Removal of alkylation products from the DNA of mammalian cells was studied in nondividing human lymphocytes damaged with 7-bromomethylbenz(a)anthracene,

Removal of alkylation products from the DNA of mammalian cells was studied in nondividing human lymphocytes damaged with 7-bromomethylbenz(a)anthracene, a carcinogenic alkylating agent that produces chemically stable products upon reaction with DNA. The reaction of 7-bromomethylbenz(a)anthracene-3 H with cells in lymphocyte culture was complete in about 20 mm. The unlabeled compound induced unscheduled (repair) DNA synthesis in lymphocytes maintained in hydroxyurea, and this process was complete in about I 2 hr. The DNA of cells treated with the radioactive bromocompound (1 pM) exhibited a 15 to 17% loss of radioactivity in I 2 hr, while exposure to a higher concentration of the agent (20 jiM) resulted in an 8 to 9% loss of radioactivity. Salmon sperm DNA alkylated by this agent exhibited no detectable loss of radioactivity after heating for 22 hr at 50°. The major sites of attack of 7-bromomethylbenz(a)anthracene on lymphocyte DNA were the amino groups of guanine and adenine. Analysis of DNA isolated from treated lymphocytes at zero time and I 2 hr later suggested that the damaged adenine nucleotides were removed more extensively than were the damaged guanine nucleotides and that participation in the cell cycle is not a precondition for removal of bound carcinogen.