Identification of Antioxidant Cysteine-stabilised Peptides of Morinda lucida Benth. Leaf

Pathogenesis of several diseases has been attributed to free radical modification of biomolecules within the body, which can be prevented or treated with antioxidants, especially those of natural origin. Some plant peptides have been reported to be effective antioxidants. Cysteine-stabilised peptide content of Morinda lucida leaf was extracted and purified using solvent extraction, column chromatography, reversephase high performance liquid chromatography and the type of cysteine-stabilised peptide present was determined using matrix-assisted laser desorption ionisation time of flight mass spectrometry analysis and ninhydrin staining. Antioxidant activities of the partially purified peptide fraction were evaluated using in vitro models. The presence of linear cysteine-stabilised peptide with masses ranging from 3.601 to 3.678 kDa and having three disulphide bonds was confirmed. Partially purified peptide fraction exhibited 2,2-diphenyl-1-picrylhydrazyl radical scavenging activity and ferric reducing antioxidant power, which is statistically compared with that of butylated hydroxytoluene but displayed statistically lower total antioxidant capacity compared to glutathione. Nitric oxide scavenging activity of partially purified peptide fraction was statistically lower compared to ascorbic acid. Hydroxyl radical scavenging activity of partially purified peptide fraction was significantly higher (p<0.05) compared with that of glutathione at higher concentrations. Cysteinestabilised peptide, identified for the first time in M. lucida leaf, possesses antioxidant properties and they may be effective in the management of reactive oxygen species-mediated pathological conditions but not reactive nitrogen species-mediated pathological conditions.