Studies on the modulation of DNA damage. 1: A simple kinetic model for radioprotection of DNA in aqueous solution.

ABSTRACT

A simple kinetic model, based on the assumption that direct and indirect interactions of ionising radiations with DNA lead to formation of DNA-radicals which are precursors of single strand break (ssb) and double strand break (dsb) is presented. Considering accessibility of OH attack on DNA and assuming homogenous kinetics, the model has been validated by using experimental data on radiation-induced strand break formation in presence of methanol (MeOH) and glutathione (GSH). Fitting the D37 values for ssb in aqueous solution of pBR322 DNA in the presence of the predominantly A-T minor groove-ligand Hoechst 33258, the value of the rate constant of OH scavenging by DNA-bound Hoechst was estimated by Martin and his group as 2.7 x 10(11) dm3 mol-1 s-1 (Martin, R.F. and Denison, L. (1992), Int. J. Radiat. Oncol. Biol. Phys., 23, 578-584). Using aqueous solution of calf thymus DNA in the presence of Hoechst this rate constant has been determined by pulse radiolysis as 1.1 x 10(9) dm3 mol-1 s-1. The yield of strand breaks has also been measured by low angle laser light scattering. Using the experimentally measured value of the rate constant of OH scavenging by DNA-bound Hoechst in the model, calculations show that OH scavenging alone could not explain the overall protection provided by Hoechst; there should be quenching of DNA-radicals by Hoechst. Fitting the yield of radiation-induced ssb (G(ssb)) obtained for CT DNA and pBR322 DNA, the rate constant of this DNA-radical quenching has been calculated as 10(6) dm3 mol-1 s-1, which agrees with pulse radiolytic data. Using these rate constant values, calculations show that the protection against radiation-induced dsb formation provided by the combination of Hoechst with GSH is more than that by the combination of MeOH with GSH.