Skin color in humans is primarily due to the production of melanin, polymers of ozidized tyrosine that form dark-colored granules in the skin. Melanogenesis is the production of melanin by specialized cells called melanocytes. Such cells are located in the skin, hair, and eyes. The enzyme that mediates the production of this pigment is a type of polyphenol oxidase known as a tyrosinase.
The melanocytes in which melanin production takes place are primarily located in the basal level of the epidermis, or skin layer. About 5-10% of the cells in this layer of the skin are melanocytes. Melanogenesis proceeds at a background, or basal, level all of the time. It can also be activated by ultraviolet (UV) radiation from the sun.
Light-skinned people, such as those of Northern European descent, have a low basal level of melanogenesis taking place. They are prone to skin damage from the sun because of their lack of melanin. People with darker colored skin produce much higher levels of the pigment and have a much greater degree of natural protection from the sun. These differences in skin color correlate with susceptibility to the skin cancer melanoma and are thought to have originated from the evolution of their ancestors in areas with differing amounts of sunlight and UV radiation.
There can be benefits to having a low level of melanogenesis. High levels of melanin interfere with the absorption of UV rays necessary for the production of important compounds such as Vitamin D. Low levels of Vitamin D can result in an array of different problems. Such levels of melanin, however, do protect against the breakdown of folic acid. Low levels of this compound can result in birth defects.
The melanogenesis process involves the enzyme tyrosinase, which introduces an atom of oxygen onto the the phenolic group of the amino acid tyrosine. This results in the presence of a hydroxyl (OH) group on the phenol. The enzyme then carries out a second reaction, adding another hydroxyl group adjacent to the first. These are then oxidized to quinones, unstable oxygen compounds that lack a hydrogen molecule. Quinones continue reacting with each other to generate the dark pigment melanin, which then accumulates in granules.
Melanogenesis is controlled by several mechanisms. It is under hormonal control because peptides activate a receptor. A number of other molecules can stimulate the production of melanin, including cholera toxin, metabolites of Vitamin D, and derivatives of retinoic acid. When UV irradiation damages the skin’s DNA, one of the breakdown products of the DNA can also trigger this process.