Free Radical Scavenging Properties and Induction of Apoptotic Effects of FA Fraction Obtained after Proteolysis of Bioactive Peptides from Microalgae <i>Synechococcus </i>sp. VDW

Abstract

This study aims to determine the antioxidant activity of bioactive peptides derived from <i>Synechococcus</i> sp. VDW cells cultured for 21 days. <i>Synechococcus</i> sp. VDW protein hydrolysates were prepared with trypsin and purified by ultrafiltration with molecular mass cut-off membranes of 10, 5 and 3 kDa. The <i>M</i><3 kDa (F_A) fraction had the highest 2,2'-azino-bis(3-ethylbenzothiazoline-6-sulphonic acid) (ABTS) and 2,2'-diphenyl-1-picrylhydrazyl (DPPH) radical scavenging activities, with IC₅₀ values of (11.5±0.3) and (13.6±0.2) µg/mL, respectively. The F_A fraction was separated by reversed phase HPLC to yield four subfractions (F_1-4). The F₄ subfraction showed the highest maximum ABTS radical scavenging activity (3.55±0.61) % and it was selected for further analysis by electrospray ionisation quadrupole time-of-flight mass spectrometry (ESI-Q-TOF-MS/MS) based on <i>de novo</i> peptide sequencing. Five antioxidant peptides were identified, of which AILESYSAGKTK had the highest ABTS radical scavenging activity. Furthermore, the F_A fraction showed high cytotoxic activities against human cancer-derived cell lines, especially the colon cancer cell line (SW620) with an IC₅₀ value of (106.6±21.5) µg/mL, but not the untransformed Wi38 cell line. The F_A fraction activated the apoptotic pathway in SW620 cells after treatment for 24, 48 and 72 h, with the highest activities of caspases-3, -8 and -9 being observed after treatment for 72 h. These findings suggested that microalgae <i>Synechococcus</i> sp. VDW may be used to develop natural anticancer drugs.