Abstract: Purpose and Methods : The aim of the study was observing the effect of endonuclease Ⅲsupplementation on the damage of DNA st rand breaks , which were measured by comet assay (SCGE) . Results : DNA stran break damage of lymphocytes and Hela cells induced by 100μmol H2O2 in cont rols reached 198. 5 and 320 arbitary units respectively. The rate of DNA st rand breaks of lymphocytes was gradually decreased f rom the beginning to 4 hour culture in cont rol group. At the end of 4 hour culture , there was a much lower rate of DNA repair in Endonuclease Ⅲgroup with 33. 4 % than cont rol group with 67. 6 % ( P < 0. 05) . The same result was found in HeLa cells. The levels of DNA st rand breaks was quickly decreased f rom 320 to 19 arbitary units in control group within 4 hour culture , whereas the DNA st rand breaks in supplementing group with Endonuclease Ⅲwas kept at a high level with 172. 0 arbitary unit s at 4 hour culture ( P < 0. 05) . Conclusion : The study suggested that the supplementation of endonuclease Ⅲcould increase the levels of DNA st rand breaks.

Key words: endonuclease Ⅲ, SCGE, lymphocyte, DNA repair, DNA damage