Haptoglobin polymorphism affects its N-glycosylation pattern in serum

Haptoglobin (Hp) is an abundant acute-phase protein secreted mainly by the liver into the bloodstream. There are three Hp protein phenotypes (Hp type 1-1, 2-1, and 2-2), which differ in the number of α- and β-chains, type of α-chain (the β-chain type remains the same in all the Hp phenotypes), and the polymers that they form via disulfide bonds. Hp has four N-glycosylation sites on the β-chain. Glycosylation of Hp has been reported frequently as a potential glycobiomarker for many diseases; however, whether Hp polymorphism affects its glycosylation has not yet been addressed extensively or in depth.

This study investigated the differences between the glycosylation patterns of Hp phenotypes using serum from 12 healthy individuals (four for each Hp phenotype).

An efficient method for isolating Hp from serum was established and subsequently the Hp phenotype of each sample was characterized by immunoblotting. Then, LC-MS/MS analysis of isolated Hp after treatment with three exoglycosidases (sialidase, α2-3 neuraminidase, Endo F3) was performed to characterize the glycosylation pattern of Hp for each individual sample.

The data reveal significant differences among the branching, sialylation, and fucosylation of Hp types, documenting the effect of Hp polymorphism on its glycosylation.

Overall, the study suggests that Hp phenotype characterization should be considered during the investigation of Hp glycosylation.