Anticardiolipin antibodies (ACA) directed not to cardiolipin but to a plasma protein cofactor

The binding of affinity-purified anticardiolipin antibodies (ACA) to liposomes that contained cardiolipin or phosphatidylserine was investigated. ACA bound to these liposomes only in the presence of plasma or serum, which indicated a requirement for a plasma component. This component-referred to as aca-cofactor-was purified& its activity to support ACA binding to liposomes that contained cardiolipin was not destroyed by heat (10 min at 90.degree.C), but was greatly diminished on incubation with trypsin. aca-cofactor bound liposomes that contained negatively charged phospholipid but had no affinity for liposomes that contained neutral phospholipid (eg, phosphatidylcholine)& this binding was independent of calcium ions. aca-cofactor was essential for ACA to bind to liposomes that contained cardiolipin or phosphatidylserine and, when coated on a microtitre plate in the absence of any phospholipid, aca-cofactor was an apparent antigen for ACA in an enzyme-linked immunosorbent assay. aca-co-factor is a single chain polypeptide with an apparent molecular weight of 50 kD (non-reduced), which increases to 70 kD upon reduction, and its properties closely resemble those of .beta.2-glycoprotein I (apolipoprotein H).