Handbook of Vitamin C Research

216J.J.G. Marin, M.A. Serrano, M.J. Perez,et al.Van Schaftingen, 2003). Down-stream in this biochemical pathway is the limiting enzyme ofthe overall process; namely, GLO (Chatterjee et al., 1960).Figure 1. Simplified schematic representation of the pathway responsible for ascorbic acid synthesis.The major product of vitamin C oxidation is semi-DHA, which is generated in manyreactions that use AA as co-factor, such as those catalyzed by mono- and di-oxygenases(Linster & Van Schaftingen, 2007). The recycling of semi-DHA to AA takes place in thecytoplasm through the activity of cytochrome B5 reductase and thioredoxine reductase, tworeactions that require NADH and NADPH, respectively (Ito et al., 1981).The catabolism of vitamin C starts with the formation of DHA from AA (Linster & VanSchaftingen, 2007). DHA is hydrolyzed to generate 2,3-diketo-L-gulonolactone, which hasno anti-scurvy activity (Bode et al., 1990). In the absence of hydrogen peroxide, thismetabolite is spontaneously degraded to oxalate and L-erythrulose (Simpson & Ortwerth,2000). Owing to the high reactivity of L-erythrulose, the accumulation of this ketose may be