Radioligand-dependent differences in the molecular properties of the mouse and rat hepatic aryl hydrocarbon receptor complexes

A comparison of the molecular properties of the male Long-Evans rat and male C57BL/6 mouse hepatic cytosolic aryl hydrocarbon (Ah) receptor complex was determined using 2,3,7,8-3H]tetrachlorodibenzo-p-dioxin (TCDD) and 2,3,7,8-3H]tetrachlorodibenzofuran (TCDF) as radioligands. In low salt buffer, the sedimentation coefficients, Stokes radii, relative molecular masses, frictional ratios, axial ratios and gel permeation chromatographic properties of the rat receptor complexes were ligand independent. In contrast, there were several ligand-dependent differences in the mouse Ah receptor complexes formed after incubation in low salt buffer and these include: sucrose density gradient analysis of the 2,3,7,8-3H]TCDF receptor complex gave a 9.5 S specifically bound peak and a 2.6 S nonspecifically bound peak whereas the corresponding 2,3,7,8-3H]TCDD receptor complex gave a single 9.6 S specifically bound peak; sucrose density gradient analysis of the two major peaks eluted from a Sephacryl S-300 column chromatographic separation of the 2,3,7,8-3H]TCDF receptor complex gave two specifically bound peaks at 9.2 and 5.1 S. The molecular properties of the rat hepatic cytosolic receptor complexes incubated in high salt (0.4 M KCl) buffer were ligand independent with one exception, namely the significant difference in the sedimentation coefficient of the specifically bound disaggregated 2,3,7,8-3H]TCDD receptor complex (6.8 S) and the corresponding 2,3,7,8-3H]TCDF receptor complex (5.0 S). The major ligand-dependent differences in the mouse receptor complexes incubated in high salt (0.4 M KCl) were associated with the sedimentation coefficients of the complexes derived after direct incubation and after gel permeation chromatography. For example, both ligands gave two specifically bound complexes after chromatography on Sephacryl S-300 column and centrifugation of these fractions gave both the approximately 9 and approximately 5 S peaks; this suggested that there was some equilibration between the aggregated and disaggregated receptor complexes. The behavior of the 2,3,7,8-3H]TCDF mouse receptor complex was similar after incubation in low or high salt buffer except that sucrose density gradient analysis of the gel permeation chromatographic fractions gave an additional specifically bound peak which sedimented at 7.2 S. These studies demonstrate that the molecular properties of the Ah receptor were dependent on the source of the cytosolic receptor preparation, the ionic strength of the incubation media, and the structure of the radioligand.