Treatment of human respiratory tract epithelial cellswith H202 led to concentration-dependent DNA strand breakage that was highly-correlated with multiple chemical modifications of all four DNA bases, suggesting that damage is due to hydroxyl radical, OH'. However, the ma|or base damage occurred to adenine. Hence, conclusions made about the occurrence and the extent of oxidative DNA damage on the basis only of changes in 8-hydroxyguanine should be approached with caution.

\bstract Treatment of human respiratory tract tracheobron&al epithelial cells with gas-phase cigarette smoke led to dosedependent DNA strand breakage that was highly correlated with multiple chemical modifications of all four DNA bases. The pattern of base damage suggests attack by hydroxyl radicals (OH'). However, by far the most important base damage in quantitative terms was formation of xanthine and hypoxanthine, presumably resulting from deamination of guanine and adenine respectively. Hence, DNA damage by cigarette smoke may involve reactive riitrogen species as well as reactive oxygen species.