Oxidation of β₂-glycoprotein I (β₂GPI) by the Hydroxyl Radical Alters Phospholipid Binding and Modulates Recognition by anti- β₂GPI Autoantibodies

Summary

We investigated whether β₂-glycoprotein I (β₂GPI), the key antigen in the antiphospholipid syndrome, is susceptible to oxidative modifications by the hydroxyl radical (°OH) that may influence its lipid-binding and antigenic properties. The effects on human and bovine β₂GPI of °OH free radicals generated by γ-radiolysis of water with ¹³⁷Cs were studied. Radiolytic °OH caused a dose-dependent loss of tryptophan, production of dityrosine and carbonyl groups, dimerization and/or extensive aggregation of β₂GPI. It ensued a reduction in affinity binding to cardiolipin liposomes and loss of β₂GPI-dependent autoanti-body binding to immobilized cardiolipin. Patient anti- β₂GPI antibodies (n = 20) segregated into two groups based on the effect in the β₂GPIELISA of β₂GPI pretreatment with °OH: enhancement (group A, n = 10) or suppression (group B, n = 10) of IgG binding. The avidities of group A antibodies for fluid-phase β₂GPI were low but increased in a dose-dependent manner upon β₂GPI irradiation, in relation to protein crosslinking. Distinguishing features of group B antibodies included higher avidities for fluid-phase β₂GPI that was no longer recognized after °OH treatment, and negative anticardiolipin tests suggesting epitope location near the phospholipid binding site. The °OH scavengers thiourea and mannitol efficiently protected against all above changes. Therefore, oxidative modifications of β₂GPI via °OH attack of susceptible amino acids alter phospholipid binding, and modulate recognition by autoantibodies depending on their epitope specificities. These findings may be of clinical relevance for the generation and/or reactivity of anti- β₂GPI antibodies.

Keywords

β₂-glycoprotein I - hydroxyl radical - antiphospholipid syndrome - atherosclerosis

• References

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