Handbook of Vitamin C Research

48Yasuo Kagawa, Shizu Higasa, Masaru Tsujimura et al.important role in the modulation of the cellular redox state [13].Oxidative stress caused byxenobiotic substances and reactive oxygen species (ROS) [14] may be removed by VC andother antioxidants [10, 15, 16]. The removal of ROS is vital to the prevention of diabetes andaging [7, 10, 14, 16], as the free radical theory of aging posits that oxidative stress is amongthe major mechanisms in aging and age-related diseases [7, 10, 14]. To address theseproblems, both metabolic studies and epidemiological surveys are needed.(1) Metabolism of VC and Oxidative StressVC is classified into a reduced form, L-ascorbic acid (AsA), and an oxidized form, L-dehydroascorbic acid (DAsA). VC plays a protective role against oxidative stress andxenobiotics in vivo [1, 10, 14]. AsA is converted into DAsA by reacting with ROS andxenobiotics via monodehydroascorbate radicals, which are rapidly reduced by variousreductases back to AsA [1, 2]. Monodehydroascorbate radicals, a major oxidation product ofAsA under oxidative stress, are reconverted to AsA in the cytosol by cytochrome b5reductase [EC 1.6.2.2] and thioredoxin reductase [EC 1.6.4.5] in reactions involving NADHand NADPH, respectively [1]. The dismutation of a pair of monodehydroascorbate radicalsproduces one molecule of AsA and one of DAsA. DAsA is spontaneously reduced to AsA byglutathione [1], as well as enzymatically in reactions mediated by glutaredoxin etc. [2]. Theirreversible step of VC catabolism is the delactonization of DAsA into 2, 3-diketo-L-gulonateby dehydroascorbatase (gluconolactonase) [EC 3.1.1.17] [3].ROS are generated continuously by intracellular oxidative events and mitochondriacontribute significantly to the production of intracellular ROS [15], which can modify thebiological activity of enzymes, modulate intracellular signaling events, and damagebiological macromolecules [16]. The mitochondrial DNA has been shown to be extremelysusceptible to the mutagenic effects of ROS [16]. Loading cells with VC reduces oxidativecell death [17, 18], inhibits FAS-induced apoptosis [19].In contrast to the antioxidant properties of VC, pro-oxidant properties have also beendescribed [12, 20, 21]. It has been amply documented that AsA added to the medium of a cellculture increases oxidative damage, and this effect of AsA has been ascribed to thegeneration of reactive oxygen intermediates in the medium during its auto-oxidation. Thiseffect is also exerted inside the cell as well [20]. This will be discussed in the section (7) ofdiscussion.VC is synthesized in most mammals (VC autotrophs) by an active five-enzyme processthat begins with an activated form of glucose, UDP-glucose and ends with L-gulonolactoneoxidase (GLO) which is missing in humans [1, 22]. Humans are VC auxotrophs and haverelatively low plasma VC levels [23] and high serum uric acid levels [24] compared to VCautotrophs due to the destructions in genes for GLO and uricase, respectively [25]. Moreover,expression of erythrocyte facilitative glucose transporter 1 (GLUT1) is a specific trait of VCauxotrophs, comprising only higher primates [22, 26], guinea pigs [27], and fruit bats [26].Human GLUT1 is a very active DAsA transporter by expressing stomatin, which increasesthe affinity of GLUT1 for DAsA, and transported DAsA is conserved as stable AsA [26].Because VC auxotrophs genetically compensate for the defective GLO [25, 26], human in