Determination of the CYP1A-inducing potential of single substances, mixtures and extracts of samples in the micro-EROD assay with H4IIE cells.

This protocol describes a quantitative and robust 96-well-plate-reader-based assay for the measurement of ethoxyresorufin-O-deethylase (EROD) activity using the rat hepatoma cell line H4IIE. The assay can be used to determine the cytochrome P450 subfamily 1A (CYP1A)-inducing potential of single substances, as well as of mixtures and extracts of samples. It is based on the aryl hydrocarbon receptor (AhR)-mediated induction of cytochrome P450 enzymes (subfamily 1A) in cells after exposure to dioxins and dioxin-like compounds. One enzymatic reaction catalyzed by CYP1A is the deethylation of the exogenous substrate 7-ethoxyresorufin to the fluorescent product resorufin, which is measured as EROD activity in the assay. The CYP1A-inducing potential of a sample can be reliably quantified by comparing the EROD activity with the concentration-response curve of the standard substance 2,3,7,8-tetrachlorodibenzo-p-dioxin, which can be detected at concentrations down to the picogram per liter range. A researcher familiar with the procedure can process up to 160 samples with four wells each within 3 d. The series described uses four plates with three concentrations per sample, which can be easily scaled to accommodate different sample sizes.