Transport into Tissues and Organs

Ascorbic acid in blood is transported by a reversible complex with serum albumin (Moloy et al., 1980). The main transport form of vitamin C seems to be the reduced molecule. The concentration of dehydroascorbic acid can, however, exceed that of ascorbic acid in vitamin C deficiency and certain diseases (Stone, 1977). The relation of ascorbic acid/dehydroascorbic acid in human plasma decreases also with age (Sasaki et al., 1983). Since the typical screening methods for the determination of vitamin C status in blood are not able to distinguish between the different circulating forms of vitamin C, it has to be clearly stated that the detailed interpretation of vitamin C status and its relation to certain disorders must reflect on both ascorbic acid and dehydroascorbic acid concentration in blood. 

Vitamin C is transported into the blood cells in its oxidized form as dehydroascorbic acid, since this form is non-ionized under physiological conditions and is therefore permeable for membranes. It has been postulated that at least human neutrophils are able to oxidize extracellular ascorbic acid for a more efficient uptake into the cells (Washko et al., 1993). This mechanism also represents a part of the biological recycling system for antioxidants. The intracellular dehydroascorbic acid is rapidly reduced again to ascorbic acid by the GSH redox system. This reduction might also be mediated by the still hypothetical dehydroascorbic acid reductase. 

The transport of dehydroascorbic acid into blood cells uses the same system as the transport of glucose. Therefore, the cellular uptake of dehydroascorbic acid depends on the presence of insulin and can be inhibited by high concentrations of glucose. These findings are important for the vitamin C metabolism of diabetics, since several diabetic risk factors (like angiopathies) might be related to a lack of ascorbic acid (Verlangieri et al., 1981). 

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). 

---