Formation of N-alkylprotoporphyrin IX after interaction of porphyrinogenic xenobiotics with rat liver microsomes.
Mechanism-based inactivation of selected cytochrome P450 (CYP) isozymes by xenobiotics can lead to N-alkylation of the heme moiety and the formation of ferrochelatase-inhibitory N-alkylprotoporphyrin IX (N-alkylPP), resulting in hepatic porphyrin accumulation and porphyria. Therefore, it is important to determine which of the CYP isozymes are responsible for N-alkylPP formation. Our objective was to determine if N-alkylPPs could be isolated from rat liver microsomes following interaction in vitro with porphyrinogenic xenobiotics, and if so, whether they are produced in amounts sufficient for further studies using microsomes containing cDNA-expressed human hepatic CYP isozymes. The in vitro formation of N-alkylPP was observed following incubation of 3-[(arylthio)ethyl]sydnone; allylisopropylacetamide; and 3, 5-diethoxycarbonyl-1,4-dihydro-2,6-dimethyl-4-ethylpyridine; but not 1-aminobenzotriazole, with rat hepatic microsomes. However the overall N-alkylPP yield per nmol CYP in vitro was much lower than previously observed in vivo. It was concluded that microsomal preparations containing cDNA-expressed CYP isozymes do not contain sufficient CYP for in vitro studies designed to isolate N-alkylPP and human liver microsomes would be a more appropriate source of N-alkylPP because they contain higher levels of total CYP.
Wong
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SG
Department of Pharmacology and Toxicology, Faculty of Health Sciences, Queen's University, K7L 3N6, Kingston, Ontario, Canada.
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D000818:Animals; D065607:Cytochrome P-450 Enzyme Inhibitors; D003577:Cytochrome P-450 Enzyme System / Q000378:metabolism; D004791:Enzyme Inhibitors / Q000378:metabolism; D006801:Humans; D007527:Isoenzymes / Q000037:antagonists & inhibitors / Q000378:metabolism; D008297:Male; D008862:Microsomes, Liver / Q000378:metabolism*; D011164:Porphyrias / Q000209:etiology / Q000378:metabolism; D011166:Porphyrins / Q000378:metabolism*; D011524:Protoporphyrins / Q000378:metabolism*; D051381:Rats; D017207:Rats, Sprague-Dawley; D015262:Xenobiotics / Q000378:metabolism*
D065607:Cytochrome P-450 Enzyme Inhibitors; D004791:Enzyme Inhibitors; D007527:Isoenzymes; D011166:Porphyrins; D011524:Protoporphyrins; D015262:Xenobiotics; C030827:N-ethylprotoporphyrin IX; D003577:Cytochrome P-450 Enzyme System