Characterisation of the non-oxidative degradation pathway of dehydroascorbic acid in slightly acidic aqueous solution

Although l-ascorbate (vitamin C) is an important biological antioxidant, its degradation pathways in vivo remain incompletely characterised. Ascorbate is oxidised to dehydroascorbic acid, which can be either hydrolysed to diketogulonate (DKG) or further oxidised. DKG can be further degraded, oxidatively or non-oxidatively. Here we characterise DKG products formed non-enzymically and non-oxidatively at 20 °C and at a slightly acidic pH typical of the plant apoplast. High-voltage electrophoresis revealed at least five products, including two novel CPLs (epimers of 2-carboxy-l-threo-pentonolactone), which slowly interconverted with CPA (2-carboxy-l-threo-pentonate). One of the two CPLs has an exceptionally low pK_a. The CPL structures were supported by MS [(C₆H₇O₇)^-] and by ¹H and ¹³C NMR spectroscopy. Xylonate and its lactone also appeared. Experiments with [1-¹⁴C]DKG showed that all five products (including the 5-carbon xylonate and its lactone) retained DKG's carbon-1; therefore, most xylonate arose by decarboxylation of CPLs or CPA, one of whose -COOH groups originates from C-2 or C-3 of DKG after a 'benzilic acid rearrangement'. Since CPLs appeared before CPA, a DKG lactone is probably the main species undergoing this rearrangement. CPA and CPL also form non-enzymically in vivo, where they may be useful to researchers as 'fingerprints', or to organisms as 'signals', indicating a non-oxidative, slightly acidic biological compartment.