For the first time, inhibitory plant serpins, including WSZ1 from wheat, BSZ4, and the previously unknown protein BSZx from barley, have been expressed in Escherichia coli, and a procedure for fast purification of native plant serpins has been developed, BSZx, BSZ4, and WSZ1 were assayed for inhibitory activity against trypsin, chymotrypsin, and cathepsin G, and cleavage sites in the reactive center loop were identified by sequencing. BSZx proved to be a potent inhibitor with specific, overlapping reactive centers either at P-1 Arg for trypsin or at P-2 Leu for chymotrypsin, At 22 degrees C, the apparent rate constant for chymotrypsin inhibition at P-2 (k(a) = 9.4 x 10(5) M(-1) s(-1)) was only four times lower than for trypsin at P-1 (k(a) = 3.9 X 10(6) M(-1) s(-1)), and the apparent inhibition stoichiometries were close to 1. Furthermore, our data suggest that cathepsin G was inhibited by BSZx (k(a) = 3.9 x 10(6) M(-1) s(-1)) at both the P-1 Arg and P-2 Leu. These results indicate a unique adaptability of the reactive center loop of BSZx. WSZ1 inhibited chymotrypsin (k(a) = 1.1 x 10(5) M(-1) s(-1)) and cathepsin G (k(a) = 7.6 x 10(3) M(-1) s(-1)) at P-2 Gln and not, as for BSZx, at the more favorable P-2 Leu. BSZ4 inhibited cathepsin G (k(a) = 2.7 x 10(4) M(-1) s(-1)) at P-1 Met but was hydrolyzed by trypsin and chymotrypsin. The three plant serpins formed stable SDS-resistant complexes with the proteinases in accordance with the kinetic data.