The oxidation of dehydroascorbic acid and 2,3-diketogulonate by distinct reactive oxygen species

L-Ascorbate, dehydro-L-ascorbic acid (DHA), and 2,3-diketo-L-gulonate (DKG) can all quench reactive oxygen species (ROS) in plants and animals. The vitamin C oxidation products thereby formed are investigated here. DHA and DKG were incubated aerobically at pH 4.7 with peroxide (H2O2), 'superoxide' (a similar to 50 : 50 mixture of O-2(-) and HO2-), hydroxyl radicals ((OH)-O-center dot, formed in Fenton mixtures), and illuminated riboflavin (generating singlet oxygen, O-1(2)). Products were monitored electrophoretically. DHA quenched H2O2 far more effectively than superoxide, but the main products in both cases were 4-O-oxalyl-Lthreonate (4-OxT) and smaller amounts of 3-OxT and OxA + threonate. H2O2, but not superoxide, also yielded cyclic-OxT. Dilute Fenton mixture almost completely oxidised a 50-fold excess of DHA, indicating that it generated oxidant(s) greatly exceeding the theoretical (OH)-O-center dot yield; it yielded oxalate, threonate, and OxT. O-1(2) had no effect on DHA. DKG was oxidatively decarboxylated by H2O2, Fenton mixture, and O-1(2), forming a newly characterised product, 2-oxo-L-threo-pentonate (OTP; '2-keto-L-xylonate'). Superoxide yielded negligible OTP. Prolonged H2O2 treatment oxidatively decarboxylated OTP to threonate. Oxidation of DKG by H2O2, Fenton mixture, or O-1(2) also gave traces of 4-OxT but no detectable 3-OxT or cyclic-OxT. In conclusion, DHA and DKG yield different oxidation products when attacked by different ROS. DHA is more readily oxidised by H2O2 and superoxide; DKG more readily by O-1(2). The diverse products are potential signals, enabling organisms to respond appropriately to diverse stresses. Also, the reactionproduct 'fingerprints' are analytically useful, indicating which ROS are acting in vivo.