A RADIOIMMUNOASSAY FOR TYPE I IODOTHYRONINE 5'- MONODEIODINASE IN HUMAN TISSUES

We have developed a sensitive, specific and reproducible 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 amminoacid synthetic peptide, corresponding to a portion of the carboxy-terminal region of the human 5'-DI (PI99). In a final dilution of 1:500, our anti-5'-DI antibody bound about 30-35% of a tracer amount of 125I-PI-99. The detection threshold of the RIA approximated 0.4 pmol PI99 or an equivalent amount of 0.4 pmol 5'-DI. The coefficient of variation averaged 5% within an assay and 14% between assays. Dose-response curves of tissue proteins were essentially parallel to that of PI99. In a total number of 35 normal human tissue samples, the mean (+SD, pmol/mg protein) 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 thyroid; it was 1.4+0.3 in Graves’ thyroid. Our data suggest that 1) 5’-DI is distributed widely among human tissues; 2) kidney is the tissue most enriched with 5'-DI; 3) 5'-DI content in the thyroid is not increased in Graves' disease.