Modification of beta 2glycoprotein I by glutardialdehyde. Conformational changes and aggregation accompany exposure...

Autoantibodies from patients with antiphospholipid syndrome (APS) recognize an epitope on beta 2glycoprotein I (beta 2 GPI) only when native beta 2 GPI is adsorbed on surfaces composed of anionic phospholipids or oxidized polystyrene. beta 2 GPI was modified with the crosslinking agent, glutardialdehyde (GDA), which induced exposure of the anti-beta 2 GPI epitope at GDA:beta 2 GPI mol ratios in the range of 500-2000. A second crosslinking agent, dimethyl-suberimidate (DMS), did not expose the epitope, which may be a consequence of its having less tendency than GDA to form intermolecular links. SDS-PAGE experiments demonstrate that GDA does promote extensive intermolecular crosslinking of beta 2 GPI, and DMS does not. Formaldehyde also reacts with the lysine residues of beta 2 GPI, but does not expose the epitope. The circular dichroism spectra of native and modified beta 2 GPI confirm that GDA induces changes in conformation that are qualitatively different from those caused by formaldehyde. These data provide evidence that binding of lysine residues is not a sufficient condition for exposure of the autoepitope, and also support the likelihood that anti-beta 2 GPI antibodies bind only to aggregates of the protein. Thus, by synthesizing an active holoantigen of beta 2 GPI, conditions were defined that are necessary for binding of human autoantibodies.