Abstract
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 (H(2)O(2)), '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, (1)O(2)). Products were monitored electrophoretically. DHA quenched H(2)O(2) 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. H(2)O(2), 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. (1)O(2) had no effect on DHA. DKG was oxidatively decarboxylated by H(2)O(2), Fenton mixture, and (1)O(2), forming a newly characterised product, 2-oxo-l-threo-pentonate (OTP; '2-keto-l-xylonate'). Superoxide yielded negligible OTP. Prolonged H(2)O(2) treatment oxidatively decarboxylated OTP to threonate. Oxidation of DKG by H(2)O(2), Fenton mixture, or (1)O(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 H(2)O(2) and superoxide; DKG more readily by (1)O(2) 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.