Intracellular forms of transferrin oligosaccharide chains in rat liver

Glycosylation of transferrin was investigated in vivo by using antibody monospecific for rat serum transferrin. Pulse-chase experiments indicated that, after intravenous injection of [35S]methionine, labeled transferrin appeared in the rough and smooth microsomes and Golgi subfractions in rapid succession in 10 min and that an additional 10 min was required for it to be secreted. Most of the intracellular transferrin (95%) immunoprecipitated from the total microsome fraction was sensitive to endo-beta-N-acetylglucosaminidase H (endo H), whereas serum transferrin was completely resistant to it. Further fractionation of the total microsomes has revealed that the intracellular transferrin immunoprecipitated from the rough and smooth microsomes and GF3 are all endo-H-sensitive and most of the endo-H-sensitive oligosaccharides were eluted at the position corresponding to Man8GlcNAc on high-resolution Bio-Gel chromatography. This finding suggests that the major form of intracellular transferrin oligosaccharide in the course of intracellular transport from the endoplasmic reticulum to the Golgi apparatus is Man8GlcNAc2. Endo-H-resistant forms were first detected in the GF2 but more in GF1, most of which were sensitive to neuraminidase. Since the heavy Golgi subfraction contains mainly cis-Golgi elements, such as cisternae, and the light subfraction mainly trans-Golgi elements, such as secretory granules, it is strongly suggested that the processing of these large mannosyloligosaccharide chains and the subsequent addition of terminal sugars to them are performed successively in the trans-Golgi region just before secretion.