Avocado Proanthocyanidins as a Source of Tyrosinase Inhibitors: Structure Characterization, Inhibitory Activity, and Mechanism

Proanthocyanidins were purified from avocado (Persea americana) fruit, and their structures were analyzed by matrix-assisted laser desorption/ionization–time-of-flight mass spectrometry (MALDI–TOF MS) and high-performance liquid chromatography–electrospray ionization–QTRAP mass spectrometry (HPLC–ESI–QTRAP MS) techniques. The results obtained from mass spectrometry (MS) analysis demonstrated that the proanthocyanidins were homo- and heteropolymers of procyanidins, prodelphinidins, propelargonidins, and procyanidin gallate. From the enzyme analysis, the results showed that they could inhibit the monophenolase and diphenolase activities of tyrosinase. The inhibition mechanism of the proanthocyanidins on the enzyme was further studied, and the results indicated that they were reversible and competitive inhibitors. Finally, the results acquired from molecular docking, fluorescence quenching, and copper ion interacting tests revealed that adjacent hydroxyl groups on the B ring of proanthocyanidins could chelate the dicopper catalytic center of the enzyme. In addtion, proanthocyanidins were proven to be an efficient quencher of substrates. This study would lay a scientific foundation for their use in agriculture, food, and nutrition industries.