Melanins (from the Greek μέλας mélas "black") are dark brown to black or yellowish to reddish pigments that color the skin , hair , feathers and eyes , except for albinism . Chemically, they are copolymers with indole compounds as subunits. They are found in vertebrates and insects, as a dye in the ink of squid (see sepia ) and also in microorganisms and plants. Melanins arise from the enzymatic oxidation of tyrosine (enzymatic browning). In vertebrates, melanin is formed in the melanocytes of the skin as well as in the choroid and iris of the eye. In humans and other primates , neuromelanin , whose function there is unclear, occurs in the substantia nigra of the brain .
Despite many years of efforts, it has not yet been possible to elucidate the exact structure of a melanin. It is considered certain that these are copolymers whose subunits are indole compounds , which are mainly linked via CC bonds. The difficulty lies in the insolubility of melanins in any solvent, in their pronounced heterogeneity and in the lack of well-defined spectral or physicochemical signals. In addition, they are difficult to separate from proteins that arise biologically at the same time . An overview of melanin formation, investigation methods and structural elements can be found in two of the following standard works on the chemistry and biology of melanins
Melanin in humans
In humans, melanin occurs mainly in two variants: a brownish-blackish variant ( eumelanin ), which is derived from the amino acids tyrosine and levodopa , and a lighter yellowish-reddish variant ( pheomelanin ) which contains sulfur. There are also different colored variants, so-called allomelanins, which are made from hydroxybenzenes . These are mainly found in plants, fungi and bacteria. Melanins almost always appear as mixed types and are also linked to lipids or protein.
The melanins in human skin and hair are mixed forms of eumelanins and the sulfur-containing pheomelanins. The mixing ratio of these two types of melanin is one of the determining factors for a person's skin type. The content of phaeomelanin in deep red hair is particularly high and decreases from brown to black hair. The formation of melanin is stimulated by UVB radiation and it presumably serves as light protection against the harmful effects of UV radiation from the sun. One of the main arguments for the UV protective function is the observation that heavily pigmented population groups suffer less from sun-induced skin cancer (“ melanoma ”) than less pigmented population groups. The photochemical processes that make melanin an excellent UV filter have now also been investigated. Melanin has been shown to convert more than 99.9% of radiant energy into harmless heat. This is done by the ultrafast internal conversion (engl. Internal conversion ) from the electronically excited state in vibrational states of the molecule. This ultra-fast conversion shortens the life of the excited state. This prevents free radicals from forming. The excited state of melanin is very short-lived and therefore it offers excellent photo protection .
Redheads are more likely to develop melanoma. Because of this, it is believed that this type of melanin protects the skin less efficiently.
The synthesis of melanin can be disturbed by genetic predisposition or by damage to the genetic material acquired over time. Decreased education leads to hypopigmentation . If production is blocked, the colorants in the skin, hair and eyes are also missing, which results in very light white skin, an unusually light hair color and blue, blue-gray or green eyes, which can appear red depending on the angle of incidence of the light . One speaks of albinism and describes the affected organisms as albinos. With overproduction ( hyperpigmentation ), more and more dark spots appear in the skin ( liver spots , freckles ), which can become malignant ( melanoma ). Melanin production can be specifically interrupted by the active ingredient rucinol .
In 2016, scientists from the universities in Mainz and Kiel uncovered further details on the molecular mechanism of the enzyme-catalyzed oxidation of melanin formation. These studies focus on the activities of the enzymes tyrosinase and catechol oxidase .
Melanin in mushrooms
It is expressly emphasized that the role of melanin in energy production in the organism is still unclear and the radioactivity is not reduced by the metabolism. The only thing that is clear is that the fungi originating from samples from the sealed nuclear reactor block 4 from Chernobyl
- a higher metabolic rate was given when they were fortified with melanin than with untreated mushrooms,
- Changes in the electron configuration of the electron shell of their melanin were detected during energy generation . This indicates a changed energy level, which can also be expected when generating energy,
- an increased fourfold reduction of NAD + can be observed when they are irradiated. This is a metabolic process.
With radiation exposure increased by a factor of 500 , the metabolic activity of Wangiella dermatitidis and Cryptococcus neoformans was significantly higher than normal activity under natural radiation exposure .
- Pschyrembel. Clinical Dictionary. De Gruyter, 255th edition. Berlin / New York 1986, ISBN 3-11-007916-X , p. 1041.
- What is albinism?
- Pezzella, Alessandro, et al. "An integrated approach to the structure of Sepia melanin. Evidence for a high proportion of degraded 5,6-dihydroxyindole-2-carboxylic acid units in the pigment backbone. "Tetrahedron 53.24 (1997): 8281-8286.
- Banerjee, Aulie, Subhrangshu Supakar, and Raja Banerjee. "Melanin from the nitrogen-fixing bacterium Azotobacter chroococcum: a spectroscopic characterization." PloS one 9.1 (2014): e84574.
- RA Nicolaus "Melanins", Hermann Verlag, Paris 1968
- G. Prota "Melanins and Melanogenesis", Academic Press 1992
- Meredith, Paul; Riesz, Jennifer: Radiative Relaxation Quantum Yields for Synthetic Eumelanin . In: Photochemistry and photobiology . 79, No. 2, 2004, pp. 211-216.
- Medical University of Vienna - AKH consilium: Skin cancer (malignant melanoma) ( Memento of the original from June 12, 2010 in the Internet Archive ) Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice.
- Even Solem, Felix Tuczek, Heinz Decker: Tyrosinase versus Catechol Oxidase. One Asparagine Makes the Difference . In: Angewandte Chemie International Edition . tape 55 , no. 8 . WILEY Online Library, February 18, 2016, ISSN 1521-3773 , p. 2884–2888 , doi : 10.1002 / anie.201508534 .
Ekaterina Dadachova et al .: Ionizing Radiation Changes the Electronic Properties of Melanin and Enhances the Growth of Melanized fungi. In: PLoS ONE 2 (5), 2007, doi: 10.1371 / journal.pone.0000457 .
Mushroom eats radioactivity. On: Wissenschaft.de from May 23, 2007.