Relative Importance Of Plasma Protease Inhibitors In The Inactivation Of Kallikrein In Human Plasma

Abstract

Human plasma contains several inhibitors of plasma kallikrein (KAL), including C1-inhibitor (C1INH), α2-macro- globulin (α2M), antithrombin III (ATIII), and α1-antitrypsin (αlAT). Studies in purified systems have allowed us to quantitate the kinetic constants of each isolated inhibitor. We also have demonstrated that high molecular weight kininogen (HMWK) is an important regulator since it decreases the inactivation rate of KAL by all inhibitors except α2M. When purified inhibitors and HMWK are present at plasma concentrations, it can be calculated that C1INH, α2M, ATIII and α1AT account respectively for 49%, 49%, 0.8% and 0.2% of the KAL inhibitory activity. To assess if this prediction derived from purified system adequately describes the inhibition of KAL under more physiological conditions, we incubated KAL with normal human plasma (NHP), C1INH-deficient plasma (C1INH-D), and α2M-deficient plasma (α2M-D). KAL activity_was assessed using H-D-Pro-Phe-Arg- Nan as a substrate. C1INH-D was obtained from a patient with hereditary angioedema. C1INH in C1INH-D was 15% of the normal value as assessed by radial immunodiffusion. α2M-D was obtained by selective and complete inactivation of α2M by methylamine. The pseudo-first-order rate constants for the inactivation of_KAL by NHP, C1INH-D, α2M-D, and plasma deficient in both C1INH and α2M were respectively 8.8, 5.0, 5.5, and 1.8 S-1(×l03). Therefore, C1INH accounted for 63% of the observed inhibition, α2M for 35%, and all the other inhibitors for 2%. When these values were corrected for the concentration of HMWK, C1INH accounted for 47% of the inhibition, α2M for 51%, and all the other inhibitors for <2%. Thus, there is an excellent agreement between the results obtained when KAL is inhibited in purified systems and those obtained when it is inhibited in plasma. Moreover, these results indicate that C1INH and α2M are the only important inhibitors of KAL in NHP.