Figure 4.

Biological anticoagulant cofactor activity of microprotein S and the effect of plasma inhibitor C4b-binding protein. (A) Prolongation of the clotting time of protein S-deficient plasma in the presence of APC because of addition of microprotein S (●), native plasma-derived protein S (▾), standard plasma calibrator (■), thrombin-cleaved microprotein S (⊙), and thrombin-cleaved protein S (▿·). In controls in the absence of APC, no significant anticoagulant effect of microprotein S (○) or protein S (▿) was observed. Protein S unicalibrator lyophilized plasma containing protein S at a level of 115% was used as a calibration curve. Molecular concentrations of plasma protein S and microprotein S were determined with an ELISA and amino acid analysis, respectively. (B) The effect of C4b-binding protein on the anticoagulant APC-cofactor activity of protein S (300 nM, ▾) and microprotein S (900 nM, ●). In controls, C4b-binding protein alone was added to protein S deficient plasma (▵).