Processing and transport of human small intestinal lactase-phlorizin hydrolase (LPH). Role of N-linked oligosaccharide modification.

The effect of glycosylation on the intracellular transport of human intestinal lactase-phlorizin hydrolase (LPH) was investigated by biosynthetic labeling of biopsy samples in the presence or absence of glycosidase inhibitors. In the presence of deoxynojirimycin (dNM) and deoxymannojirimycin (dMM), endo H sensitive LPH glycoforms of M(r) = 135,000 in both cases were produced (LPHdNM and LPHdMM). The LPH glycoform generated in the presence of swainsonine had an apparent molecular mass of 141,000 (LPHSwa) and was partially sensitive to endo H. By contrast to unmodified mature LPH (LPHm, M(r) = 160,000), these glycoforms are either not O-glycosylated (LPHdNM and LPHdMM) or partially O-glycosylated (LPHSwa) indicating that processing of N-linked carbohydrates has direct effects on the O-glycosylation of pro-LPH. Analysis of transport kinetics of the various glycoforms strongly suggested that carbohydrate modification does not affect the transport of pro-LPH from the cis-Golgi to the cell surface, but could be rate limiting at the level of the ER.