Salusin-ß, an antimicrobially active peptide against Gram-positive bacteria.

Salusin-ß has been detected in numerous mammalian tissues and has been shown to have various effects on the cardiovascular system. In this study, we showed that salusin-ß exhibited potent antibacterial activity against Gram-positive microorganisms such as Bacillus subtilis NBRC 3513, Bacillus megaterium ATCC 19213, Staphylococcus aureus NBRC 12732, and Staphylococcus epidermidis NBRC 12933. A cytoplasmic membrane-depolarizing assay using the DiSC3(5) dye revealed that the addition of 4 nmol/mL of salusin-ß caused the leakage of fluorescence dye from Staphylococcus aureus NBRC 12732. The antimicrobial potency and circular dichroism (CD) spectroscopy of five analogs related to salusin-ß were examined to determine structure-function relationships in its N- and C-terminal regions. The results obtained suggest that the N-terminal sequences of the salusin-ß molecule are important for the expression of the potent antimicrobial activity of this peptide. A profile corresponding to that of the α-helix conformation was observed in the salusin-ß solution.

Polycationic gramicidin S analogues with both high antibiotic activity and very low hemolytic activity.

The substitution of each constituent amino acid residue of gramicidin S (GS), cyclo(-Val(1,1')-Orn(2,2')-Leu(3,3')-D-Phe(4,4')-Pro(5,5')-)(2) with Lys residue indicated that each side chain structure of the constituent amino acid residues affect largely the antibiotic activity and hemolytic activity of GS. Further, the substitution of D-Phe(4,4') and Pro(5,5') residues with basic amino acid residues as a Lys residue results the high antibiotic activity and the very low hemolytic activity. Thus, we have found novel positions on the scaffold of GS at D-Phe(4,4') and Pro(5,5') residues whose modification will significantly increase the therapeutic index.

Synthesis of novel fatty-acyl gratisin derivatives.

To find candidates with high antimicrobial and low hemolytic activities, many gratisin (GR) analogues have been designed and synthesized. In the present account, we synthesized novel derivatives of GR having both the polycationic and fatty acyl groups, cyclo{-Val(1)-Orn(2)-Leu(3)-D-Phe(4)-Pro(5)-D-Lys(6)(X)-Val(7)-Orn(8)-Leu(9)-D-Phe(10)-Pro(11)-D-Lys(12)-} {X=-CO(CH(2))(6)CH(3) (1), -Lys-CO(CH(2))(6)CH(3) (2), -(Lys)(2)-CO(CH(2))(6)CH(3) (3), and -(Lys)(3)-CO(CH(2))(6)CH(3) (4)}, and examined the biological activities. Among them, we found that 2-4 have differential ionic interaction against the prokaryotic membrane and eukaryotic membrane. In other words, the dissociation with high antimicrobial activity and low hemolytic activity is caused by the addition of D-Lys(6)-{(Lys)(n)-CO(CH(2))(6)CH(3)} residues at position 6 of [D-Lys(6,12)]-GR. Our findings should be helpful in finding drug candidates with high antimicrobial activity and low hemolytic activity that are capable of combating microbial resistance.

Fatty acyl-gramicidin S derivatives with both high antibiotic activity and low hemolytic activity.

In the present study, novel eight GS derivatives having the octanoyl-(Lys)(n)- moieties, cyclo{-Val-Orn-Leu-d-Phe-Pro(4ß-NH-X)-Val-Orn-Leu-d-Phe-Pro-} {X=-H (1), and -(Lys)(n)-CO(CH(2))(6)CH(3)n=0 (2), 1 (3), 2 (4), and 3 (5)} and cyclo{-Val-Orn-Leu-d-Phe-Pro(4α-NH-X)-Val-Orn-Leu-d-Phe-Pro-} {X=-H (6), and -(Lys)(n)-CO(CH(2))(6)CH(3)n=1 (7), and 2 (8)} were synthesized. Among them, 4, 5 and 8 result the high antibiotic activity against both Gram-positive and Gram-negative microorganisms tested. In addition, 4 and 5 showed very low hemolytic activity compared with that of GS. Thus, the introduction of the excess amino groups and the fatty acyl moiety to the γ-NH(2) group of Pro(5) residue in GS molecule lowered the unwanted hemolytic activity and enhanced the desired antibiotic activity.

Novel cycloundecapeptides related to gramicidin S with both high antibiotic activity and low hemolytic activity.

To find candidates with high antimicrobial and low hemolytic activities, many gramicidin S (GS) analogs of various ring sizes have been designed and synthesized. However, syntheses of antimicrobially active analogues of GS having a disordered symmetry structure from C(2) have almost never been reported, because the stable, amphiphilic ß-sheet structure of GS with C(2) symmetry is considered essential for its strong antibacterial activity. In the present studies, novel thirteen cycloundecapeptides 1-13 related to GS were synthesized and examined. Among them, cyclo(-Va1(1)-Orn(2)-Leu(3)-D-Phe(4)-X(5)-Pro(6)-Val(7)-Orn(8)-Leu(9)-D-Phe(10)-Pro(11)-) (X=Lys (10), Orn (11), Arg (12) and Lys(Lys) (13)) resulted in high antibiotic activity against both Gram-positive and Gram-negative microorganisms tested. In addition, 11 showed low toxicity against sheep blood cells compared with that of GS. Further, circular dichroism (CD) spectra of 10-13 had a curve similar to each other, suggesting that the conformations of these analogues in methanol are similar to each other. However, CD spectra of 10-13 were different from that of GS in the 190-210 nm region. These results suggest that the presences of one added amino acid residue at position 5 of 10-13 might be partially effective through a structural change in the biological activity of 10-13. In addition, the structural modifications at position 5 lower the undesirable hemolytic activity and enhance the desirable antibiotic activity.

Novel des-fatty acyl-polymyxin B derivatives with Pseudomonas aeruginosa-specific antimicrobial activity.

Polymyxin B (PMB) is a cationic cyclic decapeptide antibiotic with a fatty acyl (FA) modification at the α-amino group of Dab¹ (Dab: L-α,γ-diaminobutyric acid). In this study, which is part of a series of PMB structure-activity relationship investigations focused on identifying clinically useful peptide antibiotics, we synthesized ten des-FA PMB derivatives whose N-terminal moieties were changed to basic or hydrophilic amino acids. The antimicrobial and lipopolysaccharide (LPS) binding activities of these synthetic analogs were tested. The analogs showed more potent antimicrobial activity against Pseudomonas aeruginosa (P. aeruginosa) compared with the PMB nonapeptide. In particular, [Ser²-Dap³]-PMB(2-10), Guanyl-[Thr²-Dab³]-PMB(2-10), Guanyl-[Dab¹-Thr²-Dab³]-PMB(1-10), and N(α,γ)-diguanyl-[Dap³]-PMB(3-10) had antimicrobial activity equivalent to PMB. In LPS binding assays, the displacement curves shifted in a manner proportional to the number of positive charges available to bind to Escherichia coli (E. coli) and P. aeruginosa. Furthermore, peptides with basic side chains were comparable to PMB in binding activity assays against E. coli and P. aeruginosa. The acute toxicities of the peptides were evaluated by intravenously administering the peptides to mice through the tail vein. The toxicities of [Ser²-Dap³]-PMB(2-10), [Dap³]-PMB(3-10), and [Ser³]-PMB(3-10) were lower that of PMB (LD50, 4.8 µmol/kg).

Novel gratisin derivatives with high antimicrobial activity and low hemolytic activity.

The substitution of each constituent amino acid residue of gratisin (GR) with Ala residue indicated that each side chain structure of the constituent amino acid residues affect largely the antibiotic and hemolytic activities of GR. Among them, the substitution of Pro residues at positions 5 and 5' with a cationic amino acid residues (Lys and Arg) results the high antibiotic activity and the low toxicity against human blood cells. Thus, we have found a novel position on the scaffold of GR at Pro(5,5') residues whose modification will significantly lower the unwanted hemolytic activity and enhance the desired antibiotic activity.

Antimicrobially active cycloundecapeptides related to gramicidin S having a novel turn structure with cis D-Phe-Pro peptide bond.

Antimicrobially active cycloundecapeptides related to gramicidin S, cyclo(-Val1-Orn2-Leu3-X4-D-Phe5-Pro6-Val7-Orn8- Leu9-D-Phe10-Pro11-) (X= Leu (1), Ala (2), Orn (3), Lys (4) and Arg (5)), were synthesized. From the CD and NMR studies, 1-5 possess antiparallel -sheet conformation linked by a type II -turn around D-Phe10-Pro11 and a novel turn structure around X4-D-Phe5-Pro6 sequence with cis D-Phe-Pro peptide bond. The structural modifications at position 4 of 1-5 are beneficial to identification of novel antibiotic candidates without hemolytic activity.

Structure-activity relationship of indolicidin, a Trp-rich antibacterial peptide.

A series of Trp and Arg analogs of antibacterial indolicidin (Ind) was synthesized and the antimicrobial and hemolytic activities were investigated. [L(9)]Ind, [L(11)]Ind, [K(8),L(9)]Ind and [K(6, 8),L(9)]Ind showed desirable characteristics, exhibiting negligible hemolytic activity while keeping strong antibacterial activity. The results indicated that the Trp residue at position 11 essentially contributes to both activities and one can not be exchanged for the other, whereas the Trp residues at positions 4 and 9 play important roles in antimicrobial and hemolytic activities, respectively. The Trp residues at positions 6 and 8 play no important roles in biological activities. We then found that the retro analog of Ind showed higher antibacterial activity than Ind against both Gram-positive and Gram-negative bacteria but remarkably lower hemolytic activity than that of Ind.

Syntheses of low-hemolytic antimicrobial gratisin peptides.

Antibiotic and hemolytic activities of gratisin (GR), cyclo(-Val(1)-Orn(2)-Leu(3)-d-Phe(4)-Pro(5)-d-Tyr(6)-)(2), and fifteen GR analogues, which have various d-amino acid residues in place of d-Tyr(6,6') residues, were examined. Among them, [d-Orn(6,6')]-GR, [d-Lys(6,6')]-GR and [d-Arg(6,6')]-GR showed the strong activity against both Gram-positive and Gram-negative bacteria. In addition, the antibiotics showed significantly reduced toxicity against human blood cells compared with gramicidin S, cyclo(-Val(1)-Orn(2)-Leu(3)-d-Phe(4)-Pro(5)-)(2).

Contribution of each amino acid residue in polymyxin B(3) to antimicrobial and lipopolysaccharide binding activity.

This study on the structure-activity relationship of polymyxin B, a cyclic peptide antibiotic, used sixteen synthetic polymyxin B(3) analogs including alanine scanning analogs to elucidate the contribution of the side chains to antimicrobial activity and lipopolysaccharide (LPS) binding. Of these analogs, [Ala(5)]-polymyxin B(3) showed greatly reduced antimicrobial activity against Escherichia coli (E. coli), Salmonella Typhimurium (S. Typhimurium) and Pseudomonas aeruginosa (P. aeruginosa) with MIC values of 4-16 nmol/ml, suggesting that the Dab (alpha,gamma-diaminobutyric acid) residue at position 5 is the most important residue contributing to bactericidal activity. The antibacterial contribution of Dab when located within the lactam ring (positions 5, 8 and 9) was greater than when located outside the ring (positions 1 and 3). [D-Ala(6)]-, [L-Phe(6)]-, [Ala(7)]-, and [Gly(7)]-polymyxin B(3) analogs retained potent antimicrobial activity, indicating that neither the reduction of hydrophobic character of the D-Phe(6)-Leu(7) region nor the D-configuration at position 6 is indispensable for antimicrobial activity. LPS binding studies showed that decreased hydrophobicity of the lactam ring had little effect, but the N(gamma)-amino function of the Dab residues at position 1, 3, 5, 8 and 9 greatly affected LPS binding, with the contribution of Dab(5) being the most significant.

A novel polycationic analogue of gratisin with antibiotic activity against both Gram-positive and Gram-negative bacteria.

A novel polycationic analogue of gratisin, cyclo(-Val-Orn-Leu-D-Phe-Pro-D-Lys-)2, was designed and synthesized, which exhibited strong activity against all Gram-positive and Gram-negative bacteria tested. Its activity against Pseudomonas aeruginosa IFO 3080 was two times higher than gramicidin S.

A novel, antimicrobially active analog of gramicidin S without amphiphilic conformation.

A novel gramicidin S analog, cyclo(-Val-Leu-Leu-Orn-Leu-D-Phe-Pro-)2, was synthesized, its antibiotic activity compared with gramicidin S and shown to be as potent as gramicidin S when compared with the susceptibility toward five Gram-positive microorganisms. It exceeded the activity of gramicidin S against Bacillus megaterium ATCC 19213 by a factor of two. Circular dichroism and NMR data suggested this analog to adopt an antiparallel beta-sheet conformation without amphiphilic character.