4. Discussion
Having identified two peptides SR11 and IR15 from typtic digest (10kDAGP) of Abrus agglutinin, it was expected that these peptides would show one or more properties that were associated with 10kDAGP. When checked for cytotoxicity by MTT and hemolytic assay, both the peptides were found to be non-toxic at higher concentrations (up to 50 μg/ml). IR15 showed toxicity at higher concentrations (IC50 = 100 μg/ml or 60 μM) while SR11 was non-toxic even at higher doses (IC50 > 100 μM). Moreover, none of the peptides had IC50 value lower than that of 10kDAGP. In our previous reports [20], it was shown that component peptides of 10kDAGP entered into the cells and co-localized with the mitochondria. Thus, it was hypothesized that these two peptides might possess CPP-like properties, facilitating the internalization of other cytotoxic peptide components of 10kDAGP. In line with this hypothesis, it was found that FITC labeled IR15 and SR11 peptides readily entered into the cells. Trypsinization before analysis by flow cytometry removes non-specific electrostatic interaction between highly cationic peptides with negatively charged membrane [35] and [36]. Hence, the uptake of peptides was not non-specific or just an artifact of electrostatic interaction. However, unlike 10kDAGP (which partly localized to the mitochondria), localization of FITC labeled peptides was partly (40%) in nucleus. As the localization experiments were performed on live cells, nuclear co-localization was due to peptide and not due to fixation artifacts [35]. Nuclear localization may be another reason behind the non-toxic nature of SR11 and IR15 in comparison to 10kDAGP which acts through mitochondria. According to the earlier reports, CPPs like TAT, antennapedia, polyarginine and transportan also do not show toxicity (IC50 > 100 μM) when treated in free form. However, except antennapedia, other CPPs showed higher toxicity when the CPPs were conjugated with small peptide sequence or Rhodamine dye [37]. Hence, further studies regarding the conjugation of small or bulky group to IR15 and SR11 peptide are necessary to evaluate their toxicity and eligibility for their clinical applications.