Some cetaceans bioaccumulate substantial concentrations of planar halogenated aromatic hydrocarbons (PHAHs) in their tissues, but little is known about the effects of such burdens on cetacean health. 2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) and related PHAHs cause toxicity via activation of the aryl hydrocarbon receptor (AHR), a member of the bHLH-PAS family of transcription factors. Differences in AHR structure and function are known to contribute to species-specific differences in susceptibility to PHAH toxicity. To ascertain the potential for PHAH effects in a cetacean, we characterized an AHR from the beluga whale, Delphinapterus leucas. The 3.2 kb cDNA encodes an 845-amino acid protein with a predicted size of 95.5 kDa. Overall, the beluga AHR shares 85% amino acid sequence identity with the human AHR and 75% identity with the mouse AHR Ah(b-1) allele. Beluga AHR protein synthesized in a rabbit reticulocyte lysate system demonstrated specific, high-affinity [(3)H]TCDD binding. Saturation binding analysis was used to compare the [(3)H]TCDD binding affinity of the in vitro-expressed beluga AHR with affinities of in vitro-expressed AHRs from a dioxin-sensitive mouse strain (Ah(b-1) allele) and humans. The beluga AHR bound [(3)H]TCDD with an affinity (K(d)= 0.43 +/- 0.16 nM) that was at least as high as that of the mouse AHR (K(d)= 0.68 +/- 0.23 nM), and significantly greater than that of the human AHR (K(d)= 1.63 +/- 0.64 nM). In electrophoretic mobility shift assays, the beluga AHR exhibited sequence-specific, Arnt-dependent binding to a dioxin responsive enhancer (DRE). Upon transient transfection into mammalian cells, the beluga AHR activated transcription of a luciferase reporter under control of a DRE-containing fragment of the mouse Cyp1a1 promoter. These results show that in an in vitro system, the beluga AHR possesses characteristics similar to those of AHRs from other mammals that are considered sensitive to toxic effects of PHAHs. Together, these results demonstrate that the use of in vitro-expressed proteins is a promising approach for addressing molecular and biochemical questions concerning PHAH toxicity in endangered or protected species.