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A Rapid and Specific Microplate Assay for the Determination of Intra- and Extracellular Ascorbate in Cultured Cells
Published on: April 12, 2014
Mechanism of the reaction catalyzed by dehydroascorbate reductase from spinach chloroplasts
Taise Shimaoka1, Chikahiro Miyake, Akiho Yokota
1Graduate School of Biological Sciences, Nara Institute of Science and Technology, 8916-5 Takayama, Ikoma, Nara 630-0101, Japan.
Abstract:
Dehydroascorbate reductase (DHAR) reduces dehydroascorbate (DHA) to ascorbate with glutathione (GSH) as the electron donor. We analyzed the reaction mechanism of spinach chloroplast DHAR, which had a much higher reaction specificity for DHA than animal enzymes, using a recombinant enzyme expressed in Escherichia coli. Kinetic analysis suggested that the reaction proceeded by a bi-uni-uni-uni-ping-pong mechanism, in which binding of DHA to the free, reduced form of the enzyme was followed by binding of GSH. The Km value for DHA and the summed Km value for GSH were determined to be 53 +/- 12 micro m and 2.2 +/- 1.0 mm, respectively, with a turnover rate of 490 +/- 40 s-1. Incubation of 10 microm DHAR with 1 mm DHA and 10 microm GSH resulted in stable binding of GSH to the enzyme. Bound GSH was released upon reduction of the GSH-enzyme adduct by 2-mercaptoethanol, suggesting that the adduct is a reaction intermediate. Site-directed mutagenesis indicated that C23 in DHAR is indispensable for the reduction of DHA. The mechanism of catalysis of spinach chloroplast DHAR is proposed.
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