Background Activated protein C (APC) is a major regulator of thrombin formation. Two major plasma inhibitors form complexes with APC, protein C inhibitor (PCI) and α 1 -antitrypsin (α 1 AT), and these complexes have been quantified by specific enzyme-linked immunosorbent assays (ELISAs). Also, complexes of APC with α 2 -macroglobulin (α 2 M) have been observed by immunoblotting. Here, we report an ELISA for APC:α 2 M complexes in plasma. Methods Plasma samples were pre-treated with dithiothreitol and then with iodoacetamide. The detection range of the newly developed APC:α 2 M assay was 0.031 to 8.0 ng/mL of complexed APC. Following infusions of APC in humans and baboons, complexes of APC with α 2 M, PCI and α 1 AT were quantified. These complexes as well as circulating APC were also measured in 121 patients with a history of venous thromboembolism (VTE) and 119 matched controls. Results In all the in vivo experiments, α 2 M was a significant APC inhibitor. The VTE case–control study showed that VTE patients had significantly lower APC:α 2 M and APC levels than the controls ( p < 0.001). Individuals in the lowest quartile of APC:α 2 M or the lowest quartile of APC had approximately four times more VTE risk than those in the highest quartile of APC:α 2 M or of APC. The risk increased for individuals with low levels of both parameters. Conclusion The APC:α 2 M assay reported here may be useful to help monitor the in vivo fate of APC in plasma. In addition, our results show that a low APC:α 2 M level is associated with increased VTE risk.
- activated protein C
ASJC Scopus subject areas