Melanin is the primary pigment responsible for determining the color of human skin, hair, and eyes. This natural substance is produced by specialized cells called melanocytes. Melanin is a diverse group of biopolymers, with the two main types being pheomelanin and eumelanin, which together influence the vast spectrum of human color variation.
Defining Pheomelanin and Eumelanin
Eumelanin and pheomelanin are fundamentally distinct, beginning with their chemical structure. Eumelanin is a dark, non-sulfur containing polymer responsible for black and brown pigmentation. Pheomelanin, in contrast, is a lighter pigment that imparts yellow and reddish-brown hues. Chemically, pheomelanin is a sulfur-containing polymer, incorporating the amino acid L-cysteine during its synthesis, which forms benzothiazine and benzothiazole units that give it its characteristic color.
Pigmentation and Physical Traits
The physical appearance associated with pheomelanin is linked to the dominance of this red-yellow pigment over the darker eumelanin. Individuals who produce high levels of pheomelanin and low levels of eumelanin exhibit red or ginger hair. This pigment profile is also associated with fair skin that burns easily in sunlight and a tendency toward freckling, which is a localized accumulation of pigmented cells.
The Biological Production Pathway
The synthesis of both eumelanin and pheomelanin begins with the amino acid tyrosine, converted to an intermediate molecule by the enzyme tyrosinase. The Melanocortin 1 Receptor (MC1R), a protein on the surface of melanocytes, acts as a switch controlling the type of melanin produced. When the MC1R is fully functional, it stimulates the production of eumelanin. However, variations in the MC1R gene can result in a receptor with reduced function, leading to a shift that promotes the production of pheomelanin.
Pheomelanin and UV Exposure Risks
The differing chemical compositions of the two pigments result in vastly different abilities to protect the skin from ultraviolet (UV) radiation. Eumelanin is an effective UV absorbent that shields DNA and acts as a free radical scavenger, while pheomelanin provides only weak photoprotection. Pheomelanin is considered phototoxic, meaning it can cause cellular damage when exposed to UV radiation. When UVA light interacts with pheomelanin, it generates reactive oxygen species (ROS), which cause oxidative stress and indirect DNA damage. This mechanism directly links high pheomelanin levels to an increased risk of melanoma and non-melanoma skin cancers, resulting in heightened vulnerability to photodamage.