Differential induction of indirect DNA breaks but no inhibition of strand-break ligation of alkylated DNA by 3-aminobenzamide in human and C3H 10T1/2 cells.

Methyl methanesulfonate (MMS; 0.5 mM, 1 h) induced the same frequency of single strand breaks (SSBs) (2.1-2.3 X 10(-8) SSBs per dalton) in NHSF6 normal human and C3H 10T1/2 mouse cells. The rate of rejoining without 3-aminobenzamide (3AB), detected by alkaline sucrose sedimentation, was rapid, with respective half-lives of 2.5 and 1.8 h for NHSF6 and 10T1/2. 3AB (5 mM) neither affected this ligation rate nor induced indirect SSBs in NHSF6 cells. In contrast, 5 mM 3AB did produce indirect SSBs through inhibition of poly(adenosine diphosphoribosyl)ation in unalkylated 10T1/2 cells in such a manner that the SSB frequency increased quickly to a maximum of 1.0 X 10(-8) SSB/dalton in 3 to 6 h and decreased thereafter. In MMS-alkylated 10T1/2 cells, 5 mM 3AB produced a complex DNA breakage/repair curve created by the time-dependent dynamic balance between the repair of MMS-induced SSBs and the 3AB-dependent production of additional SSBs. The ligation curve calibrated for such extra SSBs also revealed no delaying effect of 3AB on the rate of ligation of MMS-induced SSBs in 10T1/2 cells. Further, both induction of indirect SSBs and inhibition of de novo purine synthesis by 3AB may be associated with the 3AB-hypersensitive property of 10T1/2 cells, since there were no such side effects in NHSF6 cells.