title: Characterization of apolipoprotein M isoforms in low-density lipoprotein.
abstract: Apo M is a recently discovered human lipoprotein thought to be involved in the metabolism of lipids and lipoprotein particles. Here, a proteomic approach was applied to examine the glycosylation pattern of apo M in human LDL. We treated LDL proteins with N-glycosidase or neuraminidase, studied mobility shifts of Apo M by two-dimensional gel electrophoresis, and different isoforms were then identified with mass spectrometry. This way, we demonstrated the presence of five isoforms of apo M in LDL: three that are both N-glycosylated and sialylated, one that is N-glycosylated but not sialylated, and one that is neither N-glycosylated nor sialylated. As judged from the examination of LDL from 20 healthy human subjects, the three N-glycosylated and sialylated forms are most abundant (80-100% of the total apo M in LDL) whereas the unsialylated and unglycosylated variants constitute at most 20%. Comparative analysis showed that the same five isoforms of apo M are also present in HDL. Further studies aiming at elucidating the role of apo M in health and disease will have to take this polymorphism of apo M proteins into account.
authors.affiliation: Division of Occupational and Environmental Medicine, Department of Molecular and Clinical Medicine, Faculty of Health Sciences, Linköping University, Sweden.
authors.forename: Helen
authors.initials: H
authors.lastname: Karlsson
authors.forename: Helen
authors.initials: H
authors.lastname: Lindqvist
authors.forename: Christer
authors.initials: C
authors.lastname: Tagesson
authors.forename: Mats
authors.initials: M
authors.lastname: Lindahl
mesh_terms: D000328:Adult; D000368:Aged; D001053:Apolipoproteins / Q000032:analysis*; D000075243:Apolipoproteins M; D015180:Electrophoresis, Gel, Two-Dimensional; D005260:Female; D006026:Glycoside Hydrolases / Q000737:chemistry; D006031:Glycosylation; D006801:Humans; D054834:Lipocalins; D008075:Lipoproteins, HDL / Q000737:chemistry; D008077:Lipoproteins, LDL / Q000737:chemistry*; D008297:Male; D008875:Middle Aged; D019158:N-Acetylneuraminic Acid / Q000032:analysis; D009439:Neuraminidase / Q000737:chemistry; D020033:Protein Isoforms / Q000032:analysis; D040901:Proteomics
chemical_list: C400523:APOM protein, human; D001053:Apolipoproteins; D000075243:Apolipoproteins M; D054834:Lipocalins; D008075:Lipoproteins, HDL; D008077:Lipoproteins, LDL; D020033:Protein Isoforms; D006026:Glycoside Hydrolases; D009439:Neuraminidase; D019158:N-Acetylneuraminic Acid