[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 chlorine-tritium 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 (Ksub(d)=0.95 nM) than for [3H]-TCDD (Ksub(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. (author)