiation]] of the sun's [[ultraviolet]] rays. In general, people whose ancestors lived for long periods in the regions of the globe near the [[equator]] have larger quantities of eumelanin in their skins. This makes their skins brown or black and protects them against high levels of exposure to the sun, which more frequently result in [[melanoma]]s in lighter-skinned people.<ref>{{Cite web |date=20 June 2012 |title=Human Skin Color Variation |url=http://humanorigins.si.edu/evidence/genetics/human-skin-color-variation
Not all the effects of pigmentation are advantageous. Pigmentation increases the heat load in hot climates, and dark-skinned people absorb 30% more heat from sunlight than do very light-skinned people, although this factor may be offset by more profuse sweating. In cold climates dark skin entails more heat loss by radiation. Pigmentation also hinders synthesis of [[vitamin D]]. Since pigmentation appears to be not entirely advantageous to life in the tropics, other hypotheses about its biological significance have been advanced; for example a secondary phenomenon induced by adaptation to parasites and tropical diseases.<ref>{{
===Evolutionary origins===
[[Early humans]] evolved dark skin color, as an adaptation to a loss of body hair that increased the effects of UV radiation. Before the development of hairlessness, early humans might have had light skin underneath their fur, similar to that found in other [[primate]]s.<ref>{{Cite news |last=Wade |first=Nicholas |date=19 August 2003 |title=Why Humans and Their Fur Parted Ways |url=https://www.nytimes.com/2003/08/19/science/why-humans-and-their-fur-parted-ways.html
===Effects===
As with peoples having migrated northward, those with light skin migrating toward the equator acclimatize to the much stronger solar radiation. Nature selects for less melanin when ultraviolet radiation is weak. Most people's skin darkens when exposed to UV light, giving them more protection when it is needed. This is the physiological purpose of [[sun tanning]]. Dark-skinned people, who produce more skin-protecting eumelanin, have a greater protection against [[sunburn]] and the development of melanoma, a potentially deadly form of skin cancer, as well as other health problems related to exposure to strong [[solar radiation]], including the [[photodegradation]] of certain [[vitamin]]s such as [[riboflavin]]s, [[carotenoid]]s, [[tocopherol]], and [[folate]].<ref>{{Cite journal |last1=Jablonski |first1=Nina G. |last2=Chaplin |first2=George |date=11 May 2010 |title=Human skin pigmentation as an adaptation to UV radiation |journal=Proceedings of the National Academy of Sciences |volume=107 |issue=Supplement 2 |pages=8962–8968 |bibcode=2010PNAS..107.8962J |doi=10.1073/pnas.0914628107
Melanin in the eyes, in the [[Iris (anatomy)|iris]] and [[choroid]], helps protect from [[ultraviolet]] and [[high-energy visible light|high-frequency visible light]]; people with [[Eye color| blue, green, and grey eyes]] are more at risk of sun-related eye problems. <!--
Line 165 ⟶ 176:
--> Furthermore, the ocular lens yellows with age, providing added protection. However, the lens also becomes more rigid with age, losing most of its [[accommodation (eye)|accommodation]]—the ability to change shape to focus from far to near—a detriment due probably to [[protein]] crosslinking caused by UV exposure.
Recent research suggests that melanin may serve a protective role other than photoprotection.<ref>{{
== Physical properties and technological applications ==
Evidence exists for a highly cross-linked [[heteropolymer]] bound [[covalent bond|covalently]] to matrix scaffolding [[melanoprotein]]s.<ref>{{
=== Role in melanoma metastasis ===
Heavily pigmented melanoma cells have a [[Young's modulus]] of about 4.93 kPa, compared to non-pigmented cells, with a value of 0.98 kPa.<ref name="
== See also ==<!-- Please keep entries in alphabetical order & add a short description per [[WP:SEEALSO]] -->
Line 193 ⟶ 204:
{{Wiktionary}}
{{wiktionary|leuco form}}
* {{
* {{
* {{
{{Amino acid metabolism enzymes}}
| |||