Ascorbate (L-ascorbic acid, L-AA, Vitamin C) is essential for both plants and animals. It is an important enzymatic cofactor, as well as a primary antioxidant, that is thought to be important in preventing various oxidative stress-related conditions (Davey et al. 2000; De Tullio and Arrigoni 2004; Li and Schellhorn 2007). Because humans, and a few other animal species have lost the capacity to synthesize it, they are dependent on diet to ensure adequate ascorbate levels (Linster and Van Schaftingen 2007). Plants, especially leaves and fruits, are a good source of ascorbate; it is therefore important to consume substantial amounts of fruits and vegetables (Poiroux-Gonord et al. 2010).
In plants and animals, ascorbate detoxifies reactive oxygen species (ROS). In plants, ROS are formed during the process of lignification, cell division and the hypersensitive response (Davey et al. 2000). Ascorbate is the predominant form of vitamin C; its main oxidation product L-Dehydroascorbic acid (DHA) is usually less than 10% of total vitamin C. Only one report indicates an increase of the DHA/AA ratio during storage of crops(Lee Seung and A Kade 2000).
Ascorbate is proposed to be synthesised in plants through four biosynthetic pathways: Smirnoff-Wheeler (SW), galacturonate, myo-inositol, and gulose pathways (Ntagkas et al. 2018). The predominant one is the SW pathway, and because the others are not contributing significantly, they will not be considered here. In the SW pathway, ascorbate is synthesized from D-Glucose-6P via D-Mannose and L-Galactose to L-galactono-1,4-lactone (G14L) (Figure 3.6) (Wheeler et al. 1998; Ishikawa et al. 2006). During ROS scavenging ascorbate is firstly oxidized into monodehydroascorbate (MDHA), catalysed by ascorbate oxidase and ascorbate peroxidase (Nakano and Adada 1981). MDHA can be turned back into ascorbate by monodehydroascorbate reductase (MDHAR) (Hossain et al. 1984)or further oxidized into dehydroascorbate (DHA) in a non-enzymatic reaction. DHA can be reduced back to ascorbate by the enzyme dehydroascorbate reductase (DHAR) at the expense of reduced glutathione. These reactions occur in most subcellular components, with the exception of the apoplast (Dalton et al. 1993; Koshiba 1993; Yamaguchi et al. 1995)
Figure 3.6. Smirnoff Wheeler pathway for Ascorbate biosynthesis. Based on Ntagkas et al., (2018). Abbreviations: DHA: dehydro ascorbate; DHAR: DHA reductase; MDHA: monodehydroascorbate; MDHAR: MDHA reductase; *: conversion of MDHA to DHA is non-enzymatic.
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