melanins

Melanins are biopolymers that give color to skin, hair, and eyes and help protect against UV radiation. They are produced by melanocytes, mainly in the epidermis and hair follicles, but also in the retinal pigment epithelium and other tissues. The main forms are eumelanin (brown/black) and pheomelanin (red/yellow); a related pigment, neuromelanin, occurs in some brain regions.

Biosynthesis occurs in melanosomes within melanocytes. The enzyme tyrosinase catalyzes the rate-limiting steps: tyrosine is converted to DOPA and then to dopaquinone, after which enzymatic routing yields eumelanin or pheomelanin. Depending on enzyme activity and redox state, plus cysteine availability, the pathway is directed toward one or the other pigment; melanosomes are then transferred to keratinocytes, contributing to skin and hair color.

Functions and variation: Melanin absorbs and dissipates UV radiation, protecting cells from DNA damage, and also has antioxidant properties. Levels vary with genetics and sun exposure; higher melanin yields darker skin and greater photoprotection, while lower levels increase UV sensitivity and cancer risk. In the eye and brain, melanin-containing tissues include the iris, retinal pigment epithelium, and neuromelanin in regions such as the substantia nigra; its brain function is not fully understood but may relate to aging and disease.

Clinical relevance: Abnormal melanin production underlies conditions such as albinism (reduced melanin from TYR or related gene mutations) and vitiligo (loss of melanocytes). Melanocytes can give rise to melanoma, a malignant skin cancer. Research into melanin pathways informs approaches to pigmentation disorders and photoprotection.