Radiosensitization with Iodine Compounds

Summary

The synthesis of DNA, RNA and protein, and the rejoining of DNA strand-breaks and DNA breakdown were studied in cells of Bacillus subtilis γ-irradiated in the presence of iodoacetic acid (I. CH₂. COOH), potassium iodide (KI) or potassium iodate (KIO₃). Observations on incorporation of radioactive precursors into bacteria mildly radiosensitized by the above reagents indicated that synthesis of DNA was generally more sensitive than that of protein or RNA. A cellular radiosensitizing effect cannot be explained by increase of radiation-induced in situ damage in DNA, because a drug effect was not found on radiation inactivation of transforming DNA. Also, the number of radiation-induced breaks in DNA strand studied by means of alkaline-sucrose gradient centrifugation was very similar in the absence or presence of reagent at the time of irradiation.

Studies with KIO₃ have shown that the presence of the reagent during irradiation of bacteria considerably reduced rejoining of DNA strand-breaks during subsequent post-irradiation incubation without the drug. However, the inhibition of break-rejoining seemed to be less sensitive than that of DNA synthesis and was correlated more closely with a reduced rate of protein synthesis in radiosensitized cells. In highly radiosensitized cells, all macromolecular synthesis, rejoining of strand-breaks as well as breakdown of cellular DNA, were totally absent. Relationships between increase of cellular lethality, preferential inhibition of de novo DNA synthesis and of rejoining of DNA strandbreaks are discussed.