Rat liver type I iodothyronine deiodinase is not identical to protein disulfide isomerase

This study was done to test the recent hypothesis (Boado et al. (1988) Biochem. Biophys. Res. Commun. 155, 1297-1304) that type I iodothyronine deiodinase (ID-I) is identical to protein disulfide isomerase (PDI). Autoradiograms of rat liver microsomal proteins, labeled with N-bromoacetyl-[125I]triiodothyronine (BrAc[125I]T3) and separated by SDS-PAGE, show predominantly 2 radioactive bands of Mr 27 and 56 kDa. Substrates and inhibitors of ID-I inhibited labeling of the 27 kDa band but not that of the 56 kDa band. Treatment of microsomes with trypsin abolished labeling of the 27 kDa protein and destroyed the activity of ID-I but did not prevent labeling of the 56 kDa protein. Following treatment of microsomes at pH 8.0-9.5 or with 0.05% deoxycholate (DOC) PDI content and labeling of the 56 kDa protein were strongly diminished but ID-I activity and labeling of the 27 kDa protein were not affected. The latter decreased in parallel after treatment at pH greater than or equal to 10. Rat pancreas microsomes contain high amounts of PDI but show no ID-I activity. Reaction of these microsomes with BrAc[125I]T3 results in extensive labeling of a 56 kDa protein but no labeling of a 27 kDa protein. Pure PDI (Mr 56 kDa) was readily labeled by BrAc[125I]T3 but showed no deiodinase activity. These results strongly suggest that the 27 kDa band represents (a subunit of) ID-I while the 56 kDa band represents PDI. From these and other data it is concluded that PDI and ID-I are not identical proteins.