Effect of LA IgGs on thrombin production in static conditions. Phospholipid vesicles (5 μmol/L), prothrombin (0.1 μmol/L), factor Va (0.1 nmol/L), and control or LA IgG (5 to 50 μmol/L) in HEPES buffer was incubated for 1 hour at room temperature. Factor Xa was added to a final concentration of 20 pmol/L to initiate the reaction. Aliquots of the reaction were removed at discrete time intervals and thrombin concentration was determined by the rate of hydrolysis of N-p-Tosyl-Gly-Pro-Arg-p-nitroanilide. The concentration of LA IgG used in the experiment was that which resulted in maximal prothrombin binding (see Fig 5): 5 μmol/L for LA4 and LA7; 10 μmol/L for LA2, LA5, and LA6; 20 μmol/L for LA1; and 50 μmol/L for LA3. Data points and error bars represent the mean and SE of triplicate determinations.