化学抑制剂对果蔬食品多酚氧化酶性质影响的研究进展

多酚氧化酶(polyphenol oxidase,PPO)引起的酶促褐变会导致果蔬食品色泽劣变,营养成分降低,甚至使其丧失商品价值.化学抑制剂对PPO具有较好的抑制效果,并且使用方便,在果蔬食品中广泛使用,因此对它的研究具有理论和实际意义.该文论述了羧酸、抗坏血酸及其衍生物、含硫氨基酸、酚酸及其他抑制剂对PPO的抑制作...     

Browning prevention by ascorbic acid and 4-hexylresorcinol: different mechanisms of action on polyphenol oxidase in the presence and in the absence of substrates

ABSTRACT:  We have investigated the mechanism of action of 4-hexylresorcinol (4-HR) and ascorbic acid (AA) on the polyphenol oxidase (PPO) catalyzed oxidation of phenolic substrates. Incubation of PPO with 4-HR diminishes strongly PPO activity. This effect can be erroneously interpreted, due to the high affinity of 4-HR for PPO, as irreversible inactivation of PPO. However, PPO activity can be recovered by dialysis after incubation with 4-HR. 4-hexylresorcinol is a canonical enzyme inhibitor that binds preferentially to the oxy form of PPO. It is a mixed-type inhibitor, because it influences both apparent V _max (1.26 compared with 0.4 units in the absence and presence of 4-HR, respectively) and K _m values (0.28 mM compared with 0.97 mM in the absence and in the presence of 4-HR, respectively) of PPO. AA can prevent browning by 2 different mechanisms: In the absence of PPO substrates it inactivates PPO irreversibly, probably through binding to its active site, preferentially in its oxy form. In the presence of PPO substrates, AA reduces PPO oxidized reaction products, which results in a lag phase when measuring PPO activity by monitoring dark product formation but not when monitoring O₂ consumption. The simultaneous use of both 4-HR and AA on PPO results in additive prevention of browning.     

马铃薯贮藏期间褐变强度与多酚氧化酶活性变化的研究

测定了十个马铃薯品种在六个月窖藏期间褐变强度和多酚氧化酶活性的变化。结果表明:马铃薯的褐变强度和多酚氧化酶活性在品种间的差异达极显著水平,除渭薯8号外,其它品种在贮藏期间的变化也达到极显著水平。在贮藏期间,不同品种的褐变强度和多酚氧化酶活性的变化趋势不同,但同一品种褐变强度与多酚氧化酶活性之间存在中度的正相关,相关系数r为0.736。      

香蕉中多酚氧化酶的灭酶条件研究

比较了蒸汽、热水烫漂及微波灭酶的优劣,初步探讨了对香蕉进行微波灭酶的基本条件。结果表明,对于香蕉中的多酚氧化酶(PPO),微波灭酶较蒸汽及热水烫漂灭酶迅速,效果好;微波功率密度为4W/g时,5 min内将香蕉PPO活力降至5%以下,切片厚度对灭酶效果无显著影响,剥皮后可直接进行灭酶,单层或双层叠放。     

护色处理对冻藏香蕉片多酚氧化酶活性的影响

通过设计单一护色剂、不同浓度L-半胱氨酸、不同热处理时间及多因子护色处理四个实验研究其对冻藏香蕉片多酚氧化酶(PPO)活性的影响,结果表明,5种护色剂均能有效地抑制冻藏香蕉片PPO活性,但L-半胱氨酸对其抑制效果更佳;随着护色液中Cys浓度的增加,残余PPO活力呈下降趋势,表现为CK>0.05%Cys>0.1%Cys>0.2%Cys;热处理能有效地抑制冻藏香蕉片PPO活性,PPO活性随着热处理时间的增加而降低,但热处理却大大降低了冻藏香蕉片的品质;0.1%L-半胱氨酸、0.05%异抗坏血酸、0.1%蔗糖和0.1%氯化钙构成的护色剂组合能有效地抑制PPO活性,并起到良好的保质增脆效果。     

Inhibition of enzymatic browning in actual food systems by the Maillard reaction products

BACKGROUND: The Maillard reaction occurring between amino acids and sugars produces neo‐formed compounds having certain levels of antioxidant activity depending on the reaction conditions and the type of reactants. The objective of this study was to investigate enzymatic browning inhibition capacity of Maillard reaction products (MRPs) formed from different amino acids including arginine (Arg), histidine (His), lysine (Lys) and proline (Pro). RESULTS: The inhibitory effects of the MRPs on polyphenol oxidase (PPO) were determined. The total antioxidant capacity (TAC) of MRPs derived from different amino acids were in the order Arg > His > Lys > Pro. The TAC and PPO inhibition of MRPs were evaluated as a function of temperature (80–120 °C), time (1–6 h) and pH (2–12). Arg‐Glc and His‐Glc MRPs exhibited strong TAC and PPO inhibition. Increasing temperature (up to 100 °C) and time also increased TAC and PPO inhibition. Kinetics analysis indicated a mixed type inhibition of PPO by MRPs. CONCLUSION: The results indicate that the MRPs derived from Arg and His under certain reaction conditions significantly prevent enzymatic browning in actual food systems. The intermediate compounds capable of preventing enzymatic browning are reductones and dehydroreductones, as confirmed by liquid chromatographic–mass spectrometric analyses. Copyright © 2010 Society of Chemical Industry     

Immunogold labelling to localize polyphenol oxidase (PPO) during wilting of red clover leaf tissue and the effect of removing cellular matrices on PPO protection of glycerol‐based lipid in the rumen

BACKGROUND: The enzyme polyphenol oxidase (PPO) reduces the extent of proteolysis and lipolysis within red clover fed to ruminants. PPO catalyses the conversion of phenols to quinones, which can react with nucleophilic cellular constituents (e.g. proteins) forming protein–phenol complexes that may reduce protein solubility, bioavailability to rumen microbes and deactivate plant enzymes. In this study, we localized PPO in red clover leaf tissue by immunogold labelling and investigated whether red clover lipid was protected in the absence of PPO‐induced protein–phenol complexes and plant enzymes (lipases). RESULTS: PPO protein was detected to a greater extent (P < 0.001) within the chloroplasts of mesophyll cells in stressed (cut/crushed and wilted for 1 h) than freshly cut leaves for both palisade (61.6 and 25.6 Au label per chloroplast, respectively) and spongy mesophyll cells (94.5 and 40.6 Au label per chloroplast, respectively). Hydrolysis of lipid and C18 polyunsaturated fatty acid biohydrogenation during in vitro batch culture was lower (P < 0.05) for wild‐type red clover than for red clover with PPO expression reduced to undetectable levels but only when cellular matrices containing protein–phenol complexes were present. CONCLUSION: Damaging of the leaves resulted in over a doubling of PPO detected within mesophyll cells, potentially as a consequence of conversion of the enzyme from latent to active form. PPO reduction of microbial lipolysis was apparent in macerated red clover tissue but not in the absence of the proteinaceous cellular matrix, suggesting that the PPO mechanism for reducing lipolysis may be primarily through the entrapment of lipid within protein–phenol complexes. Copyright © 2009 Society of Chemical Industry     

The effect of fruit extracts with polyphenol oxidase (PPO) activity on the in vitro accessibility of iron in high-tannin sorghum

Dephytinized high-tannin sorghum flour was incubated with crude extracts from pear, banana or avocado, respectively, followed by investigation of the effects on the phenolic content and on in vitro accessible iron. All fruits contained polyphenol oxidase (PPO) activity and incubation resulted in significant reduction of phenolic compounds. Incubation with avocado extract resulted in the lowest levels of phenolic compounds, as well as the highest amount of in vitro accessible iron. Peroxidase activity and some organic acids in the fruit extracts might also have contributed to the positive effect on iron accessibility. Nevertheless, incubation of the sorghum flour with the fruit extracts under conditions enabling the PPO to oxidize phenolic compounds, resulted in the highest accessibility of iron. The results from this study suggest that the PPO activity in simple fruit extracts can be utilized to increase the accessibility of iron in dephytinized polyphenol-containing cereal foods.     

Effect of tannic acid on in vitro enzymatic hydrolysis of some protein sources

The pH‐stat system has been used to assess the effect of tannic acid (TA) on solubility and in vitro enzyme hydrolysis of different proteins. Added TA (from 10 to 50 g kg^?1) decreased the extent of hydrolysis of bovine serum albumin. Enzymic hydrolysis of casein, pea meal, soybean meal, and haemoglobin (HB) was increased, as measured by total amino acids released and by the degree of hydrolysis. SDS‐PAGE confirmed the results of the in vitro enzymatic hydrolysis. These findings suggest that, under in vitro conditions, when simulating the gastrointestinal environment of domestic mammals, the negative effects of TA described from in vivo experiments are not necessarily due to reduced hydrolysis of proteins. Copyright © 2003 Society of Chemical Industry     

Polyphenol oxidase activity in grass and its effect on plant‐mediated lipolysis and proteolysis of Dactylis glomerata (cocksfoot) in a simulated rumen environment

Little is known about the level or activity of polyphenol oxidase (PPO) in grasses and its potential impact on proteolysis and lipolysis. Six grass species were initially screened for PPO activity (740.6, 291.9, 213.6, 119.0, 16.3 and 6.5 U g^?1 fresh weight (FW) for cocksfoot, hybrid ryegrass, Italian ryegrass, perennial ryegrass, timothy and tall fescue respectively). Cocksfoot, which expressed the highest activity, was then used to determine the effect of PPO on plant‐mediated proteolysis and lipolysis in a simulated rumen environment. Sourced cocksfoot was macerated and incubated in an antibiotic‐containing anaerobic medium with or without ascorbate to deactivate PPO in the dark at 39 °C over five time points. At each time point (0, 1, 2, 6 and 24 h), six replicate samples were destructively harvested; three of the replicates were used for lipid analysis and the other three for protein, free amino acid and bound phenol determination. Characterisation of the herbage showed PPO activities of 649.6 and 0 U g^?1 FW, which were reflected in the extent of phenol (derived from quinones) binding to protein after 24 h of incubation, namely 65.1 and 29.6 mg bound phenol g^?1 protein (P < 0.001) for cocksfoot and cocksfoot + ascorbate respectively. Proteolysis, measured as free amino acids released into the incubation buffer, was significantly reduced (P < 0.001) with increasing PPO activity, with values after the 24 h incubation of 0.03 and 0.07 mmol L^?1 g^?1 FW for cocksfoot and cocksfoot + ascorbate respectively. Lipolysis, measured as the proportional decline in the membrane lipid polar fraction, was likewise reduced (P < 0.001) with increasing PPO activity, with values after the 24 h incubation of 0.43 and 0.65 for cocksfoot and cocksfoot + ascorbate respectively. Changes that occurred in protein and the lipid fractions (polar fraction, monoacylglycerol + diacylglycerol, triacylglycerol and free fatty acids) during the incubations are also reported and discussed. These results support the selection of forages high in PPO activity to reduce protein and lipid losses in silo and potentially in the rumen. Copyright © 2006 Society of Chemical Industry     

高压CO2处理对香蕉果肉中多酚氧化酶活性的影响

比较了CO2压力、温度和处理时间对香蕉果肉中多酚氧化酶活性的影响,并且采用二次回归正交旋转组合设计优化了高压CO2处理对香蕉果肉中多酚氧化酶的钝化条件.结果表明,影响高压CO2处理钝化香蕉果肉中多酚氧化酶活性的主次因素顺序为温度＞时间＞ CO2压力;高压CO2处理钝化香蕉果肉中多酚氧化酶的最佳条件为温度60℃、CO2压力19MPa、处理时间50min,在此条件下,香蕉果肉中多酚氧化酶的残活力仅为0.9％.     

Effects of isomerisation and oxidation on the immunomodulatory activity of chlorogenic acid in RAW264.7 macrophages

Chlorogenic acid (CA) is one of the major polyphenols in fruit that contributes to most bioactivities. However, it is susceptible to isomerisation and oxidation in processing and thus displays varied bioactivity. This study aimed to evaluate the isomerisation and oxidation effects of CA on the potential immunomodulatory activity in RAW264.7 macrophages through the NF-E2-related factor-2 (Nrf2) and nuclear factor-kappa B (NF-κB) signalling pathways. The results showed that isomerisation significantly affected the immunomodulation of CA by reducing Nrf2 and increasing NF-κB nuclear translocation. The oxidation of CA weakened the effect of immune regulation in macrophages through impacts on the nucleic translocation of Nrf2 and NF-κB, cellular reactive oxygen species (ROS) accumulation, antioxidant enzyme (superoxide dismutase, SOD) activity and cytokine expression. Consequently, isomerisation and oxidation remarkably affect the immunomodulation of CA via the Nrf2 and NF-κB pathways.     

Inhibitory effects of p-alkylbenzoic acids on the activity of polyphenol oxidase from potato (Solanum tuberosum)

In order to prevent potato from browning during preservation and processing, the effects of p-alkylbenzoic acids (p-propylbenzoic acid, p-butylbenzoic acid, p-pentylbenzoic acid, p-hexylbenzoic acid, p-heptylbenzoic acid, p-octylbenzoic acid) on the activity of potato polyphenol oxidase (PPO) have been studied. The PPO was extracted with PBS (pH 6.8), fractionated with ammonium sulphate, concentrated, and purified with Sephadex G-100 (1.8 × 100 cm) filtration chromatography. The active fractions were pooled and the PPO activity was determined to be 79.83 U/mg. The results showed that p-alkylbenzoic acids could strongly inhibit the activity of potato PPO. The values of IC₅₀ of these six inhibitors were determined to be 0.213, 0.180, 0.152, 0.106, 0.075 and 0.047 mM, respectively. Kinetic analyses showed that p-alkylbenzoic acids used in this experiment were reversible and noncompetitive inhibitors to the enzyme. The inhibitory effects were potentiated with increasing lengths of the hydrocarbon chains, indicating that the inhibitory efficiency on the enzyme was influenced by the steric effect of the substituted groups.     

Effect of konjac glucomannan hydrolysates and probiotics on the growth of the skin bacterium Propionibacterium acnes in vitro

The synbiotic ability of probiotic bacteria and konjac glucomannan hydrolysates (GMH) to inhibit acne‐inducing bacterium, Propionibacterium acnes growth was studied in vitro. All probiotic bacteria strains tested were able to inhibit the growth of this species of skin bacterium where the inhibition was significantly (P < 0.01) enhanced by the presence of the GMH prebiotic. As the current treatment of acne is based on topical or systemic drugs, it is worth examining further the biotherapeutic activities of the GMH and selected probiotics with a view to future use as prophylactic or therapeutic synbiotics for treating acne infections.     

体外模拟法研究生物利用率的影响因素

体外模拟法是目前常用的研究胃肠道营养素的生物利用率的方法。其方法简单,成本低,时间短、效果好、因而应用很广。总结了体外模拟时需考虑的各项因素,包括胃的消化和胰的消化、pH调节、蠕动频率、膜的渗透等;同时概述了目前存在的问题并对以后的发展方向进行了展望。