Every human skin tone mixes its two main pigments in nearly the same ratio
Pale or deep brown, human skin holds roughly three parts dark eumelanin to one part reddish pheomelanin, around 74 to 26 percent. What varies from person to person is the total quantity, from almost none in albinism to over ten micrograms per milligram of tissue. Our melanocytes are similarly numerous; they simply differ in output.
Melanin, from the Greek for black, is a family of pigments assembled when the amino acid tyrosine is oxidised and then linked into long chains. Five basic kinds are recognised. Eumelanin gives brown and black, and absorbs light so effectively that it dissipates more than 99.9 percent of the ultraviolet it takes in. Pheomelanin, built with sulfur from cysteine, yields yellow and red tones and reflects rather than absorbs that light. Neuromelanin sits in certain brain cells, most abundantly in humans, and allomelanin and pyomelanin contain no nitrogen.
For decades chemists knew only the first two. Neuromelanin turned up in 1955, water-soluble pyomelanin in 1972 and allomelanin in 1976. Hair colour comes from the blend: a little eumelanin alone gives blond, a little black eumelanin alone gives grey, and blond levels mixed with pheomelanin produce red or ginger. Some red hair also carries small pigments called trichochromes.
Across nature melanin takes on unexpected jobs. Many cephalopods squirt ink made from it to escape predators. Dark bird feathers resist bacteria better than pale ones, and melanin granules packed between keratin strands make them 39 percent tougher against abrasion. Invertebrates wall off invading microbes inside melanin capsules within minutes, while disease-causing fungi such as Cryptococcus neoformans use it as armour against their host's immune attack. Radiotrophic fungi seem to go further, using melanin to capture gamma rays and turn them into energy for growth.
Researchers have even swapped selenium in for sulfur. The resulting selenomelanin, made both chemically and biologically, shielded human cells and bacteria from ionising radiation in experiments, raising hopes for protecting people on spaceflights.
Source: Melanin
