Coagulation factor III (tissue factor) is a membrane glycoprotein which serves as a cofactor in the proteolytic activation of factor X and factor IX by factor VIIa. Mixing of human placental factor III apoprotein with vesicles of bovine brain phospholipids does not produce significant reconstitution of factor III activity, but, when the mixture of apoprotein and vesicles is made 5 mM with CdCI2, the apoprotein is incorporated into the vesicles. Ultracentrifugation on sucrose density gradients demonstrated that the active factor III-lipid complex formed by reconstitution with vesicles had a density indistinguishable from that of the complex formed by detergent dialysis. Vesicles isolated after centrifugation were shown to range in diameter from 20 nm to over 100 nm using the electron microscope. Gel filtration showed that factor-III activity was associated with all size-classes of vesicles. The presence of factor III activity in the smaltest vesicles argues for a specific cadmium-mediated reconstitution of the apoprotein with phospholipid vesicles.

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