Effect of cleavage of the heavy chain of human plasma kallikrein on its functional properties

Abstract

Human plasma kallikrein consists of an N-terminal heavy chain of molecular weight (mol wt) 52,000, linked by disulfide bonds to two light chain variants (mol wt 36,000 or 33,000). Although the active catalytic site of kallikrein resides on the C-terminal light chain, the role of the N-terminal heavy chain is less clear. We therefore studied an enzyme designated β-kallikrein, containing a single cleavage in the heavy chain (mol wt 28,000 + 18,000) and compared it to the enzyme, α-kallikrein, with an intact heavy chain. The rates of inactivation by C1 inhibitor of plasma α- and β-kallikreins were kinetically identical, as measured by residual amidolytic activity, after various times of incubation with the inhibitor. Both enzymes reacted completely with C1 inhibitor after 18 hours and formed identical C1 inhibitor-kallikrein complexes of mol wt 195,000. The rate of activation of factor XII by α-kallikrein and β-kallikrein was similar. In contrast, the rate of cleavage of high molecular weight kininogen (HMWK) by α-kallikrein was at least 5-fold faster and the ratio of coagulant activity to amidolytic activity was 4-fold greater than for β-kallikrein. Plasma α-kallikrein, at concentrations potentially achievable in plasma, induced aggregation of neutrophils, but β-kallikrein failed to elicit this response. In addition, human neutrophils pretreated with cytochalasin B released 2.46 ± 0.10 μg/107 cells of elastase antigen, but β-kallikrein released only 0.25 ± 0.10 μg/107 cells. These observations suggest that cleavage of the heavy chain influences the rate of cleavage of HMWK and decreases its coagulant activity. Moreover, an intact heavy chain appears to be requisite to support the ability of kallikrein to aggregate neutrophils and release elastase.