A Second Plasma Inhibitor of Activated Protein C: α₁-Antitrypsin

Summary

Inactivation of activated protein C (APC) in normal human plasma was studied in the absence and presence of heparin. In the absence of heparin APC inactivation followed pseudo-first order kinetics. In the presence of heparin the neutralization of APC was found to be biphasic. Up to 500 nM APC could be readily inactivated in normal plasma, indicating that the concentration of the APC inhibitor must be higher than previously assumed. Plasma deficient in the protein C inhibitor (PCI-I, as described by Suzuki and coworkers) and deficient in p₂-glycoprotein I still possessed APC neutralizing capacity, presumably through the formation of complexes of APC with another plasma protein as was demonstrated by immunoblotting with anti-protein C antibodies. Together these data made us to conclude that a second inhibitor of APC (PCI-II) must be present in normal human plasma. This second inhibitor should be heparin independent, have a relatively high plasma concentration and form complexes with APC.

Subsequently, we purified this PCI-II by isolating APC-PCI-II complexes from plasma deficient of vitamin K dependent proteins, PCI-I and β₂-glycoprotein-I, to which purified human APC had been added. Purified PCI-II has a molecular weight of 50,000 daltons and aminoacid analysis revealed that PCI-II is identical with α₁-antitrypsin (α₁-AT). The second order rate constant for the reaction between purified α₁-AT and APC was found to be 269 M⁻¹ min⁻¹ in the absence of calcium and 602 M⁻¹ min⁻¹ in the presence of calcium.

Finally, the analysis of the kinetics of APC inactivation in the plasma of a patient congenitally deficient in α₁-AT (3% α₁-AT antigen) revealed that there probably is a third inhibitor of APC in plasma, different from PCI-I and from α₁-AT.

Keywords

Protein C inhibitor - α₁-antitrypsin

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