Pseudin Peptides and Analogs

Definition
Pseudin is a peptide derived from Pseudis paradoxa. Pseudins have shown to have some antimicrobial function.

Analogs
A study show that analogs of the peptide with increased cationicity and a-helicity were chemically synthesized by progressively substituting neutral and acidic amino acid residues on the hydrophilic face of the a-helix by lysine. Analogs with up to three L-lysine substitutions showed increased potency against a range of gram-negative and gram-positive bacteria (up to 16-fold) whilst retaining low hemolytic activity. Increasing the number of l-lysines to four and five did not enhance antimicrobial potency further but increased hemolytic activity towards human erythrocytes. Increasing the hydrophobicity of pseudin-2, while maintaining the amphipathic character of the molecule, by substitution of neutral amino acids on the hydrophobic face of the a-helix by l-phenylalanine, had only minor effects on antimicrobial and hemolytic activities1.

Related Peptides
Pseudin-1, Pseudin-2, Pseudin-3, and Pseudin-4 are structurally related peptides with antimicrobial activity isolated from skin extracts of the South American paradoxical frog Pseudis paradoxa (Pseudidae) (Olson et al., 2001). Pseudins are a subfamily of the FSAP family (Frog Secreted Active Peptides) and belong to the class of cationic, amphipathic-helical antimicrobial peptides. Pseudin-2 is the most abundant and the most potent peptide2.

Structural Characteristics
Pseudin-2 is a naturally occurring 24 residues (GLNALKKVFQGIHEAIKLINHVQ-NH2) antimicrobial peptide and it has broad-spectrum antimicrobial activity and low cytotoxicity activity.

In a membrane-mimetic environment, the peptide exists in an amphipathic a-helical conformation. Circular dichroism (CD) studies showed that Pseudin-2 has a more amphipathic a-helical structure in negative liposome than in zwitterionic liposome. The peptide had no structural change in the increased salt concentration. It has been shown that salt did not affect peptide conformation but did influence membrane potential energy2.

Mode of Action
Pseudin-2 peptide is believed to exert its antimicrobial activity via formation of pores in the target cell membrane. A study of relationship between structure and mechanical action of pseudin-2 in microorganism and the interaction with various liposomes revealed that it causes perturbation of different liposomes and that the action of peptide is modulated to some extent by membrane lipid composition. In addition, pseudin-2 could form self-associated oligomeric structure in zwitterionic liposome and the release of fluorescent markers caused by pseudin-2 is size-dependent on liposome. It has been suggested that pseudin-2 forms pores via peptide oligomerization using the toroidal pore-forming mechanism in zwitterionic liposomes, whereas the barrel-stave model occurs in negatively charged liposomes.



Functions
Insulin-releasing properties of the frog skin peptide pseudin-2 and its [Lys18]-substituted analogue: A study was conducted to investigate the insulin-releasing properties and cytotoxicity of the peptide, together with selected analogues with increased cationicity and hydrophobicity. It has been found that at concentrations in the range 10(-9)-10(-6) m, pseudin-2, and its [Lys18], [Phe8], and [D-Lys3, D-Lys10, D-Lys14] derivatives, stimulated insulin release from the BRIN-BD11 clonal ß-cell line without increasing release of lactate dehydrogenase. Further, the [Lys18] analogue was the most potent (46% increase in insulin release at 10-9 M) and the most effective (215% increase in insulin release at 10-6 M). The more cationic [Lys3,Lys10,Lys14] and [Lys3,Lys10,Lys14,Lys21] analogues lacked insulinotropic action and the more hydrophobic [Phe16] analogue was cytotoxic at concentrations =10-7 m. Pseudin-2 and [Lys18]-pseudin-2 had no effect on intracellular calcium concentrations and stimulated insulin release in the absence of external calcium. [Lys18]-pseudin-2 (10 - 8 M) stimulated insulin release in the presence of diazoxide and verapamil. It has been suggested that pseudin-2 stimulates insulin secretion from BRIN-BD11 cells by a mechanism involving Ca2+-independent pathways and identify [Lys18]-pseudin-2 as a peptide that may have potential for development as a therapeutically valuable insulinotropic agent for the treatment of type 2 diabetes3.

Reference:
1. Pál T, Sonnevend A, Galadari S, Conlon JM. (2005). Design of potent, non-toxic antimicrobial agents based upon the structure of the frog skin peptide, Pseudin-2. Regul Pept., 129(1-3):85-91.
2. Olson L 3rd, Soto AM, Knoop FC, Conlon JM. (2001). Pseudin-2: an antimicrobial peptide with low hemolytic activity from the skin of the paradoxical frog. Biochem Biophys Res Commun., 288(4):1001-1005.
3. Abdel-Wahab YH, Power GJ, Ng MT, Flatt PR, Conlon JM. (2008). Insulin-releasing properties of the frog skin peptide pseudin-2 and its [Lys18]-substituted analogue. Biol Chemy., 389(2):143-148.

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