Characterization of inhibitory effects of perfluorooctane sulfonate on human hepatic cytochrome P450 isoenzymes: Focusing on CYP2A6

Perfluorooctane sulfonate (PFOS) is a chemically stable compound extensively used as oil and water repellent, surface active agents in our daily life. Accumulative research evidence gradually appears the toxicity of PFOS against mammals, but the whole figure remains to be elucidated. The present study was conducted to know the effects of PFOS on human hepatic drug metabolizing-type cytochrome P450 (CYP) isoenzymes such as CYP1A2 (7-ethoxyresorufin as a substrate), CYP2A6 (coumarin), CYP2B6 (7-ethoxy-4-trifluoromethylcoumarin), CYP2C8 (paclitaxel), CYP2C9 (diclofenac), CYP2C19 (S-mephenytoin), CYP2D6 (bufuralol), CYP2E1 (chlorzoxazone) and CYP3A4 (testosterone) in human livers employing their typical substrates. Although all of the oxidation reactions tested were more or less inhibited by PFOS, diclofenac 4′-hydroxylation mediated mainly by CYP2C9 was most strongly inhibited (Ki value of 40 nM), followed by paclitaxel 6α-hydroxylation mediated mainly by CYP2C8 (Ki value of 4 μM). The substrate oxidation reactions catalyzed by CYP2A6, CYP2B6, CYP2C19 and CYP3A4 were moderately (Ki values of 35 to 45 μM), and those by CYP1A2, CYP2D6 and CYP2E1 were weakly inhibited by PFOS (Ki values of 190–300 μM). The inhibition by PFOS for coumarin 7-hydroxylation mainly catalyzed by human liver microsomal CYP2A6 as well as by the recombinant enzyme was found to be enhanced by the preincubation of PFOS with human liver microsomes and NADPH as compared to the case without preincubation. The inhibition of the human liver microsomal cumarin 7-hydroxylation was PFOS concentration-dependent, and exhibited pseudo-first-order kinetics with respect to preincubation time, yielding Kinact and KI values of 0.06 min−1 and 23 μM, respectively. These results suggest that the metabolism of medicines which are substrates for CYP2C9 may be altered by PFOS in human bodies, and that PFOS is a mechanism-based inhibitor of CYP2A6.

 Type of inhibition Competitive: CYP2C9, 3A4 Noncompetitive: CYP1A2, 2C8, 2E1 Mixed: CYP2A6, 2B6, 2C19, 2D6Extent of inhibitionHuman CYP2C9 (Ki = 40 nM) ≫ 2C8 > 2C19 ≧ 2A6 ≧ 2B6 ≧ 3A4 ≫ 2D6 ≧ 2E1 > 1A2 (Ki = 303 μM).A possible mechanism-based inhibition of CYP2A6 Preincubation of PFOS with human liver microsomes in the presence of NADPH caused Inhibition of CYP2A6-catalyzed coumarin 7-hydroxylation in PFOS concentration- and time-dependent manners (Kinact 0.057 min−1; KI 23 μM).Figure optionsDownload as PowerPoint slideHighlights
► We examined in vitro effects of PFOS on human hepatic microsomal drug metabolism-type CYPs.
► All of nine CYP enzymes examined were more or less inhibited by PFOS.
► The extent of inhibition was: CYP2C9 (Ki = 40 nM) ≫ 2C8 > 2C19 ≧ 2A6 ≧ 2B6 ≧ 3A4 ≫ 2D6 ≧ 2E1 > 1A2 (Ki = 300 μM).
► PFOS caused a possible mechanism-based inactivation of CYP2A6 (Kinact = 0.06 min−1, KI = 23 μM).
► These results are the first findings of strong inhibitory effects of PFOS on human CYP enzymes.