Nitrosourea-induced DNA single-strand breaks.

Reaction of DNA with nitrosoureas in vitro results in extensive formation of alkali labile sites. Two types of single-strand scission (SSS) processes may be distinguished by their different rates: (1) type I SSS which occurs relatively fast at high pH, and (2) type II SSS which is a much slower process. Neither of these processes is affected by free radical traps. Dimethyl sulfate, which is known to alkylate DNA bases but not phosphate residues, shows no type I SSS but does show extensive type II SSS. That the latter process involves alkylation of bases followed by the formation of apurinic sites was confirmed by using endonuclease VI, an enzyme specific for apurinic positions. Reactions of chloroethylnitrosoureas with DNA produces both type I and type II SSS. Aliphatic amines produced in the decomposition of alkyl nitrosoureas do not contribute significantly to the scission of apurinic sites via Schiff base formation. However, this process may be significant for aryl nitrosoureas. Ethyl nitrosourea (ENU), 1, 3-bis(2-chloroethyl)nitrosourea (BCNU), and 3-cyclohexyl-1-(2-hydroxyethyl)-1-nitrosourea (CHNU) readily degrade poly A by phosphate alkylation, with rates that parallel their relative rates of decomposition. The relative rates of hydrolysis of triethylphosphate and β-hydroxyethyl diethyl phosphate parallel the type I SSS observed for ENU and CHNU with DNA. The type I SSS of DNA by these compounds appears to involve a similar phosphotriester formation and hydrolysis. The type I SSS is in accord with the observed extreme liability of β-hydroxyethyl diethyl phosphate which is attributed to participation of the OH group, and by the fact that methylation of the OH completely inhibits the type I SSS process.