Comparison of Inhibitory Effects of Cinnamic Acid, β-Cyclodextrin, L-Cysteine, and Ascorbic Acid on Soluble and Membrane-Bound Polyphenol Oxidase in Peach Fruit

There has been considerable interest in controlling polyphenol oxidase (PPO) activity to prevent enzymatic browning in foods. However, studies on inhibitions of different forms of PPO are very limited. Thus, this study focuses on the effects of cinnamic acid, β-cyclodextrin, L-cysteine, and ascorbic acid on soluble PPO (sPPO) and membrane-bound PPO (mPPO) in peach fruit. The activity of partially purified sPPO was 3.17 times higher than that of mPPO. However, mPPO was shown to be more stable than sPPO in the presence of inhibitors with different concentrations (i.e., 1, 3, 5 mM); activation of mPPO was found by 5 mM L-cysteine. Both sPPO and mPPO inhibitions were PPO substrate-dependent. Ascorbic acid showed the highest inhibitory effect on both sPPO and mPPO with all studied inhibitors and substrates. The inhibition of 1 mM ascorbic acid on sPPO and mPPO reached 95.42 ± 0.07% and 65.60 ± 1.16%, respectively. β-Cyclodextrin had a direct inhibitory effect only on sPPO, while the other three inhibitors had direct effects on both sPPO and mPPO. Cinnamic acid exhibited a non-competitive inhibition on sPPO and mPPO, with L-cysteine showing the same, though on sPPO. The inhibition of studied inhibitors on sPPO and mPPO is highly related to the substrate environment, type, and concentration of inhibitors. This study provides a basis for the further prevention of peach fruit browning from the perspective of different enzyme forms.

Chemical inhibition of polyphenol oxidase and cut surface browning of fresh-cut apples

Fresh-cut apples, which offer consumers health benefits and convenience, have become popular in recent years. One of the main challenges for processing fresh-cut apples is rapid development of cut surface browning, immediately after fruits are cut. Browning, a physiological response that impacts organoleptic properties and deters consumer purchase of fresh-cut fresh produce, is mainly a result of enzymatic reaction of phenolic compounds with oxygen catalyzed by polyphenol oxidase (PPO), a decapper enzyme. Many antibrowning agents have been developed and evaluated to inhibit PPO activities by using reducing agents (antioxidants), chelating agents, acidulants, etc. The present manuscript reviews the diverse characteristics of PPO (such as optimum pH and temperature, and molecular weight) in apples reported in the literature and the enzyme's latency, multiplicity and copper states in the active site. It also summarizes the latest development in the investigation and formulations of antibrowning compounds, and discusses future research needs. This review should stimulate further research to discover more effective, low cost, and natural antibrowning compounds to meet the demand of consumers as well as the food industry for clean label and long shelf-life of fresh-cut apples.

Evaluation of quercetin omega-6 and -9 esters on activity and structure of mushroom tyrosinase: Spectroscopic and molecular docking studies

Quercetin is one of the most ubiquitous dietary flavonoids widely distributed in plants and foods of plant origin, and is a potent tyrosinase inhibitor. Quercetin fatty esters could lead to an improve in quercetin lipophilicity which could positively affect its pharmacological activity. In this study, the inhibitory effect of two novel esters of quercetin-linoleic acid (ligand A) and quercetin-oleic acid (ligand B) has been investigated on structure and diphenolase activity of mushroom tyrosinase (MT) by experimental and molecular docking techniques. The inhibitory kinetics study using UV-visible spectrophotometry in the presence of its substrate 3,4-dihydroxyphenylalanine (L-dopa), revealed that both esters successfully inhibit the activity of tyrosinase and reduce the formation of dopaquinone. Results showed that the binding of ligands to MT induced rearrangement and conformational changes of the enzyme. Thermodynamic parameters of these interactions (K_a , ∆G°, ∆H° and ∆S°) were obtained at pH = 6.8 and temperatures of 298 and 310 K. Molecular docking studies further was applied to calculation of binding energies (ΔG_bA  = -21.84 kJ/mol, ΔG_bB  = -20.92 kJ/mol), inhibition constant values (K_IA  = 160 µM, K_IB  = 220 µM) and the special binding site. It can be deduced that ligands act as a potential tyrosinase inhibitor and it was found that the best possible interaction condition with binding modes visualize was achieved by ligand A and exhibited the potent tyrosinase inhibitory activity. These findings may be helpful to understand the inhibition mechanism of quercetin fatty acids esters on tyrosinase and provide a convenient screening method to differentiate phenolic tyrosinase inhibitors. PRACTICAL APPLICATIONS: Bioavailability and antioxidant activity of conjugated fatty acids with their bioequivalence in several biological effects and metabolic processes such as beta-oxidation from various forms has been reported to be highly variable and useful. Quercetin shows beneficial role in human health, but its biological effects in vivo is limited by poor bioavailability, low skin permeability and solubility. This study design new tyrosinase inhibitors which helpful to functional research of unsaturated fatty acid esters in the treatment of inflammatory diseases and hyperpigmentation disorders. In addition, undesirable enzymatic browning of plant derived-foods by tyrosinase causes a decrease in market value and economic loss of food products. The results suggest that the conjugation of quercetin with linoleic and oleic acids resulted in novel stronger tyrosinase inhibitors which may have therapeutic applications and replacement of toxic tyrosinase inhibitors and contribute as anti- browning agents in food, cosmetic and pharmaceutical industry.