Mechanism of protection against radiation-induced DNA damage in plasmid pBR322 by caffeine.
OBJECTIVE
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 gamma-irradiation of plasmid DNA, a system devoid of replication and repair, could elicit protection by scavenging free radicals.


METHODS
Plasmid pBR322 DNA was exposed to gamma-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 7.5x10(1) M(-1) s(-1) with O(2)(-) 1.05x10(8) M(-1) s(-1) with ROO(*) and 8.8x10(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.
Kumar
S S
SS
Cell Biology Division, Bhabha Atomic Research Centre, Mumbai - 400 085, India.
Devasagayam
T P
TP
Jayashree
B
B
Kesavan
P C
PC
D002110:Caffeine / Q000494:pharmacology*; D004247:DNA / Q000187:drug effects / Q000378:metabolism / Q000528:radiation effects*; D004249:DNA Damage; D004270:DNA, Circular / Q000378:metabolism / Q000528:radiation effects; D016166:Free Radical Scavengers / Q000494:pharmacology; D005720:Gamma Rays; D006861:Hydrogen Peroxide / Q000378:metabolism / Q000494:pharmacology; D007700:Kinetics; D010084:Oxidation-Reduction; D010100:Oxygen / Q000378:metabolism*; D010957:Plasmids / Q000187:drug effects / Q000235:genetics / Q000528:radiation effects; D011837:Radiation-Protective Agents / Q000494:pharmacology*; D017382:Reactive Oxygen Species / Q000378:metabolism
D004270:DNA, Circular; D016166:Free Radical Scavengers; D011837:Radiation-Protective Agents; D017382:Reactive Oxygen Species; D002110:Caffeine; D004247:DNA; D006861:Hydrogen Peroxide; D010100:Oxygen