Melanin in skin - Function of melanin

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The primary function of melanin is in skin pigmentation. Melanin has other important roles in the human body such as, sunlight absorption and sun screening, protection from ultraviolet radiation, charge-transfer redox activity, free radical scavenging, antimicrobial immune defense, immunomodulation and ion chelating.

Melanin in skin

Melanins are a broad spectrum of very common natural pigment compounds found in both the plant and animal kingdoms. It is a complex molecule derived from the amino acid tyrosine via the intermediate product L-DOPA, by the action of the tyrosinase enzymes. There are three basic types of melanin, namely, eumelanin, pheomelanin, and neuromelanin. Variation in human skin color is a result of density and relative proportions of contributing melanins.

Function of melanin

In plants and invertebrates, melanin is involved in immune defence. In certain pathogenic microbes, bacteria and fungi, these pigments function as defence mechanisms against the host's immune responses.

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Though many other functions are attributed to melanins in plant and animal kingdoms, herein we will consider their functions related to the human body.

Skin pigmentation

Melanin, whose primary function is in pigmentation, has many more important functions to perform in the human body.

Protection from UV radiation

Melanin functions as a sunscreen and as a broad spectrum sun radiation absorbent. The absorbed energy causes redox reactions, and both eumelanin and pheomelanin form free radical species. Eumelanin, being more photostable scavenges the originated oxidative species and neutralizes them. However, pheomelanin being more photolabile may not neutralize all the oxidative free radicals. Free radicals have the potential to cause DNA damage. Hence, fair individuals, having high amounts of pheomelanin, are more prone to skin cancers on sun exposure.

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Melanin in the eyes

Melanin in the iris, protects the retina from excessive light and photodamage. By absorbing the scattered light within the eyeball, it also helps in improving the visual acuity. Melanin in the retinal pigment epithelium (RPE) has antioxidant properties and neutralizes the photooxidative species. The antioxidative activity wanes with aging and one of causes of macular degeneration is oxidative stress.

Melanin in the ears

The melanin pigment is present in the inner ear with no known function. In Waardenburg syndrome, deafness occurs along with reduced inner ear pigmentation. In the inner ear, the loss of melanocytes in stria vascularis of cochlear duct is associated with loss of hearing.

Melanin in the brain

The substantia nigra pars compacta is a part of the midbrain, which appears darker than the surrounding areas due to the presence of neuromelanin in dopaminergic neurons. Degeneration of the pigmented neurons or depigmentation is found in persons affected with Parkinson's disease. It is hypothesized that neuromelanin may be neutralizing the oxidative stress and chelating the toxic metal ions in the brain. The decrease in neuromelanin, as observed in Parkinson's disease may be the cause of the associated degenerative symptoms.

Melanins also function as chelating agents. They can chelate metal ions, including toxic species, and protect the cells from damage. Melanins can also bind with a number of organic molecules and neutralize their damaging effects.

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Melanins behave as semiconductors. Thin melanin films having multifarious electrical and optical properties can be made by spray deposition. Having interesting biophysical properties, melanin opens up many new biotechnological applications.

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Reference: 1.Michaela Brenner, Vincent J. Hearing. The Protective Role of Melanin Against UV Damage in Human Skin. Photochem Photobiol. 2008; 84(3): 539–549. 2.F. Solano. Melanins: Skin Pigments and Much More—Types, Structural Models, Biological Functions, and Formation Routes. New Journal of Science, vol. 2014, Article ID 498276, 28 pages, 2014. doi:10.1155/2014/498276. 3.Burkhart, C. G. and Burkhart, C. N. The mole theory: primary function of melanocytes and melanin may be antimicrobial defense and immunomodulation (not solar protection). International Journal of Dermatology, 44: 340–342.

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