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  • Title: Plasma membrane gamma-glutamyltransferase activity facilitates the uptake of vitamin C in melanoma cells.
    Author: Corti A, Raggi C, Franzini M, Paolicchi A, Pompella A, Casini AF.
    Journal: Free Radic Biol Med; 2004 Dec 01; 37(11):1906-15. PubMed ID: 15528049.
    Abstract:
    Adequate cellular transport of ascorbic acid (AA) and its oxidation product dehydroascorbate (DHA) is assured through specific carriers. It was shown that vitamin C is taken up as DHA by most cell types, including cancer cells, via the facilitative GLUT transporters. Thus, AA oxidation to DHA can be considered a mechanism favoring vitamin C uptake and intracellular accumulation. We have investigated whether such an AA-oxidizing action might be provided by plasma membrane gamma-glutamyltransferase (GGT), previously shown to function as an autocrine source of prooxidants. The process was studied using two distinct human metastatic melanoma clones. It was observed that the Me665/2/60 clone, expressing high levels of membrane GGT activity, was capable of effecting the oxidation of extracellular AA, accompanied by a marked increase of intracellular AA levels. The phenomenon was not observed with Me665/2/21 cells, possessing only traces of membrane GGT. On the other hand, AA oxidation and stimulation of cellular uptake were indeed observed after transfection of 2/21 cells with cDNA coding for GGT. The mechanism of GGT-mediated AA oxidation was investigated in acellular systems, including GGT and its substrate glutathione. The process was observed in the presence of redox-active chelated iron(II) and of transferrin or ferritin, i.e., two physiological iron sources. Thus, membrane GGT activity-often expressed at high levels in human malignancies-can oxidize extracellular AA and promote its uptake efficiently.
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