What is the interaction between Protein S and C4b-binding protein?

The affinity of protein S to C4BP is several hundred-fold increased by calcium ions. C4BP is an acute-phase reactant, and its concentration in plasma may increase as much as four-fold. This does not result in free PS levels approaching 0 however, because C4BP exists in 2 forms: under normal circumstances, about 10 – 15% of the C4BP circulating in plasma lacks the b-chain and hence does not bind PS. During an acute phase reaction, the C4BP which lacks the b-chain is increased and therefore no significant decrease of free PS occurs.

How is functional activity of protein S tested?

Many labs perform functional clotting assays to test free PS activity. These will detect PS deficiency, but will not discriminate between Types I, II, and III. Clotting tests utilize an excess of protein S deficient plasma, thereby keeping the amount of prothrombin essentially constant. APTT and PT methods commonly used. In a FXa based method, coagulation is triggered by FXa in the presence of calcium ions and phospholipids. This method is not currently used in routine settings, however.

Are elevated levels of Protein S clinically significant?

Though not as well studied as PS deficiency, there has been shown an associated between free and total PS with cholesterol and triglycerides. Though more studies are needed, preliminary findings suggest that elevated free PS may be associated with an increased risk of ischemic heart disease.

What are the different types of protein S deficiency?

The classification of PS deficiency is as follows:

Type 1: Low Protein S Ag Total, Low Protein S Ag Free, Low Protein S Activity
Type II: Normal PS Ag Total, Normal PS Ag Free, Low PS Activity
Type III: Normal PS Ag Total, Low PS Ag Free, Low PS Activity

Type II deficiency seems to be quite rare. Acquired PS is also possible, such as with liver disease, DIC, IBD, and APS. Some research suggests that free PS rather than total PS should be measured in the diagnosis of PS deficiency.

What is the clinical significance of Protein S?

Although the exact role of PS in vivo in the protein C anticoagulant pathway has not been clarified, there is no doubt that PS is an important anticoagulant protein and that PS deficiency is primarily associated with venous thromboembolism. The reported prevalence of PS deficiency in thrombosis patients varies between 1.5-7%, the difference being due to methods used and to the selection of patients. Clinical symptoms in patients affected with PS deficiency are very similar to those with protein C deficiency (DVT). Superficial thrombosis seems to be more common than for antithrombin deficient patients.

What is the interaction between Protein S and APC?

Only the free, native form of PS binds to APC and functions as a cofactor. PS has the highest affinity for negatively charged phospholipids of all the vitamin K-dependent proteins and has been shown to increase by approximately 10-fold, the affinity of APC for membranes or vesicles containing such phospholipids. This may be of physiological importance since APC degrades preferentially membrane-bound FVa and FVIIIa, but not the circulating, inactivated co-factors. In addition to increasing the affinity of APC to membranes, PS also enhances the cleavage of FVa by APC and works in concert with FV to increase the ability of APC to inactivate FVIIIa.

How do Protein S and APC interact?

Only the free, native form of PS binds to APC and functions as a cofactor. PS has the highest affinity for negatively charged phospholipids of all the vitamin K-dependent proteins and has been shown to increase by approximately 10-fold, the affinity of APC for membranes or vesicles containing such phospholipids. This may be of physiological importance since APC degrades preferentially membrane-bound FVa and FVIIIa, but not the circulating, inactivated co-factors. In addition to increasing the affinity of APC to membranes, PS also enhances the cleavage of FVa by APC and works in concert with FV to increase the ability of APC to inactivate FVIIIa.

Protein concentrations in plasma

Component

Molecular
Weight kDa

Plasma
Concentration
mg/l

Plasma
Concentration
μmol/l

Fibrinogen 330 3000 9
Prothrombin 72 150 2
Factor V 330 20 0.05
Factor VII 50 0.5 0.01
Factor VIII 330 0.1 0.0003
Factor IX 56 5 0.09
Factor X 59 8 0.13
Factor XI 160 5 0.03
Factor XII 80 30 0.4
Factor XIII 320 10 0.03
Protein C 62 4 0.06
Protein S 70 10 (free) 0.14
Protein Z 62 2 0.03
Prekallikrein 86 50 0.6
HMW kininogen 120 70 0.6
Fibronectin 450 300 0.7
Plasminogen 92 200 2
t-PA 60 0.005 0.0001
Urokinase 53 0.004 0.0001
Antithrombin 58 145 2.5
Heparin Cofactor II 66 80 1.2
Plasmin Inhibitor 63 60 1
Protein C Inhibitor 57 4 0.07
α2-Macroglobulin 725 2000 3