Transfer of glucose in the biosynthesis of thyroid glycoproteins. I. Inhibition of glucose transfer to oligosaccharide lipids by GDP-mannose.

Thyroid rough microsomes catalyzed the synthesis of glucose-containing oligosaccharide lipids which were compared to those extracted from labeled thyroid cells and were found to be largely similar. Glucose transfer to these oligosaccharide lipids in the microsomal system was shown to be markedly depressed by an addition of GDPmannose. This sugar nucleotide, already at 1 microM, blocked dolichol-P-glucose synthesis, thus restraining further glucosylation of oligosaccharide lipids. Using this concentration of radioactive GDPmannose in the incubation medium lead to the detection of three glucose containing mannose-labeled oligosaccharide lipids. Double labeling experiments suggested a precursor-product relationship between them. Previously labeled oligosaccharide lipids, containing glucose or not were compared in their efficiency to act as donors of their oligosaccharide chain to an exogenous synthetic Asn-X-Thr containing peptide. It was found that the presence of glucose did not significantly influence the transfer. Free glucose was released during the reaction when using the glucose-labeled oligosaccharide lipid.