Antioxidant dipeptide Threonyltyrosine from Porphyridium cruentum: in silico prediction and in vitro protection against paraquat-induced HaCaT cell damage.

Marine microalgae rich in high-quality proteins are highly favored in the development of bioactive peptides. However, the scarcity of effective preparation methods still limits its application in healthy improvement. This study aimed to search for the effective natural antioxidative peptides from the microalga Porphyridium cruentum (P. cruentum) for protection skin cells against chronic oxidative stress by UPLC-Q-Exactive MS identification coupled to in silico prospection. An antioxidative dipeptide threonyltyrosine (TY) was identified among 425 peptides. The in vitro antioxidant capacity of TY was slightly higher than that of vitamin C in ABTS•+ radical scavenging assays with an IC50 of (11.37 ± 1.32) ×103 µg/L. The significant protective role of TY against paraquat-induced cytotoxicity in human keratinocyte (HaCaT) cells was observed in cell viability (+ 10.1%, P < 0.001), MDA (- 23.6%, P < 0.001) levels, and enzyme activities, i.e., superoxide dismutase (SOD, + 17.1%, P < 0.001) and catalase (CAT, + 20.5%, P < 0.001). TY showed a dose-dependent effect (30-100 µg/mL) in upregulating the proliferative activity of paraquat-injured HaCaT cells.In addition, TY stable binding to the Kelch domain of Keap1 (ΔG = -5.13 kcal/mol) was analyzed via molecular docking and molecular dynamics simulations, which gave some clues on the potential activating on Nrf2. Totally, TY exhibited a dual mechanism of free radical scavenging and antioxidant enzymes activation. These findings advanced our understanding of TY's development, properties and provided a foundation for future mechanistic investigations.