Purity ≥95% by SDS-PAGE. Human Protein S is isolated from fresh frozen plasma by a combination of conventional methods and immunoaffinity chromatography as described by Jenny, et al. Purity is determined by SDS-PAGE analysis. Domain Structure of Protein S: GLA Region: contains g-carboxyglutamic acid residues, EGF Region: contains sequences homologous to human epidermal growth factor, TSR = Thrombin sensitive region, Region of unknown function which replaces the catalytic triad found in vitamin K-dependent serine proteases. Protein S is a single chain vitamin K-dependent protein which is thought to function in both the coagulation and complement cascades (1,2). Approximately 60% of protein S circulating in plasma is complexed to C4b binding protein (C4BP). It has been suggested that g-carboxyglutamic acid (gla) dependent binding of protein S to negatively charged phospholipids may function to concentrate C4BP at cell surfaces following injury. In the coagulation system, Protein S functi