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

The Raji human lymphoma line is able to remove O⁶-methylguanine(O⁶MeG) 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 theO⁶MeG is removed in the first 10 min. The capacity of cellsfor rapid O⁶MeG removal is limited and is saturated at concentrationsof MNNG which do not saturate the systems removing 3-methyladenine.Pretreatment of cells with MNNG inhibits their ability to removeO⁶MeG produced by a subsequent dose given after 2 h. Treatmentwith N-ethyl-N'-nitro-N-nitrosoguanidine (ENNG) is effectivein diminishing cellular capacity for O⁶MeG removal, and cellsunable to remove O⁶MeG and sensitive to the cytotoxic effectsof MNNG are also more sensitive to ENNG than their removal competentcounterparts. Regeneration of the ability to remove O⁶MeG requiresincubation of cells for periods > 24 h. The O⁶MeG removalsystem is similar to that found in adapted Escherichia colialthough the capacity of the Raji lymphoma line is much lowerthan that of the induced bacteria per unit of DNA.