Study on the enzymatic 5'-deiodination of 3',5'-diiodothyronine using a radioimmunoassay for 3'-iodothyronine.

A radioimmunoassay for 3'-iodothyronine has been developed. All iodothyronine analogues (except 3,3'-diiodothyronine) showed very litte (0.02% at most) cross-reactivity, and the assay was sensitive to 1 pg 3'-iodothyronine/tube. We have studied the 5'-deiodination of 3'-5'-diiodothyronine by rat liver microsomal fraction in the presence of dithiothreitol. Production of 3'-iodothyronine at 37 degrees C was found to be linear with time of incubation up to 30 min and with concentration of microsomal protein up to 100 microgram/ml. The reaction rate reached a limit on increasing 3',5'-diiodothyronine concentration to 10 microM. The effect of pH on 3'-iodothyronine production was found to depend on 3',5'-diiodothyronine concentration. Increasing 3,5'-diiodothyronine concentration from 0.1 to 10 microM resulted in a shjift of the pH optimum from 6-6.5 to 7.5. Similar effects on the 5'-deiodination of 3,3',5'-triiodothyronine were observed, supporting the hypothesis that these reactions are catalysed by a single enzyme (iodothyronine 5'-deiodinase).