Effects of 1,3-bis(2-chloroethyl)-1-nitrosourea and related compounds upon the synthesis of DNA by cell-free systems.

Abstract

Summary Incubation of crude enzyme preparations from leukemia L1210 ascites cells with 1,3-bis(2-chloroethyl)-1-nitrosourea (BCNU) caused decreases in the DNA nucleotidyltransferase activity. 1-(2-Chloroethyl)-3-cyclohexyl-1-nitrosourea was as active as BCNU, but 1-(2-chloroethyl)-1-nitrosourea and 1-methyl-1-nitrosourea were much less active. 2-Chloroethyl isocyanate was about equal to BCNU in activity, and this fact is consistent with the possibility that a major portion of the observed inhibitory action of BCNU is due to the 2-chloroethyl isocyanate generated from it. Incubation of commercial DNA with BCNU did not significantly alter the activity of the DNA as a primer for the crude DNA nucleotidyltransferase system. Incubation of intact L1210 ascites cells with 2.5 × 10 -3 m BCNU caused a decrease in the synthesis of DNA by the cells and also a decrease in the DNA nucleotidyltransferase activity of the crude enzyme preparation obtained from these cells but did not alter the primer activity of the DNA isolated from the treated cells. With 10 -3 m BCNU, inhibition of the synthesis of DNA by the intact cells occurred without a decrease in the DNA nucleotidyltransferase activity. Treatment of mice bearing L1210 ascites cells or L1210 solid tumors with single doses (20 mg per kg) of BCNU caused decreases in the in vivo fixation of 3 H from deoxythymidine-(C 3 H 3 ) without causing decreases in the DNA nucleotidyltransferase activities of the crude preparations from these ascites cells or tumors. The priming activity of the DNA from ascites cells of treated animals was as great as that of DNA from ascites cells of untreated animals. These results indicate that, although under certain conditions BCNU can cause decreases in DNA nucleotidyltransferase activity, deactivation of this enzyme is probably not the cause for decreased synthesis of DNA in vivo .