Stabilization of nitrogen-mustard alkylations and inter-strand crosslinks in DNA by alkali

Abstract

Inter-strand crosslinks in nitrogen mustard ( N-methyl-bis(2-chloroethyl)amine) reacted DNA have an increased stability at high pH. This increased stability correlates which a reduced rate of elimination of bound 14C-labeled nitrogen mustard residues from the DNA. Mild alkali treatment of nitrogen mustard reacted DNA causes an increased stability of bound nitrogen mustard and of inter-strand crosslinks with respect to subsequent heating at neutral pH. This is due to the alkali-induced hydrolysis of the imidazole ring of alkyl-guanine residues. A method is described for estimating the extent of alkylation at the N-7 position of guanine, based on the convertibility of [8- 14C] guanine to 14CO 2 after alkaline cleavage of the imidazole ring. The findings constitute independent evidence that the major site of DNA alkylation by nitrogen mustard is the N-7 position of guanine and that at least one such position is involved in inter-strand crosslinking.