Dehydroascorbate-reducing proteins in maize are induced by the ascorbate biosynthesis inhibitor lycorine

Dehydroascorbate (DHA) reductase (glutathaone: dehydroascorbate oxidoreductase, EC 1.8.5.1) has been generally considered a specific enzyme of the ascorbate-glutathione cycle. However, at least four distinct proteins can catalyze in vitro both glutathione-dependent DHA reduction and other reactions mainly related to thiol-disulphide exchange. These data have raised questions both on the existence of specific DHA reductase and the actual physiological role of DHA-reducing proteins (DRP). We have observed characteristic electrophoretic patterns of DRP in dark-germinating embryos of different plant species. Marked differences were observed not only in the number, but also in the migration rate of DRP under non-denaturing conditions. In order to evaluate the actual contribution of DRP activity to ascorbate (ASC) regeneration under conditions limiting ASC biosynthesis, Z. mays germinating embryos excised from endosperm were either incubated in distilled water or treated with the alkaloid lycorine, an inhibitor of ASC biosynthesis. In parallel with the decrease in ASC content, a strong enhancement in DRP activity occurred. The increase in DRP activity was prevented by cycloheximide, and thus seems to be due to de novo protein synthesis. The possible involvement of DRP in avoiding DHA accumulation under adverse environmental conditions is discussed.