ELEVATED LEVELS OF BETA(2) GLYCOPROTEIN-I (BETA(2)GPI) IN ANTIPHOSPHOLIPID ANTIBODY SYNDROME ARE DUE TO INCREASED AMOUNTS OF BETA(2)GPI IN ASSOCIATION WITH OTHER PLASMA CONSTITUENTS

beta(2) glycoprotein-I(beta(2)GPI) is a 50 kDa plasma protein which associates with a number of anionic plasma constituents and has been identified as a cofactor for the binding of some antiphospholipid antibodies (aPAs). beta(2)GPI antigen levels are increased in some patients with aPAs. In order to examine the distribution of beta(2)GPI in these patients,we developed a method for measurement of free beta(2)GPI. Forty-three patients with SLE, of whom 18 had laboratory evidence of aPAs, and 22 normal healthy subjects were studied. Plasma was filtered by centrifugation at 650 x g through a 100 kDa filter in order to allow separation of free and complexed beta(2)GPI, and beta(2)GPI levels were measured in the starting plasma and filtrate by a standardized ELISA. Total beta(2)GPI levels of the SLE aPA positive patients (253.3 mg/l) were significantly increased compared with the SLE aPA negative patients and normal controls (188.0, P<0.001 and 194.9 mg/l, P< 0.01, respectively), but there were no significant differences between free beta(2)GPI levels of these groups (20.8, 24.0 and 20.5 mg/l, respectively). These results suggest that levels of complexed beta(2)GPI are increased in patients with aPAs, perhaps as a result of immune complex formation, or altered binding to other plasma constituents.