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The interaction of duodenase, a new serine protease from a small group of Janus-faced proteases, with serpins, α1-protease inhibitor (α1-PI) and antichymotrypsin (ACT) from human blood serum, was studied. The stoichiometry of the inhibition process was found to be 1.2 and 1.3 mol/mol for α1-PI and ACT, respectively. The presence of a stable enzyme–inhibitory complex duodenase–α1-PI was confirmed by SDS-PAGE. The formation of the duodenase–ACT complex was not demonstrated; instead, the band of the cleaved inhibitor indicated the ACT hydrolysis. The suicide mechanism of the duodenase interaction with the human blood serpins was proved. The association rate constants (kа × 105, М−1 s−1) were 2.4 ± 0.3 × 105 for α1-PI and 3.0 ± 0.4 × 105 for ACT. These results indicate the possibility of the regulation of duodenase activity by endogenous serpins.