Studies on Kallikrein Inhibitors from Potatoes*

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Abstract
Some properties of two isolated potato kallikrein inhibitors (PKI-56 and PKI-64) and their inhibitory activities towards kallikreins [EC 3.4.4.21] and other proteinases have been studied. The following results were obtained. 1) Twenty minutes were necessary for both PKI's to form an equilibrated complex with hog pancreatic kallikrein (Hog Pane. K., homogeneous preparation) at 37°C and pH 8.0. The optimal pH of both PKI's was pH 8.0, and inhibition was found to be negligible at pH 4 for both PKI's. 2) Both PKI's were unstable against heat treatment at more than 60°C and also in the alkaline pH range. However, both were stable in the acidic pH range. 3) Dissociation constants (Ki) of Hog Pane. K.-PKI-56 were 4.0×10−6 M (BAEE) and 4.6×10−6 M (TAME), and those of Hog Panc. K.-PKI-64 were 2.2×10−6M (BAEE) and 3.0×10−6 M (TAME) in the presence of BAEE or TAME substrates at 25°C and pH 8.0. In this case, the inhibition of both PKI's was competitive. In the absence of substrate, Ki values (calculated from [E] [I]/[E-I]) of Hog Panc. K.-PKI-56,-PKI-64, and -Trasylol were 1.2×10−6 M, 7.4×10−7 M, and 1.4×10−8M, respectively. The final concentration of Hog Panc. K. in the preincubation mixture had a strong effect on the inhibitory activities of both PKI's. This may probably be based on the high dissociation constants of both PKI's. 4) Both PKI's showed stronger inhibitory activities to human plasma kallikreins and kallikrein-like enzymes from plasma than to glandular kallikreins. The specific inhibition of plasma enzymes was equal to or stronger than those of Trasylol and soy bean trypsin inhibitor. Both PKI's also inhibited trypsin [EC 3.4.4.4], α-chymo-trypsin [EC 3.4.4.5], nagarse [EC 3.4.4.16], and so on, though inhibition of these enzymes was weak. Both PKI's failed to inhibit thrombin [EC 3.4.4.13] and papain [EC 3.4.4.10]. 5) Neither of the PKI's were digested by the above proteinases, even by pepsin [EC 3.4.4.1] at pH 2.

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