Lawrence Berkeley National Laboratory

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 ∼50 : 50 mixture of [Formula: see text] and [Formula: see text]), hydroxyl radicals (^•OH, formed in Fenton mixtures), and illuminated riboflavin (generating singlet oxygen, ¹O₂). Products were monitored electrophoretically. DHA quenched H₂O₂ far more effectively than superoxide, but the main products in both cases were 4-O-oxalyl-l-threonate (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 yield; it yielded oxalate, threonate, and OxT. ¹O₂ had no effect on DHA. DKG was oxidatively decarboxylated by H₂O₂, Fenton mixture, and ¹O₂, 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 ¹O₂ 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 ¹O₂ The diverse products are potential signals, enabling organisms to respond appropriately to diverse stresses. Also, the reaction-product 'fingerprints' are analytically useful, indicating which ROS are acting in vivo.