Measurement of cytochrome P450 activation of xenobiotics using the Ames Salmonella test

Characterisation of CYP3A gene subfamily expression in human gastrointestinal tissues

The human CYP3A subfamily is of interest due to its multiplicity, activity toward known carcinogens, and extrahepatic expression. In situ hybridisation analysis of formalin fixed, routinely processed biopsy specimens was used to localise CYP3A mRNA in human gastrointestinal tissues from several individuals. CYP3A mRNA is abundant in human liver and in mucosal epithelial cells of all segments of the human small intestine. RNA blot analyses showed that the mRNA species observed in most livers and in human small intestine represent CYP3A3/3A4 transcripts. This was confirmed at the protein level by immunoblot comparison of small intestine microsomes to in vitro expressed CYP3A4 and CYP3A5 proteins. In liver and small intestine, CYP3A mRNA is not uniformly distributed, with grain density highest in cells within the respective non-proliferative compartments. CYP3A mRNA was also observed in human oesophagus and colon. RNA blot analysis of multiple colons showed heterogeneity in the CYP3A mRNAs present. Two CYP3A mRNAs (CYP3A3/3A4 and CYP3A5) were detected in colon samples from several individuals. In addition to those localisation studies, the capacity of expressed CYP3A4 and CYP3A5 to activate the dietary heterocyclic amine MeIQ in the presence of alpha-naphthoflavone was shown. These results show that there is considerable heterogeneity in the expression of the CYP3A subfamily in human gastrointestinal tissues.

Caffeine metabolism by human hepatic cytochromes P450: contributions of 1A2, 2E1 and 3A isoforms

Caffeine (CA) N1-, N3- and N7-demethylase, CA 8-hydroxylase and phenacetin O-deethylase activities were measured in microsomes from 18 separate human livers which had been characterized previously for a range of cytochrome P450 (CYP) isoform-specific activities and immunoreactive CYP protein contents. Correlations between the high affinity components of the three separate CA N-demethylations were highly significant (r = 0.77-0.91, P < 0.001) and each of the three high affinity CA N-demethylations correlated significantly (r = 0.64-0.93, P < 0.05-0.001) with the high affinity phenacetin O-deethylase, 2-acetylaminofluorene N-hydroxylation and 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP) and 2-amino-3-methylimidazo[4,5-f]quinoline (IQ) mutagenicity (all predominantly CYP1A2-mediated reactions). Consistent with these observations, cDNA-expressed human CYP1A2 catalyzed the N1-, N3- and N7-demethylation of CA and apparent Km values were similar (0.24-0.28 mM) for all three reactions and comparable to those observed previously with human liver microsomes. The low affinity components of CA N1- and N7-demethylation correlated significantly (r = 0.55-0.85, P < 0.05-0.001) with immunoreactive CYP2E1 content and the CYP2E1-specific activities 4-nitrophenol and chlorzoxazone hydroxylation. Diethyldithiocarbamate, a selective inhibitor of CYP2E1, inhibited the low affinity CA N1- and N7-demethylation, with IC50 values of 23 microM and 11 microM, respectively. The apparent Km values for CA N1- and N7-demethylation by cDNA-expressed CYP2E1 (namely 28 and 43 mM, respectively) were of a similar order to those calculated for the low affinity microsomal activities. Significant correlations (r = 0.87-0.97, P < 0.001) were observed between CA 8-hydroxylation and immunoreactive CYP3A content and the CYP3A-mediated reactions benzo(a)pyrene hydroxylation, omeprazole sulfoxidation and aflatoxin B1 mutagenesis. Effects of alpha-naphthoflavone, erythromycin, troleandomycin and nifedipine on microsomal CA 8-hydroxylation were generally consistent with CYP3A involvement. Taken together with previous data, the results indicate a major involvement of CYP1A2 in the high affinity component of all three human hepatic CA N-demethylations. In contrast, CYP2E1 appears to be the main enzyme involved in the low affinity components of CA N1- and N7-demethylation while CA 8-hydroxylation is catalysed predominantly by a CYP3A isoform(s).