Just to add further insult, studies in birds have found evidence of a physiological trade-off: increased pheomelanogenesis (that is, birds with more red plumage) is correlated with reduced brain size. If cysteine incorporated into the pheomelanogenesis pathway comes from an excess pool, then pheomelanogenesis could represent an excretory mechanism of cysteine and an adaptive process that may explain the evolution of pheomelanin. Cysteine also reacts with dopaquinone thus participat- ing in the synthesis of pheomelanin that takes place in melanosomes. In birds, excess dietary amino acids are also diverted to the synthesis of uric acid, the main product of excretion in birds. This excess, which can be toxic, is partly eliminated by cysteine sulfoxidation, a process mainly catalyzed by cysteine dioxygenase (CDO) in which less toxic products than cysteine such as sulfate and taurine are formed. When the levels of cysteine are higher than required for these functions, especially for protein synthesis, excess cysteine occurs. It is also used for the synthesis of reduced glutathione (GSH), which is the main storage of cysteine and thus also acts as a source of cysteine. This amino acid is used for protein synthesis, and can be recovered by protein breakdown. So making red hair actually depletes the body of a protective substance, and has the side-effect of producing carcinogens.Ĭhart showing the physiological activity of dietary cysteine.

It's also synthesized biochemically by a process that consumes glutathione (GSH), an important cellular anti-oxidant. In fact, pheomelanin is prone to photodamage (exposure to light causes it to break down) that produces carcinogenic byproducts. Red hair is stuffed full of pheomelanin rather than eumelanin.īut here's the thing: pheomelanin is a lousy barrier to UV. We all make it to varying degrees - nipples and lips get their reddish color from it - but some people make it in much larger quantities than they do eumelanin. If you're dark-haired, it's because the keratinocytes are stuffed full of eumelanin, which is also an excellent barrier to UV damage. We vertebrates produce primarily two forms of melanin, eumelanin and pheomelanin.Įumelanin is a dark pigment cells loaded up with it look black or dark gray. Pigmentation is produced by the deposition of a complex light absorptive polymer called melanin in cells. The biochemical cause of these differences are known.

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