Validation of in vitro bioassays for 2,3,7,8-TCDD equivalents

The inducibility of aryl hydrocarbon hydroxylase (AHH) and ethoxyresorufin O-deethylase (EROD) in rat hepatoma H-4-II E cells culture has been utilized as a quantitative bioassay for polychlorinated dibenzo-p-dioxins (PCDDs) and dibenzofurans (PCDFs). There was an excellent linear correlation between the -log EC₅₀ (in vitro enzyme induction) for 16 PCDD and PCDF congeners and their -log ED₅₀ values for in vivo toxicity (body weight loss and thymic atrophy) in the rat. There was also a comparable linear correlation between the in vitro and in vivo AHH/EROD induction activities. Extraction of a fly ash sample with methylene chloride followed by cleanup gave a PCDD/PCDF fraction which contained a complex mixture of isomers and congeners. The total PCDD and PCDF levels were 3.83 and 5.52 ug/g fly ash respectively and 2,3,7,8-TCDD constituted 2-3% of the total tetrachlorodibenzo-p-dioxins. Using the in vitro enzyme induction bioassay procedure, the estimated 2,3,7,8-TCDD (equivalents) in this extract were 105 ng/g fly ash. The doseresponse in vivo induction of AHH was also determined and the estimated 2,3,7,8-TCDD (equivalents) in this extract were 75 ng/kg fly ash and this confirms the excellent correlation between the in vivo and in vitro bioassays.