Competitive inhibition by inducer as a confounding factor in the use of the ethoxyresorufin-O-deethylase (EROD) assay to estimate exposure to dioxin-like compounds

The ethoxyresorufin-O-deethylase (EROD) assay has been extensively used in whole animals and in cell culture as a biomarker of exposure to environment al contaminants such as dioxin-like compounds (DLCs). This paper addresses two controversial phenomena that arise when DLCs are examined by the EROD a ssay. Firstly, the maximum level of induced EROD activity varies with the i dentity of the inducing compound; secondly, the induced EROD activity reach es a concentration-dependent maximum level that is followed by an apparent reduction in activity when the concentration of inducer is further increase d. These phenomena are completely explained by competitive inhibition of th e EROD enzyme-substrate reaction by the dioxin-like compound. A kinetic mod el explains the biphasic appearance of EROD induction curves as a function of a compound's binding affinity with the Ah receptor (K-d) and its binding affinity to CYP lAl (K-i) which results in inhibition of the EROD enzyme-s ubstrate reaction. These results limit the reliability of the information o btained from calibration curves of EROD activity versus concentration of a standard DLC such as 2,3,7,8-tetrachlorodibenzo-p-dioxin. (C) 1999 Publishe d by Elsevier Science Ireland Ltd. All rights reserved.