Beta-2-Glycoprotein 1

Autoantibodies directed to negatively charged phospholipids, in particular cardiolipin, have been detected in the serum of patients with systemic lupus erythematosus (SLE) and antiphospholipid syndrome (APS). APS is characterised by venous and arterial thrombosis, recurrent spontaneous abortions and thrombocytopenia. It is now known that a serum cofactor, β₂-glycoprotein 1, is required for the binding of cardiolipin by autoantibodies in the sera of patients with APS. By contrast, anticardiolipin antibodies from patients with infectious diseases (in particular, syphilis) do not require this cofactor.

β₂-Glycoprotein 1, also known as apolipoprotein H, is a relatively abundant serum protein (present at a concentration of about 0.2 mg/ml) that may play a role in coagulation. It has been shown to bind to platelets, mitochondria and negatively charged substances such as heparin, DNA, dextran sulphate and negatively charged phospholipids.

β₂-Glycoprotein 1 is known to be very heat stable. On SDS-electrophoresis the protein displays an apparent molecular weight of ~55 kDa reduced and ~45kDa in non-reduced conditions. Isoelectric focussing has been reported to reveal genetic polymorphisms of β₂-glycoprotein 1, the multiple bands seen by this method may be due to different amounts of sialic acid. The amino acid sequence of β₂-glycoprotein 1 reveals a 326 amino acid protein of about 36 kDa. This sequence has been confirmed by sequencing the protein’s cDNA. The fact that the protein migrates on SDS-electrophoresis with a much larger apparent m.wt., suggests that it is extensively glycosylated. β₂-Glycoprotein 1 has five consensus repeats, referred to as "Sushi domains", characteristic of the complement control protein family.

 

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