THE PRESENCE OF FREE IODINATED COMPOUNDS IN THE THYROID AND THEIR PASSAGE INTO THE CIRCULATION

WHILE it is generally believed that the iodine of the thyroid is either bound in the thyroglobulin molecule or free as the iodide ion, recent evidence has indicated that other iodine containing substances are present in the gland. Thus, free thyroxine has been detected in extracts of unhydrolyzed thyroids (Leblond and Gross, 1949; Gross et al., 1950); free diiodotyrosine, monoiodotyrosine and several other, as yet unidentified, iodine compounds have also been found in the gland (Tishkoff et al., 1949; Gross et al., 1950). The thyroglobulin molecule contains these ammoacids bound in peptide linkage, as shown by Harington for thyroxine and diiodotyrosine (1933) and by Fink and Fink for monoiodotyrosine (1948).1 Therefore, it has been presumed that these free amino-acids are derived from thyroglobulin through the action of a proteolytic enzyme system such as the one demonstrated by de Robertis in the thyroid (1949). Furthermore, this pool of amino-acids appears to be the source of the plasma thyrozine,