Active Transport of Ascorbic Acid

Ascorbate is absorbed from the lumen of the human intestine by sodium-ascorbate cotransport in enterocytes, as has been shown by measuring transport activities in luminal (brush-border) membrane vesicles (Malo and Wilson, 2000). This phenomenon couples ascorbate uptake to the concentration gradient of sodinm ion across the plasma membrane that is maintained by sodium/potassium-ATPase. It is likely because of the limited capacity of enterocytes for sodium-ascorbate cotransport that large oral doses of ascorbate are absorbed less completely than are small doses. Absorption sites for ascorbate are found along the entire length of the small intestine (Malo and Wilson, 2000). 

Besides intestinal absorption, another important determinant of bioavailability is secondary active transport of ascorbate in the kidney. Most AA circulates in the blood in the form of the ascorbate anion. The ascorbate in the blood plasma is freely filtered at the renal glomerulus, but much of it is reabsorbed in the proximal tubule. Ascorbate uptake across the luminal membranes of renal proximal tubule cells occurs through sodium-ascorbate cotransport. The amount of ascorbate lost in the urine rises when the plasma ascorbate concentration exceeds the renal threshold. Above this threshold the tubular reabsorptive capacity is overwhelmed. The renal threshold for AA is reported to be slightly higher in men than in women (plasma ascorbate concentrations of 86 and 71 pM, respectively), but the underlying mechanism and physiological importance of this difference are unknown (Oreopoulos etal., 1993). 

Sodium-ascorbate cotransporters are remarkably specific for L-ascorbate (Dixon and Wilson, 1992 Franceschi et al., 1995 Liang et al., 2002 Malo and Wilson, 2000  

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As mentioned above, ascorbic acid acts as a neuroprotective agent in in vitro models of scurvy. Therefore, it is a surprise that no symptoms of brain cell damage have been reported in conditions involving severe systemic ascorbic acid deficiency. This may be explained by the fact that the scorbutic state cannot be produced in the intact animal brain because of the brain s homeostatic mechanisms such as the highly specific ascorbic acid transport system in the choroid plexus (Spector, 1989) and the inability of ascorbic acid to cross the blood-brain barrier, which effectively isolate the ascorbic acid content of the intact brain from the rest of the body s ascorbic acid pool. The active transport of ascorbic acid from blood to cerebrospinal fluid (Spector and Eells, 1984), together with cellular uptake mechanisms, represents the base for homeostasis of brain ascorbic acid concentrations (see also Section 2). This is in agreement with the report about normal ascorbic acid concentrations in brains from patients with Parkinson s disease (Riederer et al., 1989), in which free radical damages are postulated to be involved (see below). 

Dimattio, J., 1989b, Active transport of ascorbic acid into lens epithelium of the rat, Exp. Eye Res. 49 873-885. 

DiMattio, J., and Streitman, J., 1991, Active transport of ascorbic acid across the retinal pigment epithelium of the bullfrog, Curr. Eye Res. 10 959-965. 

Vitamin C Ascorbic acid is the most important redox substance of cell metabolism. The body content probably amounts to about 2-5 g, the major part being stored in the liver and muscles. Intestinal absorption (80-90%) is an active, sodium-dependent process. The transport of ascorbic acid in the blood probably takes place as an ascorbic acid-albumin complex. Cellular uptake is stimulated by insulin. 

Contrary to the active carrier-mediated transport of ascorbic acid into the cell, the intracellular transport into organelles like mitochondria follows carrier-free diffusion of both ascorbic acid and dehydroascorbic acid. Intramitochondrial dehydroascorbic acid is not reduced to ascorbic acid (Ingebretsen et al., 1982). 

Absorption of ascorbic acid in the gut is a passive process for the rat (18), while scurvy-prone animals require an active transport system with a Na -dependent, gradient-coupled carrier mechanism that is inhibited by ouabain (19,20). A transport model is favored that fea-... 

In humans there is a Na -dependent active transport system with a Km of about 1 mM. Absorption is very eflBcient at low intakes of ascorbic acid and becomes poor as stomach levels of ascorbic acid increase. The upper level of ascorbic acid in the blood is limited by kidney clearance with Tm of 1.5 mg/100 mL. Where intestinal absorption is excessive the eflSciency of the kidney clearance improves. Transfer of ascorbic acid into the central nervous system and other tissues is a facilitated saturable process. Therefore, control at all levels appears to sharply limit maximum levels of ascorbate in tissues. 

The concentration of ascorbate in the human plasma is 25 pM and above. Cells take up ascorbate by a Na -coupled uptake mechanism against a concentration gradient. A marked stereo-selectivity for L-ascorbic acid relative to D-isoascorbic acid in their cellular transport has been shown by Franceschi et al. [12]. The same transport is also important in the intestine. The nutritional supply of ascorbic acid is the only source for this vitamin in humans, primates, and guinea pigs. Other mammals are able to produce ascorbic acid. There exists sufficient evidence for an active role of ascorbate as an antioxidant in vivo. Decreased ascorbic acid will increase lipid peroxidation and decrease vitamin E and is connected with oxidative DNA damage. The supply of ascorbate in some cases will reduce the amount of oxidative damage in diseases that... 

While ascorbate is transported into isolated intact chloroplasts (Anderson et al., 1983b Beck et al., 1983 Foyer and Lelandais, 1996), the thylakoid membranes appear to have no carrier system to transport ascorbate into the thylakoid lumen (Foyer and Lelandais, 1996). This is surprising since the enzyme VDE, located inside the thylakoid, requires ascorbate to convert violaxanthin to zeaxanthin (Fig. 2). Upon illumination the pH ofthe lumen falls and VDE binds to the lumenal side of the thylakoid membrane and becomes active (Hager and Holocher, 1994 Rockholmand Yamamoto, 1996). The affinity of VDE for ascorbate is strongly dependent on pH, because ascorbic acid is the true substrate of the enzyme (Bratt et al., 1995). The addition ofHjOj to illuminated thylakoid membrane preparations induces a transient inhibition of zeaxanthin formation (Neubauer and Yamamoto, 1992), suggesting competition for ascorbate between the Mehler-peroxidase cycle and the VDE reaction. Since the ascorbate pool in the chloroplast stroma is substantial (10-50 rnM), sufficient amounts of ascorbic acid may be able to aoss the membrane by diffusion (Eoyer et al., 1983 Foyer, 1993 Foyer and Lelandais,... 

Ascorbic acid deficiency also reduces the activity of several dehydrogenases involved in the Krebs cycle. The mechanism by which the vitamin leads to such alteration is not clear, but the effect is reversed by insulin administration. Thus, miscellaneous observations on the effect of ascorbic acid on carbohydrate metabolism have been made, but they are difficult to interpret because no specific coenzyme effect of ascorbic acid has been demonstrated. Again, ascorbic acid is assumed to be directly involved in an electron transport chain that involves cytochrome and NAD. The vitamin may also affect the electron transport chain indirectly because a decrease in NADH concentration has been observed in vitamin C deficiency. But this decrease may also result from an interference with the insulin production because it is... 

The suggestion that there may be a sodium-dependent ascorbic acid transport mechanism in the pigmented layer of the ciliary epithelium (the layer which faces the blood side) fits well with a study conducted by Chu and Candia (1988), who isolated the rabbit iris-ciliary body and measured trans-tissue fluxes of labeled ascorbic acid. A net flux of ascorbic acid was observed in what would have been the blood-to-aqueous humor direction. Importantly, this net flux could be inhibited by phloridzin added to the blood side but not when added to the aqueous side. This finding is consistent with a model where active ascorbic acid is accumulated by the pigmented cells on the blood side of the ciliary epithelium bilayer, then passes via gap junctions into the nonpigmented cells, and finally diffuses into the aqueous humor. However, the situation may be more complex since cultured cells derived from the nonpigmented ciliary epithelium have also been shown to be capable of sodium-dependent ascorbic acid accumulation (Delamere et al., 1993). 

On the basis of Khatami s work it seems reasonable to guess that the high levels of ascorbic acid measured in the neural retina are the result of active transport... 

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