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The cleavage of the angiotensinogen molecule by renin is slow. The rate constant for cleavage of the enzyme-substrate complex, Kcat (turnover number) is lower than usual for enzymes (0.6/s for the homologous human renin reactions and 0.15/s for the homologous mouse renin reaction). Thus each round of catalysis occurs in 1.7 s in the human and 6.7 s in the mouse reaction. The kcat/Km values (104-106 l/mol per s; Km, Michaelis constant) are high and in the range for protein inhibitors. The association constants (Ka) are (106-107 l/mol) close to that of inhibitors. The slow rate seems to be favourable since homologous reactions are slower than heterologous. Angiotensinogen is structurally related to α1-antitrypsin. We conclude that kinetic as well as structural data indicate that angiotensinogen might be considered a protease inhibitor for renin.