Thyroglobulin biosynthesis in the rat thyroid

Radioautographic visualization of the incorporation of galactose-3H and mannose-3H by rat thyroids in vitro in relation to the stages of thyroglobulin synthesis

Rat thyroid lobes incubated with mannose-(3)H, galactose-(3)H, or leucine-(3)H, were studied by radioautography. With leucine-(3)H and mannose-(3)H, the grain reaction observed in the light microscope is distributed diffusely over the cells at 5 min, with no reaction over the colloid. Later, the grains are concentrated towards the apex, and colloid reactions begin to appear by 2 hr. With galactose-(3)H, the reaction at 5 min is again restricted to the cells but it consists of clumped grains next to the nucleus. Soon after, grains are concentrated at the cell apex and colloid reactions appear in some follicles as early as 30 min. Puromycin almost totally inhibits incorporation of leucine-(3)H and mannose-(3)H, but has no detectable effect on galactose-(3)H incorporation during the 1st hr. Quantitation of electron microscope radioautographs shows that mannose-(3)H label localizes initially in the rough endoplasmic reticulum, and by 1-2 hr much of this reaction is transferred to the Golgi apparatus. At 3 hr and subsequently, significant reactions are present over apical vesicles and colloid, while the Golgi reaction declines. Label associated with galactose-(3)H localizes initially in the Golgi apparatus and rapidly transfers to the apical vesicles, and then to the colloid. These findings indicate that mannose incorporation into thyroglobulin precursors occurs within the rough endoplasmic reticulum; these precursors then migrate to the Golgi apparatus, where galactose incorporation takes place. The glycoprotein thus formed migrates via the apical vesicles to the colloid.

Biosynthesis of thyroglobulin. Incorporation of [1-14C] galactose, [1-14C] manose and [4,5-3H2] leucine into soluble proteins by rat thyroids in vitro

1. Rat thyroid lobes were incubated for various periods of time in Krebs-Ringer bicarbonate containing [(3)H]leucine and either [1-(14)C]galactose or [1-(14)C]mannose. Radioactivity in soluble proteins was determined after their separation by sucrose-gradient centrifugation. 2. The time-course of incorporation of label from [(14)C]-mannose into soluble thyroid proteins was parallel to that observed for [(3)H]leucine. There was a lag of at least 30min. before either label appeared in non-iodinated thyroglobulin (protein 17-18s). During this time both labels were detected in two fractions known to contain subunit precursors of thyroglobulin (fractions 12s and 3-8s). Radioactivity from double-labelled fractions 12s and 3-8s was transferred to protein 17-18s during subsequent incubation in an unlabelled medium. 3. In contrast, most of the [(14)C]galactose was immediately incorporated into protein 17-18s. 4. During the first hour of incubation, puromycin almost completely inhibited the incorporation of label from [(3)H]leucine and [(14)C]mannose into all protein fractions, but had little effect on the incorporation of [(14)C]galactose into protein 17-18s. 5. These results indicate that mannose is incorporated into the carbohydrate groups of protein 17-18s at an earlier stage in its formation than galactose. It is suggested that the synthesis of the carbohydrate groups of ghyroglobulin begins soon after formation of the polypeptide components, more than 30min. before these are aggregated to protein 17-18s; carbohydrate synthesis then proceeds in a stepwise manner, galactose being incorporated at about the time of aggregation of subunits to protein 17-18s. Most, if not all, the carbohydrate is added to thyroglobulin before it is iodinated.