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 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 T 3 ‐binding properties of purified 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 1.8×10 −3 , mol T 3 /mol PDI monomer, and second at a very low affinity site that is unsaturated up to 100 μM T 3 . Thus, T 3 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 ‐T 3 exclusively; a wide variety of analogs are also bound. Refolding of reduced denatured ribonuclease A by PDI is inhibited by T 3 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 T 3 ‐binding activity, results are discussed in terms of a necessary PDI association with another component to form a T 3 receptor.