Abstract
Although coagulation factor Xa requires Ca2+ for binding to phospholipid, factor V, the other protein component in the prothrombinase enzyme complex, binds tightly to phospholipid in the absence of Ca2+. To explore the possibility that calcium might be present in the fact V molecule, the effect of several chelators, including oxalate, citrate, pyrophosphate, and EDTA, on factor V activity has been studied. A time- and concentration-dependent inhibition of factor V which reflects the respective association constants for calcium of each chelator is observed. The inhibition can be prevented by the prior addition of calcium and manganese but not magnesium. Reversal of the activity loss can be accomplished at high protein concentrations by the addition of calcium, the removal of the chelator by gel filtration, or an increase in temperature. Factor V contains 1 g atom of calcium per 300,000 daltons which is not removed by incubation with EDTA under nondenaturing conditions. Thus, the inhibition by EDTA is due to binding to calcium associated with factor V. In 8 M urea, EDTA can remove over 80% of the calcium, demonstrating the importance of the native structure in maintaining the calcium binding site. Prior binding of phospholipids to factor V prevents inhibition by EDTA. The results suggest that phospholipids complex at the calcium site on the factor V metallopretein.
