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Oxidising Enzymes. II: The Nature of the Enzymes associated with Certain Direct Oxidising Systems in Plants.
... The term oxydase was introduced by Bertrand (1896) as a general term for these water-soluble oxidising enzymes using O 2, replacing the previous term of oxidising ferments coined by Traube (1877). As these oxidases were all phenoloxidases, the two terms were used synonymously at the time (Kastle, 1910;Onslow, 1920;Szent-Györgyi, 1930). In 1903, the "activated oxygen" initially described by Schönbein was identified as hydrogen peroxide . ...
... In 1903, the "activated oxygen" initially described by Schönbein was identified as hydrogen peroxide . This result led the same authors to postulate that all phenoloxidases were two-component systems comprising an H 2 O 2 generating oxygenase and a phenol oxidising PRX Onslow, 1920). However, Szent-Györgyi (1925) rebutted this two-component model and showed that the blueing of guaiacum by a potato oxydase was independent from peroxide and PRX activity. ...
The metabolism of polyphenolic polymers is essential to the development and response to environmental changes of organisms from all kingdoms of life, but shows particular diversity in plants. In contrast to other biopolymers, whose polymerisation is catalysed by homologous gene families, polyphenolic metabolism depends on phenoloxidases, a group of heterogeneous oxidases that share little beyond the eponymous common substrate. In this review, we provide an overview of the differences and similarities between phenoloxidases in their protein structure, reaction mechanism, substrate specificity, and functional roles. Using the example of laccases (LACs), we also performed a meta-analysis of enzyme kinetics, a comprehensive phylogenetic analysis and machine-learning based protein structure modelling to link functions, evolution, and structures in this group of phenoloxidases. With these approaches, we generated a framework to explain the reported functional differences between paralogs, while also hinting at the likely diversity of yet undescribed LAC functions. Altogether, this review provides a basis to better understand the functional overlaps and specificities between and within the three major families of phenoloxidases, their evolutionary trajectories, and their importance for plant primary and secondary metabolism.
Varieties of Kaki (Diospyros Kaki) are used to be classified into two forms, pollination constant and pollination variant. In the former, the brown dots are never seen in the fruit flesh even in the seeded fruit, but in th latter, the browning of the flesh occurs in proportional intensity to the number of seed developed. The classification is applicable to the astringent as well as the non-astringent varieties. The brown dot is the tannin cell with coagulated and oxidized contents in it. The oxidation of the contents in the tannin cell may be brought about by the activity of oxidizing enzymes. The present study is carried out to accertain the relation of oxidizing enzymes to the browning of fruit flesh on the one hand, and to make oonsiderations of an analogous mechanism of oxidizing enzymes to the colour change of fruit into dull grey after artificial removal of astringency on the other.
The writer tested the peroxidase and oxidase reactions with 24 varieties. The reaction of peroxidase is measured by WILLSTÄETTER'S pyrogallol method, and of oxidase with guaiac tincture.
SZENT GYÖRGYI's hypothesis, which denotes that the peroxidase is in close relation to ascorbic acid in a respiratory system, is taken into account and ascorbic acid (reduced form) is measured by the titration method after TILLMAN's 2.6-dichlorphenol-indophenol with 41 varieties.
Further, the influence of artificial processing to remove the astringency and of the short day treatment on the oxidizing enzymes and ascorbic acid content is studied.
The experimental results are summarized as follows:
1. Ascorbic acid content is higher in the pollination constant than in pollination variant. The contrast is especialy remarkable in the non-astringent varieties. These results indicate that the ascorbic acid is found in the inverse proportion to the occurrence of brown dot.
2. The activity of peroxidase correlates to the density of brown dots, and the most intensitive reaction of peroxidase is represented in the non-astringent pollination variant.
3. The reaction of oxidase is concurrent with that of peroxidase in any case.
4. After the artificial removal of astringency of astringent fruit, ascorbic acid decreases, though the activities of oxidizing enzymes being not influenced.
5. By the short day treatment, ascorbic acid decreases, and the reaction of oxidizing enzymes increases.
The investigations outlined in ‘Browning reactions in fruits’, part of the special study of food science of the Nuffleld Advanced Chemistry course, are modified to make the material meaningful and relevant to pupils in the middle school. This article discusses a series of simple experiments on apple browning which were performed by third formers. A summary of the nature of the browning process and its control are included.
IntroductionTyrosinase PreparationThe Enzymatic Oxidation of CatecholThe Measurement of Tyrosinase ActivityThe Stability and Inactivation of TyrosinaseThe Enzymatic Oxidation of Monohydric PhenolsTyrosinase and Plant RespirationThe Nature of Tyrosinase
All the energy driving life is generated in the sun by atomic reactions. Some of this energy reaches our globe in the form of a radiation commonly called “sunshine”. A small part of this sunshine is captured by the photosynthetic apparatus of plants, which use the energy of the radiation for the splitting of water molecules into H and O. By this reaction the sunshine is traded in for a thermodynamic potential, that of H and O, and so, henceforth the sole source of the energy of life has to be this thermodynamic potential. To keep life going the energy of this potential has to be released. This can be done by reversing the reaction which produced the potential, reversing the splitting of water and bringing H and O together to form water again, that is, oxidizing the hydrogen.
Palmitic acid stimulated the activity of mango peroxidase and reversed the inhibition due to the peroxidase inhibitor present in the preclimacteric fruit. The palmitic acid effect appeared to saturate in the range of 45 to 60 muM palmitic acid. Crude fatty acid extract of the mango exerted similar effect. The percentage stimulation was pH-dependent. Palmitic acid stimulated the enzyme by 18 percent at its optimum pH (5) but the stimulation was in excess of 63 percent at pH 2.5. At pH 2.5 the enzyme concentration versus velocity plot was non-linear and the activation by palmitic acid appeared to saturate between 32 and 48 muM concentration of the effector. The inhibition of the enzyme at and above 0.86 muM concentration of substrate (H202) was not found in the presence of palmitic acid. The effector also changed the heat inactivation kinetics of the enzyme and activated only two out of the four peroxidase isoenzymes present in the climacteric fruit extracts. The results presented indicate the regulatory nature of the enzyme and support its significance in fruit ripening.
This chapter discusses the function of ascorbic acid in plants. The almost universal distribution of ascorbic acid in plant tissues, together with the fact that its occurrence there coincides quite generally with high metabolic activity, suggests that, as in animal tissues, it performs some essential functions in cellular metabolism. Of these functions, its action as a respiratory catalyst has received most attention because the most characteristic property of the molecule is the ease with which it may be reversibly oxidized and reduced. Ascorbic acid occurs both in the reduced form (AA) and in the oxidized form, dehydroascorbic acid (DHA), in plant tissues, usually about 95% of the total being present in the reduced form. On injury to the tissues in the presence of oxygen, the reduced form is oxidized, generally with great rapidity, thus indicating the presence of oxidase enzymes. The demonstration that there is in many plants an enzyme, ascorbic oxidase, capable of catalyzing a direct reaction between ascorbic acid and molecular oxygen has added weight to the idea that ascorbic acid may act like cytochrome as a respiratory catalyst.
Accurate and rapid methods for the quantitative analysis of peroxidases and polyphenolases are described which allow for mechanical deduction of nonenzymic oxidation effects commonly found in crude plant extracts. These methods were used in evaluating the successive steps for a method of partial isolation of the enzymes of two species of Impatiens.
In sensitized albino guinea pigs it was not possible to demonstrate therapeutic activity for jewelweed in poison ivy dermatitis after the outward manifestations became apparent. Neither was it possible to exhibit activity in limiting the biologic activity of poison ivy when poison ivy extracts and jewelweed juice were combined in vitro, or on the skin. This was attributable to a lesser degree of sensitivity in guinea pigs than in man and the greater amounts of ivy toxin necessary to elicit dermatitis exceeded the neutralizing powers of even concentrated preparations of jewelweed. In human subjects jewelweed was of no value in treating experimentally established dermatitis. Jewelweed juice or concentrated solutions of jewelweed enzymes were capable of inactivating more dilute poison ivy extracts when mixed in vitro and tested on human subjects under controlled conditions.
Handbuch der biochemischen Arbeitsmethoden
Chodat (1910). Handbuch der biochemischen Arbeitsmethoden. E. Abderhalden, Berlin, 3, 42.
Chodat and Bach (1903). Ber. 36, 606.