Structural elucidation of the N- and O-glycans of human apolipoprotein(a) - Role of O-glycans in conferring protease resistance

Apolipoprotein(a) (apo(a)) is a multikringle domain glycoprotein that exist s covalently linked to apolipoprotein B100 of low density lipoprotein, to f orm the lipoprotein(a) (Lp(a)) particle, or as proteolytic fragments. Eleva ted plasma concentrations of apo(a) and its fragments may promote atheroscl erosis, but the underlying mechanisms are incompletely understood. The fact ors influencing apo(a) proteolysis are also uncertain. Here we have used ex oglycosidase digestion and mass spectrometry to sequence the Asn (N)-linked and Ser/Thr (O)-linked oligosaccharides of human apo(a). We also assessed the potential role of apo(a) O-glycans in protecting thermolysin sensitive regions of the polypeptide. Apo(a) contained two major N-glycans that accou nted for 17% of the total oligosaccharide structures. The N-glycans were co mplex biantennary structures present in either a mono- or disialylated stat e. The O glycans were mostly (80%) represented by the mono sialylated core type 1 structure, NeuNAc2 alpha -3Gal beta1-3GalNAc, with smaller amounts o f disialylated and non-sialylated O-glycans also detected. Removal of apo(a ) O-glycans by sialidase and O-glycosidase treatment dramatically increased the sensitivity of the polypeptide to thermolysin digestion. These studies provide the first direct sequencing data for apo(a) glycans and indicate a novel function for apo(a) O-glycans that is potentially related to the ath erogenicity of Lp(a).