Activation of factor B of the complement system by kallikrein and its light chain.

The cleavage of factor B, a protein of the alternative pathway of complement, by kallikrein was studied. Like factor D, kallikrein can cleave B to generate the alternative pathway C3 convertase C3bBb. When this convertase was formed on erythrocytes previously coated with C3b, lysis was observed indicating that a functionally active C3 convertase was formed. B was also cleaved by kallikrein in the presence of fluid phase C3b, and this resulted in B fragments comparable in size to those generated in the presence of D. The capacity of kallikrein to cleave B is localised in the light chain of the kallikrein molecule, which is the same chain of kallikrein that is responsible for its other enzymatic activities. Since on a molar basis D is much more active then kallikrein in cleaving B, a physiological role for B activation by kallikrein is only likely under certain conditions, and still has to be established.