August 29, 2005
Chemists Find Possible Reason why Redheads Have More Skin Cancer
WASHINGTON, D.C. -- A Duke University chemist has found differences in how ultraviolet light affects the photochemistry of human pigments that he says may explain why red-haired people are more prone to skin cancer than those with black hair.
Duke chemistry professor John Simon and his collaborators used a broadly-tunable ultraviolet laser and a special microscope to distinguish between the oxidation potentials of pigments of redheaded and black-haired people. Oxidation potentials measure how likely chemicals are to activate oxygen by taking up electrons.
"We were very interested in determining if there are differences in the ability of the two kinds of human pigments to activate oxygen," Simon said in an interview. "Activating oxygen can produce compounds called radicals that put oxidative stress on cells. Such stress could ultimately lead to cancer and other diseases."
Simon will describe his work in a talk at the first session on Frontiers of Photobiology that begins at 8:20 a.m. Eastern Time on Sunday, Aug. 28, 2005, in Room 143C at the Washington D.C. Convention Center during the 230th national meeting of the American Chemical Society.
Aspects of this work were originally funded by the National Institute of General Medical Sciences and are now supported by Duke University and the Air Force Office of Scientific Research.
"Red-haired, fair skinned people have a higher instance of skin cancer than black-haired individuals," Simon said. "And the melanin pigment in the skin of red haired people differs chemically from the melanin in the skin of those with black hair. So researchers have tried to compare the red with the black pigments, but have not succeeded in comparing the isolated human pigments until now."
Melanins in pigment-producing cells known as melanocytes have proved difficult to isolate from human skin, he explained. "So scientists have developed techniques to make synthetic pigments in the laboratory," Simon explained. "However, those pigments are not structurally similar to what is found in humans."
But in 2000 an Italian research team reported a method for extracting intact melanin-containing structures, called melanosomes, from human hair. Simon's group at Duke then used that method to "isolate what I think were the first quantities of intact human black and red melanosomes amenable to study by a variety of analytical techniques," he recalled.
Their next step, to fully characterize the two pigment types, was done collaboratively with the research group of pigment cell chemist Shosuke Ito at the Fujita Health University in Japan.
While researchers confirmed that the samples were high quality and reflected those found in nature, measuring their oxidation potentials proved to be a challenge. "Basically, those measurements could not be made using the standard approaches of solution or solid state electrochemistry," Simon said.
One of his postdoctoral investigators, Alexander Samokhvalov, suggested skirting that problem with a technique called photoelectron emission. Photoelectron emission is commonly used to measure oxidation potentials in the dry thin films within solar cells when those films receive light of just the right wave length needed to release an electron.
But the researchers knew that melanosomes' oxidation potentials would still be difficult to measure with this technique, according to Simon. They would need a laser that could be adjusted, or "tuned," to a variety of wavelengths in part of the ultraviolet portion of the light spectrum. Finding a conventional laser that could be tuned so broadly in the ultraviolet spectrum would be "difficult to impossible," he said.
Fortunately, widely tunable ultraviolet light was available at Duke's Free-Electron Laser Laboratory (FELL). There, magnets manipulate electrons freed of their usual association with atoms to produce light at a large variety of wavelengths.
Simon's group also learned that a team headed by Robert Nemanich at North Carolina State University had installed a photoelectron emission microscope at FELL that could resolve the tiny pigment granules.
But that microscope operates in conditions very different from the human body, including a high vacuum. So Simon's group collaborated with Nemanich and FELL director Glenn Edwards to devise a way to study the melanosomes in such an environment.
"We found the melanosomes survived, were robust, and weren't affected by the experimental procedure," he said. "So we went ahead and measured the photoelectron emission properties of the different human pigments."
The group found that the pigment produced by cells in black-haired people has an oxidation potential "indicating that it's thermodynamically unfavorable for black melanosomes to activate oxygen," Simon said.
By contrast, "we found it is thermodynamically favorable for red melanosomes to activate oxygen," he said.
"Whether or not this is important in what happens in cellular systems is an open question and the subject of future work," Simon cautioned. "However, studies on melanocytes reported by other groups are consistent with these findings.
"This is the first measurement to ever be reported that compared the two human pigments, and also clearly links the red pigments to possible oxidative stress through their electrochemical properties."
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