Sep. 3, 1998 Despite all the warnings to avoid exposure to the sun and to wear sunscreen, scientists don't really know how the sun damages our skin. Now, they're a bit closer to the answer.
Two scientists recently discovered that sunlight triggers a harmful reaction when it strikes a molecule in our skin--ironically a molecule once thought to be "nature's sunscreen." The work suggests the science behind the sagging, leathery skin typical of long-term sun worshipers, and may also shed light on how ultraviolet light causes skin cancer.
"We studied a natural component of human skin exposed to ultraviolet light and uncovered a new chemical reaction that may contribute to aging [of the skin] and cancer," said Dr. John Simon, who led the study.
The research will appear in the September 1 issue of the Proceedings of the National Academy of Sciences. The work was conducted at the University of California, San Diego by Dr. John D. Simon (who is now at Duke University), and his graduate student Kerry Hanson (who is now a postdoctoral fellow at the University of Illinois at Urbana-Champaign).
The sun's harmful rays come in two flavors: ultraviolet A and ultraviolet B. Evidence mounts that ultraviolet A and B both play a role in causing skin cancer and photoaging, which is characterized by deep, premature wrinkles, thickened skin, and age spots.
Time, gravity, and heredity notwithstanding, "something like 90 percent of all the visible signs of aging are from ultra-violet sources," said Dr. Kerry Hanson. "Photoaging is not just a cosmetic effect. It destroys the integrity of your skin."
The focus of this study is the sun-sensitive molecule called trans-urocanic acid (t-UA). Formed in the top layer of the skin, t-UA molecules cover our bodies, acting like antennae for light. In the 1950s, urocanic acid was hailed as a "natural sunscreen" because it absorbs ultraviolet B light. It was thus thought to protect against damage by such rays, which can potentially lead to skin cancer. For a time, the pigment was even added to sunscreens and skin lotions.
When exposed to ultraviolet rays, trans-UA buckles in upon itself to form cis-UA. Most theories about urocanic acid's action focus on this structural flip and subsequent chemical reactions. But the data didn't seem to fit--urocanic acid was absorbing almost three times more energy than could be accounted for by its twist into its cis form. It must undergo some additional chemical reaction, the scientists reasoned. So they investigated the molecule in a different light--literally. What they found revealed not only the molecule's additional maneuver, but its sinister consequences.
Using a cutting-edge technique called photoacoustic spectroscopy, Drs. Hanson and Simon studied urocanic acid's activity when exposed to light near the tail end of the ultraviolet A range, where the molecule's reactivity was thought to be harmless. They discovered that when this type of light strikes t-UA, it zaps the molecule into an excited "triplet" state that sparks the creation of oxygen radicals.
Oxygen radicals are chemical rogues blamed not only for premature aging, but also for damaging DNA, suppressing the immune system, and causing some respiratory problems.
"The results certainly surprised us," said Dr. Simon. "We never expected to see [any] process that allowed oxygen to get sensitized" because the study was conducted under ultraviolet light much different than that which most excites t-UA. "It's a real serendipity case."
"What it means is that you have to be more concerned about protecting yourself from ultraviolet A radiation," he continued. "We should probably use sunscreens that block all the way out to 400 nm [the end of the ultraviolet A region]."
The SPF (solar or sun protection factor) in sunscreen refers to its ability to protect against the burning rays of ultraviolet B light. But currently there are no world-wide standards to measure protection against ultraviolet A, which accounts for 95 percent of sunlight that reaches the earth.
The work may also change the way researchers approach similar projects. Instead of studying a biological molecule exclusively under the type of light that it absorbs most, Dr. Simon suggested that scientists may need to expose the molecule to a whole spectrum of light, slice by slice, to fully understand the molecule's physiological effects.
Hanson K, Simon J. Epidermal trans-Urocanic Acid and the UV-A Induced Photoaging of the Skin. Proc. Natl. Acad. Sci. USA 1998;95(18):10576-10578.
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