Previous work has shown that member(s) of the cytochrome P450IIC sub-family play significant roles in the formation of diols of 7,12-dimethylbenz[a]anthracene (DMBA) and are particularly important in formation of the proximate carcinogen (DMBA-3,4-diol). To further characterize the role of members of this subfamily in DMBA-diol formation and to assess the part played by other P450s, DMBA metabolism has been investigated in microsomes prepared from animals pre-treated with isoenzyme selective inducers. The rates of formation of DMBA-diols in membranes from phenobarbital-treated rats were very low when NADH was used as reductant and rates were not altered when NADPH and NADH were used in combination rather than using NADPH alone. This suggests that cytochrome b5 is not involved in DMBA-diol formation in these membranes. Treatment of animals with clofibrate, pyrazole and dexamethasone produced regio-selective alterations in the rates of formation of DMBA-diols at the -3,4-, -5,6- and -8,9- positions. However, none of the inducers caused increases in the rates of DMBA-diol formation of any great magnitude suggesting that the isoforms which are the major induced proteins (P450IVA1, P450IIE1 and P450IIIA1) do not play a significant role in diol formation. The content of other P450s in these membrane are also altered and these were investigated by Western blot using antibodies to P450IIC6, P450IIB1 and P450IIIA1. The results of the Western blots show that the effects of the inducing agents on DMBA-diol formation can be explained by alterations of members of the P450IIC and P450IIB subfamilies.

The role of specific cytochrome P-450 isoenzymes in the regio-selective metabolism of 7,12-dimethylbenz[a]anthracene (DMBA) has been studied in microsomal membranes from rat and human liver. An antibody inhibition study using membranes from phenobarbital-treated rats demonstrates that a member(s) of the CYP2C family accounts for up to 90% of the formation of the proximate carcinogen, DMBA-3,4-diol, and makes significant contributions to the formation of DMBA-5,6-diol and DMBA-8,9-diol. In these membranes the formation of DMBA-5,6-diol can be entirely accounted by the combined activity of members of the CYP2C and CYP2B families. The metabolism of DMBA has been investigated in human using microsomes from 10 individuals and the metabolites formed by these membranes were found to be mainly hydroxymethyl- and -diol products. The rates of formation of each metabolite show considerable interindividual variation and there was no correlation between these rates for any pairing of metabolites. The CYP content in these membranes of specific members of families 1, 2, 3 and 4 did correlate with the rates of formation of individual metabolites. Surprisingly there was no correlation between the content of CYP2C and formation of DMBA-3,4-diol but an antibody to rat CYP2C6 partially inhibited the formation of this metabolite. The results indicate that in human both inducible sub-families of CYPs, particularly of the PB-type, and constitutively expressed CYPs may be important in DMBA metabolism and that each metabolite may be produced by the combined activity of several CYP isoforms.

Infection of mice with Leishmania donovani resulted in decreased activities of several liver enzymes involved in the metabolism of xenobiotics. Microsomal membranes from infected livers contained reduced amounts of cytochromes P450 and b5 and NADPH-cytochrome P450 reductase. Several cytochrome P450 isoenzymes (P450-PB1, P450-PB3, P450-PCN and P450-UT1) and P450-mediated reactions (aminopyrine demethylase, aniline hydroxylase, benzphentamine demethylase and ethoxycoumarin deethylase) were affected similarly. The metabolism of two carcinogens (nitrosodimethylamine and 7,12-dimethylbenz[a]anthracene) by liver microsomal membrane preparations was also reduced. Leishmania infection caused an increase of cytosolic epoxide hydrolase and microsomal epoxide hydrolase and NADH-cytochrome b5 reductase were unaffected. The results suggest that Leishmania-infected animals are likely to have altered responses to exogenous toxins compared to uninfected animals.

Conference Article| December 01 1989 Metabolic activation of 7,12-dimethylbenz(a)anthracene: role of cytochrome P-450 isoenzymes in the formation of DNA and protein adducts in vitro M. CUSACK; M. CUSACK *Department of Biological Sciences, Glasgow College, Glasgow G4 0BA, U.K. Search for other works by this author on: This Site PubMed Google Scholar A. K. BURNETT; A. K. BURNETT †Leukaemia Research Fund Laboratories, Glasgow Royal Infirmary, Glasgow G4 0SF, U.K. Search for other works by this author on: This Site PubMed Google Scholar V. M. MORRISON; V. M. MORRISON *Department of Biological Sciences, Glasgow College, Glasgow G4 0BA, U.K. Search for other works by this author on: This Site PubMed Google Scholar J. A. CRAFT J. A. CRAFT *Department of Biological Sciences, Glasgow College, Glasgow G4 0BA, U.K. Search for other works by this author on: This Site PubMed Google Scholar Biochem Soc Trans (1989) 17 (6): 1014–1015. https://doi.org/10.1042/bst0171014 Article history Received: June 19 1989 Views Icon Views Article contents Figures & tables Video Audio Supplementary Data Peer Review Share Icon Share Facebook Twitter LinkedIn MailTo Cite Icon Cite Get Permissions Citation M. CUSACK, A. K. BURNETT, V. M. MORRISON, J. A. CRAFT; Metabolic activation of 7,12-dimethylbenz(a)anthracene: role of cytochrome P-450 isoenzymes in the formation of DNA and protein adducts in vitro. Biochem Soc Trans 1 December 1989; 17 (6): 1014–1015. doi: https://doi.org/10.1042/bst0171014 Download citation file: Ris (Zotero) Reference Manager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentAll JournalsBiochemical Society Transactions Search Advanced Search Keywords: P-450, cytochrome P-450, DMBA, 7,12-dimethylbenz[a]anthracene, DO, 3,4-diol-1,2-oxide This content is only available as a PDF. © 1989 Biochemical Society1989 Article PDF first page preview Close Modal You do not currently have access to this content.