Inhibition of Aeromonas sobria serine protease (ASP) by α₂-macroglobulin

ASP is a serine protease secreted by Aeromonas sobria. ASP cleaves various plasma proteins, which is associated with onset of sepsis complications, such as shock and blood coagulation disorder. To investigate a host defense mechanism against this virulence factor, we examined the plasma for ASP inhibitor(s). Human plasma inhibited ASP activity for azocasein, which was almost completely abolished by treating plasma with methylamine, which inactivates α₂-macroglobulin (α₂-MG). The ASP-inhibitor complex in ASP-added plasma was not detected by immunoblotting using anti-ASP antibody; however, using gel fi ltration of the plasma ASP activity for an oligopeptide, the ASP substrate was eluted in the void fraction (M _w>200 000), suggesting ASP trapping by α₂-MG. Indeed, human α₂-MG inhibited ASP azocaseinolytic activity in a dose-dependent manner, rapidly forming a complex with the ASP. Fibrinogen degradation by ASP was completely inhibited in the presence of α₂-MG. α₁-Protease inhibitor, antithrombin, and α₂-plasmin inhibitor neither inhibited ASP activity nor formed a complex with ASP. Surprisingly, ASP degraded these plasma serine protease inhibitors. Thus, α₂-MG is the major ASP inhibitor in the human plasma and can limit ASP virulence activities in A. sobria infection sites. However, as shown by fluorescence correlation spectroscopy, slow ASP inhibition by α₂-MG in plasma may indicate insuffi cient ASP control in vivo.