A radioimmunoassay for type I iodothyronine 5 '-monodeiodinase in human tissues

We have developed a sensitive, specific and reproducible radioimmunoassay ( RIA) for measurement of human type I monodeiodinase (5 ' -DI) protein. Anti -5 ' -DI antibody was produced by immunization of rabbits with a conjugate of bovine serum albumin and a 16 amino acid synthetic peptide, correspondin g to a portion of the carboxy-terminal region of the human 5 ' -DI (PI-99). In a final dilution of 1:500, our anti-5 ' -DI antibody bound about 30%-35 % of a tracer amount of I-125-PI-99. The detection threshold of the RIA app roximated 0.4 pmol PI-99 or an equivalent amount of 0.4 pmol 5 ' -DI. The c oefficient of variation averaged 5%0 within an assay and 14% between assays . Dose-response curves of tissue proteins were essentially parallel to that of PI-99. In a total number of 35 normal human tissue samples, the mean (/- standard deviation [SD], picomole per milligram of protein [pmol]) 5 ' - DI content was 25 +/- 6.7 in kidney, it was significantly lower (p < 0.05) in liver at 3.9 +/- 1.1, 2.8 +/- 0.8 in intestine, 2.3 +/- 0.98 in adrenal, 4.2 +/- 2.5 in skeletal muscle, 3.8 +/- 1.4 in heart and 2.6 +/- 2.4 in th yroid; it was 1.4 +/- 0.3 in Graves' thyroid. Our data suggest that (1) 5<p rime>-DI is distributed widely among human tissues; (2) kidney is the tissu e most enriched with 5 ' -DI; (3) 5 ' -DI content in the thyroid is not inc reased in Graves' disease.