Federico J. Castillo

Extracellular ascorbic acid and enzyme activities related to ascorbic acid metabolism in Sedum album L. leaves after ozone exposure

Ascorbic acid (AA) and glutathione (GSH) contents and enzymatic activities of dehydroascorbate (DHA) reductase and glutathione reductase were measured in the apoplast and whole leaves of Sedum album L. plants after a 2-hr exposure to different ozone (O3) concentrations (0.2, 0.4 and 0.6 μl/l O3). Although the reduced AA level decreased in the apoplast of exposed plants, the total amount of ascorbic acid (AA+DHA) increased in that compartment following O3 exposure. The increase depended on the O3 concentration and could reach twice the control values. These results suggested the existence of a continuous supply of AA from cells in response to oxidant exposure. AA and GSH levels were depleted in the whole leaves during the exposure to O3 but rapidly recovered after exposure. Neither dehydroascorbate reductase nor glutathione reductase activities could be detected in the apoplast compartment. In cell extracts, dehydroascorbate reductase was activated and glutathione reductase was not affected by O3 exposure. These results are discussed in the context of a cycle of reactions involving ascorbate peroxidase, dehydroascorbate reductase, glutathione reductase and glucose 6-P dehydrogenase, which would operate in the apoplast and in the interior of the cell. The rapid recovery of AA and GSH levels after O3 exposure suggests a prominent role for these enzymes in cell protection against oxidative damage.

Peroxidase assay in plants: Interference by ascorbic acid and endogenous inhibitors in Sedum and Pelargonium enzyme extracts

Sedum album and Pelargonium zonale extracts do not show any peroxidase activity. Both extracts provoke a lag phase in the horse-radish peroxidase-catalyzed oxidation of guaiacol by H2O2. Preincubation of Sedum album extract with ascorbate oxidase eliminated completely the lag phase. Ascorbic acid has been identified as the substance responsible for this lag phase by reacting with a coloured intermediary product of the analytical reaction. In the Pelargonium zonale extract, the lag phase seems to be due to competitive inhibitors of peroxidase, which are of a phenolic nature.

Simple kinetic determination of trace amounts of ascorbic acid in plant extracts

Abstract Aa kinetic method of ascorbic acid assay based on the lag produced during the evolution of horse radish peroxidase-catalyzed oxidation of guaiacol or of homovanillic acid by hydrogen peroxide, is described and applied to ascorbic acid determination in Sedum album extracts.

The Regulation of Plant Peroxidases by Calcium

Among the functions currently attributed to plant peroxidases is the assembly of cell wall constituents, establishing the cell wall architecture. This role results from their ability to form cross-links between sugars, proteins, and phenols (Epstein and Lamport, 1984), It implies the transport of peroxidase molecules outside the cells and, most likely, the control of their activity in the wall. We present here experimental evidences showing that calcium plays an important role in these processes. There are at least three distinct modes of action of calcium on peroxidases, which are briefly described below.