On the distribution of thyroglobin and larger iodoproteins in single rat thyroid follicles.

The distribution of the large iodoproteins in the lumen of thyroid follicles was tested. Samples of the colloid in rat thyroid follicles were obtained in vivo by micropuncture of superficial follicles in the isthmus of the gland. One sample was collected per follicle and several follicles were punctured in each gland. The protein composition of the samples was analysed by microgel electrophoresis. The protein separation patterns were recorded by microdensitometry after staining of the proteins and the amount of protein in the fractions was estimated from the densitometric recordings. Thyroglobulin (TG) and the aggregates of TG--here designated the S-TG fractions--were present in all samples of colloid. It was observed that there was a significant positive co-variation between TG and S-TG when the colloid contained only TG, S-TG and a prealbumin fraction. When the albumin-like protein was present in the samples the co-variation was less significant. The results show that the relative amount of the aggregates of TG is constant in the albumin free colloid and that these aggregates most probably are randomly distributed in the follicle lumen. The results further indicate that the aggregates of TG are non-randomly distributed in the larger peripheral follicles when the colloid contains the albumin-like protein.