Abilities of extracts of human lymphocytes to remove O6-methylguanine from DNA.

O6 MeGua is a presumptive mutagenic and carcinogenic product in DNAs treated with methylating agents. The abilities of lymphocyte extracts from 34 apparently normal individuals to remove O6 MeGua from exogenous DNA have been measured. The activity in extracts is stable to freezing and so permits repeat determinations and hence high precision in the assays. The data on removal are consistent with the idea that the removal is accomplished by the transfer of a methyl group to a methyl-accepting protein and that the protein acts in a stoichiometric fashion. Extracts from lymphocytes stimulated with PHA show on the average more activity than from unstimulated ones, although some extracts show no increase as a result of PHA stimulation of cells. There are large variations in the abilities of human lymphocytes to remove O6 MeGua, but the differences are not correlated significantly with sex or age. Unstimulated lymphocytes show a bimodal distribution of removing activity, whereas stimulated ones show a predominant single peak of activity. Extracts of T lymphocytes are more proficient than those of B lymphocytes and of any other white cells. On the average the number of presumptive acceptor molecules per cell in unstimulated lymphocytes is between 14 000 and 110 000 and in stimulated lymphocytes between 40 000 and 140 000.