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Levels and histochemical localization of peroxidase and polyphenol oxidase, and levels of anthocyanins and (+)-catechin, were studied in fruit of two strawberry (Fragaria ×ananassa Duch.) cultivars (`Oso Grande' and `Chandler'), which show different degrees of susceptibility to enzymatic browning after processing. Although the levels of anthocyanins at the processing-ripe stage may be important in determining pigment stability, and therefore market suitability, the color stability of `Chandler' is apparently determined by the lower endogenous levels of peroxidase and polyphenol oxidase in the processing-ripe stage, which are also accompanied by a lower (+)-catechin content. Polyphenol oxidase was localized almost exclusively in the cortex and to a lesser extent in the pith, showing a complementary pattern to that shown by peroxidase, which was localized in the vascular bundles. Since peroxidase and polyphenol oxidase showed a complementary localization pattern in the fruit, these results strongly suggest a synergic role for these two oxidative enzymes in pigment decay and the associated browning reaction, which occurs in processed strawberry fruit and their derived foods.
The subcellular localization of a basic peroxidase (EC 1.11.1.7) isoenzyme in crisphead lettuce (Lactuca sativa L.) leaves was studied through subcellular fractionation and protoplast and vacuole isolation. This isoenzyme is mainly located in soluble fractions. Studies using protoplast isolation and vacuole purification indicated that the soluble basic peroxidase isoenzyme is found in the vacuolar sap, probably in equilibrium with the same isoenzyme attached to tonoplast membranes.
A technique has been developed to study the histochemical localization of peroxidase in Vitis vinifera by blotting freezing/thawing tissue sections on nitrocellulose membranes. After being stained with 4-methoxy- α -naphthol and H2O2, peroxidase-mediated reaction products in mature `Gamay' grapes were seen principally in the skin and, to a lesser extent, the pericarp, where discrete areas of reaction products were located in the vascular bundles. However, for immature `Gamay' and `Grenache' grapes, peroxidase activity in the skin was low and similar to that found in the pericarp. With this technique, fruit vascular bundle structure was preserved. The reliability of the technique in the histochemical localization of peroxidase in grapes was confirmed by fractionation and determining the peroxidase activity in the various tissues.