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CYP3A is the major cytochrome P450 subfamily constitutively expressed in the human liver. CYP3A4 is the predominant hepatic P450 form in adults and it is expressed at high but very variable levels among individuals. The fetal liver contains mainly CYP3A7, while the presence of the other CYP3A enzymes in fetal liver has remained controversial. In this study, the relative levels of CYP3A4, CYP3A5 and CYP3A7 expression were determined in a panel of 9–11 fetal livers with a similar gestation age (9–12 weeks) and compared to adult livers. CYP3A7 was found to be the major CYP3A form in all the fetal liver samples. The abundance of CYP3A7 varied more at the mRNA (77-fold variation) than at the protein level (4.8-fold variation). CYP3A5 mRNA was also detected in all of the fetal liver samples, but the average level was 700-fold lower than that of CYP3A7. CYP3A5 protein was detected by immunoblot analysis in only 1 fetal liver out of the 9 investigated, the level of expression being moderately high in this sample. CYP3A4 mRNA was detected in only a subset of the fetal liver samples and its level was the lowest of the CYP3A forms. This is the first study to demonstrate the polymorphic expression of CYP3A5 and the variability of CYP3A7 expression in fetal liver and suggests that significant interindividual differences in the metabolism of xenobiotics may already exist at the prenatal stage. These differences may contribute to individual pharmacological and/or toxicological responses in the fetus.

The carcinogenesis index of subcutaneous benzo[a]pyrene-initiated fibrosarcomas in the genetically ‘responsive’ C3H inbred mouse strain is more than five times higher, and about 15 times higher, than that in the ‘responsive’ C57BL/6 and the ‘nonresponsive’ DBA/2 strains, respectively. Carcinogenesis indices involving F1 hybrids of these strains indicate that additional genes besides the Ah locus may cause a particular inbred strain to be more sensitive, or resistant, to benzo[a]pyrene-initiated tumors than would be expected solely on the basis of aryl hydrocarbon (benzo[a]pyrene) hydroxylase (EC 1.14.14.2) inducibility. The DNA-bound benzo[a]pyrene metabolite complexes generated by mouse liver or skin microsomes in vitro can be resolved into at least nine distinct peaks by elution of a Sephadex LH-20 column with a water-methanol gradient. Eight peaks, shown previously to be associated with increased hepatic cytochrome Pi-450 content, are greater with liver microsomes from genetically responsive C3H and C57BL/6 inbred strains and the (C57BL/6)(C3H)F1, (C3H)(DBA/2)F1 and (C57BL/6)(DBA/2)Fι hybrids than from the genetically nonresponsive DBA/2 inbred strain. All nine peaks are greater with skin microsomes in vitro from C3H and C57BL/6 than from DBA/2 mice. Benzo[a]pyrene mutagenicity in vitro with Salmonella typhimurium tester strain TA98 is increased 5- to 6-fold with liver microsomes from C3H and C57BL/6 inbred mice and the three F1 hybrids mentioned above, compared with liver microsomes from DBA/2 mice. Similar genetic differences in benzo[a]pyrene mutagenesis with the bacterial tester strain TA100 are also seen. DNA ‘repair’ – as measured by the rate at which DNA-bound benzo[a]ρyrene 4,5-oxide and the 7,8-diol-9,10-epoxides are removed from mouse skinDNA – is not significantly different between C3H and C57BL/6 mice. The K-region oxide appears to be removed from DNA nucleosides, however, at least twice as rapidly as the 7,8-diol-9,10-epoxides. Our data thus demonstrate a strikingly good correlation between genetically determined increases in peaks representing Pι-450-catalyzed benzo[a]pyrene metabolites bound to DNA and benzo[a]pyrene mutagenesis in vitro. Neither of these in vitro parameters, nor DNA repair in mouse skin in vivo, however, explains the 5- to 6-fold difference in benzo[a]pyrene carcinogenesis index between the two genetically responsive strains, C3H and C57BL/6.

The binding of reactive benzo[a]pyrene metabolites to deproteinized DNA in vitro can be drastically changed, both quantitatively and qualitatively, in vitro by changes in the substrate concentration or the substrate/P-450 ratio, and in the intact animal by starvation or substitution of a ‘purified protein test diet’ for the regular laboratory chow. Liver, lung, and bowel microsomes from C57BL/6N and DBA/2N mice were examined. These data demonstrate the importance of dietary contaminants or nutrition during benzo[a]pyrene tumorigenesis. The profile of DNA binding of benzo[a]pyrene metabolites is also shown to be an extremely sensitive test for detecting minute amounts of induced cytochrome P1-450 and its associated aryl hydrocarbon (benzo[a]pyrene) hydroxylase (EC 1.14.14.2) activity. A direct correlation is not necessarily observed, however, between ‘aryl hydrocarbon hydroxylase activity’ and the amount of benzo[a]pyrene metabolites bound covalently to DNA. ‘Untreated’ genetically responsive mice are shown to have greater ‘control’ levels of benzo[a]pyrene metabolism in their various tissues than genetically nonresponsive mice simply on the basis that responsive mice have a lower threshold for ‘responsiveness’ to exogenous inducers inhaled and/or ingested in their crude diet, i.e., responsive ‘control’ animals already have partially induced enzymes.