Effect of swainsonine on the processing of the asparagine-linked carbohydrate chains of alpha 1-antitrypsin in rat hepatocytes. Evidence for the formation of hybrid oligosaccharides.

The biosynthesis of the proteinase inhibitor alpha 1-antitrypsin has been studied in rat hepatocyte primary cultures. Newly synthesized alpha 1-antitrypsin was found in hepatocytes as a glycoprotein of an apparent molecular weight of 49,000 carrying oligosaccharide side chains of the high mannose type. In the hepatocyte medium a secreted alpha 1-antitrypsin of an apparent molecular weight of 54,000 could be identified as a glycoprotein with carbohydrate chains of the complex type. Pulse-chase experiments revealed a precursor-product relationship for the two forms of alpha 1-antitrypsin. When the hepatocytes were treated with swainsonine, an intracellular form of alpha 1-antitrypsin with an apparent molecular weight of 49,000 indistinguishable from that of control cells was found. However, the alpha 1-antitrypsin secreted from swainsonine-treated hepatocytes was different from that present in control media. It was characterized by a lower apparent molecular weight (51,000), a higher amount of [3H]mannose incorporation, half as much incorporation of [3H]galactose, and the same amount of [3H]fucose incorporation compared to alpha 1-antitrypsin of control media. In contrast to the 54,000 complex type alpha 1-antitrypsin from control media the 51,000 alpha 1-antitrypsin from the medium of swainsonine-treated cells was found to be susceptible to the action of endoglucosaminidase H, even when fucose was attached to the proximal GlcNAc residue. alpha 1-Antitrypsin secreted from swainsonine-treated cells combines features usually associated with either high mannose or complex type oligosaccharides and therefore represents a hybrid structure. In spite of its effect on the carbohydrate part of alpha 1-antitrypsin swainsonine did not impair the secretion of the incompletely processed glycoprotein.