Solid-phase synthesis of mimosine tetrapeptides and their inhibitory activities on neuraminidase and tyrosinase

Neuraminidase is a rational target for influenza inhibition, and the search for neuraminidase inhibitors has been intensified. Mimosine, a nonprotein amino acid, was for the first time identified as a neuraminidase inhibitor with an IC(50) of 9.8 ± 0.2 μM. It was found that mimosine had slow, time-dependent competitive inhibition against the neuraminidase. Furthermore, a small library of mimosine tetrapeptides (M-A(1)-A(2)-A(3)) was synthesized by solid-phase synthesis and was assayed to evaluate their neuraminidase and tyrosinase inhibitory properties. Most of the tetrapeptides showed better activities than mimosine. Mimosine-FFY was the best compound, and it exhibited 50% neuraminidase inhibition at a low micromolar range of 1.8 ± 0.2 μM, whereas for tyrosinase inhibition, it had an IC(50) of 18.3 ± 0.5 μM. The kinetic studies showed that all of the synthesized peptides inhibited neuraminidase noncompetitively with K(i) values ranging from 1.9 -to 7.2 μM. These results suggest that mimosine could be used as a source of bioactive compounds and may have possibilities in the design of drugs as neuraminidase and tyrosinase inhibitors.     

柿叶提取物对马铃薯多酚氧化酶的抑制作用

多酚氧化酶（PPO）是生物体内黑色素合成的关键酶。研究了甜柿叶乙醇提取物、涩柿叶乙醇提取物、甜柿叶水提取物、涩柿叶水提取物对马铃薯多酚氧化酶活性的抑制作用。结果表明,上述4种柿叶提取物对马铃薯多酚氧化酶具有明显的抑制作用,使该酶活力下降50％所需的浓度（IG50）分别为0．21、0．26、0．37、0．45mg／mL。乙醇提取物较水提取物的作用效果更强,甜柿叶提取物的抑制作用比涩柿叶提取物的强,4种柿叶提取物对酶的抑制作用均为非竞争性可逆抑制,其抑制常数（KI=KIS）分别为0．18、0．23、0．34、0．45mg／mL。     

一些物质对芒果多酚氧化酶活性的影响

本文研究了一些物质对芒果多酚氧化酶活性的影响。结果表明：十二烷基硫酸钠（SDS）、氯化钢、尿素对芒果多酚氧化酶有激活作用;一些有机酸（苹果酸、柠檬酸）和还原剂（抗坏血酸、半胱氨酸）对芒果多酚氧化酶有抑制作用;有机酸具有明显促进还原剂的抑制作用。     

脉冲强光对多酚氧化酶活性及结构特性的影响

为探究脉冲强光(IPL)对果蔬内源酶活性的抑制效果及相关机理,采用分光光度法研究了IPL处理对多酚氧化酶(PPO)活性的影响,并通过ANS荧光探针法、内源荧光光谱法、凝胶电泳及圆二色(CD)光谱等方法探讨了不同能量IPL处理对PPO结构特性的影响。结果表明,PPO活性随着IPL单次脉冲能量、脉冲次数的增加及脉冲板间距的减小而降低。IPL处理后,PPO表面疏水性及游离巯基含量上升,色氨酸荧光强度下降,最佳发射波长红移,α-螺旋含量下降,β型结构含量上升。同时,凝胶电泳结果显示,PPO蛋白氧化降解。综上,IPL处理可改变PPO二级和三级结构,促使蛋白氧化变性,酶活性下降。本研究结果为IPL技术在抑制食品中内源酶活性及其相关机理的研究提供了一定的理论依据。     

Activity and Inhibition of Polyphenol Oxidase in Extracts of Bran and Other Milling Fractions from a Variety of Wheat Cultivars

Studies were conducted to compare polyphenol oxidase (PPO) specific activities in various milling fractions of a variety of wheat cultivars and determine the levels of activities in a number of cultivars from different localities and harvesting seasons. Substrate specificities were also investigated. Bran was singled out as the richest source of PPO activity, which may also influence the activity in the other milling fractions that are known to have some proportion of bran content. We showed by gel electrophoresis and spectrophotometrically that the protein responsible for PPO activity apparently exists as a single isoform in bran and that the observed enzyme activity is likely to be a tyrosinase type, not a laccase or peroxidase. The specific activity was not significantly different between the reduction shorts and break shorts from the same cultivar, indicating a similar level of bran contamination in these fractions. Very low levels of PPO activity were recorded in the flour of all cultivars studied. Bran was used, therefore, to determine the varietal differences in the PPO activities in a number of cultivars from different localities and seasons of harvest. Results showed that the most significant determinant of PPO activity was the genotype, and this may be influenced by seasonality. We also determined that, apart from substrate preferences by the PPO enzyme, some phenolic acids actually inhibit PPO. Furthermore, we found that bran of some cultivars extracted with acidified methanol inhibited PPO activity substantially, whereas other extracts had less inhibitory properties. Thus, these unknown compounds in wheat may inhibit endogenous PPO activity.     

超高压处理对南美白对虾在冷藏过程中贮藏特性的影响

该文以不同的超高压条件(200,400,600,700MPa)对南美白对虾进行处理,通过考察与比较不同压力处理组冷藏过程中感官、微生物以及各化学指标的变化情况,揭示超高压技术对南美白对虾冷藏特性的影响。结果表明:超高压处理可以有效地杀灭南美白对虾中绝大多数微生物,抑制贮藏过程中挥发性盐基氮的积累,延缓pH值的变化,从而延长南美白对虾的货架期,且处理压力越高延长效果越显著;超高压处理会给鲜虾带来不同程度上的煮熟虾的风味;400MPa和600MPa处理使虾在冷藏过程的黑变提前,而700MPa处理可以完全抑制南美白对虾黑变现象的发生;超高压能够改变腺苷三磷酸(ATP)及其代谢产物的代谢情况,但不影响腺苷酸(AMP)的代谢途径。因此超高压处理可以改善南美白对虾在冷藏过程中的品质,对南美白对虾冷藏具有潜在的应用价值。     

超高压联合高密度CO2处理钝化对虾多酚氧化酶

为了弥补超高压(UHP,ultra high pressure)钝化凡纳滨对虾多酚氧化酶(PPO,polyphenol oxidase)效果差的缺点,同时利用高密度CO_2钝化凡纳滨对虾PPO的优势,初步研究UHP+CO_2处理对凡纳滨对虾PPO的钝化效果,以探讨UHP+CO_2联合处理用于开发虾类新产品的可行性。研究结果表明:UHP+CO_2联合处理比单独CO_2处理和UHP处理更能有效地钝化PPO;100 MPa UHP+CO_2联合处理30 min,PPO相对酶活降至18.92%±1.52%;200 MPa UHP+CO_2联合处理10 min,PPO相对酶活降至10.91%±1.08%;300 MPa UHP+CO_2联合处理10 min,PPO被钝化95%;400 MPa UHP+CO联合处理5 min,PPO被钝化97%;500 MPa UHP+CO联合处理10 min,PPO100%被钝化;与单独UHP处理相比,UHP+CO_2联合缩短了处理时间,提高了钝化PPO的效果;PPO经UHP+CO_2联合处理后在4℃贮藏6 d后活性未见恢复,说明PPO在处理过程中发生了不可逆的变性失活。研究结果为虾类的贮藏和加工以及开发新产品提供基础数据和技术参考。     

Pineapple juice and its fractions in enzymatic browning inhibition of banana [Musa (AAA group) Gros Michel

The effectiveness of pineapple juice in enzymatic browning inhibition was evaluated on the cut surface of banana slices. After storage of banana slices at 15 degrees C for 3 days, pineapple juice showed browning inhibition to a similar extent as 8 mM ascorbic acid but less than 4 mM sodium metabisulfite. Fractionation of pineapple juice by a solid-phase C18 cartridge revealed that the directly eluted fraction (DE fraction) inhibited banana polyphenol oxidase (PPO) about 100% when compared to the control. The DE fraction also showed more inhibitory effect than 8 mM ascorbic acid in enzymatic browning inhibition of banana puree during storage at 5 degrees C for 24 h. Further identification of the DE fraction by fractionation with ion exchange chromatography and confirmation using model systems indicated that malic acid and citric acid play an important role in the enzymatic browning inhibition of banana PPO.     

Inhibition of apple polyphenol oxidase activity by procyanidins and polyphenol oxidation products

The rate of consumption of dissolved oxygen by apple polyphenol oxidase in cider apple juices did not correlate with polyphenol oxidase activity in the fruits and decreased faster than could be explained by the decrease of its polyphenolic substrates. The kinetics parameters of a crude polyphenol oxidase extract, prepared from apple (Braeburn cultivar), were determined using caffeoylquinic acid as a substrate. Three apple procyanidin fractions of n 80, 10.5, and 4 were purified from the parenchyma of cider apples of various cultivars. Procyanidins, caffeoylquinic acid, (-)-epicatechin, and a mixture of caffeoylquinic acid and (-)-epicatechin were oxidized by reaction with caffeoylquinic acid o-quinone in order to form oxidation products. All the fractions were evaluated for their inhibitory effect on PPO activity. Native procyanidins inhibited polyphenol oxidase activity, the inhibition intensity increasing with n. The polyphenol oxidase activity decreased by 50% for 0.026 g/L of the fraction of n 80, 0.17 g/L of the fraction of n 10.5, and 1 g/L of the fraction of n 4. The inhibitory effect of oxidized procyanidins was twice that of native procyanidins. Oxidation products of caffeoylquinic acid and (-)-epicatechin also inhibited polyphenol oxidase.     

Characterization and purification of polyphenol oxidase from artichoke (Cynara scolymus L.)

In this study, the polyphenol oxidase (PPO) of artichoke (Cynara scolymus L.) was first purified by a combination of (NH(4))(2)SO(4) precipitation, dialysis, and a Sepharose 4B-L-tyrosine-p-aminobenzoic acid affinity column. At the end of purification, 43-fold purification was achieved. The purified enzyme migrated as a single band on sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Polyacrylamide gel electrophoresis indicated that PPO had a 57 kDa molecular mass. Second, the contents of total phenolic and protein of artichoke head extracts were determined. The total phenolic content of artichoke head was determined spectrophotometrically according to the Folin-Ciocalteu procedure and was found to be 425 mg 100 g(-1) on a fresh weight basis. Protein content was determined according to Bradford method. Third, the effects of substrate specificity, pH, temperature, and heat inactivation were investigated on the activity of PPO purified from artichoke. The enzyme showed activity to 4-methylcatechol, pyrogallol, catechol, and L-dopa. No activity was detected toward L-tyrosine, resorsinol, and p-cresol. According to V(max)/K(m) values, 4-methylcatechol (1393 EU min(-1) mM(-1)) was the best substrate, followed by pyrogallol (1220 EU min(-1) mM(-1)), catechol (697 EU min(-1) mM(-1)), and L-dopa (102 EU min(-1) mM(-1)). The optimum pH values for PPO were 5.0, 8.0, and 7.0 using 4-methylcatechol, pyrogallol, and catechol as substrate, respectively. It was found that optimum temperatures were dependent on the substrates studied. The enzyme activity decreased due to heat denaturation of the enzyme with increasing temperature and inactivation time for 4-methylcatechol and pyrogallol substrates. However, all inactivation experiments for catechol showed that the activity of artichoke PPO increased with mild heating, reached a maximum, and then decreased with time. Finally, inhibition of artichoke PPO was investigated with inhibitors such as L-cysteine, EDTA, ascorbic acid, gallic acid, d,L-dithiothreitol, tropolone, glutathione, sodium azide, benzoic acid, salicylic acid, and 4-aminobenzoic acid using 4-methylcatechol, pyrogallol, and catechol as substrate. The presence of EDTA, 4-aminobenzoic acid, salicylic acid, gallic acid, and benzoic acid did not cause the inhibition of artichoke PPO. A competitive-type inhibition was obtained with sodium azide, L-cysteine, and d,L-dithiothreitol inhibitors using 4-methylcatechol as substrate; with L-cysteine, tropolone, d,L-dithiothreitol, ascorbic acid, and sodium azide inhibitors using pyrogallol as substrate; and with L-cysteine, tropolone, d,L-dithiotreitol, and ascorbic acid inhibitors using catechol as a substrate. A mixed-type inhibition was obtained with glutathione inhibitor using 4-methylcatechol as a substrate. A noncompetitive inhibition was obtained with tropolone and ascorbic acid inhibitors using 4-methylcatechol as substrate, with glutathione inhibitor using pyrogallol as substrate, and with glutathione and sodium azide inhibitors using catechol as substrate. From these results, it can be said that the most effective inhibitor for artichoke PPO is tropolone. Furthermore, it was found that the type of inhibition depended on the origin of the PPO studied and also on the substrate used.     

Variation in Polyphenol Oxidase Activity and Quality Characteristics Among Hard White Wheat and Hard Red Winter Wheat Samples

Polyphenol oxidase (PPO) has been related to an undesirable brown discoloration of wheat-based end products. Consumer acceptance and product quality are generally decreased by the darkening phenomena. Two sets of wheat samples (Triticum aestivum L.) were investigated for variation in grain and flour PPO levels. Samples included 40 advanced experimental hard white winter wheat lines grown at two Kansas locations and 10 hard red winter wheat genotypes grown at three Nebraska locations. The variability in grain and flour PPO activities was influenced by growing location and population for the hard white wheat samples. There also was a significant influence of population by growing location interactions on PPO activity in both grain and flour. Genotype and growing location both contributed to variability in flour PPO activity among the hard red wheat samples. The variation in flour PPO activities among growing locations appeared larger than variation produced by genotypes tested for the hard red wheat samples. Quality parameters, such as wheat physical properties, flour protein and ash contents, grain color, and milling yield significantly correlated with grain and flour PPO activities. Among red wheat samples, flour PPO activity was related to 100 kernel weight, first reduction flour yield, and flour ash content. Grain PPO activity was related to variation in grain color observed among hard white samples. The relationship of quality characteristics with grain and flour PPO activities varied among white and red wheat samples.     

Inhibitory effect of antioxidant-rich marinades on the formation of heterocyclic aromatic amines in pan-fried beef

The inhibitory effect of antioxidant-rich marinades containing beer and white wine (with/without alcohol) alone or mixed with herbs commonly used as meat flavoring (garlic, ginger, thyme, rosemary, and red chili pepper) on the formation of heterocyclic aromatic amines (HAs) in pan-fried beef was studied. Radical-scavenging activity was evaluated by DPPH assay, before the addition of meat to the marinade (T0) and after 4 h of meat marinating (T4). At T0, wine with herbs possessed the highest scavenging activity (73.5%), followed by wine (72.5%), dealcoholized wine with herbs (53.4%), beer and herbs (41.7%), dealcoholized wine (39.6%), and beer (25.9%). At T4, a decrease in the radical-scavenging activity of all marinades was observed, although with a similar radical-scavenging profile. All of the six marinades under the study reduced the total amount of HAs, keeping meat with good overall sensory quality. Beer marinades were more efficient than white wine marinades, and the addition of herbs provided a superior inhibitory effect, reducing around 90% of HAs. No correlation was observed between radical-scavenging activity of marinades and total or individual HAs formation. Herbs explained around 30% of inhibition of PhIP formation, whereas alcohol increased PhIP formation.     

Partial purification, characterization, and histochemical localization of fully latent desert truffle (Terfezia claveryi Chatin) polyphenol oxidase

In the present paper, a fully latent polyphenol oxidase (PPO) from desert truffle (Terfezia claveryi Chatin) ascocarps is described for the first time. The enzyme was partially purified by using phase partitioning in Triton X-114 (TX-114). The achieved purification was 2-fold from a crude extract, with a 66% recovery of activity. The interfering lipids were reduced to 13% of the original content. In addition, the purification gave rise to a reduction of phenolic compounds to only 37.5%, thus avoiding the postpurification tanning of the enzyme. Latent PPO was activated by the anionic surfactant sodium dodecyl sulfate (SDS) or by incubation with trypsin. The amount of SDS necessary to obtain a maximum activation was dependent on the nature of the substrate. The use of SDS also permitted the histochemical localization of the latent enzyme within the ascocarp. Terfezia polyphenol oxidase was kinetically characterized using two phenolic substrates (L-DOPA and tert-butylcatechol). The latter substrate presented inhibition at high substrate concentration with a K(si) of 6.3 mM. Different inhibiting agents (kojic and cinnamic acid, mimosine and tropolone) were also studied, tropolone being the most effective.     

Effects of thymol on mushroom tyrosinase-catalyzed melanin formation

The novel inhibitory mechanism of thymol (2-isopropyl-5-methylphenol) on dopachrome formation by mushroom tyrosinase (EC 1.14.18.1) was identified. The UV-vis spectrum and oxygen consumption assays showed dopachrome formation using L-tyrosine as a substrate was suppressed by thymol. This inhibitory activity was reversed by the addition of a well-known radical scavenger, butylated hydroxyanisole (BHA). Further investigations using N-acetyl-L-tyrosine as a substrate with HPLC analysis suggested that thymol inhibits chemical redox reactions between dopaquinone and leukodopachrome instead of enzymatic reaction. This redox inhibitory activity of thymol was examined by using a model redox reaction with L-dihydroxyphenylalanine (L-DOPA) and p-benzoquinone. Thymol successfully inhibited oxidation of L-DOPA to dopaquinone, coupled with reduction of p-benzoquinone. Hence, the suppression of dopachrome formation by thymol is due to the inhibition of conversion of leukodopachrome to dopachrome. The antioxidant property of thymol is a key characteristic for the inhibitory mechanism of melanin synthesis.     

Hard White Versus Hard Red Wheats: Taste Tests and Milling and Baking Properties

Molecular markers for the red grain color (R) loci controlling seed color and the polyphenol oxidase (Ppo‐A1) locus controlling polyphenol oxidase (PPO) activity in seed have recently been developed. These markers provided the opportunity to convert the hard red spring wheat cultivars Choteau and Hank to white‐seeded versions with high and low PPO levels, respectively. These sets of near‐isogenic lines provided material to test the effects of seed color and PPO activity on a range of end‐use quality traits. We tested recurrent parents Choteau and Hank, along with near‐isogenic derivatives with white seed, in two replicated trials in Bozeman, Montana, for end‐use quality parameters. The white‐seeded lines consisted of both high‐ and low‐PPO near‐isogenic lines. The primary impact of white seed was the production of whole wheat bread with a perceived sweeter taste relative to the red‐seeded lines. Noodle color was not consistently impacted by the level of PPO variation despite relatively large reductions in PPO level. The alleles for white seed color did not appear to impact agronomic traits. These results suggested that hard white low‐PPO hard spring wheat would be advantageous in terms of conferring brighter color to Asian noodles and a sweeter taste to whole wheat bread.     

Use of experimental design methodology to prepare Maillard reaction products from glucose and cysteine inhibitors of polyphenol oxidase from eggplant (Solanum melongena)

Polyphenol oxidase (PPO) from eggplant was extracted and partially purified by a two-step fractionation-precipitation using ammonium sulfate and phenylsepharose hydrophobic interaction chromatography. The eggplant PPO extract was characterized concerning its kinetic properties. Optimal conditions to obtain Maillard reaction products (MRPs) with a maximal inhibitory potency (IP) toward PPO activity were determined using the surface response methodology and a four-factor and five-level experimental design. The MRPs were prepared from cysteine (0.25 M) and glucose (0-1 M), at several initial pH values (2-6) and at differing heating times (3-19 h) and temperatures (95-115 degrees C). The maximal IP was obtained after heating a model system of glucose/cysteine (1/0.25 M) at pH 2 for 3 h 20 min at 115 degrees C. The soluble part of this MRP, called MRP(IPmax), was a noncompetitive inhibitor toward eggplant PPO. The IP of MRP(IPmax) on PPO activity was very potent as compared to that displayed by benzoic, p-coumaric, and t-cinnamic acids, as well as sorbic acid and 4-hexylresorcinol. The activity of preincubated PPO at 0 degrees C with MRP(IPmax) was only slightly restored after dialysis or gel filtration.     

Food components inhibiting recombinant human histidine decarboxylase activity

Histidine decarboxylase (HDC) catalyzes histamine formation from histidine. Histamine is a bioactive amine acting as a neurotransmitter as well as a chemical mediator. Phenolic food components have been tested for their ability to inhibit recombinant human HDC. Epicatechin gallate (ECG) was found to be a potent inhibitor as it inhibited HDC activity in a competitive manner with Ki = 10 muM against l-histidine. Epigallocatechin gallate (EGCG) showed time-dependent inhibition which disappeared under anaerobic conditions. It is probable that time-dependent inhibition could be due to the result of autoxidation of EGCG. The initial burst observed for EGCG suggests that EGCG itself is involved in HDC inhibition as observed for ECG. Our present results have shown that the tested food components can inhibit HDC activity. This inhibition likely affects histamine biosynthesis and possibly leads to controlling the biological action induced by histamine. Therefore, those food components exhibiting HDC inhibitory activity might be potentially useful in controlling histamine-induced biological actions.     

Polyphenol oxidase from yacon roots (Smallanthus sonchifolius)

Polyphenol oxidase (E.C. 1.14.18.1) (PPO) extracted from yacon roots (Smallanthus sonchifolius) was partially purified by ammonium sulfate fractionation and separation on Sephadex G-100. The enzyme had a molecular weight of 45 490+/-3500 Da and Km values of 0.23, 1.14, 1.34, and 5.0 mM for the substrates caffeic acid, chlorogenic acid, 4-methylcatechol, and catechol, respectively. When assayed with resorcinol, DL-DOPA, pyrogallol, protocatechuic, p-coumaric, ferulic, and cinnamic acids, catechin, and quercetin, the PPO showed no activity. The optimum pH varied from 5.0 to 6.6, depending on substrate. PPO activity was inhibited by various phenolic and nonphenolic compounds. p-Coumaric and cinnamic acids showed competitive inhibition, with Ki values of 0.017 and 0.011 mM, respectively, using chlorogenic acid as substrate. Heat inactivation from 60 to 90 degrees C showed the enzyme to be relatively stable at 60-70 degrees C, with progressive inactivation when incubated at 80 and 90 degrees C. The Ea (apparent activation energy) for inactivation was 93.69 kJ mol-1. Sucrose, maltose, glucose, fructose, and trehalose at high concentrations appeared to protect yacon PPO against thermal inactivation at 75 and 80 degrees C.     

微酸性电解水对虾仁的杀菌效果及其动力学

为探明微酸性电解水(slightly acidic electrolyzed water,SAEW)对南美白对虾(Litopenaeus vannamei)虾仁表面杀菌效果及其动力学规律,选取料液比1:4、1:10、1:20,将虾仁分别浸洗杀菌2、5、10 min,对杀菌过程中虾仁表面及SAEW残存液中总菌落数,与有效氯质量浓度(available chlorine concentration,ACC)的变化进行测定,建立ACC衰减与微生物减灭的动力学模型,并通过颜色、硬度、pH值,及水分横向弛豫行为分析,探讨SAEW杀菌处理对虾仁品质的影响。结果表明SAEW对虾仁表面大肠杆菌有较强杀菌效果,并随处理时间的延长、作用量的增大,SAEW的杀菌效力不断增强,处理5 min时,随着料液比的增加,虾仁表面菌落数从最初的6.6 l(CFU/mL),依次降到5.0、4.7、4.4 lg(CFU/mL),料液比为1:20时,静置浸洗10 min后,虾仁表面菌落数由最初的6.6 lg(CFU/mL)降至3.9 lg(CFU/mL);同时SAEW浸洗液中残存菌落数也随处理时间的延长、作用量的增大,而不断减少,在处理2、5和10 min时,SAEW中的残存菌落数分别为4.18、3.47、2.78 lg(CFU/mL),处理时间为5 min时,随料液比的增加,SAEW中的残存菌落数分别为3.47、2.78、2.65 lg(CFU/mL);同时SAEW中ACC的消耗随处理时间的延长、而不断变大,杀菌处理5、10 min时,ACC质量浓度从初始的20.53 mg/L分别降至7.79、10.97 mg/L。动力学分析表明:SAEW在杀灭虾仁表面大肠杆菌的过程中,ACC的衰减可以用一级动力学模型描述,拟合后决定系数R2均大于0.9,而微生物的减灭遵循更为复杂的动力学模型;此外经过SAEW杀菌处理的虾仁,其颜色、pH值、硬度、以及水分的横向弛豫行为,与未处理样品相比,基本没有显著性变化。相关结果能为SAEW在水产品加工过程中的应用提供技术指导,同时也有助于SAEW杀菌技术理论的完善。     

Tyrosinase inhibitory activity of cucumber compounds: enzymes responsible for browning in cucumber

The inhibition of mushroom tyrosinase by cucumber extracts was evaluated. The inhibitory effect was measured by both polarographic and spectrophotometric methods. The commercial aldehyde, trans,cis-2,6-nonadienal, described as a major volatile compound of cucumber, was characterized as a noncompetitive inhibitor against 4-tert-butylcatechol oxidation by mushroom tyrosinase. The K(I) obtained was 3.4 mM. Polyphenol oxidase (PPO) activity was not detected in cucumber skin extracts. However, the presence of PPO was revealed by Western blot; a single band was found with a M(r) of 53 kDa. These results support the assumption that the enzyme PPO is present in the cucumber skin, but its activity is inhibited. Peroxidase (PO) was also found in cucumber skin extracts. This enzyme was detected in the soluble fraction but not in the membrane fraction. The kinetic characterization of PO was carried out. Native isoelectric focusing revealed several acidic PO isoenzymes with a pI in the range between 5 and 6, a basic isoenzyme, and one principal neutral isoenzyme of pI = 7.2.