Activation of the iodinating system in sheep thyroid particulate fractions by flavin cofactors.

In an earlier report (l), we described the preparation, from sheep thyroid glands, of a particulate fraction consisting primarily of mitochondria and microsomes, which, when incubated with carrier-free radioactive iodide, converted as much as 30 per cent of the added 1131 to labeled iodoprotein. This thyroid particulate fraction differed from the cell-free thyroid preparations used by other workers (2,3) in the following respects: (1) it was active in the absence of added tyrosine and oxidizing agents; and (2) the major product formed was iodoprotein, not free iodotyrosine. In experiments in which the iodine metabolism of the particulate fraction was compared with that of thyroid slices, it was observed that the particulate preparation had a much lower capacity to utilize added carrier iodide; moreover, iodination by the particulate fraction led to the formation of an iodinated protein containing P31 almost exclusively in the form of monoiodotyrosine, whereas the iodinated protein formed by the slice contained almost as much P31-diiodotyrosine as 1131-monoiodotyrosine. The present report deals with our attempts to enhance the iodinating activity of the thyroid particulate fraction. It was found that the capacity of the particulate system to convert added iodide to iodoprotein could be greatly increased by adding flavin cofactors or by increasing the pH of the incubation medium, but in both instances the newly formed iodoprotein still contained 1131 almost exclusively in the form of monoiodotyrosine.