Handbook of Vitamin C Research

Molecular Bases of the Cellular Handling of Vitamin C 215species of vertebrates, because some of them, even mammals, have the ability to synthesizeAA (Linster & Van Schaftingen, 2007).Several metabolic pathways that permit the synthesis of vitamin C by species of fungi,plants and animals have been described. In vertebrates able to synthesize vitamin C, thisoccurs from D-glucuronate, which is produced by direct hydrolysis of UDP-glucuronate,catalyzed by UDP-glucuronidase (Linster & Van Schaftingen, 2007), which is located in theendoplasmic reticulum membrane (Bossuyt & Blanckaert, 1995). D-glucuronic acid isreduced to L-gulonic acid by the action of a NADPH-dependent oxido-reductase, glucuronatereductase, which belongs to the aldo-keto reductase family of enzymes (Mano et al., 1961).L-gulonic acid is then biotransformed into L-gulonolactone by the Mn 2+ -dependent cytosolicenzyme gulonolactonase (Winkelman & Lehninger, 1958; Bublitz & Lehninger, 1959),which has been identified as regucalcine or senescent marker protein 30 (SMP30) (Kondo etal., 2006), a protein probably involved in ageing because its expression in several organs,such as liver, kidney and lung decreases during ageing (Fujita et al., 1992) and because itsabsence in knock-out mice has been associated with senescence (Mori et al., 2004).Moreover, silencing the SMP30 gene in mice results in vitamin C deficiency (Kondo et al.,2006).The last step in vitamin C synthesis is the oxidation of L-gulonolactone to L-ascorbicacid, which is catalyzed by L-gulonolactone oxidase (GLO) (Chatterjee et al., 1960) (Figure1). The subcellular localization of GLO is also the membrane or endoplasmic reticulum(Koshizaka et al., 1988). Owing to mutations in the GLO gene, the functional expression ofthis enzyme does not occur in several species, such as guinea pigs, humans, and others(Nishikimi et al., 1992; Ohta & Nishikimi, 1999), for which AA is a vitamin. These geneticvariants have occurred in the primate GLO orthologue by the accumulation of randommutations (Ohta & Nishikimi, 1999), whereas in the case of guinea pigs there has been adeletion affecting two exons (Nishikimi et al., 1992).During evolution, inactivation of the GLO gene has happened often, which suggests thatthis event may confer some evolutionary advantage to certain species (Linster & VanSchaftingen, 2007). In this respect, it should be considered that GLO activity results in theproduction of hydrogen peroxyde and subsequent consumption of glutathione (Banhegyi etal., 1996). Accordingly, in species with a diet rich in vitamin C, the risk of producinghydrogen peroxide has been abolished by inactivation of the GLO gene.In species with a conserved ability to synthesize vitamin C, this may be increased inresponse to several stimuli, such as the need to inactivate xenobiotic compounds byconjugation with glucuronic acid. This enhances the hydrolysis of UDP-glucuronic acid andthe synthesis of L-ascorbic acid (Linster & Van Schaftingen, 2007) (Figure 1).Glutathione, together with vitamin C, is one of the most abundant reducing agents inmammalian cells. Glutathione is involved in recycling of DHA to AA and plays a role in thecontrol of the de novo synthesis of AA. Thus, when cells are depleted of glutathione anincrease in the synthesis of AA occurs. The actual mechanism accounting for this protectiveresponse is as yet unknown (Linster & Van Schaftingen, 2007).Regarding control of the vitamin C biosynthesis pathway, this is mainly regulated by thetransformation of UDP-glucuronic acid into D-glucuronic acid. Thus, an increase in theactivity of the enzyme responsible results in an enhanced production of vitamin C (Linster &