Big Blue@ (BB) and generic B6C3F1 mice were given one to three i.p. injections of 50 mg/kg benzo[a]pyrene (B[a]P) in DMSO every other day to achieve cumulative doses of 50 to 150 mg/kg. Three weeks after treatment, the mutation frequency at the endogenous hprt gene and lac/ transgene was measured in splenic T cells. Generic mice given 50, 100, and 150 mg/kg B[a]P displayed induced hprtfrequencies (observed hprf frequency minus control frequency) of 5.5 1 .O, 1 1 2 2.0, and 19 5 2.6 x 1 0-6, respectively (average 2 SEM).
In contrast, BB mice given 50 and 150 mg/kg B[a]P displayed induced hprf-frequencies of 0.9 T 0.6 and 9.1 5 1.5 x 1 OP. 32P postlabelling revealed that the lower hprt response in BB mice correlated with lower amounts of BP-DNA adducts in spleen, liver, and lung 24 hours after B[a]P exposure. Western blot onolysis of liver samples from B[a]P-treated mice suggests that the reduced adduct load in turn may be due to lower P450 1Al levels in BB mice.
The frequency of induced, nonsectored blue plaques (observed blue plaque frequency minus control frequency) in BB mice receiving 50 and 150 mg/kg B[a]P was 41 5 9 and 134 IT 10 x 1 0-6 (1 5to 40fold higher than the induced hprtfrequency in the same treated animals). Sectored plaques were observed in both control and B[a]P groups but their frequency showed no relationship to dose (sectored frequency in all groups was a p proximately 20 x 1 O-6). To test whether persistent DNA adducts in the packaged lambda vector were contributing to the observed blue plaque frequency, purified X-LIZ DNA was treated in vitro with B[a]P diol epoxide (BPDE), packaged, and plated on E. coli lawn cells. Treatment with BPDE did not produce significant increases in homogeneous blue plaques, suggesting that the majority of mutants ob tained from B[a]P-treated BB mice occurred in vivo.
These results indicate that B[a]P exposure produces many more mutations at the lac/ transgene than at the endogenous hprt locus.