|Budget Amount *help
¥2,000,000 (Direct Cost: ¥2,000,000)
Fiscal Year 1992: ¥700,000 (Direct Cost: ¥700,000)
Fiscal Year 1991: ¥1,300,000 (Direct Cost: ¥1,300,000)
We showed recently that a membrasne-associated 3,5,3'-L-triiodothyronine (T_3) binding protein of 55 kDa (55k-MFP), a multifunctional protein, was identical with protein disulfide isomerase (PDI), which catalizes the formation and rearrangement of disulfide bonds in various proteins. In the present study, we showed that the recombinant 55k-MFP (r55k-MFP) and placenta 55k-MFP (p55k-MFP) purified from recombinant Escherichia coli transformed with human 55k-MFP cDNA expression plasmid and from normal human placenta, respectively have similar T_3-binding activity and PDI activity determined by refolding of scrambled pancreatic RNase. Furthermore, the PDI activity of both proteins was dose-dependently inhibited by T_3 in vitro. The inhibitory effect of iodothyronines was stereospecific (T_3=D-T_3>>L-thyroxine). A significant inhibition was induced by T_3 at the same molar concentration with the purified proteins, and the ED_<50>s of T_3 were about 15 times higher than that the concentrations of the proteins. The T_3 binding site in the molecule was identified to be the active site region expressing PDI activity by the sequence analysis of the peptide labeled with Bromoacetyl [^<125>I]T_3. These results suggest that the binding of T_3 to 55k-MFP in the target cells might regulate the formation or reconstruction of protein disulfide bonds, probably having a significant effect on thyroid hormone actions on target cells.