The kallikrein inhibitor found in Bauhinia bauhinioides seeds (BbKI) differs from classical Kunitz plant inhibitors in the lack of disulfide bridges in its structure [Biochim. Biophys. Acta 1477 (2000) 64-74]. In this study, we examined whether structural properties may be involved in inhibitory specificity and, if so, whether those properties might be useful tools in designing compounds that interfere with enzyme activity. Peptides structurally related to the BbKI (RPGLPVRFESPLRINIIKE-NH(2)) reactive site were synthesized by solid-phase method and assayed for serine proteinase activity. The peptides RPGLPVRFESPLRINIIKE-NH(2), RPGLPVRFESPL-NH(2), and GLPVRFES-NH(2) were efficient tissue kallikrein inhibitors, with I(50) values of 0.54 microM, 0.87 microM, and 0.5mM, respectively. The lasting inhibitory effect was observed in incubation periods of up to 120 min. None of the studied peptides interfere with the activity of thrombin, factor Xa or trypsin, although the native protein BbKI is a potent trypsin inhibitor.