Abstract

Porphyrinogenicity of certain xenobiotics depends upon mechanism-based inactivation of specific cytochrome P450 (P450) enzymes, followed by formation of N-alkylprotoporphyrin IX (N-alkylPP). Examination of the porphyrinogenicity of xenobiotics in animals and extrapolation of the results to humans is associated with ambiguity due, in part, to differences between P450 enzymes. The goal of this study was to develop an in vitro test for the detection of N-alkylPPs, produced in human liver after administration of xenobiotics found to be porphyrinogenic in animals. This goal was achieved using fluorometry to detectN-alkylPP formation following mechanism-based inactivation by porphyrinogenic xenobiotics of single cDNA-expressed human P450 enzymes in microsomes prepared from baculovirus-infected insect cells (Supersomes) and in human liver microsomes. The following combinations of P450 enzymes were major sources of N-alkylPPs in Supersomes: CYP3A4 [3-[(arylthio)-ethyl]sydnone (TTMS)]; CYP1A2 and 2C9 [3,5-diethoxycarbonyl-1,4-dihydro-2,6-dimethyl-4-ethylpyridine (4-ethyl DDC)]; and CYP2C9, 2D6, and 3A4 [allylisopropylacetamide (AIA)]. Whereas similarities were found between results with human enzymes in Supersomes and their rat orthologs in rat liver microsomes, some differences were found. The results with TTMS and AIA, but not with 4-ethyl DDC, were the same in individual human enzymes expressed in Supersomes and human liver microsomes. We conclude that some differences exist between human liver P450 enzymes and their rat P450 orthologs in liver microsomes. It would therefore be prudent when dealing with xenobiotics in which porphyrinogenicity depends uponN-alkylPP formation to supplement animal data with studies using human P450 enzymes.