Evidence for naturally occurring free thiols in β2GPI in both serum and plasma. (A) Human serum samples (n = 18) were incubated with and without MPB, subjected to acetone precipitation (to remove free MPB), incubated on a streptavidin plate, and probed with the murine monoclonal anti-β2GPI antibody 4B2E7. MPB- and non–MPB-labeled samples probed with a murine immunoglobulin G2 isotype control consistently showed low background OD (405 nm) ≤ 0.1. (B) C57BL/6 β2GPI^+/+ and β2GPI^−/− murine sera (pooled samples; n = 4 mice in each group) were incubated with and without MPB and probed with a polyclonal rabbit anti-β2GPI antibody. (C) Serum and plasma drawn from the same venipuncture (healthy volunteer, age 37 years) were labeled with MPB, and the amount of reduced β2GPI was quantified with the use of the assay described above. Both serum and plasma samples gave the same readings as to amount of reduced β2GPI present. ***P < .001.