Thyroglobulin and 27-S ioprotein. Iodination and ultracentrifugal heterogeneity

Abstract

The time course of iodination of individual thyroid iodoproteins has been investigated by “equilibrium” labeling in rats and “pulse” labeling in rats and man. Although the “equilibrium” experiments demonstrated little difference between the rate of incorporation of iodine into these poteins after two days, pulse label experiments at earlier times indicated that thyroglubulin (19 S) was labeled at a faster rate than the 27-S iodoprotein. The 27-S protein, which contains more iodine, becomes more highly labeled than thyroglobulin during “equilibrium” labeling. Radiochromatography after hydrolysis revealed no significant difference in the relative distribution of iodoamino acids in the 19-S and 27-S proteins at varied times after labeling. Ultracentrifugal heterogeneity of thyroglobulin has been demonstrated by density-gradient centrifugation. A slower-sedimenting fraction containing less iodine was separated from a more highly iodinated, faster-sedimenting one. A correlation seemed also to exist between the chromatographic behavior of thyroglobulin on DEAE-cellulose and the sedimentation properties of thyroglobulin. The faster- sedimenting molecules were more strongly retained on DEAE-cellulose, and labeling in vivo showed a positive correlation between the time of iodination and the sedimentation rate. It is concluded that the faster-sedimenting thyroglobulin molecules are more dense, contain more iodine, are more negatively charged, and are iodinated later than the slower-sedimenting ones.