Abstract
The partially purified monodehydroascorbate reductase (EC 1.6.5.4) from Euglena gracilis Z showed a pH optimum of 7.0 and a temperature optimum of 41 °C, and had an M r of 52000. Activity was threefold higher with N ADPH than with N ADH; the K m values for N ADPH and NADH were 7 and 210 μm, respectively. The partially purified dehydroascorbate reductase (EC 1.8.5.1) had a pH optimum of 7.0, a temperature optimum of 38 °C and an M r of 28000. The enzyme was specific for reduced glutathione as an electron donor, with a K m of 0.85 mM, and for dehydroascorbate, with a K m of 0.26 mM. The enzyme reaction proceeded in either an ordered or a random manner. Both reductases were markedly inhibited by thiol inhibitors, and the inhibition was overcome by thiol compounds such as 2-mercaptoethanol and dithiothreitol. Both reductases were cytosolic. The activities of monodehydroascorbate and dehydroascorbate reductases could account for the regeneration of ascorbate from monodehydroascorbate and dehydroascorbate produced by ascorbate peroxidase (EC 1.11.1.11) for scavenging hydrogen peroxide.
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