# The Metabolic Stability of Antimicrobial Peptides IK8 in Plasma and Liver S9
###### Abstract  
In the face of mounting global antibiotic resistance, which has become a critical clinical problem, antimicrobial peptides (AMPs) have received considerable interest as new therapeutics with the efficacy for the treatment of multidrug-resistant (MDR) infections due to their novel mechanism. However, certain inherent shortcomings such as instability seriously limit their systemic applications in the clinic. In this study, we intend to clarify the connection between three configurations of IK8 and their stability in plasma and liver S9 of various species by confirming the metabolites. The structural information of these metabolites was scanned and identified using HPLC and Q-TOF, respectively. The results found that IK8-D exhibits superior stability, compared with IK8-2D and IK8-L in plasma and liver S9 incubation, which indicated D- type amino acids could significantly increase the stability of antimicrobial peptides. antimicrobial peptide; IK8; metabolite; Q-TOF +
Footnote †: journal: Applied Sciences  
AMP, albopeptide, in the culture broth of Streptomyces albofaciens, which displays a narrow-spectrum activity against vancomycin-resistant Enterococcus faecium. AMPs are bioactive small proteins, as the first-line defense against microbial attacks in Eukaryotes or a competitive strategy in Prokaryotes, generally containing both cationic and hydrophobic amino acids [7]. AMPs exhibit their antimicrobial effects mainly through two different mechanisms. The membrane-targeting AMPs destroy the structural integrity of the cell membrane, while the AMPs that use non-membrane targeting mechanisms principally inhibit the synthesis of nucleic acids, essential enzymes, and other functional proteins [8]. For the former, the action of AMPs is believed to be initiated by cationic charges that specifically target negatively charged microbial membrane rather than mammalian cells through electrostatic interaction [9]. The difference in membrane composition between microbial and mammalian cells serves as the cornerstone for the selectivity of AMPs. The outer monolayer of microbial lipid bilayer often partially consists of phosphatidylserine (PS) or phosphatidylglycerol (PG), which provides net negative charges for microbial surfaces. In mammalian cells, however, the outer-monolayer-possessing zwitterionic phospholipids are typically not absorbed by AMPs. Subsequently, the hydrophobic portion of the peptide penetrates into the lipid bilayer, forming pores on the bacterial surface or destabilizing the plasma membrane, finally killing the bacteria [10]. The non-membrane-targeting mechanism, which is capable of translocating across the membranes without damaging them but destabilizing normal cell functions [11], has received considerable attention lately, e.g., with Api137, a derivative of the insect-produced AMP apidaecin that inhibits translation by arresting the release factor on the ribosome [12]. Owing to such unique bactericidal mechanisms, it is hard for pathogens to develop resistance toward AMPs [13], rendering them promising candidates for addressing the detrimental issue of multidrug-resistant pathogens. Compared with classical antibiotics, multiple advantages are observed for AMPs--namely, (i) they are prone to synthesize since they consist usually of short amino acid sequences, which is also conducive to put into production at a relatively low cost [14]; (ii) some AMPs can bring about rapid death in Gram-positive, Gram-negative, fungi, parasites, encapsulated viruses or tumor cells merely within a few minutes [15]; (iii) they have no adverse influence on the microbiota, which is of vital significance in various aspects of human biology and is generally disrupted by conventional broad-spectrum antibiotics [16]. The above-mentioned advantages make it possible to have several AMPs that are now in medical use, such as gramicidin, polymyxins, daptomycin, and melittin [17]. Nevertheless, AMPs also have drawbacks that need to be overcome.