The use of prothrombin(S525C) labeled with fluorescein to directly study the inhibition of prothrombinase by antithrombin during prothrombin activation

Serine 525 of human prothrombin was mutated to cysteine and covalently labeled with fluorescein to make II(S525C)-fluorescein. Kinetics of cleavage of this derivative by prothrombinase are identical to those of wild-type prothrombin, Cleavage is coincident with a 50% increase in fluorescence intensity and the product is catalytically inactive. Thus, it allows convenient monitoring of prothrombin activation without generating active thrombin, The kinetics of inhibition of factor Xa (FXa) by antithrombin (AT) and AT-heparin were measured by monitoring activation of II(S525C)-fluorescein and the hydrolysis of the chromogenic substrate S2222 in the presence of AT. With S2222 as the substrate the rate constant for inhibition of FXa, Ca2+, and unilamellar vesicles of phosphatidylcholine and phosphatidylserine (75:25) (PCPS) vesicles by AT was 3.51 x 10(3) M-1 s(-1); when factor Va (FVa) was included the rate constant was 1.55 x 10(3) M-1 s(-1). In the absence of FVa, II(S525C)-fluorescein had no effect on inhibition. When II(S525C)-fluorescein was the substrate, however, FVa at saturating concentrations profoundly protected FXa from inhibition by AT, increasing the half-life from 3 min with FXa, Ca2+, PCPS, and II(S525C)-fluorescein, to greater than 69 min when FVa was included. Thus, both FVa and prothrombin are necessary for this level of protection. In the absence of prothrombin, FVa decreased the second order rate constant for inhibition by the AT-heparin complex from 1.58 x 10(7) M-1 s(-1), for FXa, Ca2+, and PCPS, to 7.72 x 10(6) M-1 s(-1). II(S525C)-fluorescein and factor Va together reduced the rate constant to less than 1% of that for FXa, Ca2+, and PCPS, At a heparin concentration of 0.2 unit/ml, this corresponds to a half-life increase from 1 s to 136 s.