Summary
Since Ac-globulin (factor V) is involved in the formation of prothrombin activator,
its ability to complex with phospholipids was studied. Purified bovine Ac-globulin
was complexed to asolectin, there being presumably a fixed number of binding sites
on the phospholipid micelle for Ac-globulin. In contrast to the requirement for calcium
ions in the formation of complexes between asolectin and autoprothrombin C, calcium
ions were not required for complex formation between asolectin and Ac-globulin to
occur ; in fact, the presence of calcium prevented complex formation occurring, the
degree of inhibition being dependent on the calcium concentration. By treating isolated,
pre-formed aso- lectin-Ac-globulin complexes with calcium chloride solutions, Ac-globulin
could be recovered in a much higher state of purity and essentially free of asolectin.
Complete activators were formed by first preparing the asolectin-calcium- autoprothrombin
C complex and then reacting the complex with Ac-globulin. A small amount of this product
was very effective as an activator of purified prothrombin without further addition
of calcium or any other cofactor. If the autoprothrombin C preparation used to prepare
the complex was free of traces of prothrombin, the complete activator was stable for
several hours at room temperature. Stable preparations of the complete activator were
centrifuged, resulting in the sedimentation of most of the activity. Experimental
evidence also indicated that activator activity was highest when autoprothrombin C
and Ac-globulin were complexed to the same phospholipid micelle, rather than when
the two clotting factors were complexed to separate micelles. These data suggested
that the in vivo prothrombin activator may be a sedimentable complex composed of a thromboplastic
enzyme, calcium, Ac-globulin and phospholipid.
