Inhibition of radiation-induced DNA damage in plasmid pBR322 by chlorophyllin and possible mechanism(s) of action

Naturally occurring compounds capable of protecting DNA against ionizing radiation and chemical mutagens have considerable potential for prevention of mutation-based health impairment including cancer and other degenerative diseases. Chlorophyllin (CHL), a water-soluble derivative of chlorophyll, has been examined for its ability to protect DNA against radiation induced strand breaks using an in vitro plasmid DNA system. Gamma-radiation, up to a dose of 6 Gy (dose rate 1.25 Gy/min), induced a dose-dependent increase in single-strand breaks (ssbs) in plasmid pBR322 DNA. CHL per se did not induce, but inhibited radiation-induced ssbs in a concentration-dependent manner; 500 mu M giving about 90% protection. The protection afforded by CHL was comparatively less than that of trolox, a water-soluble analogue of alpha-tocopherol. To elucidate the underlying mechanism(s), reaction of CHL with the radiation-derived hydroxyl radical ((OH)-O-.) and deoxyribose peroxyl radical (ROO.) was studied by pulse radiolysis. CHL exhibited a rate constant of 6.1 +/- 0.4 x 10(9) M-1 s(-1) with (OH)-O-. and 5.0 +/- 1.3 x 10(7) M-1 s(-1) with ROO.. To our knowledge, this is the first report providing direct evidence of free radical-scavenging properties of CHL. The results showed that CHL, effectively protects plasmid DNA against ionizing radiation, in an in vitro system independent of DNA repair or other cellular defense mechanisms. The ability of CHL to scavenge (OH)-O-. and ROO., may contribute to its protective effects against radiation induced DNA damage in the pBR322 system. (C) 1999 Elsevier Science B.V. All rights reserved.