1. INTRODUCTION
A heart of palm is rich in essential minerals, antioxidants, carbohydrates, protein, lipids, and vitamins [1]. The Mediterranean palm (Chamaerops humilis. L), named Doom in North Africa, is an endemic palm of the Mediterranean basin. The “Doum” has shown anti-inflammatory, anabolic, antiseptic, urinary, antilithic and diuretic activities [2]. In Morocco, the heart of this palm is very appreciated by the consumer for its antioxydant content and anti-aging effect but it is marketed by the rural population in its raw state without any processing, which decreases its commercial value. This is because it is very sensitive to air contact and undergoes rapid browning when peeled due to the existence of the polyphenol oxidase (PPO) enzyme, which reacts with endogenous polyphenol [3].
PPOs are enzymes widely present in plants. They belong to a group of copper-containing metalloproteins. The presence of the copper atom allows the enzyme to transform o-diphenols into o-quinones. The reaction continues with the polymerization of the resulting quinine products to give colored substances [4]. Two types of polyphenols are involved in this reaction: monophenol, which gives cresolase activity, and o-diphenol, which gives catecholase activity [5]. This reaction plays a great role in the food industry’s transformation. Thus, PPO from several plant tissues has been intensively studied, such as from apple [6], tadela (Phoenix dactylifera L.) [7], buriti [8], cucumber [9], sweet potato [10,11], blackberry [12], Kirmizi Kismis grape [13], and African bush mango [14].
The control of PPO activity of the vegetable from harvest to the consumer is necessary to minimize browning of the vegetable and maintain its economic value. Enzymatic browning of vegetables can be limited by several techniques, including the use of reaction inhibitors, physical treatments such as high pressure [15], microwave treatment [16], and the use of modified atmosphere packages [17], and the use of chemical agents such as phitic acid [18], ascorbic acid [19], essential oils [20], sulfiting agents, cysteine, glutathione, inorganic halides, acidulants such as citric, malic, and phosphoric acids [21], kojic acid [22], captopril [23], and thiopronine [24]. Finally, thermal treatment is an important physical technique used in the food industry. This process was finding able to inactivate PPO from mango slices and chili powder by heating at 80°C for 10 min [25,26].
In this study, PPO was extracted and purified from the heart of Mediterranean palm, and its kinetic parameters were determined for 4-methyl catechol, catechol, gallic acid, chlorogenic acid, pyrogallol, and tyrosine. Because the majority of the well-known PPO inhibitors have been shown to be toxic or possibly detrimental to biologic systems, their use in the food industry must be limited. The present study was the first on thermal treatment and kinetic inhibition of PPO from the heart of Mediterranean palms in the presence of a non-toxic sulfhydryl amino acid (L-cysteine). The effects of temperature, pH, and other chemical inhibitors on PPO activity, the inactivation rate constant (k), half-life time (t1/2), reduction time (D), and some thermodynamic quantities were also determined.
2. MATERIALS AND METHODS
2.1. Materials
4-methyl catechol, catechol, galic acid, chlorogenic acid, pyrogallol, tyrosine, L-cysteine, sodium florid, citric acid, sodium dihydrogen phosphate, sodium hydrogen phosphate, poly (vinyl-pyrrolidone), ascorbic acid, ammonium sulfate, and triton X100 were purchased from Sigma-Aldrich Inc. PPO was extracted from the heart of the Mediterranean palm (C. humilis L.) by blender and centrifuge (Sigma 3–18 K). Absorbance was measured using an ultraviolet (UV)-MINI 1240 Shimadzu spectrophotometer.
2.2. Enzyme Extraction
PPO extraction was realized as described in the previous studies [6-8]. The used solutions are 100 mM phosphate buffer (pH = 6.8) containing 10 mM ascorbic acid, 0.5% polyvinylpyrrolidone, and 1% of Triton X100. The enzyme solution was precipitated with 80% saturation ammonium sulfate. The pellet was dissolved in 0.1 M sodium phosphate buffer, pH 6.8, and dialyzed at 4°C against phosphate buffer for 24 h.
2.3. Protein Quantity
The Bradford method, using bovine serum albumin as a standard, was used to determine the protein contents of the enzyme extracts [27].
2.4. Assay of PPO Activity
Spectrophotometry was used to determine PPO activity. The assay mixture was performed at room temperature with 0.2 mL of enzyme and 2.8 mL of substrate dissolved in a 100 mM phosphate buffer solution. 3 min after mixing the reagents, the change in absorbance was measured using a UV-MINI 1240 Shimadzu spectrophotometer. The linear part of the plot of absorbance (A) against time (t) was used to calculate PPO activity. The enzyme activity unit (U) was taken as an increase of 0.001 units of absorbance per minute per mL of enzyme [28,29].
2.5. Enzyme Kinetics and Substrate Specificity
A mixture containing 0.2 ml of enzyme and 2.8 ml of substrate (4-methyl catechol, catechol, galic acid, chlorogenic acid, pyrogallol, or tyrosine) was prepared at 25°C at the different finale concentrations of 5, 10, 20, 30, 40, and 50 mM C in a phosphate buffer solution (100 mM, pH = 6.8). Then, PPO activity was measured by spectrophotometry. The Lineweaver–Burk Method (plot of 1/V vs 1/[S]) was used to determine KM and Vmax, and the Vmax/KM ratio, referred to as “catalytic efficiency’’, was calculated [28,29].
2.6. Effect of pH on PPO Activity
A reaction mixture containing 0.2 mL of enzyme, 0.5 mL of catechol, and 2.3 mL of buffer solution of 4.0–8.0 using 0.1M acetate (pH = 4–6) and 0.1 M phosphate (pH = 6–8) was prepared, and the enzyme activity was assayed as above. Then the optimal pH for PPO activity was determined [30].
2.7. Effect of Temperature on PPO Activity
The effect of temperature on enzyme activity was determined by increasing the temperature from 20°C to 80°C using a shaking water bath. The reaction mixture contained 0.2 mL of enzyme, 0.5 mL of catechol (30 mM), and 2.3 mL of phosphate buffer (100 mM). The phosphate buffer and PPO extract were heated in test tubes. The temperature of the reaction mixture was monitored until the predetermined temperature was reached. One minute later, catechol was added, and the enzyme activity was assayed as above [26,28].
2.8. Effect of Inhibitors on the Enzyme Activity
PPO activity of substrate was determined in the presence of inhibitors (L-cysteine, citric acid, and sodium floride) at the different final concentrations. A plot of 1/V vs. 1/[S] for different concentrations of inhibitor was used to estimate the inhibition constant (Ki) of each inhibitor [11].
2.9. Thermal Treatment of PPO
The thermal study of PPO was realized according to the Benaceur et al. [7] method in the range of 45–80°C for 0, 10, 20, 30, 40, and 50 min, respectively. A mixture containing 0.2 ml of enzyme, 0.5 ml of catechol (30 mM), and 2.3 ml of phosphate buffer was heated until the predetermined temperature and for the predetermined time. Then, residual enzyme activity was measured [7]. The slope of the natural logarithm (ln) of the At/A0 vs. time graph was used to estimate the first-order inactivation constant (k), where At is the enzyme activity after thermal treatment and A0 is the enzyme activity before thermal treatment [7]. The following equation, t1/2= ln(2)/k, was used to calculate the half-life of the enzyme (t1/2). Decimal reduction time (D value) was calculated from the relationship between k and D value: D = ln(10)/k [7].
2.10. Thermodynamic Analysis
Arrhenius’ law, expressed by ln(k) = ln(A) − (Ea/R) * (1/T), was used to evaluate the relation between the values of k and the temperature [7], where R is the universal gas constant (8.314 J mol−1.K−1) and A is the Arrhenius constant. The slope of the line, obtained using the Arrhenius diagram, was used to estimate activation energy (Ea).
The following formula, ΔH = Ea – RT, was used to calculate the enthalpy of activation (ΔH) for each temperature. Free energy of inactivation (ΔG) was calculated using the expression: ΔG = −R * T * ln(k * hP/KB * T), where KB is the Boltzmann constant (1.38 × 10−23 J/K), hP is the Planck constant (6.626 × 10−34 J s), T is the absolute temperature, and k (s−1) is the inactivation rate constant of each temperature [7].
2.11. Statistical Analyses
Statistical analyses of all experimental data on PPO activity for different parameters for all assays were performed in three replicates, and values were expressed as mean ± SD. The collected data were analyzed using the analysis of variance procedure. Statistical analyzes were performed using the statistical software Statistical Analysis System (SAS, version 9.1, 2002).
3. RESULTS AND DISCUSSION
3.1. Extraction and Purification of PPO
Ammonium sulfate precipitation was used to remove sugars and high-molecular-weight proteins and to concentrate the sample [18]. Protein concentrations of the crude extract estimated using the Bradford method were 17.2 mg (0.2 ml of enzyme) and 8.6 mg/100 g. At subsequent steps of purification, the PPO activity per ml increased and, as expected, the protein (PPO) content decreased. After ammonium sulfate fractionation, 44.6% of activity remained (1234 U/min), while 83.7% of proteins were removed. After dialysis, 20% of activity remained (571 U/min), while 95% of proteins were removed [Table 1].
Table 1: Partial purification of polyphenol oxidase from Mediterranean palm heart.
| Steps | Total volume (ml) | Total protein (mg) | Total activity (U/min) | Specific activity (U/mg protein) | Yield (%) | Purification (fold) |
|---|---|---|---|---|---|---|
| H2O extract | 100 | 8.6±0.56 | 2763±123 | 321±23 | 100±0 | 1 |
| 80% (NH4)2SO4 | 35 | 1.4±0.16 | 1234±56 | 881±67 | 44.6±3.8 | 2.7 |
| Fraction dialyzed | 15 | 0.45±0.032 | 571±32 | 1268±73 | 20±2 | 3.9 |
3.2. PPO Kinetics Parameters
PPO from Mediterranean palm hearts showed no activity using mono-hydroxyl phenol, low activity using trihydroxy phenol, but showed much greater activity with the o-dihydroxy phenol [Table 2]. It can therefore be concluded that the Mediterranean palm heart PPO has diphenolase activity but does not exhibit monophenolase activity. Similar results were observed with PPO obtained from other plants such as mango [31], atemoya fruit [32], apple (Malus domestica) [33], and broccoli [34]. PPO from other sources has both mono and diphenol oxidase activities, as mushrooms [28,35]. The lowest Km value was observed using 4-methyl catechol as substrate (3.53 mM), followed by catechol 5.0 mM, and the highest value was observed with pyrogallol, 28.8 mM [Table 2]. This result is consistent with those reported in the literature, where 4-methylcatechol usually has the best affinity for PPOs [29,36,37].
Table 2: Substrate specificity of polyphenol oxidase from Mediterranean palm heart.
| Substrats | Km (mM) | Vmax (Unit/min) | Ks (Unit.min-1.mM-1) |
|---|---|---|---|
| Catechol | 5.0±0.4 | 2932±57 | 575±51.2 |
| Chlorogenic acid | 9.5±1.7 | 6200.6±656 | 658±73 |
| 4-methyl catechol | 3.53±0.38 | 11402±973 | 3203±101 |
| Galic acid | 24±3.4 | 4184±521 | 182±21 |
| Pyrogallol | 28.8±4 | 7407±589 | 257±18.5 |
| Tyrosine | -- | -- | -- |
3.3. Optimum pH
The relative activity of PPO from Mediterranean palm hearts was determined at different pH values ranging from 2 to 8 using catechol as substrates [Figure 1]. PPO activity decreases when pH decreases. The optimal pH value is about 6.5, and activity was 3132 ± 57 (Units/min). Using catechol as substrate, the optimum pH of PPO activity from plants is usually in a pH range of 4–7.4. The optimum pH is 6.5 for rooster potato (Solanum tuberosum cv. rooster) [38] PPO, 6.8 for prawns [7,9] for buriti [8], and 5 for potato tuber PPO [39].
 | Figure 1: Effect of pH on Mediterranean palm heart polyphenol oxidase activity. [Click here to view] |
3.4. Optimum Temperature
The optimal temperature of PPO activity was determined at different temperatures ranging from 10°C to 80°C using catechol as substrates [Figure 2]. PPO has the highest activity at 35°C and was found to increase from 25°C to 35°C and then decline up to 40°C. At 60°C, where activity was about 40% of the maximum value. Literature data show that the optimum temperature of PPO activity depends on several factors, such as the source of the enzyme, the method of its extraction, and the type of substrate used to measure its activity [5]. The optimal temperature activity of PPO is mostly obtained between 30°C and 45°C. Rooster potato have optimum temperature of 30°C [38], 35°C for Buriti [8] and it is 45°C for PPO from prawns [40].
 | Figure 2: Effect of temperature on Mediterranean palm heart polyphenol oxidase activity. [Click here to view] |
3.5. Effects of Inhibitors on the Enzyme Activity
Among the inhibitors used, L-cysteine was the most effective for Mediterranean palm heart PPO. Catechol activity was 50% when the concentration of L-cystein was 10 mM (IC50=10 mM), declined to 5% when the concentration was 25 mM, and fell to 1% when the concentration of the inhibitor was 75 mM [Figure 3]. Thus, we conclude that L-cysteine may be used to prevent browning by PPO and to control PPO action in the Mediterranean palm heart.
 | Figure 3: Percent inhibition of some chemical inhibitors on Mediterranean palm heart polyphenol oxidase activity: (a) L-cystein, (b) sodium florid, and (c) citric acid. [Click here to view] |
To determine the type of inhibition, Lineweaver and Burk graphs for three concentrations of each inhibitor were used [Figure 4]. Litterateur results showed that the type of inhibition is related to the source of PPOs [5]. The present study determined that L-cysteine and sodium fluoride were non-competitive type inhibitors for PPO of the Mediterranean palm heart [Figure 4a and b]. A similar result was obtained for PPO from apples [41] and from Whangkeumbae pear [42] using L-cysteine as an inhibitor. Citric acid is an uncompetitive inhibitor [Figure 4c]. A similar result was obtained for PPO extracted from purslane, where citric acid was found as uncompetitive inhibitor [43]. Citric acid gives mixed inhibition of eggplant PPO [29]. However, citric acid was a competitive inhibitor of PPO in Chinese parsley [44].
 | Figure 4: (a) Lineweaver-Burk plots using catechol as a substrate in the presence of L-cystein. (b) Lineweaver-Burk plots using catechol as a substrate in the presence of sodium florid (c) Lineweaver-Burk plots using catechol as a substrate in the presence of citric acid. [Click here to view] |
Lineweaver-Burk graphs were used to determine inhibition constants (Ki). Following Ki values, L-cysteine was the most effective inhibitor with the lowest Ki value of 56 μM, followed by citric acid with a Ki value of 60 μM and sodium fluoride with a Ki value of 79 μM. Several studies concerning the inhibition of PPO have shown different values of the inhibition constant, dependent on the PPO source and pH. For L-cysteine, the Ki value was 1.28 μM with PPO from Ispir sugar bean [45] and 1.13 mM with PPO from lettuce tissues [46]. L-cysteine at higher concentrations (≥1.0 %), reacted with the resulted quinone to give colorless products, and the formation of quinone-sulfite complexes prevents the quinones polymerization [47]. Citric acid has a Ki value of 2.67 μM with PPO from dill [48] and 2.074 mM with PPO from lettuce tissues [46]. Sodium fluoride has a Ki value of 98 μM with PPO from apples at pH = 5 and decreases to 14 μM at pH = 3.5 [41].
3.6. Effects of Heat Treatments on PPO Stability
The effect of heat treatment on PPO stability was investigated from 45°C to 80°C [Figure 5]. Enzyme activity decreased with increasing temperature and treatment time. At 45°C and after 5 min of incubation, it remains 85% of the enzyme activity, and after 20 min, it remains 55%. At 65°C, PPO activity decreases significantly. At 80°C, no activity is observed after 5 min of incubation. The resistance of PPO activity to thermal treatment depends on the source of PPO [49,50]. PPO from Guankou grape was completely inactivated at 75°C after 10 min of incubation [51], and thermal inactivation of “Prata” and “Cavendish” banana peel PPO was achieved at 90°C after 5 and 15 min, respectively [49, 52].
 | Figure 5: Effects of heat treatments on Mediterranean palm heart polyphenol oxidase residual activity. [Click here to view] |
3.7. Inactivation Constants and Thermodynamic Quantities of the Mediterranean Palm Heart PPO
Rate constants k of inactivation were calculated from linear regression of experimental enzyme retention data of the slopes of log % residual activity against time [19]. The kinetic parameters k, t1/2, and D are presented in Table 3. The activation energies, Ea, were calculated using the Arrhenius equation [7] [Figure 6]. Then ΔG, Gibbs free energy, and ΔH, the change in enthalpy, were calculated [Table 3].
Table 3: Inactivation parameters of Mediterranean palm heart PPO.
| Temperature (°C) | K (min-1) | t1/2 (min) | D (min) | ΔH (Kj) | ΔG (Kj) |
|---|---|---|---|---|---|
| 45 | 0,0075±0.0007 | 92.4±6.7 | 307 | 154.3 | 101.83 |
| 55 | 0.018±0.008 | 38.07±0.65 | 127.9 | 154.2 | 102.66 |
| 65 | 0.027±0.005 | 25.16±0.023 | 85.2 | 154.18 | 104.7 |
| 70 | 0.306±0.07 | 2.26±0.19 | 7.5 | 154.14. | 99.5 |
| 75 | 1.60±0.15 | 0.43±0.19 | 1.42 | 154.05 | 96.12 |
| 80 | 1.65±0.31 | 0.42±0.21 | 1.39 | 154.01 | 91.7 |
 | Figure 6: Arrhenius plot for catechol oxidation catalyzed by polyphenol oxidase (PPO) from Mediterranean palm heart PPO. [Click here to view] |
The results showed that k values increased with increasing temperature, indicating that PPO is less thermostable at higher temperatures. The enzyme showed a steep decrease in t1/2 with increasing temperature, and D values declined with temperature [Table 3]. The D value, or time at a given temperature and pressure needed for 90% reduction of the initial activity, was <2 min at 80°C. The activation energy (Ea) value for heat-inactivation of PPO from Mediterranean palm heart was 157 kJ/mol, higher than those reported for rice (23.3 kJ/mol) [53], banana (155 kJ/mol) [54], but lower than that for Victoria grape (221.5 kJ/mol) [55], table grape (295.5 kJ/mol) [56], and cranberry (502 kJ/mol) [57]. High activation energy reflects a greater sensitivity of PPO to temperature [9]. Thus, we can conclude that the following k values, D, and activation energy show that the PPO is very sensitive to temperature change.
ΔG, Gibbs free energy, considered the energy barrier of the inactivation of the enzyme; ΔH, the change in enthalpy that measures the number of bonds destroyed during inactivation [19]. The results of ΔH show that the enthalpy is practically independent of temperature. So there is no change in the heat capacity of the enzyme. The mean value of ΔH found in this study (154 kJ/mol) is higher than those found for other PPOs, such as the PPO from vanilla bean (89 kJ/mol) [58]. The value of ΔG is directly related to the stability of a protein; a higher ΔG corresponds to great stability of the enzyme. When incubation temperature increases from 65°C to 80°C, there is a reduction in the ΔG values for the PPO from 104 kJ/mol to 91 kJ/mol [Table 3], indicating the destabilization of this protein. All ΔG values are in agreement with the value of 100 kJ/mol characteristic of the protein denaturation reaction as found for PPO from other plants [58].
4. CONCLUSION
In this study, characterization, inhibition, and the effect of thermal treatment on the PPO enzyme from Mediterranean palm hearts were described. Our study has reported that the purified PPO enzyme has no activity with the phenol monohydroxy substrate. Kinetic parameters show that the PPO enzyme was very effective towards 4-methylcatechol as a substrate, followed by catechol. PPO activity decreases when pH decreases. The optimum pH and temperature values were determined, and the inhibition kinetics of the PPO show that the most effective inhibitor was L-Cysteine. The D, k values, and activation energies of the PPO enzyme were very sensitive to temperature change and had a rapid loss of its activity. We conclude from the present study that enzymatic browning of Mediterranean palm hearts can be reduced by thermal treatment, pH adjustment, and the use of non-toxic sulfhydryl amino acid (L-cysteine). However, further study can be performed in order to determine the optimal conditions for inactivation of the PPO enzyme and the effect of the residual enzyme on the stability of Mediterranean palm hearts during storage should also be carried.
5. ACKNOWLEDGMENT
We would like to thank Dr Gavira for the lab measurements in the Laboratorio de Estudios, Cristalogra´ficos, IACT (CSIC–UGR), Armilla, Granada, Spain.
6. AUTHORS’ CONTRIBUTIONS
All authors made substantial contributions to conception and design, acquisition of data, or analysis and interpretation of data; took part in drafting the article or revising it critically for important intellectual content; agreed to submit to the current journal; gave final approval of the version to be published; and agreed to be accountable for all aspects of the work. All the authors are eligible to be authors as per the International Committee of Medical Journal Editors (ICMJE) requirements/guidelines.
7. FUNDING
There is no funding to report.
8. CONFLICTS OF INTEREST
The authors report no financial or any other conflicts of interest in this work.
9. ETHICAL APPROVALS
This study does not involve experiments on animals or human subjects.
10. DATA AVAILABILITY
All the data obtained in the study are represented as table or figures.
11. PUBLISHER’S NOTE
This journal remains neutral with regard to jurisdictional claims in published institutional affiliations.
REFERENCES
1. Trabzuni DM, Ahmed SE, Abu-Tarboush HM. Chemical composition, minerals and antioxidants of the heart of date palm from three Saudi cultivars. Food Sci Nutr 2014;14:1379-86. [https://doi.org/10.4236/fns.2014.514150]
2. Blumenthal M, Busse W, Goldberg A, Gruenwald J, Hall T, Riggins CW, et al. The Complete German Commission E Monographs:Therapeutic Guide to Herbal Medicines. Boston, MA, Austin, TX, USA;Integrative Medicine Communications, American Botanical Council;1998.
3. Bouchaib A, Abdellah AB, Bakkali M, Laglaoui A, Arakrak A, Carboni C, et al. Optical method for detecting oxygen via the chromogenic reaction catalyzed by polyphenol oxidase. Enzyme Microb Technol 2018;114:1-6. [https://doi.org/10.1016/j.enzmictec.2018.02.009]
4. Zhao Y, Huang ZH, Zhou HM, Zhu KX, Guo XN, Peng W. Polyphenol oxidase browning in the formation of dark spots on fresh wet noodle sheets:How dark spots formed. Food Chem 2020;329:126800. [https://doi.org/10.1016/j.foodchem.2020.126800]
5. Queiroz C, Mendes Lopes ML, Fialho E, Valente-Mesquita VL. Polyphenol oxidase:Characteristics and mechanisms of browning control. Food Rev Int 2008;24:361-75. [https://doi.org/10.1080/87559120802089332]
6. Oktay M, Küfreviolu I, Kocaçaliskan I, Saklrolu H. Polyphenoloxidase from Amasya apple. J Food Sci 1995;60:494-6. [https://doi.org/10.1111/j.1365-2621.1995.tb09810.x]
7. Benaceur F, Gouzi H, Meddah B, Neifar A, Guergouri A. Purification and characterization of catechol oxidase from Tadela (Phoenix dactylifera L.) date fruit. Int J Biol Macromol 2019;125:1248-56. [https://doi.org/10.1016/j.ijbiomac.2018.09.101]
8. de Oliveira Carvalho J, Orlanda JF. Heat stability and effect of pH on enzyme activity of polyphenol oxidase in buriti (Mauritia flexuosa Linnaeus f.) fruit extract. Food Chem 2017;233:159-63. [https://doi.org/10.1016/j.foodchem.2017.04.101]
9. Atrooz OM, AlKhamaisa NK, AlRawashdeh IM. Determination of the activity and kinetic parameters of polyphenol oxidase enzyme in crude extracts of some Jordanian plants. J Appl Biol Biotech 2020;8:69-74.
10. Li F. Purification, kinetic parameters, and isoforms of polyphenol oxidase from “Xushu 22”sweet potato skin. J Food Biochem 2020;44:e13452. [https://doi.org/10.1111/jfbc.13452]
11. Torres A, Aguilar-Osorio G, Camacho M, Basurto F, Navarro-Ocana A. Characterization of polyphenol oxidase from purple sweet potato (Ipomoea batatas L. Lam) and its affinity towards acylated anthocyanins and caffeoylquinic acid derivatives. Food Chem 2021;l356:129709. [https://doi.org/10.1016/j.foodchem.2021.129709]
12. Azzouzi N, Bouchaib A, Britel MR, Maurady A. Characterization of polyphenol oxidase (PPO) from blackberry thorny wild Rubus fruticosus and its inhibition using natural extracts. Curr Res Nutr Food Sci 2022;10:1205-21. [https://doi.org/10.12944/CRNFSJ.10.3.33]
13. Kaya ED, Bagci O. Purification and biochemical characterization of polyphenol oxidase extracted from Kirmizi Kismis grape (Vitis vinifera L.). J Food Biochem 2021;45:e13627. Erratum in:J Food Biochem 2021:e13811. [https://doi.org/10.1111/jfbc.13627]
14. Adeseko CJ, Sanni DM, Salawu SO, Kade IJ, Bamidele SO, Lawal OT. Purification and biochemical characterization of polyphenol oxidase of African Bush Mango (Irvingia gabonensis) fruit peel. Biocatal Agric Biotechnol 2021;36:102119. [https://doi.org/10.1016/j.bcab.2021.102119]
15. Zhang W, Shen Y, Li Z, Xie X, Gong ES, Tian J, et al. Effects of high hydrostatic pressure and thermal processing on anthocyanin content, polyphenol oxidase and b-glucosidase activities, color, and antioxidant activities of blueberry (Vaccinium Spp.) puree. Food Chem 2021;342:128564. [https://doi.org/10.1016/j.foodchem.2020.128564]
16. Bulhões Bezerra Cavalcante TA, Santos Funcia ED, Wilhelms Gut JA. Inactivation of polyphenol oxidase by microwave and conventional heating:Investigation of thermal and non-thermal effects of focused microwaves. Food Chem 2021;340:127911. [https://doi.org/10.1016/j.foodchem.2020.127911]
17. Wang D, Li D, Xu Y, Li L, Belwal T, Zhang X, et al. Elevated CO2 alleviates browning development by modulating metabolisms of membrane lipids, proline, and GABA in fresh-cut Asian pear fruit. Sci Hortic 2021;281:109932. [https://doi.org/10.1016/j.scienta.2021.109932]
18. Fang T, Yao J, Duan Y, Zhong Y, Zhao Y, Lin Q. Phytic acid treatment inhibits browning and lignification to promote the quality of fresh-cut apples during storage. Foods 2022;11:1470. [https://doi.org/10.3390/foods11101470]
19. Zhou FH, Xu DY, Liu CH, Chen C, Tian MX, Jiang AL. Ascorbic acid treatment inhibits wound healing of fresh-cut potato strips by controlling phenylpropanoid metabolism. Postharvest Biol Technol 2021;181:111644. [https://doi.org/10.1016/j.postharvbio.2021.111644]
20. Yousuf B, Wu S, Siddiqui MW. Incorporating essential oils or compounds derived thereof into edible coatings:Effect on quality and shelf life of fresh/fresh-cut produce. Trends Food Sci Technol 2021;108:245-57. [https://doi.org/10.1016/j.tifs.2021.01.016]
21. Yoruk R, Marshall M. Physicochemical properties and function of plant polyphenol oxidase:A review. J Food Biochem 2003;27:361-422. [https://doi.org/10.1111/j.1745-4514.2003.tb00289.x]
22. Chen JS, Wei C, Rolle RS, Otwell WS, Balaban MO, Marshall MR. Inhibitory effect of kojic acid on some plant and crustacean polyphenol oxidases. J Agric Food Chem 1991;39:1396-401. [https://doi.org/10.1021/jf00011a001 https://doi.org/10.1021/jf00008a008]
23. Espín JC, Wichers HJ. Effect of captopril on mushroom tyrosinase activity in vitro. Biochim Biophys Acta 2001;1544:289-300. [https://doi.org/10.1016/S0167-4838(00)00230-2]
24. Girelli AM, Mattei E, Messina A. HPLC study of tyrosinase inhibition by thiopronine. Biomed Chromatogr 2004;18:436-44. [https://doi.org/10.1002/bmc.333]
25. Mukhtar A, Latif S, Mueller J. Effect of heat exposure on activity degradation of enzymes in mango varieties Sindri, SB Chaunsa, and tommy atkins during drying. Molecules 2020;25:5396. [https://doi.org/10.3390/molecules25225396]
26. Schweiggert U, Schieber A, Carle R. Inactivation of peroxidase, polyphenoloxidase, and lipoxygenase in paprika and chili powder after immediate thermal treatment of the plant material. Innov Food Sci Emerg Technol 2005;6:403-11. [https://doi.org/10.1016/j.ifset.2005.05.001]
27. Bradford MM. A rapid and sensitive method for the quantitation of microgram quantities of protein utilizing the principle of protein-dye binding. Anal Biochem 1976;72:248-54. [https://doi.org/10.1016/0003-2697(76)90527-3]
28. Gouzi H, Depagne C, Coradin T. Kinetics and thermodynamics of the thermal inactivation of polyphenol oxidase in an aqueous extract from Agaricus bisporus. J Agric Food Chem 2012;60:500-6. [https://doi.org/10.1021/jf204104g]
29. Mishra BB, Gautam S, Sharma A. Purification and characterisation of polyphenol oxidase (PPO) from eggplant (Solanum melongena). Food Chem 2012;134:1855-61. [https://doi.org/10.1016/j.foodchem.2012.03.098]
30. Palma-Orozco G, Marrufo-Hernández NA, Sampedro JG, Nájera H. Purification and partial biochemical characterization of polyphenol oxidase from mango (Mangifera indica cv. Manila). J Agric Food Chem 2014;62:9832-40. [https://doi.org/10.1021/jf5029784]
31. Adeseko CJ, Sanni DM, Lawal OT. Biochemical studies of enzyme-induced browning of African bush mango (Irvingia gabonensis) fruit pulp. Prep Biochem Biotechnol 2022;52:835-44. [https://doi.org/10.1080/10826068.2021.1998113]
32. Chaves IR, Ferreira ES, Silva MA, Neves VA. Polyphenoloxidase from atemoya fruit (Annona cherimola mill. Annona squamosa L.). JFood Biochem 2011;35:1583-92. [https://doi.org/10.1111/j.1745-4514.2010.00508.x]
33. Ni Eidhin DM, Murphy E, O'Beirne D. Polyphenol oxidase from Apple (Malus domesticaBorkh. CV Bramley's Seedling). Purification strategies and characterization. J Food Sci 2006;71:51-8. [https://doi.org/10.1111/j.1365-2621.2006.tb12388.x]
34. Gawlik-Dziki U, Szymanowska U, Baraniak B. Characterization of polyphenol oxidase from broccoli (Brassica oleracea var. botrytis Italica) florets. Food Chem 2007;105:1047-53. [https://doi.org/10.1016/j.foodchem.2007.05.012]
35. Kaur R, Sharma S, Kaur S, Sodhi HS. Biochemical characterization with kinetic studies of melanogenic enzyme tyrosinase from white button mushroom, Agaricus bisporus. Indian. J Biochem Biophysics 2022;59:718-25.
36. Dincer B, Colak A, Aydin N, Kadioglu A, Guner S. Characterization of polyphenoloxidase from medlar fruits (Mespilus germanica L., Rosaceae). Food Chem 2002;77:1-7. [https://doi.org/10.1016/S0308-8146(01)00359-4]
37. Rocha AM, Morais AM. Characterization of polyphenoloxidase (PPO) extracted from “Jonagored”apple. Food Control 2001;12:85-90. [https://doi.org/10.1016/S0956-7135(00)00026-8]
38. Duangmal K, Owusu Apenten RK. A comparative study of polyphenoloxidases from taro (Colocasia esculenta) and potato (Solanum tuberosum var. Romano). Food Chem 1999;64:351-9. [https://doi.org/10.1016/S0308-8146(98)00127-7]
39. Bojer Rasmussen C, Enghild JJ, Scavenius C. Identification of polyphenol oxidases in potato tuber (Solanum tuberosum) and purification and characterization of the major polyphenol oxidases. Food Chem 2021;365:130454. [https://doi.org/10.1016/j.foodchem.2021.130454]
40. Lv Y, Cai L, Yang M, Liu X, Hui N, Li J. Purification, characterisation, and thermal denaturation of polyphenoloxidase from prawns (Penaeus vannamei). Int J Food Prop 2017;20:3345-59. [https://doi.org/10.1080/10942912.2017.1354019]
41. Janovitz-Klapp AH, Richard FC, Goupy PM, Nicolas JJ. Inhibition studies on Apple polyphenol oxidase. J Agric Food Chem 1990;38:926-31. [https://doi.org/10.1021/jf00097a001 https://doi.org/10.1021/jf00094a002]
42. Zhou X, Xiao Y, Meng X, Liu B. Full inhibition of Whangkeumbae pear polyphenol oxidase enzymatic browning reaction by l-cysteine. Food Chem 2018;266:1-8. [https://doi.org/10.1016/j.foodchem.2018.05.086]
43. Guven RG, Guven K, Bekler FM, Acer O, Alkan H, Dogru M. Purification and characterization of polyphenol oxidase from purslane. Food Sci Technol 2017;37:356-62. [https://doi.org/10.1590/1678-457x.03216]
44. Lin H, Ng AW, Wong CW. Partial purification and characterization of polyphenol oxidase from Chinese parsley (Coriandrum sativum). Food Sci Biotechnol 2016;25:91-6. [https://doi.org/10.1007/s10068-016-0103-x]
45. Sakiroglu H, Yilmaz E, Erat M, Öztürk AE. Selected properties of polyphenol oxidase obtained from Ispir Sugar Bean. Int J Food Prop 2013;16:1314-21. [https://doi.org/10.1080/10942912.2011.584258]
46. Heimdal H, Larsen LM, Poll L. Characterization of polyphenol oxidase from photosynthetic and vascular lettuce tissues (Lactuca sativa). J Agric Food Chem 1994;42:1428-33. [https://doi.org/10.1021/jf00043a008]
47. Ali HM, El-Gizawy AM, El-Bassiouny RE, Saleh MA. Browning inhibition mechanisms by cysteine, ascorbic acid and citric acid, and identifying PPO-Catechol-cysteine reaction products. J Food Sci Technol 2014;52:3651-9. [https://doi.org/10.1007/s13197-014-1437-0]
48. Sakiroglu H, Öztürk AE, Pepe AE, Erat M. Some kinetic properties of polyphenol oxidase obtained from dill (Anethum graveolens). J Enzyme Inhib Med Chem 2008;23:380-5. [https://doi.org/10.1080/14756360701587201]
49. Sabarre DC Jr., Camila Flor YL. Extraction and characterization of polyphenol oxidase from plant materials:A review. J Appl Biol Biotechnol2021;8:83-95.
50. Zawawi NA, Hazmi NA, How MS, Kantono K, Silva FV, Sulaiman A. Thermal, high pressure, and ultrasound inactivation of various fruit cultivars'polyphenol oxidase:Kinetic inactivation models and estimation of treatment energy requirement. Appl Sci 2022;12:1864. [https://doi.org/10.3390/app12041864]
51. Wang Z, Yuan J, Yang J, Dong Z, Yan X, Yuan C, et al. Effects of Guankou grape polyphenol oxidase on enzymatic browning. J Food Process Preserv 2021;46:16127. [https://doi.org/10.1111/jfpp.16127]
52. Wohlt D, Schwarz E, Schieber A, Bader-Mittermaier S. Effects of extraction conditions on banana peel polyphenol oxidase activity and insights into inactivation kinetics using thermal and cold plasma treatment. Foods 2021;10:1022. [https://doi.org/10.3390/foods10051022]
53. Owusu-Ansah YJ. Polyphenol oxidase in wild rice (Zizania palustris). J Agric Food Chem 1989;37:901-4. [https://doi.org/10.1021/jf00088a015]
54. Ünal MÜ. Properties of polyphenol oxidase from Anamur banana (Musa cavendishii). Food Chem 2007;100:909-13. [https://doi.org/10.1016/j.foodchem.2005.10.048]
55. Râpeanu G, Van Loey A, Smout C, Hendrickx M. Thermal and high pressure inactivation kinetics of Victoria grape polyphenol oxidase:From model systems to grape must. J Food Process Eng 2006;29:269-86. [https://doi.org/10.1111/j.1745-4530.2006.00063.x]
56. Fortea MI, Lopez-Miranda S, Serrano-Martinez A, Carreno J, Nunez-Delicado E. Kinetic characterization and thermal inactivation study of polyphenol oxidase and peroxidase from table grape (Crimson seedless). Food Chem 2009;113:1008-14. [https://doi.org/10.1016/j.foodchem.2008.08.053]
57. Chan HT, Yang HY. Identification and characterization of some oxidizing enzymes of the McFarlin cranberry. J Food Sci 2006;36:69-73.
58. Waliszewski KN, Márquez O, Pardio VT. Quantifcation and characterization of polyphenol oxidase from vanilla bean. Food Chem 2009;117:196-203. [https://doi.org/10.1016/j.foodchem.2009.03.118]