CL(14-25), a dodecapeptide, exhibits antimicrobial activity against Porphyromonas gingivalis with the 50% growth-inhibitory concentration (IC50) value of 145 mu M, and arginine-specific gingipain (Rgp)-inhibitory activity. Kinetic analysis revealed that CL(14-25) is a mixed-type inhibitor, with inhibition constants (K-i and K-i' values) of 1.4 x 10(-6) M and 4.3 x 10(-6) M, respectively. To elucidate the contributions of four cationic amino acid residues at the N- and C-termini of CL(14-25) toward Rgp-inhibitory activity, we investigated the Rgp-inhibitory activities of truncated and alanine-substituted analogs of CL(14-25). Rgp-inhibitory activities significantly decreased by truncated analogs, CL(15-25) and CL(1625), whereas those of CL(14-24) and CL(14-23) were almost as high as that of CL(14-25). Rgp-inhibitory activities of alanine-substituted analogs, CL(R14A) and CL(R14A, R15A) also significantly decreased, whereas those of CL(K25A) and CL(R24A, K25A) were higher than that of CL(14-25). These results suggest that the arginine residue at position 15 substantially contributes to the Rgp-inhibitory activity and that the arginine residue at position 14 plays important roles in exerting Rgp-inhibitory activity. In this study, we demonstrated that CL(K25A) was a potent, dual function, peptide inhibitor candidate, exhibiting Rgp-inhibitory activity with Ki and K-i(l) of 9.6 x 10(-7) M and 1.9 x 10(-6) M, respectively, and antimicrobial activity against P. gingivalis with an IC50 value of 51 mM. (C) 2014 Wiley Periodicals, Inc.