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Retrocyclin-1, a theta-defensin, is a circular octadecapeptide containing an internal trisulfide ladder. In addition to its antimicrobial activity, it has a significant in vitro ability to protect human cells from infection with the T- and M-tropic strains of HIV-1. Retrocyclin is a small molecule HIV-1 entry inhibitor.

Functional Peptides
Catalog number
CAS number
Molecular Formula
Molecular Weight
RC-100; Cyclo(-Gly-Ile-Cys-Arg-Cys-Ile-Cys-Gly-Arg-Gly-Ile-Cys-Arg-Cys-Ile-Cys-Gly-Arg) (Disulfide bridge: Cys3-Cys16, Cys5-Cys14, Cys7-Cys12); cyclo[L-arginyl-L-cysteinyl-L-isoleucyl-L-cysteinyl-glycyl-L-arginyl-glycyl-L-isoleucyl-L-cysteinyl-L-arginyl-L-cysteinyl-L-isoleucyl-L-cysteinyl-glycyl-L-arginyl-glycyl-L-isoleucyl-L-cysteinyl] (2->11),(4->9),(13->18)-tris(disulfide)
White Powder
Cyclic-(GICRCICGRGICRCICGR) (Disulfide bridge: Cys3-Cys16, Cys5-Cys14, Cys7-Cys12)
Store at -20°C
Soluble in Acetic Acid
InChI Key
Canonical SMILES
1. Reawakening retrocyclins: ancestral human defensins active against HIV-1
Amy L Cole, Alexander M Cole, Piotr Ruchala, Olga Stuchlik, Alan J Waring, Jan Pohl, Nitya Venkataraman, Robert I Lehrer PLoS Biol . 2009 Apr 28;7(4):e95. doi: 10.1371/journal.pbio.1000095.
Human alpha and beta defensins contribute substantially to innate immune defenses against microbial and viral infections. Certain nonhuman primates also produce theta-defensins-18 residue cyclic peptides that act as HIV-1 entry inhibitors. Multiple human theta-defensin genes exist, but they harbor a premature termination codon that blocks translation. Consequently, the theta-defensins (retrocyclins) encoded within the human genome are not expressed as peptides. In vivo production of theta-defensins in rhesus macaques involves the post-translational ligation of two nonapeptides, each derived from a 12-residue "demidefensin" precursor. Neither the mechanism of this unique process nor its existence in human cells is known. To ascertain if human cells retained the ability to process demidefensins, we transfected human promyelocytic cells with plasmids containing repaired retrocyclin-like genes. The expected peptides were isolated, their sequences were verified by mass spectrometric analyses, and their anti-HIV-1 activity was confirmed in vitro. Our study reveals for the first time, to our knowledge, that human cells have the ability to make cyclic theta-defensins. Given this evidence that human cells could make theta-defensins, we attempted to restore endogenous expression of retrocyclin peptides. Since human theta-defensin genes are transcribed, we used aminoglycosides to read-through the premature termination codon found in the mRNA transcripts. This treatment induced the production of intact, bioactive retrocyclin-1 peptide by human epithelial cells and cervicovaginal tissues. The ability to reawaken retrocyclin genes from their 7 million years of slumber using aminoglycosides could provide a novel way to secure enhanced resistance to HIV-1 infection.
2. Inhibition of dengue NS2B-NS3 protease and viral replication in Vero cells by recombinant retrocyclin-1
Shatrah Othman, Hussin A Rothan, Rohana Yusof, Thamil Selvee Ramasamy, Noorsaadah Abd Rahman, Heh Choon Han BMC Infect Dis . 2012 Nov 21;12:314. doi: 10.1186/1471-2334-12-314.
Background:Global resurgence of dengue virus infections in many of the tropical and subtropical countries is a major concern. Therefore, there is an urgent need for the development of successful drugs that are both economical and offer a long-lasting protection. The viral NS2B-NS3 serine protease (NS2B-NS3pro) is a promising target for the development of drug-like inhibitors, which are not available at the moment. In this study, we report retrocyclin-1 (RC-1) production in E. coli as a recombinant peptide to test against dengue NS2B-NS3pro.Methods:Dengue NS2B-NS3pro was produced as a recombinant single chain protein in E. coli and purified by Ni+ affinity chromatography. The RC-1 peptide was produced in E. coli and the tri-disulphide bonds were reformed in a diluted alkaline environment. Protease assay was performed using a fluorogenic peptide substrate and measured by fluorescence spectrometry. Real-time PCR was used for quantification of dengue serotype 2 (DENV-2) viral RNA produced in Vero cells.Results:The RC-1 peptide inhibited the activity of recombinant NS2B-NS3pro with different values at 50% inhibitory concentration (IC50) which are temperature dependent (28°C, 46.1 ± 1.7 μM; 37°C, 21.4 ± 1.6 μM; 40°C, 14.1 ± 1.2 μM). The presence of RC-1 significantly reduced viral replication in Vero cells infected with DENV-2 at simultaneous treatment after 48 hrs (70%) and 75 hrs (85%). Furthermore, moderate reduction in viral replication was observed at pre-treatment mode after 48 hrs (40%) and 72 hrs (38%) and post-treatment at 48 hrs (30%) and 72 hrs (45%).Conclusion:Recombinant RC-1 inhibits DENV-2 replication in Vero cells by interfering with the activity of its serine protease. Thus, we propose that recombinant RC-1 is a potent, cost-effective dengue virus inhibitor. Therefore, it is suitable to consider RC-1 as a new candidate for drug development against dengue infection.
3. Theta defensins protect cells from infection by herpes simplex virus by inhibiting viral adhesion and entry
Bushra Yasin, Wei Wang, Mabel Pang, Betsy C Herold, Natalia Cheshenko, Elizabeth A Wagar, Alan J Waring, Teresa Hong, Robert I Lehrer J Virol . 2004 May;78(10):5147-56. doi: 10.1128/jvi.78.10.5147-5156.2004.
We tested the ability of 20 synthetic theta defensins to protect cells from infection by type 1 and type 2 herpes simplex viruses (HSV-1 and -2, respectively). The peptides included rhesus theta defensins (RTDs) 1 to 3, originally isolated from rhesus macaque leukocytes, and three peptides (retrocyclins 1 to 3) whose sequences were inferred from human theta-defensin (DEFT) pseudogenes. We also tested 14 retrocyclin analogues, including the retro, enantio, and retroenantio forms of retrocyclin 1. Retrocyclins 1 and 2 and RTD 3 protected cervical epithelial cells from infection by both HSV serotypes, but only retrocyclin 2 did so without causing cytotoxicity or requiring preincubation with the virus. Surface plasmon resonance studies revealed that retrocyclin 2 bound to immobilized HSV-2 glycoprotein B (gB2) with high affinity (K(d), 13.3 nM) and that it did not bind to enzymatically deglycosylated gB2. Temperature shift experiments indicated that retrocyclin 2 and human alpha defensins human neutrophil peptide 1 (HNP 1) to HNP 3 protected human cells from HSV-2 by different mechanisms. Retrocyclin 2 blocked viral attachment, and its addition during the binding or penetration phases of HSV-2 infection markedly diminished nuclear translocation of VP16 and expression of ICP4. In contrast, HNPs 1 to 3 had little effect on binding but reduced both VP16 transport and ICP4 expression if added during the postbinding (penetration) period. We recently reported that theta defensins are miniature lectins that bind gp120 of human immunodeficiency virus type 1 (HIV-1) with high affinity and inhibit the entry of R5 and X4 isolates of HIV-1. Given its small size (18 residues), minimal cytotoxicity, lack of activity against vaginal lactobacilli, and effectiveness against both HSV-2 and HIV-1, retrocyclin 2 provides an intriguing prototype for future topical microbicide development.
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