How UV light causes cancer and wrinkles.

Skin can be its own worst enemy. Scientists have learned in recent years that two substances which protect skin from the harmful effects of the sun can also, under certain conditions, have a hand in causing that damage. Now, researchers at the University of California, San Diego (UCSD) in La Jolla have found some clues as to why.

The physical properties of the substances, melanin and urocanic acid, change with exposure to ultraviolet (UV) light. The variability of their response to different wavelengths may explain the observed biological effects of UV exposure, such as the initiation of skin cancers and wrinkles.

Melanin, a skin pigment, protects against UV exposure by absorbing the light and then dissipating the energy as heat. In test-tube studies of this reaction, chemists Susan E. Forest and John D. Simon found that at short wavelengths in the UVC range, melanin retains 27 percent of that heat. Such an excess could allow melanin to interact with oxygen to create destructive free radicals, they suggest. Whether such ill effects occur depends on what melanin does with the stored energy, Forest says. The molecule may dissipate the energy as heat, or the energy may go into chemical reactions that create oxygen radicals. Further studies should be able to tell how much energy is lost through each mechanism, she says.

Kerry M. Hanson, working with Simon at UCSD, looked at urocanic acid, a compound found in the outermost layer of skin. Previously, scientists had suggested that urocanic acid causes wrinkling during exposure to longer wavelength, UVA radiation but suppresses the immune system when stimulated with short wavelength, UVC light.

Hanson's findings support those assertions. Exposed to UVA, urocanic acid creates reactive oxygen radicals that can cause wrinkles by breaking down collagen and elastin in the skin, she says. Larger quantities of oxygen radicals that are produced under UVC exposure, however, could damage the immune system. Exposure to UVB radiation does not produce radicals.

Sunscreens and cosmetics once contained urocanic acid, until scientists discovered it might harm rather than protect, Hanson says. Most UVC is blocked by the atmosphere, but ozone depletion may make short wavelength UV light more of a concern.

The use of physical chemistry techniques to investigate how photobiology occurs in the skin makes this project "a nice blend between the two fields," Hanson says.