In Vivo Incorporation of Labelled Amino Acid into Different Protein Fractions of the Rat Thyroid Gland

Abstract

Comparative evaluation of the incorporation of labelled amino acid into different protein fractions of thyroid glands from normal rats was studied after in vivo injection of the isotope (³H-tyrosine).

The maximum of incorporation into total thyroidal proteins was observed 30 min after injection of the label and was maintained with small fluctuations during 5 hr. The maximum of incorporation of the label into total thyroglobulin was obtained later at 2 hr after injection. For the particulate thyroglobulin the maximum was observed earlier than for soluble thyroglobulin.

Total thyroglobulin (soluble + particulate) represent 45-50% of newly formed thyroidal proteins. The major part of thyroglobulin was found in the soluble protein fraction from the shortest time after injection (30 min). A relative accumulation of thyroglobulin in this fraction (% of the total thyroglobulin) was observed during the first 24 hr following injection (70% at 30 min and 90% at 24 hr).

Among proteins other than thyroglobulin 35-40% were found in the fraction of soluble proteins, while 60-65% were particulate in origin. By digitonin extraction 20-25% of the latter proteins can be solubilized.

About 35% of newly formed proteins were degraded during the first 24 hours following injection. Total thyroglobulin and the totality of proteins other than thyroglobulin were degraded to the same proportions. It seems that thyroglobulin can be degraded before being completely halogenated.

Key words

Thyroglobulin Synthesis - Protein Synthesis - Amino Acid Uptake - Thyroid Gland