SPECIFIC BINDING-SITES FOR 3,3'-DIIODO-L-THYRONINE (3,3'-T-2) IN RAT-LIVER MITOCHONDRIA

Specific binding sites for 3,3'-T-2 can be detected in swollen and osm otically treated mitochondria (OTM) from normal and hypothyroid rat li ver. In hypothyroid animals, maximal values of binding were obtained a t 0 degrees C and 20 degrees C while values were lower at 37 degrees C and no specific binding could be observed at 60 degrees C. Binding wa s maximal at pH 6.4 and the mean values for the apparent association c onstant (K-a) 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 mitocho ndrial protein). The specificity of 3,3'-T-2 binding, examined in comp etition studies, followed this order: 3,3'-T-2 > 3,5-T-2, > rT(3). The other iodothyronines (3',5'-T-2, T-3, T-4, 3-T-1, 3'-T-1, 3,5-Diac an d 3,3'-Diac) showed only a small competition or none at all.