Interspecies homology of cytochrome P-450: toxicological significance of cytochrome P-450 cross-reactive with anti-rat P-448-H antibodies in liver microsomes from dogs, monkeys and humans

Effect of some cooked food mutagens on unscheduled DNA synthesis in cultured precision-cut rat, mouse and human liver slices

Precision-cut liver slices were prepared from male Fischer 344 rats, female CDF1 mice and humans (both male and female subjects). Liver slices were cultured for 24 hr in medium containing [3H]thymidine and either PhIP, IQ, MeIQ, MeIQx, Glu-P-1 or Trp-P-1, and then processed for auto-radiographic evaluation of unscheduled DNA synthesis (UDS). All six cooked food mutagens examined produced concentration-dependent increases in UDS in human liver slices. PhIP was the most potent compound examined, followed by MeIQx, IQ and then MeIQ, Glu-P-1 and Trp-P-1. Significant increases in UDS were observed with PhIP, IQ and MeIQx at concentrations as low as 5 microM in the culture medium. The same rank order of potency was not apparent in either rat or mouse liver slices. In rat liver slices only MeIQ significantly induced UDS, although positive results were obtained with two other genotoxins, namely 2-acetylaminofluorene and aflatoxin B1. Apart from MeIQx, all the cooked food mutagens produced significant increases in UDS in mouse liver slices. This study demonstrates the usefulness of precision-cut liver slices to evaluate species differences in xenobiotic-induced genotoxicity. Both marked compound and species differences in induction of UDS were observed. The data provide further evidence that dietary cooked food mutagens are potential human carcinogens.

Regulation of CYP1A and CYP3A mRNAs by ascorbic acid in guinea pigs

In previous studies, we found that the ascorbic acid (AsA) deficiency caused changes in the amounts of the various forms of cytochrome P450 (P450) in liver microsomes from guinea pigs in a form-specific manner. Thus, the aim of this study was to clarify whether the changes seen in the protein contents of the various forms of P450 were associated with the levels of the expression of their mRNAs. Prior to determining the mRNA level, we isolated four cDNA clones, encoding CYP1A2, CYP3A14, CYP3A15, and CYP3A17, from guinea pig liver cDNA libraries to use them as probes in further experiments. The amino acid sequence of the guinea pig CYP1A2 showed identity ranging from 73 to 77% with those of other mammalian P450s. The amino acid sequences among guinea pig CYP3As had about 94% identities with each other. The AsA deficiency apparently decreased the expression of mRNA for CYP1A1 and CYP1A2. These results were in agreement with the decrease in the content of CYP1A1 and CYP1A2 proteins. The amount of P450 protein(s) immunochemically cross-reactive with antibodies to human CYP3A4 was likely unaffected while that of human CYP3A7 tended to be decreased by the AsA deficiency. It suggested that the expression of each CYP3A isozyme was regulated differently by AsA. In fact, the level of mRNA for CYP3A14 was unaffected by the AsA deficiency, while those for CYP3A15 and CYP3A17 were significantly decreased by the AsA deficiency, clearly indicating that the expression of each isozyme within the CYP3A subfamily is differently regulated by AsA. These results support the idea that the transcription of P450 is regulated by AsA in guinea pigs.

Possible occurrence of P450 related to P450 HFLb in extrahepatic tissues of human fetuses and its contribution to metabolic activation of promutagens

P450 HFLb purified from human fetal livers has been shown to be constitutively expressed in fetal livers. In the present study, the occurrence of proteins immunochemically related to P450 HFLb in extrahepatic tissues of human fetuses and their contribution to mutagenic activation of promutagens were investigated. The mutagenic activation of aflatoxin B1 (AFB1), 2-amino-3-methylimidazo[4,5-f]quinoline (IQ), 2-amino-3,4-dimethylimidazo[4,5-f]quinoline (MeIQ) and benzo[a]pyrene were observed in human fetal extrahepatic tissues, including adrenal glands, kidneys and lungs, at varying rates. Immunoblot analysis of homogenates of extrahepatic tissues with antibodies to P450 HFLb revealed the occurrence of proteins immunochemically related to P450 HFLb in adrenal glands, kidneys and lungs. Immuno-inhibition studies suggested that in fetal adrenal gland and kidney, the proteins cross-reactive with antibodies to P450 HFLb were capable of activating IQ and MeIQ to mutagens.

Food-borne heterocyclic amines. Chemistry, formation, occurrence and biological activities. A literature review

This review summarizes the abundant literature on food-borne heterocyclic amines, their chemistry and formation, their occurrence in food, their biological activities including mutagenicity, induction of DNA damage and carcinogenicity. Pharmacokinetics and biotransformation are also discussed. Factors that influence these effects are given consideration, with special emphasis on dietary factors that might counteract detrimental biological effects. The annual per capita intake of heterocyclic amines via food is estimated. Risk extrapolations that have been published suggest that food-borne heterocyclic amines are relevant for human cancer etiology.

Studies on the mutagenic activation of heterocyclic amines by cynomolgus monkey, rat and human microsomes show that cynomolgus monkeys have a low capacity to N-oxidize the quinoxaline-type heterocyclic amines

A number of mutagens and carcinogens have been isolated from cooked meats. In the current study we investigated the ability of hepatic microsomes from cynomolgus monkeys, Fischer-344 rats and humans to metabolically activate these compounds. Monkeys had almost no capacity to activate the quinoxaline-type compounds to mutagens in the Ames test relative to rats and humans but were able to activate the quinoline, pyridoindole and pyridoimidazole compounds. Differences in the mutagenicity of the quinoline and quinoxaline compounds by monkeys and rats was related to differences in cytochrome P-450-mediated N-oxidation between the species. This suggests that monkeys and rats may have different hepatic cytochrome P-450 isozymes, which are important for the metabolic activation of quinolines and quinoxalines, or that the orthologous monkey cytochromes show a select substrate specificity for the quinolines over the quinoxalines.

Evaluating the relationship of metabolic activation system concentrations and chemical dose concentrations for the Salmonella spiral and plate assays

A factorial experimental design was used within this study to evaluate the influence of multiple metabolic activation system concentrations on the dose-response exhibited by promutagens (indirect-acting mutagens) in the Salmonella spiral and plate assays. The mutagenic activity of the three compounds used spanned three orders of magnitude. The mutagenic activity of the compounds ranged from 10 to 100 revertants/micrograms for acetylaminofluorene (2AAF) to more than 1000 revertants/micrograms for 2-aminoanthracene (2AA). Benzo [a] pyrene (BaP) activity was within an intermediate range (100-1000 revertants/micrograms). During a single experiment, a mutagen was tested in TA100 at 13 doses plus a negative control dose. Each dose was tested at 10 S9 concentrations. The S9 concentrations ranged from 0.1 mg protein/plate to 4 mg protein/plate in the standard plate assay and from 0.25 to 4.90 mg-equivalents in the spiral assay. The spiral Salmonella assay, an automated version of the standard assay, generates dose-response data from a concentration gradient on a single agar plate, thereby providing a straightforward approach to this type of study. This study demonstrates not only that even small differences in S9 concentrations can affect the measurement of mutagenic potency but that S9/compound interactions cannot be generalized through the use of interaction studies. This study also shows that spiral assay data and plate assay data for promutagens cannot be compared directly unless the S9 concentrations for all chemical doses are also comparable.

Four forms of cytochrome P-450 in human fetal liver: purification and their capacity to activate promutagens

Four forms of cytochrome P-450 were separated and purified to electrophoretic homogeneity from human fetal livers. These forms of cytochrome P-450, termed P-450HFLa, P-450HFLb, P-450HFLc and P-450HFLd, were distinguishable from each other in their molecular weights, spectral properties, immunochemical properties and mutagen-producing activities from promutagens. The molecular weights of P-450HFLa, b, c and d were estimated to be 51,500, 49,000, 51,500 and 50,000, respectively. Antibodies to P-450HFLa recognized P-450HFLc but not P-450HFLb or d, and antibodies to rat P-448-H (P-450IA2) cross-reacted with P-450HFLb but not with other forms of cytochrome P-450. The N-terminal amino acid sequence of P-450HFLc was highly homologous, but not identical, to that of P-450HFLa. Each form of cytochrome P-450 catalyzed mutagenic activation of aflatoxin B1 (AFB1), 2-amino-3-methylimidazo[4,5-f]quinoline (IQ) and 2-amino-6-methyldipyrido-[1,2-a:3',2'-d]imidazole (Glu-P-1) at different rates. P-450 HFLa showed activities to produce mutagen(s) from AFB1, IQ and to a lesser extent from Glu-P-1. P-450 HFLb activated IQ at a faster rate than did the other forms. P-450 HFLc produced a mutagen from AFB1 and Glu-P-1 but not from IQ. P-450 HFLd did not activate these promutagens at significant rates.