Mechanism of protection against radiation-induced DNA damage in plasmid pBR322 by caffeine

Abstract

Purpose : Caffeine (1,3,7-trimethyl xanthine), a dietary component, has been shown to have widely varying effects on DNA damage induced by UV and ionizing radiation, depending upon pre- or post-irradiation administration and its concentration. Caffeine administered post-UV irradiation is known to inhibit enzymatic repair of DNA lesions, leading to potentiation of damage, whereas its presence before or during irradiation elicits protection in a wide range of test systems: bacteria, cultured human cells, plant seeds and mouse. The purpose of this study is to test whether caffeine present during γ-irradiation of plasmid DNA, a system devoid of replication and repair, could elicit protection by scavenging free radicals. Materials and methods : Plasmid pBR322 DNA was exposed to γ-radiation in the presence or absence of caffeine at a dose-rate of 1.20 Gy min -1 and damage measured as single-strand breaks. To understand the mechanisms of the observed protection, especially under oxic conditions, reaction of caffeine with superoxide radical (O 2 -) , hydrogen peroxide (H 2 O 2) and the deoxyribose peroxyl radical (ROO •) were studied. Results : Irradiation of pBR322 was observed to induce a dose-dependent increase in single-strand breaks. Caffeine itself did not induce strand breaks but reduced radiation-induced strand breaks at micromolar to millimolar concentrations. Caffeine has been shown to react with the radiation-derived oxidants. The reaction rate constants observed were 7.5 ×10 1 M -1 s -1 with O 2 - 1.05 ×10 8 M -1 s -1 with ROO •and 8.8 ×10 1 M -1 s -1 with H 2 O 2. Conclusions : Caffeine effectively protects DNA against ionizing radiation in a system devoid of repair and replication machinery. Thus, DNA protection shown by caffeine is possibly due to the scavenging of radiation-derived primary as well as secondary reactive oxygen species, and this physicochemical protective pathway possibly pre-empts any subsequent inhibitory effect of caffeine on the enzymatic repair of DNA.