Function and localization of cytochromes P450 involved in the metabolic activation of food-derived heterocyclic amines.
Using a variety of approaches including purified forms of cytochrome P450, cDNA expression systems, hybridization histochemistry, immunochemical techniques and metabolic inhibition studies, we have demonstrated that CYP1A proteins are expressed in rabbit and human liver and are primarily responsible for the activation of heterocyclic amines by this tissue in both species. However, in the human gastrointestinal tract which represents the major portal of entry for heterocyclic amines into the body, CYP1A proteins are negligibly expressed. CYP1A1 is constitutively expressed in the rabbit small intestine. In contrast to the CYP1A subfamily, expression of the CYP3A subfamily was demonstrated in both the rabbit and human gastrointestinal tract. In both species, CYP3A expression was highest in the duodenum and declined down the remainder of the gastrointestinal tract. CYP3A expression could be demonstrated in some human colons using hybridization histochemistry and RNA blotting. Members of the human CYP3A subfamily (CYP3A4 and CYP3A5) were capable of activating the heterocyclic amine, 2-amino-3,4-dimethylimadazo[4,5,f]quinoline (MeIQ), to a mutagen in the Ames test in the presence of the modulating flavonoid, alpha-napthoflavone. In addition, Rifampicin-induced rabbit liver microsomes which are enriched for CYP3A6, show enhanced activation of MeIQ in the presence of alpha-napthoflavone. In contrast to the CYP1A and CYP3A subfamilies, CYP4B1 expression is easily demonstrated in rabbit colon using immunochemical and RNA detection techniques. Moreover, the activation of 2-aminofluorene was comparable in microsomes from the small and large intestine of rabbits. Only low and variable expression of CYP4B1 was observed in human colon at the mRNA level. From these results, it is clear that CYP1A, CYP3A and CYP4B1 expression is negligible in human colon. The possibility exists that heterocyclic amines are first activated in the liver and are transported to the colon as either hydroxylamines or glucuronides, where they undergo further activation by beta-glucuronidases, sulfotransferases and N-acetyltransferases.
McKinnon
R A
RA
Department of Physiology and Pharmacology, University of Queensland, Australia.
McManus
M E
ME
D000123:Acetyltransferases / Q000378:metabolism; D000588:Amines / Q000378:metabolism*; D000818:Animals; D001711:Biotransformation; D002273:Carcinogens / Q000378:metabolism*; D003106:Colon / Q000201:enzymology; D003577:Cytochrome P-450 Enzyme System / Q000378:metabolism*; D004790:Enzyme Induction; D005449:Fluorenes / Q000378:metabolism; D005966:Glucuronidase / Q000378:metabolism; D006571:Heterocyclic Compounds / Q000378:metabolism*; D006801:Humans; D007421:Intestine, Small / Q000201:enzymology; D007527:Isoenzymes / Q000378:metabolism*; D008862:Microsomes, Liver / Q000201:enzymology*; D009153:Mutagens / Q000378:metabolism*; D011804:Quinolines / Q000378:metabolism; D011817:Rabbits; D015238:Sulfotransferases / Q000378:metabolism
D000588:Amines; D002273:Carcinogens; D005449:Fluorenes; D006571:Heterocyclic Compounds; D007527:Isoenzymes; D009153:Mutagens; D011804:Quinolines; C012177:2-aminofluorene; D003577:Cytochrome P-450 Enzyme System; D000123:Acetyltransferases; D015238:Sulfotransferases; D005966:Glucuronidase; C036989:2-amino-3,4-dimethylimidazo(4,5-f)quinoline