Ascorbate and dehydroascorbic acid (DHA) both serve as vitamin C in the human diet. They are interconvertible within the cells of both animals and plants. However, a cell’s vitamin C pool can be diminished by various irreversible pathways which start out from DHA. This is significant in governing the vitamin content of living plants and of plant-derived foods and beverages (e.g. ‘smoothies’). Our laboratory is working to understand routes that lead to vitamin loss, both in living plants in vitro. The present project will further define the metabolic fate of vitamin C and the biological and nutritional consequences of these pathways.
There are two competing routes that irreversibly diminish vitamin C levels by consuming DHA :
(a) Oxidative degradation of DHA to oxalyl-threonates. This route is promoted by oxidative stress, when reactive oxygen species (ROS) are present. Downstream of route (a), the oxalyl-threonates can attach their oxalyl groups to carbohydrates , including cell-wall polysaccharides.
(b) Hydrolysing DHA to diketogulonate (DKG). This route is promoted at pH values above ~6 and when ROS are not prevalent. Downstream of route (b), DKG can undergo several further oxidative, reductive and non-redox reactions.
Most of these pathways are of unknown biological significance, and a major aim of the present project is to explore their significance.
Specific topics to be addressed in experiments by the student include:
• Defining the effect of ozone on ascorbate, DHA and DKG. These are thought to ‘mop up’ ozone from atmospheric pollution, but the by-products thus formed are unknown. We will continue our work on ROS-driven oxidation of ascorbate and its derivatives, adding ozone to the list of ROS already studied.
• Exploring when and where oxalyl groups from route (a) are attached to cell-wall polysaccharides  and whether wall-bound oxalate groups affect the cell wall’s properties.
• Documenting the pathways of vitamin C loss in smoothies (e.g. blended spinach leaves), and helping to define factors that could be recommended to minimise such loss.
• Discovering how much of a plant cell’s total ascorbate is secreted through the plasma membrane into the apoplast (aqueous solution that permeates the cell wall). This is significant because the wall contains an enzyme, ascorbate oxidase, that converts ascorbate to DHA (Green & Fry, 2005).
• Defining the routes via which vitamin C is lost when spinach leaves are commercially washed for the salad industry. The mechanical agitation suffered by such leaves diminishes ascorbate levels  by unknown means.
• Testing whether routes (a) and (b) are catalysed to some extent by enzymes. Hydrolysis of DHA to DKG can occur non-enzymically but, in addition, there is early preliminary evidence that ‘DHA hydrolase’ enzymes exist (Tewari & Krishnan, 1960). This project will investigate this hypothesis as a contribution to understanding the control of vitamin C levels in vivo. We will also look for novel enzyme activities that catalyse the oxidation of DHA and/or DKG.
Possibly unfamiliar techniques, e.g. separation of ascorbate metabolites and radiolabelling, will be taught by Professor Fry during the project.
For more information on the Edinburgh Cell Wall Group, please see http://fry.bio.ed.ac.uk/index.html
Potential funding sources include:
The Darwin Trust, Edinburgh.
NERC (Natural Environment Research Council)
BBSRC (Biotechnology & Biological Sciences Research Council)
See also http://fry.bio.ed.ac.uk/links.html
 M.A. Green, S.C. Fry (2005) Vitamin C degradation in plant cells via enzymatic hydrolysis of 4-O-oxalyl-L-threonate. Nature 433: 83–88.
 R.A. Dewhirst, G.J.J. Clarkson, S.D. Rothwell, S.C. Fry (2017). Novel insights into ascorbate retention and degradation during the washing and post-harvest storage of spinach and other salad leaves. Food Chemistry, 233, 237–246.
 R.A. Dewhirst, S.C. Fry (2018) Oxalyltransferase, a plant cell-wall acyltransferase activity, transfers oxalate groups from ascorbate metabolites to carbohydrates. Plant Journal 95, 743–757.