Radiation Inactivation of Molecules in Cells

The mechanisms by which molecules in cells are inactivated by ionizing radiations are discussed. Experiments on the inactivation of enzymes and of desoxyribosenucleic acid in dry cells are in good agreement with the hypothesis that direct action requires an ionization either within the molecule, or very close to it. When wet cells are irradiated, the increased radiosensitivity can be explained quantitatively by assuming an indirect effect caused by the diffusion of radiation-produced radicals which diffuse a distance the order of magnitude of 30 angstroms. These mechanisms explain the experimental fact that biological macromolecules in cells are very resistant to radiation, and require doses the order of megarads, or greater, to inactivate half of the molecules of a given kind. They also imply that the initial biochemical effects are produced very close to the sites of the ionizations, providing a justification for the use of target theory. It thus appears that if the inactivation of a living cell is to be ascribed to the inactivation of molecules, then a number the order of a thousand molecules must be assumed, each indispensable for the process of cell division, to account for the observed effective doses.