Inhibited Polyphenol Oxidase Activity Research Articles

Biochemical and genetic characterization of wheat ( Triticum spp.) kernel polyphenol oxidases

Genetic variability of polyphenol oxidase (PPO) activity was evaluated in hexaploid, tetraploid, and aneuploid wheat ( Triticum spp.). A DNA probe to kernel-specific PPO hybridized to PPO clones from an immature wheat kernel cDNA library but not to clones from a seedling cDNA library. Recombinant protein and antibody were developed from the kernel-specific PPO. PPO transcript and antigenic proteins were most abundant during kernel development and decreased rapidly at physiological maturity. PPO proteins were more abundant in immature kernels, but PPO activity was greater in mature kernels. Wheat kernel antigenic protein bands of ∼58, 60, 62, and 75 kD were identified; immunoprecipitated proteins of ∼60 and ∼62 kD were confirmed to be PPOs by LC-MS/MS. The 2D chromosome was implicated as a major locus of PPO activity in cultivar (cv.) ‘Chinese Spring’ based on antigenic staining intensity of ∼60–62 kD proteins and kernel PPO activity in aneuploid lines. The ∼58 kD protein was not well-correlated with kernel PPO activity, while the ∼75 kD protein was likely a PPO precursor. Tropolone completely inhibited PPO activity extracted from mature kernels but not from immature kernels. Total PPO activity in mature kernels is thus a function of specific PPO isoforms present, their abundance, and activation levels.

Inhibition of Browning on Fresh-cut Pear Wedges by Natural Compounds

ABSTRACT: Mechanical operations such as peeling and cutting during minimal processing involve enzymatic browning of fruit tissue. The objective of this work was to evaluate the individual and combined effects of N-acetyl-L-cysteine, reduced glutathione, ascorbic acid, and 4-hexylresorcinol to control pear browning. Browning of fresh-cut pears was prevented by a minimum concentration of 0.75% N-acetyl-L-cysteine up to 28 d at 4°C. Reduced glutathione treatments were also effective along the storage time although browning was observed after 21 d of storage with a dip of 0.75% reduced glutathione. However, ascorbic acid or 4-hexylresorcinol treatments did not seem to completely prevent browning of pear wedges throughout the storage period. An enhanced antibrowning effect was observed when combining both N-acetyl-L-cysteine and reduced glutathione, considering hue angle as color change index. Thus, hue angle reached maximum levels at 1.5% N-acetyl-L-cysteine (NAC) or 1.5% glutathione (GSH) and 1% NAC with 1% GSH for 28 d. Besides, N-acetyl-L-cysteine, reduced glutathione and 4-hexylresorcinol completely inhibited polyphenol oxidase activity as well as browning inhibitors slightly reduced firmness of fresh-cut pears.

Delineating the Role of Polyphenol Oxidase in the Darkening of Alkaline Wheat Noodles

This study evaluated the effects of inhibitors on polyphenol oxidase (PPO) activity, the effect of the PPO inhibitor tropolone on noodle darkening, and the correlation of PPO activity with darkening of alkaline noodles. The PPO inhibitors tropolone and salicylhydroxamic acid (each at 1 microM) reduced kernel PPO activity by approximately 50% in three hexaploid wheat cultivars but did not inhibit PPO activity in the two very low PPO cultivars, durum Langdon, and the synthetic hexaploid-derived ID580. Tropolone (100 microg/g flour) inhibited alkaline noodle darkening (deltaL*) by 13-25% in the low PPO wheat cultivar, ID377s, and by 39-54% in the high PPO wheat cultivar, Klasic. Alkaline noodle darkening among 502 wheat samples was correlated with kernel PPO activity (r = 0.64). Results substantiate the hypothesis that PPO plays a major role in darkening of alkaline noodles. However, results also indicate that substantial darkening would occur even at zero PPO activity, as measured in the kernel PPO assay. Therefore, darkening of alkaline noodles is probably due to the cultivar-specific level of PPO activity and the presence of at least one additional darkening mechanism. Further investigation is required to identify the phenolic discoloration agent(s) and to determine the potential roles of non-PPO discoloration mechanisms, both enzymatic and nonenzymatic, in wheat products.

Inhibition of polyphenol oxidases activity by various dipeptides.

In an effort to develop natural and nontoxic inhibitors on the activity of mushroom polyphenol oxidase (PPO) the effect of various glycyl-dipeptides (GlyAsp, GlyGly, GlyHis, GlyLeu, GlyLys, GlyPhe, GlyPro, GlyTyr) was investigated. The inhibition study with dihydroxyphenylalanine (DOPA) as substrate is based on separation of the enzymatic reaction components by reversed phase HPLC and the UV detection of the dopachrome formed. The results have evidenced that several of tested dipeptides inhibited PPO activity in the range of 20-40% while GlyPro and GlyLeu had no effect. The study has also permitted the characterization of the following kinetic pattern: a linear-mixed-type mechanism for GlyAsp, GlyGly, GlyLys, and GlyPhe and a hyperbolic-mixed-type for GlyTyr. It was not possible to identify the inhibition mechanism for GlyHis, although it affects PPO activity. In addition the effects of GlyAsp, GlyLys and GlyHis were evaluated for lessening the browning of fresh Golden Delicious apple and Irish White Skinned potato. The effectiveness of such inhibitors was determined by the difference between the colors observed in the dipeptide-treated sample and the controls using the color space CIE-Lab system. The % browning inhibition on potato (20-50%) was greater than of apple (20-30%) by the all tested dipeptides. Only GlyLys presented the significant value of 50%.

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.

Browning control, shelf life extension and quality maintenance of frozen litchi fruit by hydrochloric acid

Litchi fruit can be stored at −18 °C without obvious adverse effects on the aril, but the pericarp darkens and browns when they are frozen or thawed. Experiments were conducted to test effects of hydrochloric acid on browning inhibition of litchi fruit pericarp for displacement of sulphur application. Litchi fruit were soaked in 0.5%, 1% and 2% HCl for 2–10 min. Fruit appearance and membrane leakage were assessed after 1 day of storage under ambient conditions of 25 °C and 80–90% relative humidity. In terms of browning control of and fruit pericarp damage, treatment with 1% HCl for 6 min gave the best red colour and minimal damage. 1% HCl treatment markedly inhibited polyphenol oxidase activity and maintained high anthocyanin content in litchi fruit pericarp tissue. Fruit treated with 1% HCl and stored at −18 °C for 12 months had a shelf life of 12 h at ambient temperature, with a uniform red colour and acceptable quality. Use of 1% HCl could be considered for commercial application in extending shelf life and maintaining quality of frozen litchi fruit following storage and during marketing.

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

ABSTRACTStudies 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.

Characterization of polyphenol oxidase in coffee

Polyphenol oxidase (PPO) was characterized in partially purified extracts of leaves (PPO-L) and fruit endosperm (PPO-E) of coffee ( Coffea arabica L.). PPO activity was higher in early developmental stages of both leaves and endosperm of fruits. Wounding or exposure of coffee leaves to methyl jasmonate increased PPO activity 1.5–4-fold. PPO was not latent and was not activated by protease treatment. PPO activity was stimulated 10–15% with sodium dodecyl sulphate (SDS) at 0.35–1.75 mM, but at higher concentrations activities were similar to the control samples, without detergent. Prolonged incubation of extracts with trypsin or proteinase K inhibited PPO activity but pepsin had no effect. Inhibition of PPO with proteinase K was increased in the presence of SDS. PPO activity from both tissues was optimal at pH 6–7 and at an assay temperature of 30°C. Activity was highest with chlorogenic acid as substrate with a K m of 0.882 mM (PPO-L) and 2.27 mM (PPO-E). Hexadecyl trimethyl-ammonium bromide, polyvinylpyrrolidone 40, cinnamic acid and salicylhydroxamic acid inhibited PPO from both tissues. Both enzymes were inactivated by heat but the activity in endosperm extracts was more heat labile than that from leaves. The apparent M r determined by gel filtration was 46 (PPO-L) and 50 kDa (PPO-E). Activity-stained SDS–polyacrylamide gel electrophoresis (PAGE) gels and western blots probed with PPO antibodies suggested the existence of a 67 kDa PPO which is susceptible to proteolytic cleavage that generates a 45 kDa active form.

Hot water brushing: an alternative method to SO2 fumigation for color retention of litchi fruits

Abstract Distribution of high-quality litchi ( Lychee chinensis Sonn.) fruits to global markets depends exclusively on postharvest treatments to suppress peel browning. The current standard treatment of litchi fruits in Israel includes fumigation with sulfur dioxide (SO 2 ) followed by dipping the fruits in hydrochloric acid containing the fungicide prochloraz. As part of the effort to reduce the use of potentially hazardous chemicals in agriculture, we developed a new procedure that may enable SO 2 to be avoided. Instead of fumigation, litchi fruits are sprayed with hot water while being brushed in a revolving drum, after which the fruits are subjected to hydrochloric acid treatment. Fruits that are processed in this way maintain a uniform red color for at least 35 days, without apparent deterioration in external or internal quality, or taste. Physiological studies demonstrate that polyphenol oxidase (PPO) activity is reduced by the hot water brushing (HWB) procedure as compared with controls but not to the same extent as inhibition by SO 2 treatments. In addition to its effect on PPO activity, HWB may lead to reduced pH of the pericarp, or more uniform distribution of the acid in it. This result suggests that HWB may act by bruising the external layer of the pericarp allowing the peel to be uniformly exposed to the acid which may inhibit PPO activity and maintain the anthocyanins in their red-pigmented form.

Purification and some properties of polyphenol oxidase of longan fruit

Polyphenol oxidase (PPO) was isolated from longan ( Dimocarpus longan Lour.) fruit peel, with a 46-fold purification of PPO by ammonium sulfate, Sephadex G-200 and Phenyl Sepharose being achieved. Pyrogallol, 4-methylcatechol, and catechol were good substrates for the enzyme, and activity with chlorogenic acid, p-cresol, resorcinol, or tyrosine was not observed. The optimal pH for PPO activity was 6.5 with 4-methylcatechol. The enzyme had a remarkably temperature optimum (35°C) and was relatively stable, requiring a little more than 20 min at 50°C for 50% loss of activity. Reduced glutathione, l-cysteine, thiourea, FeSO 4 and SnCl 2 markedly inhibited PPO activity, whereas MnSO 4 and CaCl 2 enhanced PPO activity. Data obtained in this study might help to better understand longan fruit peel browning.

Are there connections between phenol metabolism, ascorbate metabolism and membrane integrity in leaves of boron‐deficient sunflower plants?

The influence of soluble phenol concentration and polyphenoloxidase activity in leaves of both B‐deficient and B‐sufficient sunflower plants (Helianthus annuus L. cv. Frankasol) on plasma membrane permeability was investigated, A study was also undertaken as to whether or not the incubation of B‐deficient leaves in ascorbate‐ and calcium‐containing solutions has a beneficial effect on plasma membrane integrity. Plants were cultivated under controlled environmental conditions with deficient and sufficient B supply and different light intensity to provoke changes in phenol metabolism. Analysis of membrane permeability (measured by potassium efflux), soluble phenol concentration and polyphenoloxidase (EC 1.10.3.1) activity of leaves showed that there was no correlation between these parameters. Furthermore, incubation in solutions containing ascorbate and calcium did not decrease the enhanced membrane permeability due to B deficiency, which could, however, be lowered by boric acid application. In summary, the results suggest that B does not maintain plasma membrane integrity by complexing phenols or inhibiting polyphenoloxidase activity, thereby preventing damage by oxygen free radicals. Ascorbate metabolism or calcium‐related disorders seem also not to be involved. It is therefore likely that B has a direct function at the membrane, possibly by stimulating membrane‐related enzymes, or in a structural role similar to that reported for the cell wall.

Comparison of leaf susceptibility to enzymatic blackening in Protea neriifolia R. Br. and Leucospermum ‘Rachel’

Abstract Leaf susceptibility to blackening differs between Protea neriifolia R. Br. and Leucospermum ‘Rachel’ [( L. lineare × L. vestitum ) × L. glabrum ]; members from two different genera of the family Proteaceae. Protea leaf discs had a greater dry weight percentage and a higher protein and total phenolic content, than Leucospermum leaf discs. No significant difference was observed in ascorbic acid content in fresh leaf tissues of the two genera. Polyphenol oxidase (PPO) activity was high in Protea leaf homogenates with none being detected in Leucospermum leaf tissue at any pH. A significantly higher pH was observed in Protea leaf homogenate (pH 5.2) than in Leucospermum leaf homogenate (pH 4.4), with pH 5.2 being the optimum pH for PPO activity in Protea leaf. Mixing Protea leaf extract with Leucospermum leaf extract at pH 4.4–6.8 inhibited PPO activity, suggesting that Leucospermum leaf extract contained an unknown PPO inhibitor. The absence of leaf blackening in Leucospermum could be due to either or both the lower leaf homogenate pH and the presence of an inhibitor.

Maillard reaction products inhibit apple polyphenoloxidase

The effects of Maillard reaction products (MRP) on apple polyphenol oxidase (PPO) were evaluated spectrophotometrically at 400 nm after oxidation of 4-methylcatechol. MRP, synthesized under different conditions including heating time, type of amino acids, pH and concentration of both amino acid and glucose, showed different effects on apple PPO. The inhibitory effects of MRP. heated for 7 h at 90 °C and synthesized from various amino acids with glucose, were as follows: arginine = histidine > ysteine > lysine. The MRP synthesized from glutamic acid or valine enhanced rather than inhibited PPO activity. Increasing the heating time of histidine-glucose solutions also enhanced the inhibitory effect. However, a slight decrease in the inhibitory effect was observed in the cysteine-glucose solutions after 3 h of heating. The MRP synthesized at low pH levels was a more effective inhibitor than the MRP synthesized at higher pH levels.

Contribution of Enzymic Browning to Color in Sugarcane Juice

The contribution of polyphenol oxidase (PPO) and peroxidase POD) to enzymic browning in sugarcane juice was investigated. Inactivation of these enzymes with heat resulted in juice of lower color (absorbance measured at 420 nm), but POD was found to be more heat stable than PPO. Salicylhydroxamic acid (SHAM) completely inhibited PPO activity and markedly reduced juice color but had no effect on POD activity. Removal of oxygen in the presence of the substrate chlorogenic acid also stopped color formation. Upon subsequent addition of oxygen, browning continued, indicating that the process was oxygen dependent. Color development in juice was complete after 20-30 min even though PPO was still active. Addition of chlorogenic acid at this point restarted browning, suggesting that color development was limited by the availability of phenolic substrates. Varietal differences were observed in levels of PPO activity, phenolics, and color. There was a correlation between juice color and phenolic content but not between juice color and PPO activity. The sugarcane PPO enzyme was most active with chlorogenic acid. It was not active with p-diphenols and was inhibited by SHAM, suggesting that it is a catechol oxidase-type enzyme (EC 1.10.3.1)and not a laccase (EC 1.10.3.2). It is atypical in that it was inhibited by SDS. The results suggest that enzymic browning contributes significantly to color formation in sugarcane juice and that PPO is the major enzyme involved.

Differential effects of condensed and hydrolyzable tannin on polyphenol oxidase activity of attine symbiotic fungus

The leaf-cutting antAtta cephalotes is a generalist herbivore of the neotropics and collects leaf material to cultivate a fungus. It appears that this fungus, a Basidiomycete, is responsible for the ability of the ants to utilize most of the available woody plant species. Tannins and other phenolics are ubiquitous secondary chemicals in woody plants, and Basidiomycete fungi produce enzymes, such as polyphenol oxidase, that are capable of polymerizing and inactivating the phenolics. This study evaluates the effects of a condensed and a hydrolyzable tannin on the activity of polyphenoi oxidase and the growth of the fungus. I hypothesized that low concentrations of tannin would not inhibit polyphenol oxidase activity but high concentrations would inhibit the enzyme. Consequently, I predicted that only high concentrations of tannin would inhibit fungal growth. Laboratory assays with the fungus indicated that hydrolyzable tannin (tannic acid) and condensed tannin (quebracho tannin) differ in the mechanism of inhibition. Tannic acid does not inhibit polyphenol oxidase activity but does inhibit fungal growth. Quebracho tannin, however, inhibits both polyphenol oxidase activity and fungal growth. As predicted, both tannic acid and quebracho tannin primarily inhibit the fungus at high concentrations.

Reassessment of the role of gut alkalinity and detergency in insect herbivory

Previously it was reported that significant amounts of the tomato phenolic, chlorogenic acid, were oxidized in the digestive system of generalist feedersSpodoplera exigua andHelicoverpa zea. The covalent binding of the oxidized phenolic (i.e., quinone) to dietary protein exerts a strong antinutritive effect against larvae. In this study, we examined the fate of ingested chlorogenic acid in larvalManduca sexta, a leaf-feeding specialist of solanaceous plants. Significant amounts of chlorogenic acid were bound to excreted protein byM. sexta when larvae fed on tomato foliage. However, in the case ofM. sexta we suggest that the strong alkalinity and detergency of the midgut may minimize the antinutritive effects of oxidized phenolics. The solubility of tomato leaf protein is significantly greater at pH 9.7, representative of the midgut ofM. sexta, than at pH 8.0, representative of the midguts ofH. zea and S. exigua. We suggest that this increase in solubility would compensate for any loss in bioavailability of essential amino acids caused by the covalent binding of chlorogenic acid to amino acids. Furthermore, lysolecithin, a surfactant likely to contribute to the detergent properties of the midgut fluid, was shown to enhance protein solubility as well as inhibit polyphenol oxidase activity. The adaptive significance of gut alkalinity and detergency is discussed.

Condensed tannins, attine ants, and the performance of a symbiotic fungus

Field experiments indicate that the foliar concentration of condensed tannin affects the selection of leaf material ofInga oerstediana Benth., a tropical legume tree, by leaf cutter ants. In one study an increase in tannin concentration was correlated with a decrease in the acceptability of leaves to leaf-cutter ants, except at low tannin concentrations. Protein concentration was not correlated with acceptability nor was the ratio of protein to tannin. Results from a second study suggest that when the concentration of tannin was low the ants appear to select leaves on the basis of nutrient availability. Laboratory assays with the ants indicated that quebracho tannin, a commercially available condensed tannin, inhibits foraging ants. Again, at lower concentrations, quebracho tannin appeared to have little affect on the ants. The fungus the ants cultivate is a wood-rotting Basidiomycete that produces enzymes, such as polyphenol oxidase (PPO), that are capable of inactivating tannins. The activity of these PPOs may explain why leaf-cutter ants are undeterred by low concentrations of condensed tannins. I hypothesized that PPO activity would be absent from fungal cultures without tannin and that only high concentrations of tannin would inhibit the fungus. Cultures with and without tannin showed similar PPO activity. Thus PPO activity is constitutive. In fact, as fungal biomass increased, so did PPO activity. As hypothesized, only high concentrations of quebracho tannin inhibited PPO activity and fungal growth. However, it is not clear whether the ants can discriminate between concentrations that do and do not inhibit the fungus.

Substrate specificity, de novo synthesis and partial purification of polyphenol oxidase derived from the wood-decay fungus,Coriolus versicolor

Coriolus versicolor, a white-rot Basidiomycete, secretes cellulolytic and ligninolytic enzymes as well as polyphenol oxidase (PPO). Whereas the former degrade wood polymers, the latter can convert diphenols to diquinones and oligomerize syringic acid, a lignin derivative. Certain phenolic compounds can serve as disease-resistance factors controlling the proliferation of wood-decay fungi within host tissues. BecauseC. vesicolor can be ‘batch-cultured’, overproduction and enhanced secretion of enzymes of biological and commercial interests are feasible. Reported here are the results of attempts to define the timed appearances of intracellular and extracellular PPO, to assess substrate specificity as well as distinguish synthesis versus activation of intracellular PPO and to partially purify extracellular PPO. These efforts were to provide data enabling cell-free synthesis of PPO, cloning of the gene(s) for the oxidase and the establishment of its subcellular route of secretion. Whereas two protein peaks (6 and 12 days in a 16 day time-course) were observed for dialyzed mycelial homogenates, the homogenates' PPO specific activity rose between 4 and 12 days and then declined. Total extracellular protein content climbed from 6 to 15 days for dialyzed growth medium and the medium's PPO specific activity rose at 4 days post-inoculation and except at 9 days increased linearly to 15 days. When aliquots of dialyzed 12 and 15 day media were added to PPO assay mixtures containing catechol and either syringic or gallic acids, statistically significant differences in PPO specific activity between phenolic substrates were noted. Supplementation of cultures with 1.91 μg cycloheximide ml growth medium−1 (control, growth medium only) together with 0.5 μCi [14C]-leucine revealed that cycloheximide inhibited PPO activity and suppressed [14C]-leucine incorporation into TCA-insoluble cytoplasmic protein. As for PPO partial purification, growth medium dialysis followed by 0–30% (NH4)2SO4 fractionation and subsequent 12 000×g dialyzate centrifugation yielded a 3.27-fold enhancement in PPO specific activity within the 12 000×g supernatant. Chromatography of the latter upon DEAE-Sephadex indicated that PPO exchanged with the DEAE counterion as it could be eluted with high ionic strength salt. These results suggest that: the occurrences of intracellular and extracellular PPO are time-dependent, intracellular PPO is de novo synthesized, the preferred substrate for extracellular PPO appears to be catechol and extracellular PPO can be partially purified by a combination of dialysis and ammonium sulfate fractionation as well as possibly DEAE chromatography and/or Sephadex G-150 gel filtration.

Effects of catechin concentration on the formation of black tea polyphenols during in vitro oxidation

An in vitro model fermentation system, containing purified catechins and partially purified polyphenol oxidase (EC 1.14.81.1) from green tea shoots, has been used to determine the efrect of catechin mixtures of different concentration and proportions on the formation of theaflavin and thearubigin. Increases in total catechin concentration, 25% above that typical in green tea shoots of Malawi-grown bushes, inhibited polyphenol oxidase activity and, consequently, depressed theaflavin levels. Individual or combined concentrations of epicatechin gallate and epigallocatechin gallate in excess of 110 mM were shown to be responsible for enzyme inhibition, whereas epicatechin and epigallocatechin had no effect. Fermentation of a catechin mixture, containing the four major catechins, epicatechin, epigallocatechin, epigallocatechin gallate and epicatechin gallate, at equal individual concentrations (55 mM), produced, after 3O min, total theaflavin levels 68% higher and thearubigin levels only 25% higher than those from a standard catechin mixture fermented under similar conditions. Continued fermentation of this mixture produced no further theaflavin, but the thearubigin fraction increased significantly, due to subsequent oxidation of the excess of simple catechins. A new catechin mixture was, therefore, calculated to give a similar level of theaflavin to that of the previous mixture without leaving an excess of unoxidized simple catechins. The catechin proportions and concentrations of the latter mixture agree well with those of the green shoots of quality Kenyan teas or similar quality Malawi teas grown during the dry cold season. The results indicate that a high ratio of simple to gallocatechins will facilitate a high theaflavin-thearubigin ratio in black tea.

Mode of action of phenylthiourea, a therapeutic agent for cucumber scab

AbstractIn laboratory tests, phenylthiourea (PTU) induced resistance in cucumber seedlings to infection by spores of cucumber scab (Cladosporium cucumerinum). At the same time it inhibited polyphenol oxidase activity in the plant, increased peroxidase activity, and caused lignification of the cell walls in the parenchyma around the penetrating hyphae of Cladosporum. Possible association and interaction between these effects are discussed.