Peptide conjugates of 18β-glycyrrhetinic acid as potent inhibitors of α-glucosidase and AGEs-induced oxidation

Abstract

18β-Glycyrrhetinic acid (18β-GA) is known for several biological activities, and has been the focus of extensive research for the development of therapeutic agents. In the current study, 18β-GA-peptide conjugates 2-11 were evaluated for their in vitro α-glucosidase inhibitory and antiglycation activities. Structure-activity relationship (SAR) established and molecular interactions of active bioconjugates with the enzyme's binding sites were predicted through molecular modeling approach. In tripeptide moiety of conjugates 2−11, peptide residue at position 1 was found to have a significant role on α-glucosidase inhibition. The most active 18β-GA-peptide conjugates 5 (18β-GA-Cys1-Tyr2-Gly3), and 8 (18β-GA-Pro1-Tyr2-Gly3) exhibited several-fold potent α-glucosidase inhibition (IC50 values 20–28 μM), as compared to standard drug acarbose (IC50 = 875.8 ± 2.10 µM). Kinetic studies of potent compounds, 4–8 revealed that conjugate 5 exhibits competitive-type of inhibition, while conjugates 6–8 showed a non-competitive type of inhibition. The simulation studies also supported the kinetic results that conjugate 5 (18β-GA-Cys1-Tyr2-Gly3) inhibits the α-glucosidase enzyme by blocking its substrate binding site. AGEs-induced NO• inhibitors play an important role in controlling the inflammation associated with diabetes mellitus. The peptide conjugates 2–11 were also evaluated in vitro for AGEs-induced NO• inhibition using RAW 264.7 macrophage cell line. Our data revealed that conjugates 7–10 were the more potent AGEs-induced NO• inhibitors, comparable to standards rutin, and PDTC. The peptide conjugate 5 (a competitive inhibitor of α-glucosidase) also exhibited a strong inhibitory activity against AGEs-induced NO• production. Furthermore, peptide conjugates 2–11 were found non-cytotoxic to mouse fibroblast NIH-3T3, and murine macrophages RAW 264.7 cell lines. In conclusion, our data demonstrates that besides possessing strong α-glucosidase inhibition, the newly synthesized peptide conjugates also alleviated the AGEs-induced NO• production in RAW macrophages. Dual inhibition of α-glucosidase enzyme, and AGEs-induced NO• production by 18β-GA-peptide conjugates qualify them for further research in anti-diabetic drug discovery.