A molecule in the skin that converts some of the sun's energy to damaging free radicals could be a major contributor to wrinkles and skin cancer. In tomorrow's issue of the Proceedings of the National Academy of Sciences, two researchers point the finger at chemical changes in the molecule transurocanic acid--which at one time was thought to protect against ultraviolet (UV) radiation damage--as a possible controlling agent in the process.
Transurocanic acid, which is found in the top layer of skin cells, is very good at absorbing UV-B light. For this reason, it was thought to be a natural sunscreen and was even added to skin lotions until about 15 years ago, when it was found to suppress the immune system. But the molecule also reacts differently to light of different wavelengths. For example, on exposure to UV rays, transurocanic acid contorts itself to change into its mirror image called cis-urocanic acid. Chemists Kerry Hanson and John Simon, both then at the University of California, San Diego, decided to probe the changes in the molecule in the previously unexplored UV-A region.
First they blasted the molecule with short bursts of UV laser light. The molecule gets rid of some of the absorbed energy by heating up the material around it, and this generates a pressure wave. By measuring the strength of the pressure wave, the researchers could tell how much energy the molecule retained. To their surprise, they found that it was retaining much more energy than it needed to twist from the trans to the cis variety. This extra energy, they deduced, must go into forming a new, highly excited state of the trans molecule, which in turn transfers its energy to surrounding oxygen molecules. These oxygen molecules then become highly reactive "free radicals," which can damage or even kill cells. Indeed, the very wavelengths that were most likely to generate oxygen free radicals were the wavelengths that do the most harm to the skin of mice.
Although a direct link to skin cancer has not yet been made, there is a lot of evidence that oxygen free radicals are involved in a variety of cancers, says Janna Wehrle, a biochemist at the National Institute of General Medical Sciences in Bethesda, Maryland. If the link is proven, she says, a new generation of sunscreens that block the longer UV-A wavelengths could "generate a real improvement in public health."