Oxidation of beta(2)-glycoprotein I (beta(2)GPI) by the hydroxyl radical alters phospholipid binding and modulates recognition by anti-beta(2)GPI autoantibodies

We investigated whether beta (2)-glycoprotein I (beta (2)GPI), the key anti gen in the antiphospholipid syndrome, is susceptible to oxidative modificat ions by the hydroxyl radical (degrees OH) that may influence its lipid-bind ing and antigenic properties. The effects on human and bovine beta (2)GPI o f degrees OH free radicals generated by gamma -radiolysis of water with Cs- 137 were studied. Radiolytic degrees OH caused a dose-dependent loss of try ptophan. production of dityrosine and carbonyl groups, dimerization and/or extensive aggregation of beta (2)GPI. It ensued a reduction in affinity bin ding to cardiolipin liposomes and loss of beta (2)GPI-dependent autoantibod y binding to immobilized cardiolipin. Patient anti-beta (2)GPI antibodies ( n = 20) segregated into two groups based on the effect in the beta (2)GPI-E LISA of beta (2)GPI pretreatment with degrees OH: enhancement (group A, n = 10) or suppression (group B, n = 10) of IgG binding. The avidities of grou p A antibodies for fluid-phase beta (2)GPI were low but increased in a dose -dependent manner upon beta (2)GPI irradiation, in relation to protein cros slinking. Distinguishing features of group B antibodies included higher avi dities for fluid-phase beta (2)GPI that was no longer recognized after degr ees OH treatment, and negative anticardiolipin tests suggesting epitope loc ation near the phospholipid binding site. The degrees OH scavengers thioure a and mannitol efficiently protected against all above changes. Therefore, oxidative modifications of beta (2)GPI via degrees OH attack of susceptible amino acids alter phospholipid binding, and modulate C recognition by auto antibodies depending on their epitope specificities. These findings may be of clinical relevance for the generation and/or reactivity of anti-beta (2) GPI antibodies.