6-Bromo-6-deoxy-l-ascorbic Acid

Abstract

Vitamin C intracellular accumulation is mediated by Na؉ -dependent vitamin C transporters SVCT1 and -2 and dehydroascorbic acid transporters GLUT1 and -3. It is unclear which pathways dominate in vivo. As a new step to resolve this issue, we identified and tested 6-bromo-6-deoxy-L-ascorbic acid as a specific candidate for SVCTs. In high performance liquid chromatography and electron paramagnetic resonance analyses, the reduced compounds ascorbic acid and 6-bromo-6-deoxy-L-ascorbic acid were similar. The oxidized products 6-bromo-6-deoxy dehydroascorbic acid (BrDHA) and dehydroascorbic acid (DHA) had comparable stabilities, based on reduction recoveries. Upon expression of GLUT1 or GLUT3 in Xenopus oocytes, BrDHA was neither transported nor bound, in contrast to robust transport of DHA. The findings were not explained by differences in the oocyte reduction of DHA and BrDHA because lysed oocytes reduced both compounds equally. Further, there was no transport of the reduced compound, 6-bromo-6-deoxy-L-ascorbic acid, by GLUT1 or GLUT3. As a prerequisite for investigating 6-bromo-6-deoxy-L-ascorbic acid transported by SVCTs, SVCT2 transport activity in oocytes was enhanced 14-fold by construction and use of a vector that added a fixed poly(A) tail to the 3 end of cRNA. For SVCT1 and SVCT2 expressed in oocytes, similar K m and V max values were observed for ascorbic acid and 6-bromo-6-deoxy-Lascorbic acid. In human fibroblasts, predicted to have SVCT-mediated ascorbate accumulation, K m and V max values were again comparable for ascorbic acid and 6-bromo-6-deoxy-L-ascorbic acid. Using activated human neutrophils, predicted to have ascorbate accumulation mediated predominantly by DHA and GLUT transporters, 6-bromo-6-deoxy-L-ascorbic acid accumulation was <1% of accumulation when compared with ascorbic acid. We conclude that 6-bromo-6-deoxy-L-ascorbic acid is the first transport substrate identified as completely specific for SVCTs, but not GLUTs, and provide a new strategy to determine the contribution of each pathway to ascorbate accumulation.

6-Deoxy-6-[¹³¹I]iodo-<small>L</small>-ascorbic Acid for the <i>in Vivo</i> Study of Ascorbate: Autoradiography, Biodistribution in Normal and Hypolipidemic Rats, and in Tumor-Bearing Nude Mice

“…32,48,49) In the present study we confirmed the limited transfer of IAsA in the rat brain, as observed in mouse brain, 34) indicating the unsuitability of this agent for brain studies in vivo. These observations are consistent with the mechanistic hypothesis of Rumsey et al 33) and Corpe et al 50) that ascorbate analogs modified at C-6, such as 6-…”

5-O-(4-[125I]Iodobenzyl)-L-ascorbic Acid: Electrophilic Radioiodination and Biodistribution in Mice

“…There was a somewhat slower clearance of the radioactivity from the blood when compared to 6-18 FAsA 10-13) and 6-131 IAsA 14,15) studied previously. The accumulation of radioactivity in selected organs was highest in the kidneys>liver>lungs>adre nals>heart at 2 min postinjection and then the radioactivity levels gradually decreased with time, similar to those seen for [6][7][8][9][10][11][12][13][14][15][16][17][18] FAsA and 6-131…”

“…Moreover, tumor accumulation in the fibrosarcoma was not significant, also similar to those of [6][7][8][9][10][11][12][13][14][15][16][17][18] FAsA and IAsA. Recent studies have shown that high-grade tumor tissue has reduced capacity to accumulate AsA relative to normal tissue.…”

“…This 5-O-substituted AsA analog can be expected to have the hydrated bicyclic hemiketal structure like AsA in aqueous solution (Chart 1), which appears to be a molecular species capable of interacting with the GLUTs. 17,18) Recently, we developed a synthetic route for the preparation of 5-O-(4-iodobenzyl)-L-ascorbic acid through a multi-step sequence starting from AsA, such that it would be amenable to preparation of the corresponding labeled analog, and this compound was found to have almost the same reducing activity as AsA itself. 19) We report here the radiosynthesis of 5-O-(4-…”

“…In previous studies, we reported the synthesis of several AsA analogs, focusing on the introduction of radioisotopes such as 18 F and 131 I at the C-6 position of the AsA core, and their potential imaging characteristics were also evaluated in rodents. Among them, 6-deoxy-6-[ 18 F]fluoro-L-ascorbic acid (6)(7)(8)(9)(10)(11)(12)(13)(14)(15)(16)(17)(18) FAsA) [10][11][12][13] and 6-deoxy-6-[ 131 I]iodo-L-ascorbic acid IAsA) 14,15) showed the expected distribution of highest uptake in the adrenal glands in rodents, a neuroendocrine organ that is known to highly express the SVCT-2 transporter, 16) thus demonstrating their suitability as radiotracer analogs of AsA. However, the application of these radiotracers as brain-targeted imaging probes for in vivo studies was found to be limited because of their poor delivery from blood circulation into the brain.…”