Hormone binding by protein disulfide isomerase, a high capacity hormone reservoir of the endoplasmic reticulum

Protein disulfide isomerase (PDI) is a folding assistant of the eukaryotic endoplasmic reticulum, but it also binds the hormones, estradiol, and 3,3', 5-triiodo-L-thyronine (T-3), Hormone binding could be at discrete hormone b inding sites, or it could be a nonphysiological consequence of binding site (s) that are involved in the interaction PDI with its peptide and protein s ubstrates, Equilibrium dialysis, fluorescent hydrophobic probe binding (4,4 '-dianilino-1,1 '-binaphthyl-5,5'-disulfonic acid (bis-ANS)), competition b inding, and enzyme activity assays reveal that the hormone binding sites ar e distinct from the peptide/protein binding sites. PDI has one estradiol bi nding site with modest affinity (2.1 +/- 0.5 muM). There are two binding si tes with comparable affinity for T-3 (4.3 +/- 1.4 muM), One of these overla ps the estradiol site, whereas the other binds the hydrophobic probe, bis-A NS, Neither estradiol nor T-3 inhibit the catalytic or chaperone activity o f PDI, Although the affinity of PDI for the hormones estradiol and T-3 is m odest, the high local concentration of PDI in the endoplasmic reticulum (>2 00 muM) would drive hormone binding and result in the association of a subs tantial fraction (>90%) of the hormones in the cell with PDI, High capacity , low affinity hormone sites may function to buffer hormone concentration i n the cell and allow tight, specific binding to the true receptor while pre serving a reasonable number of hormone molecules in the very small volume o f the cellular environment.