Iodine content and density of thyroglobulin investigated by isopycnic centrifugation

The density of in vitro iodinated thyroglobulin (Tgb) molecules, measured by isopycnic centrifugation, increases linearly with their iodine content. In contrast, the iodine to density (I:D) regression for differently iodinated native rat Tgbs is not constant but varies, depending on experimental conditions: For LID-fed rats the slope of the individual I:D regression line of each Tgb decreases linearly with the decreasing mean degree of iodination of the sample (r = 0.9104, P less than 0.01). On the other hand, in Tgb samples synthesized by propylthiouracil-blocked glands, the I:D regression is no longer linear. Rather, a bimodal pattern is obtained revealing a preexisting highly iodinated Tgb pool together with newly synthesized, non-iodinated protein. The results indicate: (a) the mean degree of iodination of a native Tgb cannot be estimated accurately if only its mean density is known, and (b) the degree of heterogeneity of iodination is identical for any in vivo produced Tgb sample with the same iodine content.