Activation of the food-derived mutagen 2-amino-3-methylimidazo[4, 5-f]quinoline by human-liver microsomes

Localization of cytochromes P450 in human tissues: implications for chemical toxicity

Cytochromes P450 comprise a remarkably diverse superfamily of heme-thiolate proteins critical in the metabolism of numerous endogenous ligands and xenobiotics. Among the myriad of P450 substrates are many compounds of toxicological and pharmacological significance. The precise complement of cytochrome P450 isoforms in any given tissue may therefore be an important determinant of susceptibility to chemical-mediated toxicity. We have used a histological approach to study the distribution of individual P450s in human and rabbit gastro-intestinal tissues. We have focused primarily on P450 enzymes of importance in the metabolism of carcinogens, namely CYP1A1, CYP1A2, CYP2E1, CYP3A4/3A5 and CYP4B1. Here we give an overview of the distribution of these enzymes in human and rabbit tissues and discuss the possible toxicological implications of the results. In addition we will discuss the value of archival human tissue specimens for histological analysis of P450 distribution.

Extrapolation of heterocyclic amine carcinogenesis data from rodents and nonhuman primates to humans

Twenty different heterocyclic amines have been isolated and identified from cooked foods especially beef, fish, pork and fowl. Other HCAs have also been isolated but their structure remains to be elucidated and new HCAs are likely to be identified in the future. The HCAs are highly mutagenic and all ten HCAs that have been tested for carcinogenic activity, produce tumors in mice and rats. For humans the average daily intake of HCAs is in quantities of 10-20 mg/person/day. The HCAs are procarcinogens and are activated by the cytochrome P450 system especially CYP 1A2. Rodents, monkeys and humans have the capacity to activate HCAs. Studies using hepatic microsomes demonstrated that humans have a greater capacity to activate the majority of HCAs tested than rodents or cynomolgus monkeys. Three HCAs are currently under evaluation in nonhuman primates for carcinogenic activity and one, IQ, is highly carcinogenic inducing primary hepatocellular carcinomas in the majority of cynomolgus monkeys treated. Epidemiological studies, although not definitive, are supportive of an association of HCAs intake to the etiology of human cancer. Risk assessments from animal data show a risk of HCAs to humans in the range of 10(-3) to 10(-4) which is an order of magnitude greater than compounds currently regulated by the U.S. Food and Drug Administration or the Environmental Protection Agency. Taken together evidence from mutagenicity data, activation by various species including humans, carcinogenicity in animals, human consumption data, epidemiological studies and risk assessment, supports the conclusion that HCAs are probable human carcinogens.

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.

Metabolic differences in colon mucosal cells

The colonic expression of cytochromes P450 from the CYP1A, CYP3A and CYP4B subfamilies has been characterized in rabbit and human tissues using RNA blotting, immunoblotting, immunohistochemistry and hybridization histochemistry. These studies demonstrate negligible expression of the CYP1A subfamily in either rabbit or human colon. The CYP3A6 gene is expressed in rabbit colon although at markedly reduced levels relative to liver and small intestine. Whilst at least two CYP3A genes are expressed at the mRNA level in human colon tissue from some individuals, no expression was demonstrated in others. Where expression was observed, this expression was continuous throughout the length of the colon. In rabbits, CYP4B1 represents a major colon P450 enzyme, expressed at levels in colon comparable to liver and small intestine. In contrast, the human CYP4B1 gene is expressed at low levels in some individuals. These studies highlight individual differences in the expression of cytochrome P450 enzymes of importance in procarcinogen metabolism.

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.

Micronucleus induction in bone-marrow cells following consumption of cooked beef in mice. Preliminary investigations

The bone-marrow micronucleus assay was used to investigate whether the consumption of cooked meat could induce chromosome damage. There was no difference in the micronucleus frequency of mice on normal diet (cereal-based, non-purified diet) and mice on normal diet supplemented with microwaved meat. However, supplementing the normal diet with well-done pan-fried meat or rare charcoal-barbecued meat or well-done charcoal-barbecued meat produced significant increments in the micronucleus frequency of polychromatic erythrocytes--the increments were of the order of 73% (p = 0.045), 90% (p = 0.047) and 136% (p = 0.001) respectively and they were observed after a 21-day feeding trial. These results suggest that ingestion of well-done pan-fried or barbecued meat may increase genetic damage, however, the accompanying decreased intake of vegetable constituents may have also contributed to the observed changes.