The non-classical N-glycan processing pathway of bovine brain ecto-nucleotide phosphodiesterase/pyrophosphatase 6 (eNPP6) is brain specific and not due to mannose-6-phosphorylation

Ecto-nucleotide phosphodiesterase/pyrophosphatase 6 (eNPP6) is a glycosylphosphatidylinositol (GPI)-anchored alkaline lysophospholipase C which is predominantly expressed in brain myelin and kidney. Due to shedding of the GPI-anchor eNPP6 occurs also as a soluble isoform (s-eNPP6). eNPP 6 consists of two identical monomers of 55 kDa joined by a disulfide bridge, and possesses four N-glycans in each monomer. In brain s-eNPP6 the N-glycans are mainly hybrid and high mannose type structures, reminiscent of processed mannose-6-phosphorylated glycans. Here we completed characterization of the site-specific glycan structures of bovine brain s-eNPP6, and determined the endo H-sensitivity glycan profiles of s-eNPP6 from bovine liver and kidney. Whereas in brain s-eNPP6 all of the N-glycans were endo H-sensitive, in liver and kidney only one of the glycosylation sites was occupied by an endo H-sensitive glycan, likely N406, which is located within the cleft formed by the dimer interface. Thus, the non-classical glycan processing pathway of brain eNPP 6 is not due to mannose-6-phosphorylation, suggesting that there is an alternative Golgi glycan-processing pathway of eNPP6 in brain. The resulting brain-specific expression of accessible hybrid and oligomannosidic glycans may be physiologically important within the cell–cell communication system of the brain.