The identification of thyroglobulin within a rat thryroid subcellular fraction

A particulatte fraction obtained from a rat thyroid homogenate by centrifugation at 27 000 × g was compared to the soluble fraction used for the preparation of thyroglobulin. By sodium dodecyl sulfate polycrylamide gel electrophoresis analysis after reduction, the soluble fraction inculded, mainly, three high molecular weight componenents which became strongly radioactive after an in vivo radioiodine injection. The particulate fraction on the same gels after reduction inlcude several low molecular weight components and a single high molecular weight component containing little radioiodine and showing, by electron microscopy, at least three types of structure: rough surfaced and smooth surfaced vesicles and apical vesicles (the secretory granules of thyroid follicular cells). The substrate present in the particulate fraction behaved antigenically like thyrolobulin. It is concluded that the particulate fraction contained nascent thyroglobulin inits undegraded form and that this material is probably the precursor to the various components encountered in the soluble fraction.