How pheomelanin in red hair may neutralise excess cysteine, as zebra finches show

A pigment linked with red hair may be doing more than providing colour - it may help the body deal with a potentially toxic chemical build-up.

Scientists examining the orange-to-red melanin found in bird feathers report that making this pigment can reduce cellular harm.

Pheomelanin, cysteine and oxidative damage

The pigment in question is pheomelanin. To synthesise pheomelanin, cells need cysteine, an amino acid. However, when cysteine accumulates to excessive levels inside cells, it can trigger oxidative damage.

Researchers at Spain’s National Museum of Natural Sciences suggest that people who carry genetic variants associated with red hair may have specialised cells capable of turning surplus cysteine - taken in through diet or encountered in the environment - into pigment.

What zebra finches reveal about pheomelanin

To test whether pheomelanin production can protect cells, the team used zebra finches as an experimental model.

In their experiments, male finches that were unable to produce pheomelanin showed greater oxidative damage after being fed extra cysteine for a month, compared with males that could still make the pigment.

Female zebra finches do not naturally produce pheomelanin, and the drug used to block its production did not affect them. Even so, when given additional cysteine, females showed signs of slightly higher oxidative damage than females not receiving extra cysteine - but the researchers judged the difference to be insignificant.

Taken together, the results indicate that excess cysteine can contribute to cellular damage, and that producing pheomelanin can offer protection against at least some of that harm.

Red hair, pheomelanin and melanoma risk in humans

In humans, pheomelanin production is largely concentrated in the lips, nipples and genitals, while people with red hair also have pheomelanin in their hair and skin.

Although pheomelanin is linked with a higher risk of melanoma, the researchers argue there may be an advantage to genetic variants that promote its production: those variants likely help cells keep cysteine levels in balance by channelling excess cysteine into pheomelanin synthesis.

"These findings represent the first experimental demonstration of a physiological role for pheomelanin, namely avoiding the toxicity of excess cysteine, leading to a better understanding of melanoma risk and the evolution of animal coloration," write the study authors.

The study was published in PNAS Nexus.