Melanin is a group of pigments found in human skin and hair. The term “melanism” comes from the Latin word for black or dark brown coloration. Melanins are responsible for the color of humans’ skin and hair. They contribute to the appearance of lightening, deepening, whiteness, grayish white, redness and other colors. Humans produce melanin in their dermis (outermost layer) and epidermis (innermost layer). Melanins are produced mainly through ultraviolet B (UVB) radiation from sunlight. Melanin production is triggered by certain hormones including the pituitary gland.
When a person is exposed to sunlight, his or her skin begins to tan. This is caused by melanocytes (cells that produce melanin) in the upper layer of the skin turning to non-functional form. Then, when the skin is exposed to sunlight, the cells rapidly produce melanin. Melanin protects humans from UVA and UVB radiation.
It also helps regulate human body temperature. Melanin can also protect humans from skin cancer.
UVB rays (the kind that cause sunburn) are absorbed by the DNA in human skin cells and create a chemical reaction that produces melanin. UVB rays also cause tanning. UVB rays are less penetrating than UVA rays (which cause more skin damage but do not lead to tanning), so tanning beds use UVB rays to produce a tan. Additionally, the longer a person stays in the sun, the darker the skin will get.
This is because the skin darkens due to sun damage and the layers of melanin building up.
Melanin gives humans their pigmentation, and it protects them from the sun’s dangerous rays.
Melanin has many diverse functions, and daily activities can be significantly impaired without adequate melanin.
Melanin synthesis occurs in two layers, the upper layer being the dopa-mine oxidase-positive (P) cells and the lower layer being the dopa-mine oxidase-negative (N) cells. Both layers react to sunlight by generating oxygen free radicals and hydroxyl free radicals, which are then involved in the transfer of electrons from one molecule to another. These free radicals initiate a chain reaction of reactions called the tyrosine nitration- glycosylation- ubiquitination- intracellular transport- enzymatic modification pathway. The reactions end up producing the final product, DOPA, which is finally converted into melanin in a three-step process.
DOPA is a precursor of the neurotransmitter dopamine. It is also the precursor of the amino acid tyrosine, which forms the pigment molecules Lutein and Zeaxanthin. This pathway may be significantly altered in patients with Parkinson’s disease.
Melanin also protects the body from the sun’s harmful rays.
It has been shown that when exposed to UVA radiation, melanin absorbs some of the rays. This helps protect the skin and the human body from cancer-causing agents, such as
In addition, the body gets a tan.
Melanocytes are located in the basal cell layer, spinous cell layer, and lentiform cell layer of the skin. Melanocytes produce and store melanin in structures called melanosomes. Melanosomes are spherical structures made up of non-fluorescent material that are about a tenth of a micron in diameter. These structures are produced by the unrolling and expansion of an existing melanosome.
Melanosomes’ spherical shape is due to the hydrophobic lipid bilayer that covers the spherical structure. When the hydrophobic nature of the surrounding environment causes the spherical structure to become exposed, the bilayer tears and the material inside becomes more hydrophilic. This expansion and tearing of the hydrophobic layer is called desmostilation. Melanosomes are transported through the melanocyte’s cytoplasm by specialized structures called melanosomes transport vesicles. These vesicles help the melanosomes navigate through the cytoplasm. Cytoplasm is mostly water, so it would be fairly hydrophilic. However, there are other specialized membrane structures, called melanosome transport proteins, that help to add an additional hydrophobic layer to the cytoplasm, creating a more hydrophobic environment for the transport of the melanosome.
Melanocytes are equipped with a protein called tyrosinase, which is responsible for catalyzing the conversion of L-Dopa to dopaquinone (DOPA). This reaction is an oxidative process. When melanocytes are exposed to UVA radiation, a reactive oxygen species is produced which can damage tyrosinase. In order to prevent oxidative damage, melanocytes increase the production of tyrosinase into a giant, multimeric protein.
This multimeric tyrosinase can form ternary and quaternary structures.
The desmostilliation of the hydrophobic lipid layer that encases the hydrophilic core results in the formation of radicals. These radicals, when exposed to tyrosinase, can damage the enzyme. It is suggested that the formation of these multimeric tyrosinase proteins may be a protective mechanism against oxidative stress. It has also been suggested that the multimeric tyrosinase may be involved in melanosome transport.
In addition, it is probable that the multimeric tyrosinasy do have other roles as well.
However, when melanocytes are exposed to UVA, the production of tyrosinase is not increased and the multimeric tyrosinase is not formed. This allows for the melanocytes to preserve their enzymatic activity, as well as retaining the ability to create the pigment melanin. It is believed that the multimeric tyrosinase is formed as a result of the tyrosine-based hydrophobic nature of the surrounding environment.
Furthermore, multimeric tyrosinase is formed when the hydrophobic core of the melanosome interacts with the hydrophobic cytoplasm. This multimeric tyrosinase is unable to enter the hydrophilic domain of the melanosome and interact with the tyrosinase. As a result, it is not known what role multimeric tyrosinase plays within the tyrosinase-containing melanosome.
- Melanin (PA Riley – The international journal of biochemistry & cell biology, 1997 – Elsevier)
- The chemistry of melanin (HS Mason – J. biol. Chem, 1948 – pdfs.semanticscholar.org)
- Biochemistry of melanin formation (AB Lerner, TB Fitzpatrick – Physiological reviews, 1950 – journals.physiology.org)
- Melanin and fungi (BL Gómez, JD Nosanchuk – Current opinion in infectious diseases, 2003 – journals.lww.com)
- Retinal pigment epithelial lipofuscin and melanin and choroidal melanin in human eyes. (JJ Weiter, FC Delori, GL Wing… – … ophthalmology & visual …, 1986 – iovs.arvojournals.org)