Ultrasonic Processing Induced Activity and Structural Changes of Polyphenol Oxidase in Orange ( Citrus sinensis Osbeck)

Apart from non-enzymatic browning, polyphenol oxidase (PPO) also plays a role in the browning reaction of orange ( Osbeck) juice, and needs to be inactivated during the processing. In this study, the protein with high PPO activity was purified from orange ( Osbeck) and inactivated by ultrasonic processing. Fluorescence spectroscopy, circular dichroism (CD) and Dynamic light scattering (DLS) were used to investigate the ultrasonic effect on PPO activity and structural changes on purified PPO. DLS analysis illustrated that ultrasonic processing leads to initial dissociation and final aggregation of the protein. Fluorescence spectroscopy analysis showed the decrease in fluorescence intensity leading to the exposure of Trp residues to the polar environment, thereby causing the disruption of the tertiary structure after ultrasonic processing. Loss of α-helix conformation leading to the reorganization of secondary structure was triggered after the ultrasonic processing, according to CD analysis. Ultrasonic processing could induce aggregation and modification in the tertiary and secondary structure of a protein containing high PPO activity in orange ( Osbeck), thereby causing inactivation of the enzyme.