27S THYROID IODOPROTEIN. ISOLATION AND PROPERTIES.

thyroglobulin prepared by salting-out methods, of an ultracentrifugal component sedimenting faster than thyroglobulin (i.e. more than 198) has been known for some years (1, 2). Even though not proven, it has been suggested that this component is an iodoprotein (3). It was postulated to be “an aggregated or otherwise altered form of thyroglobulin itself” (4) or an artifact produced during the preparation of thyroglobulin (5). According to Ui and Tarutani (6), however, the so-called F (fast) component is not an artifact and might play a physiologically important role in the thyroid gland. Actually, the nature and significance of the faster sedimenting thyroidal component was unknown, since all prior attempts to isolate this protein had been unsuccessful (3). The purification of the faster sedimenting thyroidal protein has now been achieved by means of two techniques recently used (7) for the preparation of highly purified thyroglobulin: filtration through granulated agar gel and ultracentrifugation in a linear density gradient. Both techniques were designed to isolate thyroid proteins with discrete size and shape characteristics. The purification as well as some molecular and chemical properties of this newly isolated protein are reported in this paper.