Kinetic Peculiarities of Human Tissue Kallikrein: 1—Substrate Activation in the Catalyzed Hydrolysis of H-d-Valyl-l-leucyl-l-arginine 4-Nitroanilide and H-d-Valyl-l-leucyl-l-lysine 4-Nitroanilide; 2—Substrate Inhibition in the Catalyzed Hydrolysis of Nα-p

Hydrolysis of d-valyl-l-leucyl-l-lysine 4-nitroanilide (1), d-valyl-l-leucyl-l-arginine 4-nitroanilide (2), and Nα-p-tosyl-l-arginine methyl ester (3) by human tissue kallikrein was studied throughout a wide range of substrate concentrations. At low substrate concentrations, the hydrolysis followed Michaelis–Menten kinetics but, at higher substrate concentrations, a deviation from Michaelis–Menten behavior was observed. With the nitroanilides, a significant increase in hydrolysis rates was observed, while with the ester, a significant decrease in hydrolysis rates was observed. The results for substrates (1) and (3) can be accounted for by a model based on the hypothesis that a second substrate molecule binds to the ES complex to produce a more active or an inactive SES complex. The deviation observed for substrate (2) can be explained as a bimolecular reaction between the enzyme–substrate complex and a free substrate molecule.