Structural Characterization and Pro-inflammatory Activity of a Thaumatin-Like Protein from Pulp Tissues of Litchi chinensis

The objective of this study was to extract and purify proteins from the pulp tissues of litchi and evaluate their structure and pro-inflammatory activity. The results showed that a highly pure litchi protein was identified as the litchi thaumatin-like protein (LcTLP) by nano LC–MS/MS and verified by sequencing of the LcTLP gene. The molecular weight was 24 kDa, and the main secondary structure was a β sheet (33.00 ± 2.86%). Small-angle X-ray scattering results showed that LcTLP was a spherical particle (diameter of approximately 140 to 165 nm) with a close internal and rough surface in solution. The assay of pro-inflammatory activity in vitro revealed that the expression of inducible nitric oxide synthase and cyclooxygenase-2 genes reached 9.71 ± 0.64 and 7.05 ± 1.00 after 200 μg/mL LcTLP stimulation, which were 7.05-fold and 9.61-fold that of the blank control, respectively. LcTLP promoted the gene expression and production of pro-inflammatory cytokines, including tumor necrosis factor-α and interleukin-1β, and it also enhanced the expression of p65, which is a key component of nuclear factor-κ B signaling pathways. Additionally, the levels of anti-inflammatory cytokines interleukin-10 and transforming growth factor-β1 increased after LcTLP stimulation.