Formation and subsequent metabolism of ascorbate oxidation products in vitro and in plant cells

Abstract

Vitamin C (ascorbate and dehydroascorbic acid) is vital for plants and found throughout the plant cell including in the apoplast. The structure of ascorbate was determined eighty years ago; however, many of its degradation pathways remain unclear. Numerous degradation products of ascorbate have been reported to occur in the apoplast but many still remained unidentified. Ascorbate is well known as an antioxidant, and acts to quench reactive oxygen species (ROS), such as hydrogen peroxide and ozone in the plant apoplast. The immediate oxidation product of ascorbate is dehydroascorbic acid (DHA), which may be quickly hydrolysed to diketogulonic acid (DKG). The further reactions of radiolabelled and non-radiolabelled DHA and DKG with various ROS have been investigated. Differences were observed in the products formed from the various ROS, allowing a unique fingerprint of oxidation products to be described for each ROS. Equally, different compounds were produced depending on the starting substrate; for example cyclic oxalyl threonate was only observed in the reactions of DHA and not DKG. A major oxidation product of DHA is OxT. A novel enzyme activity involving the transfer of the oxalyl group from OxT to an acceptor substrate such as a sugar has been detected. This enzyme activity could have potential cell wall modification roles, in the formation of oxalate cross-linkages between cell wall components. This would provide a novel role for ascorbate derivatives in cell growth. Vitamin C is also a vital component of the human diet, and most dietary ascorbate comes from plants such as salads. The degradation of ascorbate during post-harvest processing and storage of salad leaves has been investigated. Spinach leaves were found to be particularly prone to losing ascorbate during the industrial washing process. The use of radiolabelled ascorbate has allowed the determination that the major degradation product formed from ascorbate during spinach washing was oxalate.