Melanin biosynthesis occurs as the skin’s defense reaction to UV rays. It includes both the preliminary pigmentation activated by UVA rays and the series of chain reactions that generate the real and longer duration pigmentation, defined as indirect.
The melanins elaborated from melanocytes can be precisely defined as eumelanins, feomelanins and tricochromes. These three different types of pigments derive from a common forerunner, L-tyrosine, a nonessential amino acid synthesized in the body from phenylalanine.
L-tyrosine’s functions are several. It is a precursor of neurotransmitters, such as norepinephrine and dopamine, both of which regulate mood, adrenalin (adrenocortical hormone) and thyroxine (thyroid hormone). Tyrosine deficiency has been also linked to hypothyroidism, low blood pressure, low body temperature and restless leg syndrome.
Because tyrosine binds unstable molecules that can potentially cause damage to the cells and tissues, it could also be considered a mild antioxidant.
L-Tyrosine is the starting material for melanin’s biosynthesis, so it is the pigment responsible for hair and skin color.
Tyrosinase, an enzyme containing copper, catalyzes the initial stages of melanogenesis. More precisely, it catalyzes the hydroxylation of tyrosine to DOPA and a further oxidation of DOPA to DOPA-quinone. The next steps, which lead to the synthesis of the different types of melanins, occur spontaneously without an enzymatic catalyst.
Melanin biosynthesis depends not only the activity of tyrosinase, but also the bioavailability of tyrosine. In fact, it is well known that in a biologic synthesis, increasing the concentration of the starting substance also increases the quantity of the synthesized molecule.