Tris inhibits both proteolytic and oligosaccharide processing occurring in the Golgi complex in primary cultured rat...

Tris caused the distention of the Golgi cisternae in primary cultured rat hepatocytes and perturbed the functions occurring there. Proteolytic cleavage of precursors of both albumin and complement C3 was inhibited, whereas that of prohaptoglobin was not affected by Tris. These effects on the proteolytic cleavages resemble those of acidotropic amines (Oda, K., and Ikehara, Y. (1985) Eur. J. Biochem. 152, 605-609; Oda, K., Koriyama, Y., Yamada, E., and Ikehara, Y. (1986) Biochem. J. 240, 739-745). However, the effects of Tris significantly differed from acidotropic amines on the basis of its effects on the processing of N-linked oligosaccharides of glycoproteins. Both alpha 1-protease inhibitor and haptoglobin secreted from the Tris-treated cells were found to contain almost equal amounts of endo-beta-N-acetylglucosaminidase H-sensitive and -resistant oligosaccharides, whereas the glycoproteins from both the control and methylamine-treated cells were resistant to the enzyme. The endo-beta-N-acetylglucosaminidase-sensitive oligosaccharides were analyzed to be Man8-5GlcNAc by high resolution gel permeation chromatography, suggesting that trimming of alpha-mannose residues from the precursor Man9GlcNAc2 is incomplete in the Tris-treated cells. On the other hand, Tris did not significantly inhibit incorporation of radioactive monosaccharides (N-acetylglucosamine, galactose, and fucose) into the glycoproteins. However, two-dimensional gel electrophoresis in combination with neuraminidase digestion demonstrated that sialylation was markedly inhibited by Tris. Taken together, our results reveal that Tris inhibits not only the sialic acid addition which takes place in the trans Golgi region, but also the trimming step of high mannose-type oligosaccharides, which is thought to occur before glycoproteins reach the trans Golgi region.