The binding and regulation of protein S by neutrophils

The degradation of the coagulation cofactors Va and VIIIa by activated protein C (APC), is dependent on calcium, a suitable surface and protein S. The latter is a vitamin K-dependent protein which functions as a cofactor in the reaction. In this study we investigated the role of neutrophils in regulating the activity of protein S. Protein S specifically bound to neutrophils pretreated with di-isopropyl fluorophosphate (DFP) in a time-dependent and saturable reaction. Double reciprocal plot analysis indicated 5 ± 10 protein S binding sites per neutrophil with half saturable binding occurring at a protein S concentration of 18 nM. Binding was unaffected by the presence of APC, factor V or factor Va. Failure to preincubate the cells with DFP allowed a surface protease to rapidly cleave protein S resulting in its dissociation from its binding site. The cleavage was associated with loss of protein S cofactor activity. The major neutrophil surface associated enzyme that caused this cleavage was identified as elastase. Exposure of neutrophils to ionophore A23187, soluble heat aggregated IgG and serum opsonized zymosan caused the release of a protease/s which cleaved and inactivated protein S. Purification of the protease revealed that it was elastase. This was verified by incubating the neutrophil releasate with anti-neutrophil elastase antibody which inhibited protein S cleavage. Purified neutrophil elastase in concentrations observed under physiological conditions resulted in rapid cleavage and inactivation of protein S. We propose that neutrophils by binding and inactivating protein S serve as an important control for the activity of this natural anticoagulant.