Purification, characterisation, and thermal denaturation of polyphenoloxidase from prawns (Penaeus vannamei)

Halophytes as a potential source of melanosis-inhibiting compounds. Mechanism of inhibition of a characterized polyphenol extract of purslane (Portulaca oleracea)

The market value of crustaceans depreciates during storage due to the appearance of melanosis caused by polyphenol oxidases. Sulfite derivatives are used as melanosis-inhibiting agents, but their unhealthy effects make it preferable to replace them with natural preservatives. In this work, a crude enzymatic extract from whiteleg shrimp (Penaeus vannamei) was characterized and used to test the diphenol oxidase-inhibiting activity of polyphenol extracts of five underutilized halophyte plants, namely crystalline ice plant, seaside arrowgrass, purslane, sea fennel, and seashore aster. The extracts inhibited diphenol oxidase activity more efficiently than sodium sulfite. The purslane extract was rich in isoorientins, isovitexin, and apigenin, and showed the highest inhibiting effect, being this classified as mixed or non-competitive. Hydroxyl groups in the phenyl B ring could be responsible for the inhibitory activity of the extract. The polyphenol extracts tested in this work could be promising melanosis-inhibiting agents of interest for seafood industries.

Effects of metal ions on activity and structure of phenoloxidase in Penaeus vannamei

Phenoloxidase (PO) is a typical metal enzyme, which requires metal ions as prosthetic groups to enable the full exertion of its activity. To study how metal ions affected the activity and structure of PO enzymes, while providing reference materials for in-depth investigations, we examined the effects of different metal ions (Cu²⁺, Zn²⁺, Mg²⁺, Ca²⁺, and Ba²⁺) on their activities. Furthermore, Cu²⁺ and Mg²⁺ were selected for further investigation through UV spectra, intrinsic fluorescence spectroscopy, AFM, and FTIR. It was revealed that Cu²⁺ had a more obvious effect on PO compared to Mg²⁺. The PO could be activated when the concentrations of Cu²⁺ and Mg²⁺ were lower than 10^-3 and 10^-2 mol/L, respectively, and maximum PO activities (182.14% and 141.02%) were observed at 10^-4 mol/L concentrations of Cu²⁺ and Mg²⁺. When the concentrations of Cu²⁺ and Mg²⁺ were higher than 10^-2 and 10^-1 mol/L, the activities PO were inhibited. The results of the UV-vis and fluorescence spectra revealed that Cu²⁺ shaped the tertiary structure of PO, whereas the effect of Mg²⁺ was slight. The AFM results demonstrated that high concentrations of Cu²⁺ and Mg²⁺ resulted in PO aggregation. FTIR analysis indicated that the total content of PO α-helices and β-sheets decreased with higher concentrations of Cu²⁺ and Mg²⁺.

Changes in amino acids and bioactive compounds of pigmented rice as affected by far-infrared radiation and hot air drying

Far infrared radiation (FIR) and hot air drying (HA) were applied to unpolished and polished pigmented rice varieties. Ferulic acid is a major phenolic acid in pigmented rice, being three-fold greater in bound form than in soluble form. Overall, FIR increased total phenolic and flavonoid contents, anthocyanin, tocopherols as well as antioxidant capacity, whereas the opposite results were observed for HA in all samples. Gallic, and ferulic acids were significantly (p < 0.05) increased in FIR dried samples. Quercetin content was significantly increased in unpolished rice dried by FIR and HA. Cyanidin-3-glucoside was greatly increased by FIR. Amino acids were decreased less by HA than did FIR. Our findings suggest that HA and FIR may have a significant effect on the internal structure of the grain, which may increase the yield of extraction of some components, including the bioactives.