Magazine article Science News

Studies Show DNA Damage by Long UV

Magazine article Science News

Studies Show DNA Damage by Long UV

Article excerpt

Warnings about the sun's burning, aging and potentially carcinogenic rays are usually directed againat overexposure to a portion of the ultraviolet (UV) spectrum termed UV-B--the medium-length UV wavelengths ranging from about 250 to 320 nanometers. For decades scientists believed longer-wave UV radiation had little biological effect. But research at Argonne (Ill.) National Laboratory is now demonstrating clearly identifiable damage to DNA--both in bacteria and in human cells--irradiated with the longer wavelengths known as UV-A. Moreover, this research is showing that the mechanisms causing UV-A's damage is quite different from what occurs with exposure to shorter UV wavelengths.

The importance of UV-A-caused DNA damage has not been determined, says Argonne group leader Meyrick Peak. Though long UV produces DNA damage with only perhaps 1/1,000 the efficiency of mid-range UV-B, he points out that UV-A penetrates light-colored skin more deeply than UV-B does. "So my feeling is that the [longwave UV-A] might prove important in the formation of skin damage," he told SCIENCE NEWS.

Shorter UV wavelengths--in the B and C portions of the spectrum--will damage DNA directly. According to Peak, "The DNA of the cells absorbs far UV [UB-B and -C] very strongly," transferring the photons' energy to the electrons in the DNA. This damage is powerful "and can clearly mutate cells and cause cancer very quickly--even at low doses," he explains.

By contrast, UV-A's effect on DNA are indirect. "DNA doesn't absorb near-UV [UV-A] as far as we can tell," Peak says. However, his group has identified cellular chemicals, which they call sensitizers, that do absorb that UV-A's photon energy. This absorption excites, or raises the energy level of, the sensitizers' electrons. But even they do not damage DNA, it appears. That's oxygen's role.

"We believe that these excited sensitizers can transfer energy directly to molecular oxygen," creating powerful, chemically reactive oxygen species, write Meyrick Peak and his wife, biochemist Jennifer Peak, in the latest (autumn) issue of LOGOS, an Argonne publication. …

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