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).
D000818:Animals; D000918:Antibody Specificity; D004104:Diiodothyronines / Q000378:metabolism*; D007453:Iodide Peroxidase / Q000378:metabolism*; D008862:Microsomes, Liver / Q000201:enzymology; D007470:Monoiodotyrosine / Q000276:immunology*; D010544:Peroxidases / Q000378:metabolism*; D011817:Rabbits; D011863:Radioimmunoassay / Q000379:methods*; D051381:Rats; D013970:Thyronines / Q000378:metabolism*; D014285:Triiodothyronine, Reverse / Q000378:metabolism
D004104:Diiodothyronines; D013970:Thyronines; D014285:Triiodothyronine, Reverse; D010544:Peroxidases; D007453:Iodide Peroxidase; D007470:Monoiodotyrosine