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Discovery of weak links may yield future measures to prevent radiation damage

DOE/Brookhaven National Laboratory : 17 September, 2003  (Technical Article)
While attempting to 'photograph' the chemical reactions of an important enzyme of the nervous system, an international team of scientists found that the 'flash' they were using, a high-intensity X-ray beam, was systematically destroying their target. The resulting 'movie' of molecular images is the first-ever direct observation of how proteins break apart when exposed to high-energy X-rays.
'The observation was stunning,' said collaborator Joel Sussman, formerly on the staff and now a visiting biologist at the U.S. Department of Energy's Brookhaven National Laboratory, where some of this research took place. Stunning because, previously, scientists believed radiation damage was nonspecific, or random. But the Brookhaven work and studies with other enzymes elsewhere confirm that the X-rays selectively break particular chemical bonds.

'It looks like we are seeing 'weak points' in protein structures that are particularly sensitive to ionizing radiation,' says Sussman, who is now principally affiliated with the Weizmann Institute of Science in Israel. Certain disulfide bonds, which often bridge protein chains, and carboxyl acids, such as those found at an enzyme's 'active site' where reactions take place, seem particularly vulnerable. Understanding these weak links may lead to improved methods of preventing high-dose radiation damage.

Organisms are constantly exposed to radiation, mainly from natural sources, such as sunlight and cosmic rays, as well as man-made sources such as diagnostic X-rays. 'The ability to visualize the specific damage caused by radiation at a 'test-tube' level offers an important diagnostic tool for developing pharmacological means to protect against radiation damage,' says Israel Silman, also a guest scientist at BNL's Biology Department. The Weizmann team and European collaborators, together with Brookhaven scientists, plan to examine the anti-radiation potential of various substances that might be used to offer general protection or in an emergency.

The findings of X-ray-induced damage, published in the January 18, 2000, issue of the Proceedings of the National Academy of Science, will also have implications for the use of X-ray techniques to decipher molecular structures. In X-ray crystallography, scientists bombard crystalline samples of proteins with high-intensity X-rays and, by analyzing how the rays diffract, or bend, they work backward to decipher the protein's molecular structure. This common technique is an important research topic at Brookhaven's National Synchrotron Light Source, where some of the research described in the paper was carried out.

Scientists typically perform these experiments at very cold (cryogenic) temperatures to minimize the damage caused by X-rays. But the current research, conducted at cryogenic temperatures, shows that these techniques do not completely prevent the introduction of inadvertent changes into experimental samples. It is well known in science that the mere act of observation may change what you are trying to observe.

The current findings may lead to changes in procedure to minimize this effect. For example, 'Less intensive radiation may provide more accurate results,' says Gitay Kryger, a Weizmann biologist.

The initial experiments were conducted at the European Synchrotron Radiation Facility in Grenoble, France, using acetylcholinesterase taken from the Torpedo fish. The work at Brookhaven used the same enzyme from humans and fruit flies. Additional experiments on a different enzyme, lysozyme, from hen egg white, were performed at the ESRF. Additional collaborators included scientists from Holland's Bijvoet Center for Biomolecular Research, and the European Molecular Biology Laboratory Outstation in Grenoble, France.
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