Evidence for a reactive sulfhydryl in the DNA binding domain of the 1,25-dihydroxyvitamin D3 receptor

Evidence is presented here that organomercurial binding to a reactive sulfhydryl group is capable of altering the DNA-binding characteristics of the 1,25-dihydroxyvitamin D receptor (D-receptor). Accordingly, hormone-free receptor (R_o) binding to DNA-cellulose is inhibited in a concentration-dependent fashion with both HgCl₂ and p-chloromercuribenzene sulfonate (pCMBS) with complete inhibition evident at 1.0 mM. Further, low concentrations (0.5 mM) of mercurials are also capable of dissociating preformed DNA-receptor complexes, a process reversible with excess thiol reagent such as monothioglycerol. These findings are in contrast to alkylating reagents such as iodoacetamide, which is capable of only partially inhibiting the formation of the receptor-DNA duplex (37% at 25 mM). Once created, however, the duplex is completely insensitive to dissociation (even at 25 mM). These results imply that in addition to the association of a cysteine(s) moiety in or near the sterol binding site, modification of a similarly reactive group(s) can also alter the D-receptor's DNA-binding domain.