Individual Reaction Steps in the Release of Kallidin from Kininogen by Tissue Kallikrein

Summary

At low pH values (around 6), porcine pancreatic β-kallikrein B attacks at first the C-terminal ARg bond of the kinin moiety in bovine HMW kininogen. Arg-cleaved kininogen accumulates as an intermediate in the solution. Kallidin is released by cleavage of the aminoterminal Met-Lys bond in a second step. At pH values between 7.6 and 9, however, Arg-cleaved kininogen does not occur as a free intermediate. The participation as a (free, not only enzyme-bound) intermediate of Arg-cleaved kininogen in a short-lived especially reactive conformation or of Met-cleaved kininogen is also unlikely. Probably, both the Met and the Arg bonds are hydrolyzed in one enzyme-substrate complex which does not dissociate between these two events.

Kinetic constants for the release of kallidin from native single-chain HMW kininogen and from Arg-cleaved kininogen (even if this Arg residue is removed) remarkably have the same values. Evidently, the rate of the reaction is determined by steps leading to the hydrolysis of the Met bond. As the state of the C-terminal Arg residue has no influence, the efficient cleavage of the Met bond by tissue kallirkein is probably not due to some strain in the kininogen molecule in the region of this bond. As modification of Arg residues of kininogen prevents cleavage also of the Met bond, some Arg residue(s) appear(s) to play a crucial role in this process. K_cat/Km (1.4 x 10⁶ M^-1 sec^-1 at pH 9, 25° C) is very high for a proteolytic reaction, mainly because of the low value of (0.6 μM). K_cat/Km for the hydrolysis of the Met bond in kininogen is 3 500 times higher than in the peptide Ser-Leu-Met-Lys-brady-kinin with a partial kininogen sequence. Important interactions with the enzyme thus occur in regions of the kininogen molecule outside this sequence. K_cat/K_m for the hydrolysis of the Arg bond in the kininogen peptide Pro-Phe-Arg-Ser-Val-Gln is also 14 times lower than this constant for the release of kallidin from kininogen, but k_cat is 6 times higher. This can explain why cleavage of the Arg bond is not reflected in the kinetic constants of single-chain kininogen.