Polyphenol Oxidase from Dendrobium officinale Flowers: Characterization and Browning-Promoting Mechanism

Polyphenol oxidase (PPO) triggered browning during the storage and processing of Dendrobium officinale flower (DoF), impairing their nutritional and aesthetic qualities. This study aimed to investigate the enzymatic properties and enzymatic browning mechanisms of PPO in DoF, providing a theoretical foundation for developing effective strategies to inhibit enzymatic browning. PPO with high purity was obtained by ammonium sulfate precipitation and ion-exchange chromatography, further identified by electrophoresis and LC-MS/MS. The PPO exhibited optimal activity at 35 °C and pH 6.5, while ascorbic acid, glutathione, and L-cysteine significantly inhibited its activity and furanones demonstrated potential inhibitory effects. Moreover, caffeic acid, identified as the most active PPO substrate in DoF, was determined to be the primary contributor to browning. Molecular docking revealed that caffeic acid exhibited stronger interactions with PPO compared with other phenolic compounds, which further revealed the substrate selection mechanism of PPO and the molecular basis of DoF browning.