CDNA-EXPRESSED HUMAN CYTOCHROME-P450 ISOZYMES - INACTIVATION BY PORPHYRINOGENIC XENOBIOTICS

A number of xenobiotics are known to exert their porphyrinogenic effec ts in rodents and chick embryos through mechanism-based inactivation o f certain cytochrome P450 (P450) isozymes. To facilitate the extrapola tion of results from test animals to humans, we have assessed the abil ity of three prototype porphyrinogenic compounds-namely, ycarbonyl-1,4 -dihydro-2,6-dimethyl-4-ethylpyridine (DDEP), -(2,4,6-trimethylphenyl) thioethyl]-4-methylsydnone (TTMS), and allylisopropylacetamide (AIA)-t o cause mechanism-based inactivation of cDNA-expressed human P450s 1A1 , 1A2, 2C9-Arg(144) (2C9), 2D6-Val(374) (2D6), and 3A4 in microsomes f rom human lymphoblastoid cell lines (Gentest Corp., Woburn, MA). The f ollowing catalytic markers of human P450 isozymes were used: ethoxyres orufin O-deethylase (P450s 1A1 and 1A2), diclofenac 4-hydroxylation (P 4502C9), dextromethorphan O-demethylase (P4502D6), and testosterone 6 beta-hydroxylation (P4503A4). We found that DDEP and TTMS caused mecha nism-based inactivation of cDNA-expressed human P450s 1A1, 1A2, and 3A 4, whereas only DDEP was able to cause mechanism-based inactivation of cDNA-expressed human P4502C9; neither xenobiotic caused mechanism-bas ed inactivation of cDNA-expressed human P4502D6. A comparison of the h uman P450 isozyme data with results previously obtained in rat and chi ck embryo liver showed a close correspondence between the results obta ined with P450s 1A and 3A, but not the P4502C subfamily. Because sever al rat isozymes (P450s 2A1, 2B1, 2C6, 2C11, and 3A1) undergo inactivat ion by AIA, it was noteworthy that AIA did not inactivate any of the c DNA-expressed human P450 isozymes. Because mechanism-based inactivatio n of P450 isozymes is related to the porphyrinogenicity of xenobiotics , our results demonstrate the importance of supplementing studies of m echanism-based inactivation of P450 isozymes in animal models with sim ilar studies on cDNA-expressed human P450 isozymes.