CHARACTERIZATION OF THE BINDING OF 3,3'-DI-IODO-L-THYRONINE TO RAT-LIVER MITOCHONDRIA

The binding of labelled 3,3'-di-iodo-L-thyronine (3,3'-T-2) to isolate d rat liver mitochondria has been characterized. Specific binding coul d be detected only in the inner mitochondrial membrane, not in other m itochondrial subfractions. The composition of the incubation medium in fluenced the binding capacity, the best combination of high specific b inding and low non-specific binding being observed in phosphate buffer , pH 6.4. The specific binding of 3,3'-T-2 to mitochondria requires lo w ionic strength: concentrations of K+ and Na+ higher than 10 mmol/l a nd 0.1 mmol/l respectively resulted in a decreased binding capacity. T he optimal calcium ion concentration was in the range 0.01-1.0 mmol/l. Varying magnesium ion, over the range of concentrations used (0.1-100 mmol/l), had no effect. Both ADP and ATP, at over 1 mmol/l, resulted in an inhibition of the specific binding. Incubation with protease res ulted in a decrease in specific binding and an increase in non-specifi c binding, thus indicating the proteic nature of the binding sites. In addition to the above factors in the local environment the thyroid st ate of the animal might influence the 3,3'-T-2-binding capacity. In fa ct, the thyroid state of the animal seemed not to have an influence on the affinity constant, but it did affect binding capacity.