Partial characterization of lettuce (Lactuca sativa L.) polyphenol oxidase

Immobilization, optimization, characterization and kinetic properties of polyphenol oxidase to multi-walled carbon nanotube

In this study, the kinetic properties of polyphenol oxidase (PPO) extracted from Satureja cuneifolia were investigated using catechol and 4-methylcatechol as substrates. Optimal pH and temperature values were determined at each purification step. Subsequently, the optimum immobilization conditions were established as 2 hours of stirring time and 0.05 g of multi-walled carbon nanotubes (MWCNTs). Characterization by BET, FTIR, DTA/TG, TEM, and SEM/EDX analyses confirmed the successful immobilization of PPO onto mesoporous MWCNTs, with notable changes in surface morphology and thermal degradation behavior. The optimum pH for the free enzyme remained constant across purification methods but varied with the substrate, while the optimum temperature was consistently found at 30 °C. Upon immobilization, the optimum temperature shifted to higher values, indicating enhanced thermal stability. Catalytic efficiency (Vmax/KM) for catechol decreased significantly after immobilization (from 2.5 × 106 to 5 × 104 min-1), whereas for 4-methylcatechol, the immobilized enzyme retained a high catalytic efficiency (Vmax/KM =1 × 106 min-1), comparable to that of the free enzyme. This shift suggests that immobilization favored substrate specificity toward 4-methylcatechol. Overall, the MWCNT-PPO system demonstrated enhanced stability, improved reusability, and altered substrate selectivity, making it a strong candidate for industrial biocatalytic applications where operational durability and efficiency are critical.

Purification and Characterization of Polyphenol Oxidase in the Fruits of Opuntia ficus-indica

Firstly, polyphenol oxidase (PPO) was purified from the fruits of Opuntia ficus-indica using Sepharose 4B-L-tyrosine-p-aminobenzoic acid affinity chromatography, and the enzyme was characterized. The PPO was purified 20.59-fold. Thereafter, PPO was performed on sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE). The kinetic parameters, optimum pHs, and optimum temperatures were investigated for three substrates. Opuntia ficus-indica PPO's optimum pH and optimum temperature were 9.0 and 20 °C; 7.5 and 20 °C; and 7.5 and 30 °C, respectively, when using pyrogallol, catechol, and 4-methyl catechol as substrates. For the pyrogallol, catechol, and 4-methyl catechol, the Km, Vmax, and Vmax/Km values were determined as 16.67 mM, 833.33 U/mLmin, and 50 U/mLminmM; 6.33 mM, 126.58 U/mLmin, and 20 U/mLminmM; and 5.38 mM, 107.53 U/mLmin, and 20 U/mLminmM, respectively. As a result, pyrogallol was a more appropriate substrate than catechol and 4-methyl catechol for the PPO from Opuntia ficus-indica.

Structural and expression analysis of polyphenol oxidases potentially involved in globe artichoke (C. cardunculus var. scolymus L.) tissue browning

Globe artichoke capitula are susceptible to browning due to oxidation of phenols caused by the activity of polyphenol oxidases (PPOs), this reduces their suitability for fresh or processed uses. A genome-wide analysis of the globe artichoke PPO gene family was performed. Bioinformatics analyses identified eleven PPOs and their genomic and amino acidic features were annotated. Cis-acting element analysis identified a gene regulatory and functional profile associated to plant growth and development as well as stress response. For some PPOs, phylogenetic analyses revealed a structural and functional conservation with different Asteraceae PPOs, while the allelic variants of the eleven PPOs investigated across four globe artichoke varietal types identified several SNP/Indel variants, some of which having impact on gene translation. By RTqPCR were assessed the expression patterns of PPOs in plant tissues and in vitro calli characterized by different morphologies. Heterogeneous PPO expression profiles were observed and three of them (PPO6, 7 and 11) showed a significant increase of transcripts in capitula tissues after cutting. Analogously, the same three PPOs were significantly up-regulated in calli showing a brown phenotype due to oxidation of phenols. Our results lay the foundations for a future application of gene editing aimed at disabling the three PPOs putatively involved in capitula browning.

Comparison of Phytochemicals and Antioxidant Activities of Berries Cultivated in Korea: Identification of Phenolic Compounds in Aronia by HPLC/Q-TOF MS

Aronia, blueberry, Korean raspberry, blackberry, mulberry, and red raspberry fruits cultivated in Korea were evaluated for total phenol content (TPC), total flavonoid content (TFC), total anthocyanin, and ascorbic acid content. All berries were assayed for antioxidant activities determined as 2,2-diphenyl-1-picrylhydrazyl free radical scavenging activity, 2,2'-azino-bis(3-ethylbenzothiazoline)-6 sulphonic acid free radical scavenging activity, and ferric reducing antioxidant power. Individual phenolic compounds in aronia were also identified using high-performance liquid chromatography/quadrupole-time of flight mass spectrometry. TPC, TFC, total anthocyanin, and ascorbic acid contents of the fruit samples ranged from 17.05 to 135.55 mg of gallic acid equivalent/g dry weight (dw), 1.0 to 8.59 mg of rutin equivalent/g dw, 2.55 to 24.43 mg of cyanidin-3-O-glucoside equivalent/g dw, and 3.14 to 19.45 mg of ascorbic acid equivalent/g dw, respec-tively. Aronia and Korean raspberry showed the highest TPC, TFC, and total anthocyanin while red raspberry had the high-est ascorbic acid content. Antioxidant activities showed positive correlations to phenolic and anthocyanin contents suggesting antioxidant activity of berry samples is due to these compounds. Aronia had the highest antioxidant value among fruits.

Inhibitory effects of organic acids on polyphenol oxidase: From model systems to food systems

Organic acids are widely utilized in the food industry for inhibiting the activity of polyphenol oxidase (PPO) and enzymatic browning. This review discusses the mechanisms of inhibition of PPO and enzymatic browning by various organic acids based on studies in model systems, critically evaluates the relevance of such studies to real food systems and assesses the implication of the synergistic inhibitory effects of organic acids with other physicochemical processing techniques on product quality and safety. Organic acids inhibit the activity of PPO and enzymatic browning via different mechanisms and therefore the suitability of a particular organic acid depends on the structure and the catalytic properties of PPO and the physicochemical properties of the food matrix. Studies in model systems provide an invaluable insight into the inhibitory mechanisms of various organics acids. However, the difference in the effectiveness of PPO inhibitors between model systems and food systems and the lack of correlation between the degree of PPO inhibition based on in vitro assays and enzymatic browning imply that the effectiveness of organic acids can be accurately evaluated only via direct assessment of browning inhibition in a particular food system. Combination of organic acids with physical processing techniques is one of the most viable approaches for PPO inhibition since the observed synergistic effect helps to reduce the undesirable organoleptic quality changes from the use of excessive concentration of organic acids or intense physical processing.

Purification and biochemical characterization of polyphenol oxidase from soursop (Annona muricata L.) and its inactivation by microwave and ultrasound treatments

The soursop (Annona muricata L.) is a climacteric fruit that may undergo enzymatic browning during ripening, mainly by the activity of polyphenol oxidase (PPO). Soursop PPO was purified 160-fold by hydrophobic interaction and ion-exchange chromatography. The native structure has a molecular weight of 112 kDa corresponding to a dimeric structure. The protein has an optimum pH and temperature of 6.5 and 25°C, respectively; and activation energy of 40.97 kJ·mol^-1 . The lowest K_m value was observed for caffeic acid (0.47 mM); the best substrate was 4-methyl-catechol (1,067 U·mM^-1  min^-1 ). Inactivation assays showed that PPO was completely inactivated by tropolone, Na₂ S₂ O₅ and ascorbic acid, and thermally at 55°C for <5 min, microwave exposure reduced activity to 57% at 70 W in 30 s and ultrasound treatment diminished activity to 43% at 120 W in 220 s. This study allows a better understanding of soursop PPO behavior and provides inactivation information. PRACTICAL APPLICATIONS: The conservation of fresh fruits is complicated due to the enzymatic reactions that are present in fruits, such as enzymatic browning. The enzymes responsible for these reactions can be inactivated by, different chemical compounds as well as by the use of emerging technologies, such as microwaves and sonication, which seek to satisfy the consumer needs to obtain fresh products with good nutritional characteristics and adequate safety.

pH dependent antioxidant activity of lettuce (L. sativa) and synergism with added phenolic antioxidants

Influence of pH on the antioxidant activities of combinations of lettuce extract (LE) with quercetin (QC), green tea extract (GTE) or grape seed extract (GSE) was investigated for both reduction of Fremy's salt in aqueous solution using direct electron spin resonance (ESR) spectroscopy and in L-α-phosphatidylcholine liposome peroxidation assay measured following formation of conjugated dienes. All examined phenolic antioxidants showed increasing radical scavenging effect with increasing pH values by using both methods. QC, GTE and GSE acted synergistically in combination with LE against oxidation of peroxidating liposomes and with QC showing the largest effect. The pH dependent increase of the antioxidant activity of the phenols is due to an increase of their electron-donating ability upon deprotonation and to their stabilization in alkaline solutions leading to polymerization reaction. Such polymerization reactions of polyphenolic antioxidants can form new oxidizable -OH moieties in their polymeric products resulting in a higher radical scavenging activity.

Selected biochemical properties of polyphenol oxidase in butter lettuce leaves (Lactuca sativa L. var. capitata) elicited with dl-β-amino-n-butyric acid

The study concentrated on changes in certain biochemical parameters of polyphenol oxidase (PPO) from lettuce leaves caused by dl-β-amino-n-butyric acid (BABA) elicitation. PPO from control plants demonstrated the highest affinity toward catechol, whereas PPO from BABA-elicited lettuce showed the highest affinity to 4-methylcatechol. The optimum temperature for enzymes from control plants was 35°C, whereas from plants elicited with 1mM BABA this was 25°C. PPO from plants elicited with BABA was also more sensitive to the tested inhibitors than PPO from control plants. l-Cysteine was the most effective inhibitor. Native gel stained for PPO activity in control samples showed two isoforms. However, in BABA-treated lettuce three bands visualising PPO activity were observed. The information obtained in this study will be valuable for the development of treatment technology and storage conditions to control undesirable browning reactions in elicited lettuce.

Role of ascorbic acid in the inhibition of polyphenol oxidase and the prevention of browning in different browning-sensitive Lactuca sativa var. capitata (L.) and Eruca sativa (Mill.) stored as fresh-cut produce

BACKGROUND

Polyphenol oxidase (PPO) and, to a minor extent, peroxidase (POD) represent the key enzymes involved in enzymatic browning, a negative process induced by cutting fresh-cut produce such as lettuce (Lactuca sativa) and rocket salad (Eruca sativa). Although ascorbic acid is frequently utilised as an anti-browning agent, its mechanism in the prevention of the browning phenomenon is not clearly understood.

RESULTS

The activity of PPO and POD and their isoforms in lettuce (a high-browning and low-ascorbic acid species) and rocket salad (a low-browning and high-ascorbic species) was characterised. The kinetic parameters of PPO and in vitro ascorbic acid-PPO inhibition were also investigated. In rocket salad, PPO activity was much lower than that in lettuce and cutting induced an increase in PPO activity only in lettuce. Exogenous ascorbic acid (5 mmol L(-1)) reduced PPO activity by about 90% in lettuce. POD did not appear to be closely related to browning in lettuce.

CONCLUSION

PPO is the main enzyme involved in the browning phenomenon; POD appears to play a minor role. The concentration of endogenous ascorbic acid in rocket salad was related to its low-browning sensitivity after cutting. In lettuce, the addition of ascorbic acid directly inhibited PPO activity. The results suggest that the high ascorbic acid content found in rocket salad plays an effective role in reducing PPO activity.