Melanin exposure

Hydroquinone inhibits melanin production when applied to the skin. The effect is reversible by exposure to ultraviolet light. 

Skin lighteners, freckle and age spot removers, and other remedies for hyperpigmentation are not actually bleaches like the products listed so far. The active ingredient is hydroquinone, which inhibits melanin formation when applied to the skin. Since the effect is easily reversed by exposure to sunlight or ultraviolet light, a sunscreen is usually included in the formula. 

Among these factors, sun exposure remains the most important, although all of them significantly increase the activity of tyrosinase in producing melanin. 

Generally, the phenotype that predisposes an individual to an increased risk of skin cancer is red or blond hair, blue eyes, and fair skin. These characteristics are surrogate measure of the sensitivity of the skin to sun exposure and the tendency to develop nevi, freckles, and sunburns based on the skin type. Freckles, which may appear abruptly after the first high dose of UV radiation sun exposure, represent clones of mutated melanocytes, and their presence is associated with an increased risk of melanoma.12 The Fitzpatrick classification of skin type is used to determine the response pattern of the skin to UV radiation and assess the risk for melanoma. There are six Fitzpatrick skin types Type I skin always burns and never tans, type II skin burns easily and tans rarely, type III skin burns sometimes and tans usually, type IV skin burns rarely and always tans, type V skin always tans and is moderately pigmented (brown), and type VI skin always tans and is darkly pigmented (black). Fitzpatrick I and II skin types are commonly affected by NMSC and MM. The susceptibility to skin cancer, both NMSC and MM, is related to the melanin content of the skin and the skin s response to UV radiation. 

Figure 1.6 (A) Changes in c-Fos IR in hypocretin/orexin(HCRT+)- containing neurons in response to BIC treatment as a function of distance from the microdialysis probe. The percentage of double-labeled cells was greatest closer to the probe (grids 1 and 2, compared with 3 and 4) and was greater with higher concentrations or exposure times. Contralateral cells were not affected. (B) Much lower percentages of melanin-concentrating hormone (MCH+) exhibited c-Fos IR activation in response to BIC. From Alam et al. (2005).

Melanin is formed by a photochemical reaction, so the concentration of melanin within the human epidermis (the outer layer of the skin) increases following exposure to photochemical reactions with tyrosine in the skin. This increase is seen readily by the formation of a suntan. This increase in the concentration of melanin following exposure to the sun is more obvious for fair-skinned people, although darker people usually tan more quickly because melanin is produced more efficiently in their skins. 

Potentials problems with the use of hair include a strong influence of hair pigmentation on nicotine and cotinine binding and uptake (Dehn et al. 2001). Nicotine and cotinine are bound to melanin. As a result, dark hair binds much more nicotine than does blond or white hair. This makes comparison across individuals difficult. Also, hair is exposed to nicotine and cotinine from sweat and from sebaceous gland secretions, and to nicotine from environmental tobacco smoke exposure. Washing the hair before analysis may reduce this problem of environmental contamination, but it is not likely to remove all environmental nicotine and cotiiune. 

Chemicals in sunscreens work much like melanin to absorb UV radiation. The most common is para-aminobenzoic acid or PABA, but there are others. Most glass, but not clear plastic, will block UV radiation. Relatively simple measures, such as hats and clothing, will greatly reduce exposure. About 90% of UV radiation is reflected by snow, making snow blindness a significant concern. 

The eye may be a target organ as a result of its external position in the organism and direct exposure, but also from systemic exposure. Thus, the various components of the eye may be specifically damaged. The presence of pigments in the eye such as melanin has been suggested as the cause of chloroquine toxicity to the retina. 

Many marine species also possess the tyrosinase-mediated pathway to synthesize the UV-absorbing pigment melanin. Melanin occurs in a wide range of taxa including bacteria, fungi, invertebrates, and chordates. While much is known about the role of melanin in the UV protection of mammalian skin, very little research has been conducted to examine the efficiency of melanin as a UV-protective mechanism in aquatic taxa.9 It is known that melanin levels in juvenile hammerhead sharks, Sphyrna lewini, are directly correlated to solar UV exposure in the freshwater crustacean Daphnia pulex, melanin concentrations are genetically determined within populations and are correlated to UV sensitivity.50-51 The few studies that have been undertaken suggest that melanin has an important role in UV protection in aquatic environments. 

Clonogenic survival of B16 melanoma cells measured after exposure to graded doses of 211 At-AMT for 45 minutes is represented in Figure 12, showing that this radiolabelled modified melanin precursor is exceptionally cytotoxic. The linear cell survival curve, without a shoulder in the low-dose region, is characterisitc for high-LET a radiation125. 

The substance melanin is found in the germinative layer, where it is formed by cells called melanocytes. The number of melanocytes is fairly constant and differences in skin colour are due to the amount of melanin produced. Exposure to sunlight promotes the synthesis of increased amounts of melanin. The function of melanin is to protect the skin from the harmful effects of some of the sun s rays. 

Prolonged exposure to the sun can allow more UV radiation to reach your skin than melanin can absorb. A commercial sunscreen can offer added protection, however, because it contains conjugated compounds that absorb UV light, thus shielding your skin (for a time) from the harmful effects of UV radiation. Two sunscreens that have been used for this purpose are para-aminobenzoic acid (PABA) and padimate O. 

---