Induction of DNA-Protein Crosslinks in Chinese Hamster V79-4 Cells Exposed to High- and Low-Linear Energy Transfer Radiation

The induction of DNA-protein crosslinks (DPCs) in Chinese hamster V79-4 cells after irradiation under hypoxic and aerobic conditions at 277 K with⁶⁰ Co γ rays,${}^{238}{\rm Pu}$ α particles and aluminum K (${\rm Al}_{{\rm K}}$) ultrasoft X rays has been determined using a nitrocellulose filter binding assay. The dose dependences for the induction of DPCs, which involves covalent linkage, are linear over the absorbed dose range used (0-400 Gy with α-particle and γ radiation, 0-600 Gy with${\rm Al}_{{\rm K}}$ X rays). The yield of DPCs induced under hypoxic conditions is 55, 51 and 25 DPCs per gray per cell for⁶⁰ Co γ rays, α particles and${\rm Al}_{{\rm K}}$ X rays, respectively. The yield of DPCs is significantly reduced in the presence of oxygen by 20, 50 and 79% for⁶⁰ Co γ rays, α particles and${\rm Al}_{{\rm K}}$ X rays, respectively. Since the mean size of the DNA attached to the protein is uniform for⁶⁰ Co γ rays and α particles, variations in the DNA size do not influence the yields of DPCs. Although a DPC may be considered as a complex lesion combining two macromolecules, the dependence of the yield of DPCs on LET does not reflect the ionizing density of the radiations used. Further, this dependence on LET and the effect of oxygen do not reflect the corresponding dependences determined for a variety of biological responses. From these findings and knowledge of the radiation tracks, it is proposed that DPCs induced particularly under aerobic conditions with⁶⁰ Co γ rays are formed mainly in the sparsely ionizing segments of the radiation track.