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• EGCG exhibited high affinity for and mixed-type inhibition on PPO. • Multispectral experiments showed the structural changes of PPO after binding with EGCG. • Molecular docking and dynamic simulation revealed EGCG directly bind to the active center of PPO. • The effect of inducing aggregation of EGCG to PPO was identified. • The PPO inhibitory mechanism of EGCG included competing with substrates and induced aggregation. Polyphenol oxidase (PPO)-mediated enzymatic browning occurs in fruit, vegetables and aquatic products and causes huge economic losses every year. In this study, epigallocatechin-3-gallate (EGCG) displayed high affinity for and efficient inhibitory capacity against PPO. To explore the inhibition mechanism, multispectroscopic methods and computational simulations were implemented. Initially, EGCG inhibited PPO activity reversibly in a mixed-type manner. Then, the conformation and secondary structure changes of PPO after binding with EGCG were discovered by fluorescence emission spectra and circular dichroism. Molecular docking and dynamic simulation results revealed that EGCG could tightly bind with the binuclear copper domain of PPO through hydrophobic stacking and hydrogen bonds. Moreover, EGCG might act as a linker to interact with different PPO molecules at another binding site. Transmission electron microscopy observation suggested that EGCG induced the aggregation of PPO. Therefore, the inhibition mechanism of EGCG on PPO included competition for catalytic centers and induced aggregation. [ABSTRACT FROM AUTHOR]

Abstract: This paper investigates polyphenol oxidase (PPO) activity, reduced weight percentage after sun drying, and the changes in colour and appearance of green pepper (Piper nigrum Linnaeus) berries after blanching and sun drying. The results show that the degree of reduced weight percentage and browning in green pepper berries after blanching for 10min is greater at 100°C than at 90 and 80°C. Moreover, the samples blanched at 100°C for 10min had the fastest water loss, but the lowest PPO activity. Thus, the PPO enzymatic oxidation of polyphenols might not be the only reason for the browning of green pepper berries. This result is significantly different from that of Variyar, Pendharkar, Banerjeea, and Bandyopadhyay (1988) and therefore deserves further study. [Copyright &y& Elsevier]

Abstract: Eggplant (Solanum melongena) is a very rich source of polyphenol oxidase (PPO), which negatively affects its quality upon cutting and postharvest processing due to enzymatic browning. PPO inhibitors, from natural or synthetic sources, are used to tackle this problem. One isoform of PPO was 259-fold purified using standard chromatographic procedures. The PPO was found to be a 112kDa homodimer. The enzyme showed very low Km (0.34mM) and high catalytic efficiency (3.3×106) with 4-methyl catechol. The substrate specificity was in the order: 4-methyl catechol>tert-butylcatechol>dihydrocaffeic acid>pyrocatechol. Cysteine hydrochloride, potassium metabilsulphite, ascorbic acid, erythorbic acid, resorcylic acid and kojic acid showed competitive inhibition, whereas, citric acid and sodium azide showed mixed inhibition of PPO activity. Cysteine hydrochloride was found to be an excellent inhibitor with the low inhibitor constant of 1.8μM. [Copyright &y& Elsevier]

Abstract: The inhibitory effect of onion extract on polyphenol oxidase and enzymatic browning of taro was investigated. The polyphenol oxidase from taro was strongly inhibited by various reducing agents, such as l-ascorbic acid, l-cysteine, dithiothreitol, glutathione and sodium pyrosulfite. The enzyme was also inhibited by addition of onion extract. Regardless of substrates used, the addition of heated onion extract at 100°C for 10min, gave a stronger inhibitory effect on taro polyphenol oxidase activity than did fresh unheated extrtact. The inhibitory effect of onion extract was dependent on heating temperature and time. The addition of glucose, glycine, or both to the onion extract, during heating, stimulated the inhibitory effect of the onion extract, suggesting that non-enzymatic browning products, produced during heating, might be responsible for the stronger inhibitory action of the heated onion extract. [Copyright &y& Elsevier]

Abstract: Polyphenol oxidase (PPO) was extracted from bayberry (Myrica rubra Sieb. et Zucc. cv. Biqi), and its partial biochemical characteristics were studied. Stable and highly active PPO extracts were obtained using insoluble polyvinylpolypyrrolidone (PVPP) in sodium phosphate, pH 7.0, buffer. The highest PPO activity was observed in the ripe fruits. Optimum pH and temperature for bayberry PPO activity were pH 6.0 and T =30°C with 0.1M catechol as substrate. PPO showed activity using the substrates of catechol, gallic acid and protocatechuic acid, but no activity with the substrates (+)-catechin, p-coumaric acid or caffeic acid. K m and V max values were 313mM and 3.26ΔOD/min/g FW, respectively, with catechol as the substrate. Bayberry PPO did not act directly on cyanidin 3-glucoside but the rate of anthocyanin degradation was stimulated by the addition of gallic acid. [Copyright &y& Elsevier]

• Salicylic acid inhibited PPO activity via acidification and competitive inhibition. • The acidification effect played a leading role in weak buffer systems. • The competitive inhibition was predominant in strong buffer systems. • Salicylic acid bind to PPO mainly by hydrogen bond and hydrophobic interaction. • Acid pH enhanced binding effect by added hydrogen bond and electrostatic interaction. Salicylic acid is generally considered to combine with polyphenol oxidase (PPO) to inhibit activity and enzymatic browning, while its acidification effect on PPO activity was usually neglected. In this study, the inhibitory mechanism of salicylic acid on PPO was examined from acidification and binding effects by altering the buffer conditions. As the buffer concentration increased, contribution of acidification decreased while the binding effect became more predominant. Salicylic acid exhibited competitive inhibition on PPO, inducing the changes in secondary structure with a reduction in α-helix. Molecular docking results showed that salicylic acid interacted with residues HIS61, HIS85, HIS259, HIS263 and VAL283 through hydrogen bond and hydrophobic interaction. Furthermore, acidic pH enhanced the binding of salicylic acid to PPO with lower binding energy, additional hydrogen bond and electrostatic interactions. Therefore, both acidification and binding effects were important for salicylic acid on PPO inhibition and enzymatic browning control in fruit and vegetables. [ABSTRACT FROM AUTHOR]