Again, what I was able to make out from johnwen's link to the liver cell SDVC transporter paper - no mention of GLUT - was that ascorbic acid (with NA+ present) enters cells (depending on pH, temperature, NA+, etc.)
Steve Brown wrote:I've done further reading on this topic, and it appears that both GLUT and SVCT are involved in the transport of vitamin C into various cells. GLUT transports dehydroascorbic acid (DHA), and SVCT transports ascorbate. However, only DHA can be transported into the mitochondria within cells, and only DHA can enter the brain through the blood/brain barrier. DHA competes with glucose for transport into cells, and so the amount of DHA that is absorbed depends on the concentration of glucose in the blood. This may cause a person who consumes so much sugar that he or she has too much glucose in the blood to have cellular deficiency of vitamin C, at least in terms of supplying the mitochondria with vitamin C.
As you say DHA, although short-lived, can be dangerous, so having GLUT transporters remove it from the blood makes biological and evolutionary sense.
I am not certain about the mitochronria - (Russell Jaffe in his lectures mentions that Alpha Lipoic Acid is the "natural" antioxidant in the mitochondria, and that vitamin E is the antioxidant generally protecting the cellular membrane, etc.) but again, it makes no common sense that vitamin C only enters the brain as the "dangerous" DHA. I (think I) know from Sherry Lewin, that DHA is more permeable to lipid membranes, but I would be a Quarter that there are SDVC Transporters, a slew of them, at the blood-brain barrier.