Studies on the mechanism of iodination by the thyroid gland: Iodide-activating enzyme and an intracellular inhibitor of iodination

Abstract

1. 1. The organic iodine formation by pig thyroid cell-free preparations was investigated, by using the mitochondria and microsome system (“T” system) and the system with enzymatic H 2O 2 generation (“A” system). 2. 2. Iodination activity was found in mitochondria and microsome fractions, and the higher activity resided in microsome fraction in the case of “A” system. 3. 3. “T” system iodinated proteins and lipids of mitochondria and microsomes, and also a minute quantity of added proteins. On the other hand, “A” system iodinated added amino acids (mono- and diiodotyrosine and monoiodohistidine), proteins such as albumin, and proteins and lipids which were contained in the particulate fraction. 4. 4. Iodinated lipids formed in both the systems were identified as nonphosphatide lipid, phosphatidylserine, and sphyngomyelin; others remained unidentified. 5. 5. Similarity of the “iodide-activating enzyme” in the “T” system with that in the “A” system is indicated by the effects of KCN, NaN 3, methylthiouracil, and NaF. 6. 6. Iodinating activity of the “T” system is suggested to be a complex activity which consists of an iodide-activating system and probably a H 2O 2 generating system. 7. 7. In the formation of organic iodine compounds, the activity of the “A” system was not replaced by catalase and peroxidase, nor could H 2O 2 itself be substituted for the enzymatic system generating it. 8. 8. In the soluble fraction, a potent inhibitor of the iodination reaction was found. It was identified as ascorbic acid.