Phosphotriester formation by the haloethylnitrosoureas and repair of these lesions byE.coliBS21 extracts

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Abstract
The alkylation of phosphates in DNA by therapeutically active haloethylnitrosoureas was studied by reacting N-chloroethyl-N-nitrosourea (CNU) with dTpdT, separating the products by HPLC, and identifying them by cochromatography with authentic markers. Both hydroxyethyl and chloroethyl phosphotriesters of dTpdT were identified; a similar reaction between CNU and dTR yielded 3-hydroxyethyl and 3-chloroethyl dTR as the major products of ring alkylation. A DNA-like substrate for repair studies was synthesized by reacting 14C-labelled N-(2-chloroethyl)-N′-cyclohexyl-N-nitrosourea (14C-CCNU) with poly dT and annealing the product to poly dA. An extract of E. coll strain BS21 selectively transferred a chloroethyl group from one of the chloroethyl phosphotriester isomers in this substrate to the bacterial protein; chemical Instability of the hydroxyethyl phosphotriesters precluded definite conclusions about the repair of this product.