Expression and activities of several drug-metabolizing enzymes in LLC-PK1 cells

LLC-PK1 cells are frequently used in toxicology research, but little information is available concerning the capacity of these cells to metabolize xenobiotics. We examined the expression and activities of cytochromes P450 (P450) 1A1/1A2 (CYP 1A1/1A2), 2E1 (CYP 2E1), flavin monooxygenase (FMO), 5-lipoxygenase (5-LO) and prostaglandin H synthase (PHS)-associated cyclooxygenase-1 (COX-1). We prepared S9 fractions from LLC-PK1 cells, rat liver, and rat kidney, and measured enzyme activities using ethoxyresorufin O-deethylation (EROD) for CYP 1A1/1A2 and ethoxycoumarin O-deethylation (ECOD) for CYP 2E1, benzydamine N-oxidation (BNO) for FMO, leukotriene B₄ (LTB₄) formation for 5-LO, and thromboxane B₂ (TXB₂) formation for COX-1 activities. To assure that product formation was due to enzymatic activity, we used the following inhibitors: 1-aminobenzotriazole (ABT) for P450, methimazole for FMO, caffeic acid for 5-LO and acetylsalicylic acid (ASA) for COX-1. We also performed Western blot analysis to confirm our observations. All five enzyme activities were demonstrable in rat liver at much greater levels than in rat kidney S9 fractions. Activities in LLC-PK1 cells were significantly lower than activities in rat liver S9 fraction and generally less than activities in rat kidney S9 fraction. Enzyme inhibitors decreased product formation in all three tissues and Western blot analysis supported our observations of low enzyme activity in LLC-PK1 cells. These results indicate that LLC-PK1 cells have very low content of relevant drug-metabolizing enzyme activities.