The Effect of Age on the Glycosylation of Human Alpha-2-Macroglobulin

Introduction: Alpha-2-macroglobulin (a2m) is a plasma glycoprotein capable of inhibiting all classes of serine proteases. Within the coagulation and fibrinolytic systems, a2m inhibits several critical factors, including thrombin, plasmin, and activated protein C. Previous studies have shown that coagulation factor concentrations are highly variable with age. Notably, a2m levels are approximately twice as high in newborns compared to adults. Elevated levels of a2m in newborns may, in part, contribute to a resistance towards thrombotic events observed in this population. Protein glycosylation is known to affect protein activity as well as pharmacodynamics. The glycosylation profile of adult a2m has previously been analyzed and shown to contain 8 potential sites of N-linked glycosylation. Information regarding glycosylation of a2m in other age groups has yet to be elucidated. Therefore, the purpose of this study is to examine the differences in the glycosylation profiles between newborn and adult a2m.

Methods: Normal adult pooled platelet-poor plasma was obtained commercially. Newborn platelet-poor plasma was isolated by standard methods from umbilical cord blood obtained immediately after delivery of healthy full-term infants. Pooled newborn plasma was constructed from 30 different donors. To evaluate the degree of N-linked glycosylation, plasma samples were enzymatically deglycosylated by peptide N-glycosidase F (PNGaseF). Samples were subjected to SDS polyacrylamide (5%) gel electrophoresis (SDS PAGE) and western blotting to detect a2m in plasma. To evaluate a2m sialic acid content, plasma samples were incubated with Neuraminidase from Clostridium perfringens. Following incubation, samples were subjected to native polyacrylamide (4%) gel electrophoresis and western blotting. To detect the presence of non-sialylated terminal galactose residues, plasma samples were incubated with immobilized Ricinus communis lectin, and lectin-bound molecules were separated from unbound molecules by centrifugation. Bound and unbound fractions were subjected to SDS-PAGE and western blotting as described above to detect a2m.

Results: Deglycosylation of both newborn and adult a2m with PNGaseF resulted in a change in migration and apparent molecular weight on SDS-PAGE. There was no significant difference (p=0.28, n=3) between the change in apparent molecular weight for newborn a2m (16.1 ± 0.42 kDa) versus adult a2m (14.5 ± 0.99 kDa). On native polyacrylamide gel electrophoresis, newborn a2m exhibited slightly increased migration compared to adult a2m. This difference in migration was abolished following treatment with neuraminidase, indicating the variation in migration was due to differing sialic acid content. Additionally, a lower proportion of newborn a2m was bound to Ricinus communis compared to adult a2m. This indicates newborn a2m has fewer non-sialylated galactose residues available for binding to this lectin.

Conclusions: To our knowledge, this is the first study investigating potential glycan heterogeneity between newborn and adult a2m molecules. The results from PNGaseF analyses indicate that there is no macroheterogeneity in total N-glycan content apparent between newborn and adult a2m. However, the observed increase in sialic acid content of newborn a2m compared to adult a2m may be responsible for age-related differences in pharmacodynamic and pharmacokinetic properties of this important protease inhibitor.