Limited capacity for the removal of O6-methylguanine and its regeneration in a human lymphoma line

Abstract

The Raji human lymphoma line is able to remove O ⁶ -methylguanine (O ⁶ MeG) lesions introduced by treatment of cells with N-methyl-N'-nitro-N-nitroso-guanidine (MNNG). The reaction has a rapid phase in which ∼ 40% of the O ⁶ MeG is removed in the first 10 min. The capacity of cells for rapid O ⁶ MeG removal is limited and is saturated at concentrations of MNNG which do not saturate the systems removing 3-methyladenine. Pretreatment of cells with MNNG inhibits their ability to remove O ⁶ MeG produced by a subsequent dose given after 2 h. Treatment with N-ethyl-N'-nitro-N-nitrosoguanidine (ENNG) is effective in diminishing cellular capacity for O ⁶ MeG removal, and cells unable to remove O ⁶ MeG and sensitive to the cytotoxic effects of MNNG are also more sensitive to ENNG than their removal competent counterparts. Regeneration of the ability to remove O ⁶ MeG requires incubation of cells for periods > 24 h. The O ⁶ MeG removal system is similar to that found in adapted Escherichia coli although the capacity of the Raji lymphoma line is much lower than that of the induced bacteria per unit of DNA.