Two Forms of the Human Prothrombin Converting Enzyme (Active Factor X)1

Chromatographic analysis and concomitant coagulation measurements have revealed two forms of active Factor X (Xa). Both have the ability to convert prothrombin to thrombin. Measurements at different stages during the activation of human Factor X demonstrated that the first Xaformed is eluted from DEAE-cellulose at relatively high ionic strength. Its enzymic activity is greatly enhanced by Factor V in the presence of calcium and phospholipid. Upon further incubation, this material is converted to a second form of Xawhich is eluted at lower ionic strength and which shows little acceleration of prothrombin conversion in the presence of calcium, phospholipid, and Factor V.