Dose- and cell cycle-dependent O6-methylguanine elimination from DNA in regenerating rat liver after [14C]dimethylnitrosamine injection.

To study relations between increased transformation sensi tivity during DMA synthesis and carcinogen-induced molecular DMA alterations in regenerating liver, the kinetics of formation and the persistence of alkylated DMA bases were determined 10 to 240 min after a single [14C]dimethylnitrosamine (DMN) injection (1.43 and 4.0 mg/kg) during G, (12 h after partial hepatectomy) and in synchronized S phase (4 hr after contin uous infusion of hydroxyurea after partial hepatectomy; about 80% hepatocytes in DMA synthesis). At 120 min after administration of DMN, the molar fraction of 7-methylguanine reached a maximum which was proportional to the injected DMN dose in G t but lower by about 25% in Sphase cells. During G,, the molar fraction of O6-methylguanine revealed the same formation kinetics as 7-methylguanine; but in S-phase cells, it was reduced by 60% after 1.43 mg/kg and by 35% after 4.0 mg/kg. 3-Methyladenine reached its maxi mum earlier in S phase than in Gì, with a faster decline in S than in Gt. As soon as 10 min after DMN was given, the O6-methylguanine/7-methylguanine ratio was significantly lower in S-phase cells than in Gìcells, with a rapid further decrease during 30 min shortly after carcinogen exposure during DNA synthesis. This rapid repair was less expressed after 4.0 mg/kg, indicat ing an exhaustion of this process after higher carcinogen doses. Cell cycle-dependent differences of the kinetics of O6-methylguanine and 3-methyladenine in DNA in vivo confirm earlier observations in vitro that these adducts are released by differ ent enzymatic mechanisms the activity of which appears to be enhanced during DNA synthesis in rat liver (Pegg ef a/., Biochem. J., 797. 195-201,1981 ; Gombaref a/., Carcinogenesis, 2. 595-599, 1981). The faster elimination of the promutagenic O6-methylguanine during DNA synthesis could represent an important but, be cause of its saturation at higher carcinogen dosages, not sufficiently effective mechanism to protect proliferating cells from initiation during the most critical period of the cell cycle.