Isolation and properties of thyroglobulin-related 4S protein in the soluble fraction of thyroid tissue.

A 4S protein which is thyroglobulin-related and which is immunologically distinguished from unusually iodinated albumin-like proteins or impaired substances of thyroglobulin synthesis, was found in the thyroid-soluble fraction even in the normal gland, though in minute amounts. In the present study, purification and characterization of the substance has been carried out using human and hog thyroid glands. This protein contains some of the immunospecific determinants of thyroglobulin, although the amino acid composition of the protein differs from that of thyroglobulin. The 4S protein has a molecular weight of about 58,500 as determined by the sedimentation equilibrium method. Interestingly, the present 4S protein is eluted just in front of, or together with, thyroglobulin by gradient elution chromatography on a DEAE-cellulose column, and it is precipitated with thyroglobulin at 1.9 M ammonium sulfate. Therefore, it was concluded that a molecular-sieving process is necessary for the purification of thyroglobulin, in addition to a complex procedure which consists of ammonium sulfate fractionation and DEAE-cellulose chromatography.