Specific binding sites for 3,3'-diiodo-L-thyronine (3,3'-T2) in rat liver mitochondria

Specific binding sites for 3,3'-T2 can be detected in swollen and osmotically treated mitochondria (OTM) from normal and hypothyroid rat liver. In hypothyroid animals, maximal values of binding were obtained at 0 degrees C while values were lower at 37 degrees C and no specific binding could be observed at 60 degrees C. Binding was maximal at pH 6.4 and the mean values for the apparent association constant (Ka) and the binding capacity were on average 0.5 x 10(8) M-1 and 2.4 pmol/mg mitochondrial protein, respectively. No differences were observed between normal and hypothyroid rats with the exception of the capacity that was higher in normal animals (5.5 pmol/mg mitochondrial protein). The specificity of 3,3'-T2 binding, examined in competition studies, followed this order: 3,3'-T2 > 3,5-T2 > rT3. The other iodothyronines (3',5'-T2, T3, T4, 3-T1, 3'-T1, 3,5-Diac and 3,3'-Diac) showed only a small competition or none at all.