Effect of zinc(II) and other divalent cations on binding of 3,5,3'-triiodo-L-thyronine to nuclear receptors from cultured GC cells

The effect of Zn(II) in 3,5,3'-triiodo-L-thyronine (T₃) binding to nuclear receptors was studied in dialyzed 0.4 M NaCl extracts of nuclei from cultured GC cells. Addition of ZnCl₂ to nuclear extracts resulted in a time- and concentration-dependent dissociation of T₃ from nuclear receptors. Half-maximal dissociation occurred at 6 μM ZnCl₂. Addition of ZnCl₂ also resulted in a concentration-dependent inhibition of binding of T₃ to nuclear receptors. Half-maximal inhibition of binding occurred at 1-3 μM ZnCl₂. Scatchard analysis indicated that Zn(II) addition decreased k(A) and did not alter receptor concentration. These effects of Zn(II) were prevented when ZnCl₂ was added to nuclear extracts in the presence of 5 mM EDTA or 5 mM dithiothreitol. Moreover, Zn(II)-induced inhibition of T₃ binding was reversed by the addition of 5 mM EDTA. The inhibitory effect of Zn(II) on T₃ binding seemed specific for nuclear receptors; no effect of Zn(II) on the binding of T₃ to proteins in rat serum or GC cell cytosol or to rabbit anti-T₃ serum was observed. Cd(II) had a similar concentration-dependent inhibition of T₃ binding to nuclear receptors which was reversible. Our findings suggest that Zn(II) may play a role in T₃ binding to nuclear receptors as well as its putative role in the binding of receptor to DNA.