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 (T3)-binding protein. As pointed out by several authors, the physiological significance of this hormone-binding property has not been fully addressed. To clarify this point, we have analyzed the T3-binding properties of purified PDI. At equilibrium, T3 binds PDI at two binding sites: first, at a high-affinity site with a Kd of 21 nM and a Bmax of 1.8 x 10(-3) mol T3/mol PDI monomer, and second at a very low affinity site that is unsaturated up to 100 microM T3. Thus, T3 binding is mainly non-specific and the specific part represents only about 0.2% of the protein monomer. Cross-linking experiments at a concentration where mainly specific binding occurs indicate that PDI does not bind L-T3 exclusively; a wide variety of analogs are also bound. Refolding of reduced denatured ribonuclease A by PDI is inhibited by T3 and analogs, and the inhibition profile reflects the binding properties very closely. Since purified PDI displays neither the specificity expected for a physiological receptor, nor significant T3-binding activity, results are discussed in terms of a necessary PDI association with another component to form a T3 receptor.