Assessment of biological activities of mixtures of polychlorinated dibenzo-p-dioxins: comparison between defined mixtures and their constituents.

As a first step to assess biological activities of complex mixtures of polychlorinated dibenzo-p-dioxins (PCDDs), induction of 7-ethoxyresorufin O-deethylase (EROD) by defined mixtures and their constituents has been analysed in vitro. Two cell systems have been compared: primary hepatocyte cultures and hepatoma H4IIE cells. EC50 values of PCDDs were compared with that of the most potent compound, 2,3,7,8-Cl4DD (2,3,7,8-tetrachlorodibenzo-p-dioxin) and expressed as 2,3,7,8-Cl4DD equivalents (TEs). TEs for three defined mixtures containing up to 49 PCDDs could be predicted from the sum of TEs for the 2,3,7,8-substituted congeners. Efficacies (maximal enzyme induction) of less potent PCDDs (1,2,3,4-Cl4DD, Cl8DD and of a mixture containing 86% Cl8DD and of benz(a)anthracene were lower in hepatocytes (by 33%) and in H4IIE cells (by 50%). The results suggest that biological activities of complex PCDD mixtures are largely due to additive effects of their 2,3,7,8-substituted constituents.