Ascorbic acid vitamin transport

Ascorbic acid is transported in the plasma in association with the protein albumin (see Chapter 4). The reduced form of the vitamin normally predominates... 

Ascorbic acid (vitamin C) has no action on aluminium. It can be prepared and transported in aluminium equipment. 

Absorption, Transport, and Excretion. The vitamin is absorbed through the mouth, the stomach, and predominantly through the distal portion of the small intestine, and hence, penetrates into the bloodstream. Ascorbic acid is widely distributed to the cells of the body and is mainly present in the white blood cells (leukocytes). The ascorbic acid concentration in these cells is about 150 times its concentration in the plasma (150,151). Dehydroascorbic acid is the main form in the red blood cells (erythrocytes). White blood cells are involved in the destmction of bacteria. 

Since many essential nutrients (e.g., monosaccharides, amino acids, and vitamins) are water-soluble, they have low oil/water partition coefficients, which would suggest poor absorption from the GIT. However, to ensure adequate uptake of these materials from food, the intestine has developed specialized absorption mechanisms that depend on membrane participation and require the compound to have a specific chemical structure. Since these processes are discussed in Chapter 4, we will not dwell on them here. This carrier transport mechanism is illustrated in Fig. 9C. Absorption by a specialized carrier mechanism (from the rat intestine) has been shown to exist for several agents used in cancer chemotherapy (5-fluorouracil and 5-bromouracil) [37,38], which may be considered false nutrients in that their chemical structures are very similar to essential nutrients for which the intestine has a specialized transport mechanism. It would be instructive to examine some studies concerned with riboflavin and ascorbic acid absorption in humans, as these illustrate how one may treat urine data to explore the mechanism of absorption. If a compound is... 

Wang, Y., et al. Human vitamin C (L-ascorbic acid) transporter SVCT1. Biochem. Biophys. Res. Commun. 2000, 267, 488-494. 

Vitamin C (VC, L-ascorbic acid) is known to be essential for many enzymatic reactions. Sodium-dependent VC transporters (SVCT), SVCT1 and SVCT2, were recently identified and reported to be localised in the apical cell membrane of AECs in the lung of adult rats. These results suggest that SVCT proteins could transport the reduced form of VC from the airway/alveolar surface liquid into respiratory epithelial cells [106],... 

GSH synthesis. GLUT1 transports dehydroascorbic acid, an oxidized form of vitamin C, to supply the retina with ascorbic acid [47],... 

Vitamins are chemically unrelated organic compounds that cannot be synthesized by humans and, therefore, must must be supplied by the diet. Nine vitamins (folic acid, cobalamin, ascorbic acid, pyridoxine, thiamine, niacin, riboflavin, biotin, and pantothenic acid) are classified as water-soluble, whereas four vitamins (vitamins A, D, K, and E) are termed fat-soluble (Figure 28.1). Vitamins are required to perform specific cellular functions, for example, many of the water-soluble vitamins are precursors of coenzymes for the enzymes of intermediary metabolism. In contrast to the water-soluble vitamins, only one fat soluble vitamin (vitamin K) has a coenzyme function. These vitamins are released, absorbed, and transported with the fat of the diet. They are not readily excreted in the urine, and significant quantities are stored in Die liver and adipose tissue. In fact, consumption of vitamins A and D in exoess of the recommended dietary allowances can lead to accumulation of toxic quantities of these compounds. 

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. 

The functions and fate of i.-ascorbic acid in humans and other primates are reviewed in this chapter. Topics included are use of subhuman primates for research in nutrition evolution and subsequent loss of ascorbic acid biosynthesis absorption, tissue transport, and distribution of ascorbic acid and catabolism, functions, and requirements of ascorbic acid. In retrospect, the insight provided by this chapter suggests new work areas of emphasis for developing better understanding of the vitamins role in human health. 

The oxidative product of ascorbic acid, dehydroascorbic acid, is the preferred form of the vitamin for uptake by neutrophils, erythrocytes, and lymphocytes (27). Once within the erythrocyte, dehydroascorbic acid is reduced to ascorbic acid by a glutathione-dependent, dehydro-ascorbic-acid-reducing enzyme (20,28). However, the reduced form of ascorbic acid is found in most other tissues, that is, liver, lungs, kidneys, skin, and pituitary and adrenal glands (20,29). From these studies, ascorbic acid is taken up by several tissues by an energy-dependent and Na -sensitive process, but the transport of the oxidized vitamin form follows the principles of diflFusion. 

Figure 5. Reversal of decreased electron transport components in vitamin C deficient guinea pigs with ascorbic acid (15).

Disposable transport-facilitating moieties are also used to enhance the absorption of the water-soluble vitamins used as food additives, such as thiamine, ascorbic acid, and riboflavine. The vitamin derivatives obtained are poorly water-soluble and therefore are less extracted during the preparation of the food, which also gives some protection against oxidative decomposition. The increased lipophilicity enhances absorption from the intestinal tract (Fig. 32)151 155>. 

Uptake. Ascorbic acid is absorbed from the intestine by a sodium-dependent active transport system that is saturable. As the concentration of vitamin C increases in the intestinal tract, the absorption changes to passive diffusion. Once in systemic circulation, there are specific transporters based on cell types. 

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... 

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