The structure and properties of 27S and larger iodoproteins in the thyroid gland.

Iodoproteins larger than 19S were isolated from hog and calf thyroid glands using gel filtration on Sepharose 6B and one or two centrifugations in glycerol density gradients. Purified protein fractions were analysed in the analytical ultracentrifuge and characterized by the combined use of electrophoresis in continuous polyacrylamide gel gradients and electron microscopy. Three bands migrating more slowly than 19S could be identified in the polyacrylamide gels. Electron microscopy of the fastest of these species, having a sedimentation constant of 27S, showed pairs of 19S thyroglobulin molecules which had the same size and the same ovoid shape as normal, well-iodinated thyroglobulin molecules. The ovoids were randomly attached side to side, end to end. The more slowly migrating proteins were shown to consist of similar aggregates of three, four or more randomly attached molecules. Iodine and sialic acid determinations in 27S and 19S separated from the same pool of well iodinated protein showed no difference in iodine content but a larger amount of sialic acid in 27S than in 19S.