Biological Effects of Auger Processes of Bromine on Yeast Cells Induced by Monochromatic Synchrotron X-rays

Summary

The biological effects of inner-shell ionization in bromine atoms incorporated into DNA in the form of bromodeoxyuridine monophosphate (BrdUMP), induced by monochromatized synchrotron X-rays, were studied using a deoxythymidine monophosphate (dTMP)-permeable mutant of yeast, Saccharomyces cerevisiae. The BrdUMP-incorporated yeast cells were irradiated with monochromatic X-rays of 13·51 or 13·45 keV, between which the bromine K-absorption edge (13·47 keV) is located. The cells were 1·07 times more sensitive to irradiation by 13·51 keV X-rays than at 13·45 keV, while dTMP-incorporated cells did not show any difference in sensitivity. In the presence of a radioprotector during irradiation, BrdUMP-incorporated cells showed a larger enhancement (1·20). These enhancements observed in the bromine-incorporated cells cannot be explained only by an increase of the absorbed dose due to a substitution of CH₃ group of thymine by bromine. It may be concluded that a major part of the enhancement was caused by inner-shell photoionization, followed by an Auger cascade of the bromine in the DNA. The quantum yield of lethality caused by the photoabsorption of bromine K-shell is not affected by the presence of cysteamine, suggesting the biological enhancement by the Auger processes may not be influenced by chemical protection.