Incorporation of N-Acetylmannosamine and N-Acetylglucosamine into Thyroglobulin in Rat Thyroid in Vitro

Abstract The incorporation of N-acetylmannosamine ([3H]ManNAc) and N-acetylglucosamine ([3H]GlcNAc and [14C]GlcNAc) has been studied in rat thyroid hemilobes and homogenates. The radioactive label is incorporated into thyroglobulin (TG), [3H]ManNAc showing a time course more rapid than [3H]GlcNAc. [3H]ManNAc is incorporated directly into 19 S thyroglobulin showing no lower molecular weight-labeled precursors of TG either in the soluble or in the solubilized proteins; [3H]GlcNAc is incorporated into 19 S TG and into precursors both in soluble and solubilized proteins. [3H]ManNAc is converted only into N-acetylneuraminic acid (sialic acid). [3H]GlcNAc and [14C]GlcNAc are incorporated mainly as GlcNAc, and an additional small proportion is converted into sialic acid. Both labeled GlcNAc and sialic acid are found in soluble TG but only GlcNAc is found in membrane-bound TG. [3H]ManNAc is particularly useful to study the last step of carbohydrate incorporation while [3H]GlcNAc is a better tool to study the initiation of carbohydrate incorporation. The presence of the label as sialic acid only in soluble TG suggests that sialic acid is incorporated at the time of TG release from endoplasmic membranes. It may, therefore, serve as an indicator of TG secretion.