Editing Melanin (section)

== Melanin types ==

=== 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 ({{lit|true melanin}}) 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">{{Cite journal |last1=Alaluf |first1=Simon |last2=Heath |first2=Alan |last3=Carter |first3=Nik |last4=Atkins |first4=Derek |last5=Mahalingam |first5=Harish |last6=Barrett |first6=Karen |last7=Kolb |first7=Ria |last8=Smit |first8=Nico |date=2001 |title=Variation in Melanin Content and Composition in Type V and VI Photoexposed and Photoprotected Human Skin: The Dominant Role of DHI |journal=Pigment Cell Research |volume=14 |issue=5 |pages=337–347 |doi=10.1034/j.1600-0749.2001.140505.x |issn=0893-5785 |pmid=11601655}}</ref> A small amount of black eumelanin in the absence of other pigments causes grey hair. A small amount of eumelanin in the absence of other pigments causes blond hair.<ref>{{Cite journal |last1=Ito |first1=S. |last2=Wakamatsu |first2=K. |date=December 2011 |title=Diversity of human hair pigmentation as studied by chemical analysis of eumelanin and pheomelanin |journal=Journal of the European Academy of Dermatology and Venereology |volume=25 |issue=12 |pages=1369–1380 |doi=10.1111/j.1468-3083.2011.04278.x |issn=1468-3083 |pmid=22077870 |s2cid=5121042}}</ref> Eumelanin is present in the skin and hair, etc.

=== 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, from Greek φαιός ''phaios'', "grey") impart a range of yellowish to reddish colors.<ref>{{Cite journal |vauthors=Slominski A, Tobin DJ, Shibahara S, Wortsman J |date=October 2004 |title=Melanin pigmentation in mammalian skin and its hormonal regulation |journal=Physiological Reviews |volume=84 |issue=4 |pages=1155–228 |doi=10.1152/physrev.00044.2003 |pmid=15383650 |s2cid=21168932}}</ref> Pheomelanins are particularly concentrated in the lips, nipples, glans of the penis, and vagina.<ref>{{Cite web |year=2010 |title=pheomelanin |url=https://metacyc.org/META/NEW-IMAGE?type=COMPOUND&object=CPD-12380 |website=MetaCyc Metabolic Pathway Database |access-date=24 August 2019 |archive-date=11 February 2020 |archive-url=https://web.archive.org/web/20200211114047/https://metacyc.org/META/NEW-IMAGE?type=COMPOUND&object=CPD-12380 |url-status=live }} {{full citation needed|date=August 2014}}</ref> When a small amount of eumelanin in hair (which would otherwise cause blond hair) is mixed with pheomelanin, the result is orange hair, which is typically called [[red hair|"red" or "ginger" hair]]. Pheomelanin is also present in the skin, and redheads consequently often have a more pinkish hue to their skin as well. Exposure of the skin to ultraviolet light increases pheomelanin content, as it does for eumelanin; but rather than absorbing light, pheomelanin within the hair and skin reflect yellow to red light, which may increase damage from UV radiation exposure.<ref>{{Cite journal |last1=Thody |first1=A. J. |last2=Higgins |first2=E. M. |last3=Wakamatsu |first3=K. |last4=Ito |first4=S. |last5=Burchill |first5=S. A. |last6=Marks |first6=J. M. |date=August 1991 |title=Pheomelanin as well as eumelanin is present in human epidermis |url=https://pubmed.ncbi.nlm.nih.gov/2071942/ |journal=The Journal of Investigative Dermatology |volume=97 |issue=2 |pages=340–344 |doi=10.1111/1523-1747.ep12480680 |pmid=2071942 |access-date=25 September 2022 |archive-date=25 September 2022 |archive-url=https://web.archive.org/web/20220925140609/https://pubmed.ncbi.nlm.nih.gov/2071942/ |url-status=live }}</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"/> In chemical terms, pheomelanins differ from eumelanins in that the oligomer structure incorporates [[benzothiazine]] and [[benzothiazole]] units that are produced,<ref>{{Cite journal |vauthors=Greco G, Panzella L, Verotta L, d'Ischia M, Napolitano A |date=April 2011 |title=Uncovering the structure of human red hair pheomelanin: benzothiazolylthiazinodihydroisoquinolines as key building blocks |journal=Journal of Natural Products |volume=74 |issue=4 |pages=675–82 |doi=10.1021/np100740n |pmid=21341762}}</ref> instead of DHI and [[DHICA]], when the amino acid [[L-cysteine]] is present.

Pheomelanins, unlike eumelanins, 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>{{Cite journal |vauthors=Fedorow H, Tribl F, Halliday G, Gerlach M, Riederer P, Double KL |year=2005 |title=Neuromelanin in human dopamine neurons: comparison with peripheral melanins and relevance to Parkinson's disease |journal=Prog Neurobiol |volume=75 |issue=2 |pages=109–124 |doi=10.1016/j.pneurobio.2005.02.001 |pmid=15784302 |s2cid=503902}}</ref> The biological function remains unknown, although human NM has been shown to efficiently bind [[transition metals]] such as iron, as well as other potentially toxic molecules. Therefore, it may play crucial roles in [[apoptosis]] and the related [[Parkinson's disease]].<ref>{{Cite journal |last=Double KL |year=2006 |title=Functional effects of neuromelanin and synthetic melanin in model systems |journal=J Neural Transm |volume=113 |issue=6 |pages=751–756 |doi=10.1007/s00702-006-0450-5 |pmid=16755379 |s2cid=23096297}}</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="Cao_2021" />

==== 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 |volume=9 |issue=34 |pages=36353–36370 |language=en |doi=10.1021/acsomega.4c03704 |pmid=39220543 |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>{{Cite journal |last1=Cao |first1=Wei |last2=McCallum |first2=Naneki C. |last3=Ni |first3=Qing Zhe |last4=Li |first4=Weiyao |last5=Boyce |first5=Hannah |last6=Mao |first6=Haochuan |last7=Zhou |first7=Xuhao |last8=Sun |first8=Hao |last9=Thompson |first9=Matthew P. |last10=Battistella |first10=Claudia |last11=Wasielewski |first11=Michael R. |last12=Dhinojwala |first12=Ali |last13=Shawkey |first13=Matthew D. |last14=Burkart |first14=Michael D. |last15=Wang |first15=Zheng |date=2020-07-22 |title=Selenomelanin: An Abiotic Selenium Analogue of Pheomelanin |url=https://pubs.acs.org/doi/10.1021/jacs.0c05573 |journal=Journal of the American Chemical Society |language=en |volume=142 |issue=29 |pages=12802–12810 |doi=10.1021/jacs.0c05573 |issn=0002-7863 |pmid=32638590 |s2cid=220413025 |access-date=30 August 2024 |archive-date=6 September 2023 |archive-url=https://web.archive.org/web/20230906193055/https://pubs.acs.org/doi/10.1021/jacs.0c05573 |url-status=live }}</ref> Due to selenium's higher atomic number, the obtained selenomelanin can be expected to provide better protection against ionising radiation as compared to the other known forms of melanin. This protection has been demonstrated with radiation experiments on human cells and bacteria, opening up the possibility of applications in space travel.<ref>{{Cite web |last=Mark Heiden |date=8 July 2020 |title=New biomaterial could shield against harmful radiation |url=https://news.northwestern.edu/stories/2020/07/new-biomaterial-could-shield-against-harmful-radiation-selenomelanin/ |access-date=29 January 2023 |publisher=Northwestern University |archive-date=30 August 2024 |archive-url=https://web.archive.org/web/20240830064612/https://news.northwestern.edu/stories/2020/07/new-biomaterial-could-shield-against-harmful-radiation-selenomelanin/ |url-status=live }}</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>{{Cite journal |last1=Prota, G. |last2=Searle, A. G. |year=1978 |title=Biochemical sites of gene action for melanogenesis in mammals |url=http://www.gse-journal.org/10.1051/gse:19780101/pdf |journal=Annales de Génétique et de Sélection Animale |volume=10 |issue=1 |pages=1–8 |doi=10.1186/1297-9686-10-1-1 |pmc=2757330 |pmid=22896083 |doi-access=free |access-date=24 August 2019 |archive-date=30 August 2024 |archive-url=https://web.archive.org/web/20240830064741/http://www.gse-journal.org/10.1051/gse:19780101/pdf |url-status=live }}</ref>