Molecular and Cellular Effects of Fast Charged Particles

Abstract

The radiosensitivity of enzymes, bacteriophage, bacteria, and yeast cells was studied as a function of the linear energy transfer (LET) of a wide variety of ionizing radiations. Very high LET values were obtained with monoenergetic beams of stripped nuclei with atomic number up through 10, accelerated to a maximum energy of Mev/amu in the Heavy Ion Linear Accelerator. With dried, crystalline enzymes exposed in vacuum, the radiosensitivity could be increased or decreased by mixing the enzymes with various inert substances prior to irradiation. It is postulated that intermolecular energy transfer processes account for these effects as well as for the unexpectedly high radiosensitivity of pure enzymes exposed to very densely ionizing radiations. The relative biological effectiveness (RBE) for inactivation of dried crystalline enzymes and for inhibition of plaque formation of T-1 bacteriophage exposed in dry state and in nutrient broth, decreases with increasing LET. The RBE for inhibition of colony formation of haploid Saccharomyces cerevisiae exposed in O2 and N2 atmosphere, and for inhibition of colony formation of Shigella sonnei exposed in N2 atmosphere, as well as for induction of mutants in diploid Saccharomyces cerevisiae exposed in O2 and N2 atmospheres increases with increasing LET and then decreases when the LET exceeds a certain value characteristic of the system studied. A track segment analysis is given for the LET dependence of the various effects studied, which also shows how much more energy is required locally for radiation injury under anoxia than is needed when oxygen is present. For all the biological systems studied thus far, the RBE was declining when the LET exceeded 3000 Mev gm-1 cm2.