Jr. Petrulis et al., Application of the ethoxyresorufin-O-deethylase (EROD) assay to mixtures of halogenated aromatic compounds, ENVIRON TOX, 16(2), 2001, pp. 177-184
The ethoxyresorufin-O-deethylase (EROD) assay monitors the induction of the
xenobiotic-metabolizing enzyme cytochrome P-450 (CYP) 1A1 and is a widely
used biomarker for exposure of wildlife to substances that bind the aryl hy
drocarbon (Ah) receptor. in this work the induction of EROD activity by sin
gle compounds and binary mixtures in primary rat hepatocytes was compared w
ith the predictions of a kinetic model involving mixtures of inducers. The
inducing agents were also examined for their ability to activate the Ah rec
eptor to its DNA-binding form and for their ability to act as competitive i
nhibitors for CYP 1A1. Xenobiotics that failed to activate the Ah receptor
did not induce EROD activity. Competitive inhibition for CYP 1A1 between th
e xenobiotic and 7-ethoxyresorufin caused EROD activity to fall at high xen
obiotic concentrations. Competition for a limited number of Ah receptor sit
es depressed the EROD activity of a strong inducer such as 2,3,7,8-tetrachl
orodibenzo-p-dioxin at high concentrations of a weak inducer. Application o
f the kinetic model to the example of a mixture of low concentrations of di
benzo-p-dioxins and much higher concentrations of polychlorinated biphenyls
indicated that EROD assays often seriously underestimate the true potency
of an environmental sample. Hence the EROD assay cannot be used for determi
ning dioxin equivalent concentrations using the toxic equivalence factor ap
proach. (C) 2001 by John Wiley & Sons, Inc.