[³H]-2,3,7,8-Tetrachlorodibenzofuran (TCDF)-synthesis and characterization as a radioligand for the Ah receptor

2,6-Dichloroaniline was reacted with 2,5-dichlorophenol in the presence of isoamyl nitrite to give a mixture of chlorinated biphenylols which were purified by thin-layer chromatography. The resulting products were cyclized to give 1,4,9-trichlorodibenzofuran (purity > 98%) which was converted into 1,4,9-[³H₃]-dibenzofuran by catalytic chlorinetritium exchange. The [³H]-dibenzofuran was chlorinated in chloroform solution and the crude chlorination products were separated by preparative HPLC to give [³H]-2,3,7,8-TCDF, specific activity, 57 Ci/mmol. The binding of [³H]-2,3,7,8-TCDF to the cytosolic Ah receptor resembles that of [³H]-TCDD. The receptor binding affinity as determined from the Scatchard plot is slightly less for [³H]-TCDF (K_d=0.95 nm) than for [³H]-TCDD (K_d=0.60 nM). The mechanism of binding each radioligand to the Ah receptor is similar and occurs in the absence of detectable positive cooperativity. However, for quantification, the use of [³H]-TCDF is distinctly advantageous over [³H]-TCDD because the ease of dissociation of the [³H]-TCDF-Ah receptor complex is much greater than that of the [³H]-TCDD-Ah receptor complex. Because of the ease of reversibility of binding, [³H]-TCDF appears to be the superior radioligand for the in vitro characterization of the Ah receptor.