SUPPRESSION OF CROSS-LINK FORMATION IN CHLOROETHYLNITROSOUREA-TREATED DNA BY AN ACTIVITY IN EXTRACTS OF HUMAN-LEUKEMIC LYMPHOBLASTS

Abstract

Pulse treatment of DNA with any of several chloroethylnitrosoureas [CNU, 1-(2-chloroethyl)-1-nitrosourea; BCNU, 1,3-bis(2-chloroethyl)-1-nitrosourea; CCNU, 1-(2-chloroethyl)-3-cyclohexyl-1-nitrosourea; MeCCNU, 1-trans-(2-chloroethyl)-3-(4-methylcyclohexyl)-1-nitrosourea] presently in clinical use [for cancer therapy] leads to formation of monoadducts. Further incubation of these monoadducts in the absence of drug leads to DNA interstrand cross-links; however, this cross-link formation is suppressed by a partially purified extract of cultured human leukemic lymphoblasts [CEM-CCRF cells]. The cross-link-suppressing activity copurifies with O6-methylguanine-DNA methyltransferase, shows similar kinetics for heat inactivation, and shows similar responses to inhibitors. Reactions for both the cross-link-suppressing and methyltransferase activities reach completion within 5 min at 37.degree., and both are stoichiometric rather than catalytic. The number of cross-links induced by the chloroethylnitrosoureas and, therefore, their cytotoxicity, should be inversely related to O6-methylguanine-DNA methyltransferase content of a cell.