In the present study the hypothesis was tested that 
N-bromoacetyl-3,3',5-[125I]triiodothyronine (BrAc[125I]T3) is a useful affinity 
label for both type I and type III iodothyronine deiodinases (ID-I and ID-III). 
Therefore, the microsomal fractions of various rat tissues were tested for ID-I 
and ID-III activities, and microsomal proteins were labeled with BrAc[125I]T3 
and analyzed by SDS-PAGE. In agreement with previous observations, high ID-I 
activities were found in liver, kidney and thyroid, and high ID-III activities 
in brain, in particular fetal brain, and placenta. SDS-PAGE of 
BrAc[125I]T3-labeled microsomes showed a prominent radioactive approximately 27 
kDa protein (p27) in liver, kidney and thyroid, which was previously identified 
as ID-I, and a approximately 32 kDa protein (p32) in brain, in particular fetal 
brain, and placenta. A good correlation was found between the affinity labeling 
of p32 and the inactivation of ID-III by BrAcT3, suggesting that p32 represents 
ID-III or a subunit thereof. After treatment of microsomes with 0.05% 
deoxycholate or carbonate buffer (pH 11.5) p32 was still labeled by 
BrAc[125I]T3, indicating that p32 is a transmembrane protein. Although 
3,3',5'-triiodothyronine (rT3) is not a substrate for ID-III, p32 was readily 
labeled with BrAc[125I]rT3. Labeling of p32 in rat brain microsomes by 
BrAc[125I]rT3 was not affected by addition of 100 microM unlabeled thyroxine 
(T4) or T3, whereas deiodination of [125I]T3 by ID-III was inhibited by 91 and 
96% in the presence of 1 microM T4 and T3, respectively.(ABSTRACT TRUNCATED AT 
250 WORDS)