Dehydroascorbate transport

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. 

Blood-to-retina influx transport GLUT1 D-Glucose/Dehydroascorbic acid + TR-iBRB Isolated retinal capillary [5, 13, 14, 43, 47]... 

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

Dehydroascorbate, the oxidized form of vitamin C (Box 18-D) is also transported into cells by GLUT1 and GLUT3.409 A related transporter carries L-fucose into mammalian cells 410 Another facilitates the uptake of galactose in yeast.411... 

Dehydroalanine 755, 757s Dehydroalanine residue 754, 756 Dehydroascorbate 787 transport 416... 

Vera JC, Reyes AM, Carcamo JG, Velasquez FV, Rivas Cl, Zhang RH, Strobel P, Iribarren R, Scher HI, Slebe JC, et al. 1996. Genistein is a natural inhibitor of hexose and dehydroascorbic acid transport through the glucose transporter, GLUT1. J Biol... 

By reaction with ascorbate to yield the monodehydroascorbate radical, which in turn can either be reduced to ascorbate or can undergo dis-mutation to yield dehydroascorbate and ascorbate (Section 13.4.7.1). In vitro, the formation of the tocopheroxyl radical can be demonstrated by the appearance of its characteristic absorbance peak, which normally has a decay time of 3 msec in the presence of ascorbate, the tocopheroxyl peak has a decay time of 10 /rsec, and its disappearance is accompanied by the appearance of the monodehydroascorbate peak. There is an integral membrane oxidoreductase that uses ascorbate as the preferred electron donor, linked either directly to reduction of tocopheroxyl radical or via an electron transport chain involving ubiquinone (see no. 4 below May, 1999). 

Dehydroascorbate enters cells byway of the (insulin-dependent) glucose transporters (GLUT), and is reduced to ascorbate intracellularly. 

Ascorbate oxidase is a tetramer each subunit has 552 amino acids and contains 4 copper ions, the type-I blue copper center and the adjacent trinuclear center (arranged as a type-n center and a type-in dinuclear center) separated by /S-sheets (Figure 20) °. Ascorbate is oxidized to dehydroascorbate by dioxygen however, it is not bound directly to the metal center to be oxidized, but is proposed to bind near the type-I Cu site which may facihtate electron transfer to oxygen, presumably in the tri-Cu cluster site. Since humans cannot synthesize ascorbic acid, conservation of this important compound is highly regulated. For example, the oxidized ascorbate can be transported into red blood... 

Luminal thiol oxidation is facilitated by ascorbate (vitamin C) (45) or FAD (12, 13), so the physiologic role of their transport has been proposed. ER membrane is permeable selectively to dehydroascorbate, the oxidized form of ascorbate (10, 11). Luminal reduction of dehydroascorbate to ascorbate is associated with thiol oxidation and leads to ascorbate entrapment (46). 

Banhegyi G, Marcolongo P, Pusk F, Fulceri R, Mandl J, Benedetti A. Dehydroascorbate and ascorbate transport in rat liver microsomal vesicles. J. Biol. Chem. 1998 273 2758-2762. 

Rose, R. C, Choi, J.-L., and Koch, M. J, (1988), Intestinal transport and metabolism of oxidized ascorbic acid (dehydroascorbic acid). Am. ]. Phifiloi, 254, OS24r-C828. 

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. 

There must also be a second mechanism of ascorbic acid transport through certain specialized cells, difierent from the diffusion of dehydro-ascorbic acid followed by intracellular reduction. There are no indications that dehydroascorbic acid is involved in the renal tubular absorption of ascorbic acid, and, contrary to earlier conclusions, it appears that the concentration of ascorbic acid into the aqueous humors of the eye occurs in the form of ascorbic acid itself. 

This system cannot now be fitted into known pathways of electron transport, principally because a system that oxidizes ascorbic acid in animal tissues is unknown. The fact is clear, however, that ascorbic acid is oxidized to dehydroascorbic acid in animal tissues and that dehydro-ascorbic add can be reduced. At the very least, this system emphasizes the potential eflBciency of a form of ascorbic acid as an electron acceptor (K3). 

Reduction of dehydroascorbic acid is one of the steps of a potential electron transport system in plants involving ascorbic add oxidase as the terminal oxidation reaction (Eqs. 19). 

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