Kinetic Parameters of Kallikrein-Kinin System Undergoing Cold Activation

Human kallikrein-kinin system (KKS) is one of the shortest proteolytic cascades with two serine-protease zymogen couples (Factor XII and its activated form, FXIIa, prekallikrein and kallikrein) releasing bradykinin by cleavage of native high molecular weight kininogen (nHK) into cleaved HK. This cascade is usually triggered ex vivo on ice in human plasma for KKS investigation.FXIIa and kallikrein were found 5-times less active on ice than at 37°C: k cat =0.133±0.034s -1 and 0.0119±0.0027s -1 , K M =672±150nM and 115±24nM, respectively. A progress curve analysis of in vitro KKS reconstitution using 14 different sets of initial concentrations showed parameters of nHK cleavage kinetics that differed from a Michaelis-Menten mathematical simulation by a faster initial rate and a slower late rate. These two features were also observed ex vivo by using dextran-sulphate activated plasma and could help reinforce in KKS activation a maximal local effect (bradykinin release) and a minimal systemic consequence (prekallikrein preservation).