REEXAMINATION OF HORMONE-BINDING PROPERTIES OF PROTEIN DISULFIDE-ISOMERASE

Protein disulfide-isomerase (PDI), an abundant multifunctional protein , has been described as a 3,3',5-triiodo-L-thyronine (T-3)-binding pro tein. As pointed out by several authors, the physiological significanc e of this hormone-binding property has not been fully addressed. To cl arify this point, we have analyzed the T-3-binding properties of purif ied PDI. At equilibrium, T-3 binds PDI at two binding sites: first, at a high-affinity site with a K-d of 21 nM and a B-max of 1.8x10(-3) mo l T-3/mol PDI monomer, and second at a very low affinity site that is unsaturated up to 100 mu M T-3. Thus, T-3 binding is mainly non-specif ic and the specific part represents only about 0.2% of the protein mon omer. Cross-linking experiments at a concentration where mainly specif ic binding occurs indicate that PDI does not bind L-T-3 exclusively; a wide variety of analogs are also bound. Refolding of reduced denature d ribonuclease A by PDI is inhibited by T-3 and analogs, and the inhib ition profile reflects the binding properties very closely. Since puri fied PDI displays neither the specificity expected for a physiological receptor, nor significant T-3-binding activity, results are discussed in terms of a necessary PDI association with another component to for m a T-3 receptor.