We have continued our synthetic work with 6-halo-6-deoxyacorbic acids, analogues that have proven to be useful tools for transport studies, done in a collaborative project by Dr. Mark Levine. The reduced forms of the 6-halo ascorbic acid analogs are structurally similar to ascorbic acid, and it was predicted they would be transported by the vitamin C transporters SVCT1 and SVCT2. The only form of oxidized ascorbic acid (DHA) in aqueous detected by 13C NMR spectroscopy solution is a hydrated bicyclic hemiketal, formation of which requires the presence of the C6 hydroxyl group. The overall geometry of this molecule and the presence of multiple hydroxyl groups has suggested a similarity of this structure to that of glucose. This structural similarity in part led to the hypothesis nearly 30 years ago that DHA and glucose would share the same transport mechanisms, a hypothesis that was subsequently verified for several facilitated glucose transporters. When 6-bromo-6-deoxy-L-ascorbic acid is oxidized to 6-bromo-6-deoxyacorbic acid (BrDHA), the hydrated bicylcic hemiketal cannot form since the C-6 halogen has replaced the 6-OH group. The 2,3-diketolactone, or more likely the hydrated form, would represent the structure of BrDHA in solution. This compound lacks structural similarity to glucose, and we predicted that it would not be transported or bound by GLUTs. When GLUT1 or GLUT3 were expressed in Xenopus oocytes, BrDHA was neither transported nor bound. This contrasted to robust transport of DHA by Xenopus oocytes expressing glucose transporters GLUT1 and GLUT3. In addition, 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 compared to 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 provides a new strategy to determine the contribution of each pathway to ascorbate accumulation. We are continuing synthetic work to prepare new flavonoid analogues to study sturtural parameters of these inhibitors of ascorbate and glucose transport. Inculded will be potential affinity labels for the transport protein(s). Effects of fluorine substitution biolgical begavior of the flavonoids also will be examined. A series of fluorinated chalcones have been prepared and will be cyclized under conditions that produce the flavonoid structures.
| Chen, Qi; Espey, Michael Graham; Sun, Andrew Y et al. (2008) Pharmacologic doses of ascorbate act as a prooxidant and decrease growth of aggressive tumor xenografts in mice. Proc Natl Acad Sci U S A 105:11105-9 |
| Corpe, Christopher P; Lee, Je-Hyuk; Kwon, Oran et al. (2005) 6-Bromo-6-deoxy-L-ascorbic acid: an ascorbate analog specific for Na+-dependent vitamin C transporter but not glucose transporter pathways. J Biol Chem 280:5211-20 |
| Rumsey, S C; Welch, R W; Garraffo, H M et al. (1999) Specificity of ascorbate analogs for ascorbate transport. Synthesis and detection of [(125)I]6-deoxy-6-iodo-L-ascorbic acid and characterization of its ascorbate-specific transport properties. J Biol Chem 274:23215-22 |
