Structural characterization of a L-dehydroascorbic acid-L-homocysteine thiolactone reaction product: Intracellular formation in neuronal cells.

Homocysteine thiolactone (HTL) has been implicated in cardiovascular and neurological pathologies. While homocysteine can S-homocysteinylate thiol groups in proteins, the chemical properties of HTL facilitates covalent binding to protein ε-amino groups on lysine residues, which can initiate protein aggregation. Ascorbate plays an important role in the prevention of oxidative stress. Ascorbate is easily oxidized by the loss of two electrons to dehydroascorbate (DHA), which can be reduced back to ascorbate by thiol-containing smaller and larger molecules. In the present study, reaction products between two aldehydes, formaldehyde and propionaldehyde as well as DHA with the physiological relevant isomer of homocysteine thiolactone i.e. L-HTL were purified, and their structure was determined by 1D and 2D-nuclear magnetic resonance. In all three cases the reaction products are likely formed by initial imine condensation, subsequent formation of a hemiaminal product followed by HTL ring opening and intramolecular nucleophilic attack of the resulting thiol anion to form a six-member thiazinane ring with a carboxylic acid group. The structure of the DHA, L-HTL reaction product was confirmed by high resolution accurate ESIMS/MS in negative mode. Formation of the reaction product between DHA and HTL prevented N-homocysteinylation of cytochrome c by HTL, confirming earlier observations. The reaction product is formed in human neuroblastoma cells (SH-SY5Y) when exposed to HTL and DHA or ascorbate, potentially preventing protein aggregation. The consequences associated with formation of a reaction product between DHA and HTL suggest that DHA could protect against protein N-homocysteinylation.