How Does Vitamin C as Ascorbic Acid Enter Cells?

[ofonorow]

After all these years, this topic will discuss the elusive issue of how ordinary ascorbic acid enters cells.

For Steve Brown and other chemists, I read somewhere that "DHAA, the oxidized form of ascorbic acid is similar to the glucose molecule and shares the GLUT transporter (insulin mediated transport) through the cell membrane"

Can a chemist explain why merely missing an electron (presumably donated to a free radical) makes the molecule more like glucose?

The current dogma seems to be that only the reduced form of AA (DHA) uses the GLUT transporters.

We'll forget Sodium transporters for now - concentrating on AA (unless they can also transport ascorbic acid).

From biochemist Sherry Lewin, I think we know that the reduced form of AA - DHAA - is more permeable through lipid membranes (such as the blood brain barrier) and perhaps this was noticed before the GLUT receptors were understood?

But I personally find it difficult to believe that ordinary ascorbic acid doesn't utlitise the GLUT (insulin) receptors/transporters. If it doesn/cannot - then perhaps Jay Patrick was correct - that "vitamin C/ascorbate" only travels in the blood in the form of a salt, usually sodium ascorbate. (A counter-point is the 1976 Lewin book that says vitamin C travels in the blood as AA, and in the lymph as SA.)

So, again the question for this thread is understanding the mechanism by which ascorbic acid enters cells. And if the literature doesn't answer the question, what experiments could/should be run to determine this?

[skyorbit]

This is a quote from Dr Levy in his GSH, Master Defender. Page 74.

"Lyposomal delivery is . . . energy-sparing. . . . "Similarly, liposomes can directly deliver active, reduced vitamin C directly into the cells. Most commonly, vitamin C must be in its oxidized (spent) form to penetrate the cell membrane. Cellular electron stores must then be tapped to convert the oxidized vitamin C back to its active form." (His parenthesis, and bold/italics.)

Why don't you just e-mail Dr Levy and ask him.

Tracy

PS, perhaps this is why AA is better for Viral infections and SA is better for rebuilding tissue damage. This would be in line with Cathcart.

[ofonorow]

I appreciate the post, however, liposomal vitamin C (commercial forms) are sodium ascorbate.

How liposomal encapsulated C enters cells is another question/issue.

Sounds simple, how does ascorbic acid enter a cell? What is the mechanism?

I think there are 3 known mechanisms (GLUT, SVCT, and "dispersion" - and not one of those (if you believe what we read) is for ordinary ascorbic acid!

As Levy says, it is commonly thought that only the reduced form can penetrate cells. Common sense comes into play. How can vitamin C be a "rate unlimited antioxidant" if only the oxidized form (DHA) penetrates the cell membrane? So I am questioning the assertion he (and everyone else) makes.

If it is true, it might explain the 5 g Lypo functional equivalence to 25 g IV/C...

[Johnwen]

Here's a pretty comprehensive report on how things work but as you'll see that's not what their testing for but they take a broad look at the transport issue. Might help with your understanding.

http://ajpgi.physiology.org/content/295 ... 7.full.pdf

[Steve Brown]

I doubt that a molecule of ascorbic acid can exist for long in the bloodstream before reacting with the bicarbonate buffering system and being converted to sodium ascorbate. The concentration of sodium bicarbonate in the bloodstream is many times higher than the concentration of ascorbic acid that can be attained by oral dosage. I recall posting the exact numbers in another thread, years ago.

[ofonorow]

Hi Steve, I was hoping for your input. If you are correct - then Jay Patrick was essentially correct (I believe Patrick wrote that the sodium attached in the liver) and thus, logically, the major way for Ascorbate (unreduced) to enter cells are the SVCTs.

So this raises several questions.

1. Why did biochemist/exert Sherry Lewin write in Vitamin C: Its Biology and Medical Potential that vitamin C traveled in the blood as ascorbic acid? (And as SA in the lymph?)

2. Why did Cathcart report that he could only achieve the 'ascorbate effect' orally, with ascorbic acid?

3. Why would vitamin C and glucose 'compete' for entry into cells? (Ely's GAA theory)

4. And I still don't understand how donating an electron "changes the molecule" so that it can use the GLUT receptor. Why couldn't AA use the GLUT receptor?

One scenario might be that "Free" AA does rapidly break down into DHA in the blood (that which doesn't attach to sodium) and this resulting DHA uses the GLUT transporters as reported. However, Sherry Lewin also notes that DHA is short-lived, and further breaks down quickly into substances that can not be recyled back into vitamin C.

Steve, is there an experiment that could be run in water to test the idea that "free" AA will combine with sodium, and verify how much, etc?

[Steve Brown]

1. Why did biochemist/exert Sherry Lewin write in Vitamin C: Its Biology and Medical Potential that vitamin C traveled in the blood as ascorbic acid? (And as SA in the lymph?)

The statement "vitamin C travels in the blood as ascorbic acid" is true only as long as it does not mean "vitamin C travels in the blood only as ascorbic acid." Sodium ascorbate circulates in the blood as the result of ascorbic acid reacting with sodium bicarbonate in the blood, or as the result of sodium ascorbate being absorbed into the bloodstream from the digestive tract. Furthermore, it is well known that dehydroascorbic acid is present in the blood, because that is the form of vitamin C than can be transported into cells by means of glucose transporters, whereas ascorbate cannot.

2. Why did Cathcart report that he could only achieve the 'ascorbate effect' orally, with ascorbic acid?

I can only guess at the answer to that question. It may be that ascorbic acid is more rapidly absorbed from the digestive tract than sodium ascorbate. I recall reading that ascorbic acid can be absorbed into the bloodstream directly from the stomach, and that vitamin C is aborbed from the small intestine by means of sodium-dependent transporters. For that reason, I would expect that sodium ascorbate is more readily absorbed than ascorbic acid from the small intestine, but ascorbic acid may be more rapidly absorbed from the stomach. I doubt the "ascorbate effect" depends on ascorbic acid entering the bloodstream, because in that case, it could not be attained by intravenous administration of sodium ascorbate.

3. Why would vitamin C and glucose 'compete' for entry into cells? (Ely's GAA theory)

Dehydroascorbic acid is chemically similar to glucose, so it can use the same cellular transport mechanism. There is competition because cells have limited capacity and a limited number of glucose transporters. To draw an analogy, let men represent dehydroascorbic acid and women represent glucose. Suppose there are 50 men waiting for a bus. An empty bus arrives, and all 50 men can board. Now suppose there are 50 men and 50 women waiting for the same bus. When the bus arrives, in a non-Islamic country, the people queue up without regard to gender. In that case, it is possible the bus will be filled to capacity before all 50 men have boarded.

4. And I still don't understand how donating an electron "changes the molecule" so that it can use the GLUT receptor. Why couldn't AA use the GLUT receptor?

It is probably because dehydroascorbic acid is chemically more similar to glucose than ascorbic acid or ascorbate is. The oxidation of ascorbic acid to dehydroascorbic acid results in a different molecule, having a different chemical formula. Ascorbic acid is C6H8O6, and dehydroascorbic acid is C6H6O6; it has lost two hydrogen atoms. In the process of donating two electrons, ascorbic acid also loses two hydrogen nucleii, so the chemical formula changes. In the case of sodium ascorbate, it donates two electrons and loses one hydrogen nucleus and one sodium nucleus, becoming dehydroascorbic acid, converting from a neutral salt back to an acid.

It is enlightening to go to Wikipedia and compare the molecular diagrams of ascorbic acid, sodium ascorbate, and dehydroascorbic acid. The differences occur at the bottom side of the pentagon at the bottom of the molecule, while the rest of the molecule remains the same. The corners of the pentagon represent carbon atoms, which each have four bonds with neighboring atoms. In the case of ascorbic acid, the bottom two carbon atoms have a double bond with each other, each has a single bond to an upper carbon atom in the pentagon, and each has a bond extending down to an OH (hydroxyl) group. In the case of sodium ascorbate, one of the lower OH groups has the H replaced by Na, which is sodium. In the case of dehydroascorbic acid, the bottom two carbon atoms have a single bond with each other, and each has a double bond extending down to an oxygen atom; the two hydrogen atoms (or hydrogen atom and sodium atom) are gone.

One scenario might be that "Free" AA does rapidly break down into DHA in the blood (that which doesn't attach to sodium) and this resulting DHA uses the GLUT transporters as reported. However, Sherry Lewin also notes that DHA is short-lived, and further breaks down quickly into substances that can not be recyled back into vitamin C.

I suspect that is the case, but note that either ascorbic acid or sodium ascorbate can be doubly oxidized to dehydroascorbic acid. The oxidation likely occurs as the result of both ascorbic acid and sodium ascorbate quenching free radicals in the blood. It seems that ascorbic acid probably has a short life span in the blood, as it can quench a free radical directly, or it can react with sodium bicarbonate, becoming sodium ascorbate, which then quenches a free radical. I suspect that there is a need for both ascorbic acid and sodium ascorbate in the blood, involving different reactions with free radicals.

Steve, is there an experiment that could be run in water to test the idea that "free" AA will combine with sodium, and verify how much, etc?

There probably is. We know from experience that when you combine ascorbic acid with sodium bicarbonate in water, a fizzing reaction occurs in which the ascorbic acid molecule gives up an H and takes an Na from the sodium bicarbonate molecule. Carbon dioxide and water are byproducts of the reaction, and the pH of the solution is raised (the acid is converted to a salt). If the solution is dilute, the reactions proceed at a slower rate, while unreacted reactants remain in solution. Blood is a fairly dilute solution of sodium bicarbonate, compared to the concentration we use in a container to make sodium ascorbate, but the concentration of ascorbic acid in blood is far lower than the concentration of sodium bicarbonate. Whenever there is an overabundance of one reactant in a solution, it tends to scavenge all of the other reactant. An experiment in water could be set up that simulates the concentrations of sodium bicarbonate and ascorbic acid in blood. Measuring the pH of the solution would be only a first step in analyzing the result, but I would expect it to be at or near normal physiologic pH. The relative concentrations of ascorbic acid and sodium ascorbate could be ascertained by very accurate pH measurement and complex calculations. One could check for carbon dioxide. If bubbles are visible, that would be a certain indicator of the reaction. There are probably other techniques that could be used to corroborate the results.

[Steve Brown]

Considering that the uptake of vitamin C by cells is preferentially for dehydroascorbic acid raises two questions in my mind. The first is, what happens to vitamin C when a person's diet is so abundant in antioxidants that there are few free radicals in the blood? One possible answer is that most of it remains as ascorbate, and little of it gets absorbed into cells. In that case, perhaps dehydroascorbic acid would be a good form of vitamin C to take.

The other question concerns potassium ascorbate. Cells have a sodium-potassium transmembrane pump, which concentrates potassium inside the cell and sodium outside the cell. Is it possible that this pump enables potassium ascorbate to sneak into the cell, bypassing the glucose transporter?

[Steve Brown]

Thinking along the lines of an experiment...

The normal range of concentration of bicarbonate ion (HCO3-) in the blood is 22 to 30 millimoles per liter. For our purposes, let's say it's 25 mmol/L. The molar mass of sodium bicarbonate (NaHCO3) is 84 grams per mole. 25 millimoles of NaHCO3 would be 0.025 x 84 = 2.1 grams. One tenth that amount, 210 mg, dissolved in 100 ml of warm water would give a solution having a concentration of sodium bicarbonate approximately equal to that of human blood. Adding ascorbic acid to that solution would tend to lower the pH and decrease the concentration of bicarbonate ion, whereas blood would maintain the concentration of bicarbonate ion as ascorbic acid is added, owing to the effects of respiration and the enzyme carbonic anhydrase. Therefore, the simulation is valid only for a small amount of ascorbic acid added to the solution. If there is any visible sign of reaction, that would be a good indication of what could be observed if adding the same amount of ascorbic acid to blood.

I prepared a solution of 210 mg of sodium bicarbonate in 100 ml of water at approximately 100 degrees Fahrenheit. Into that solution I sprinkled a few milligrams of ascorbic acid. It sank to the bottom of the beaker, and there was no apparent fizzing. I stirred it, and the ascorbic acid powder dissolved. Carbon dioxide may have been generated, in which case the gas was too diffuse to manifest as visible bubbles, going instead into solution.

[ofonorow]

I want to digest Steve's post before responding, but I did take the time to read the paper johnwen provided the link to: Mechanisms and regulation of vitamin C uptake: studies of the hSVT systems in human liver epithelial cells (Thanks johnwen!)

Trying to resolve the following statement with what I just read.

Considering that the uptake of vitamin C by cells is preferentially for dehydroascorbic acid raises two questions in my mind.

The liver cell uptake study was of ascorbic acid - not sodium ascorbate w/r to the SVT (1 and 2) transporters.

In fact there was no mention of the GLUT (glucose transporters) in the paper as far as I can see.

NA+ (sodium ions in the medium) did increase cellular uptake, and there was an optimal pH range and optimal low/high concentrations of aa.

If as Steve Brown suggests - sodium as ubiquitous in the blood - it may be that the primary way ascorbic acid enters cells is not "preferentially dehydroascorbic acid" but via these "sodium dependent" vitamin C transporters. In point of fact, a "bound" sodium ascorbate molecule itself may not be transportable! (I think we know the two ions disassociate/separate to some extent in solution though.)

Now to go back and read Steve Brown's posts.

[Steve Brown]

"Active transport is the main mechanism of vitamin C distribution within the body. Simple diffusion may occur in the mouth and stomach but accounts for only a very small percentage of uptake (13). Sodium-independent transport systems shuttle vitamin C across the basolateral membrane of the intestinal cells. In the plasma absorbed ascorbic and dehydroascorbate (DHAA) can either be transported freely or be bound to albumin. Ascorbate can also move into body cells and tissues (13). As previously mentioned DHAA is the primary form of vitamin C that crosses cellular membranes. The adrenal and pituitary glands, red blood cells, lymphocytes, and neutrophils all receive vitamin C in the form of DHAA (13,17)."

http://www.exrx.net/Nutrition/Antioxida ... aminC.html

[Steve Brown]

It occurred to me that the benefits claimed for taking hydrogen peroxide may be due to it oxidizing some of the vitamin C in a person's system, converting it to dehydroascorbic acid which is readily taken up by cells. If that is the case, my opinion is that it would be safer to react hydrogen peroxide with ascorbic acid outside the body, then take the resulting dehydroascorbic acid, rather than ingesting hydrogen peroxide, which is toxic to cells because it is a strong oxidizer. The ratios of reactants could be adjusted so that all of the hydrogen peroxide is reacted before the dehydroascorbic acid is taken.

[ofonorow]

I prepared a solution of 210 mg of sodium bicarbonate in 100 ml of water at approximately 100 degrees Fahrenheit. Into that solution I sprinkled a few milligrams of ascorbic acid. It sank to the bottom of the beaker, and there was no apparent fizzing. I stirred it, and the ascorbic acid powder dissolved. Carbon dioxide may have been generated, in which case the gas was too diffuse to manifest as visible bubbles, going instead into solution.

Thank you for running this experiment.

And from this, what do you think we can conclude?

It would seem that you showed that ascorbic acid can be in the blood at the combination/concentrations found in the blood and not necessarily react with the sodium bicarbonate.

Thank you for the help on the difference in molecules between AA and DHAA, but I still do not understand the often made statement

uptake of vitamin C by cells is preferentially for dehydroascorbic acid

especially in light of that paper johnwen cited on ascorbate uptake by liver cells which doesn't even mention glucose uptake. In fact, while "sodium" dependent transporters are involved, the experimenters used ascorbic acid! I think your discussion above helps explain how SDVT work - with ascorbic acid - because of the NA+ ions in solution.

So far the mechanism, in my mind, from this discussion is that your statement should be modified to

"the uptake of vitamin C by GLUT transporters in cells is preferentially for dehydroascorbic acid"

and from my reading of the other paper, I propose (for discussion) that

"the uptake of vitamin C by cells is preferentially for ascorbic acid from SDVT transporters"

and this

"The uptake of vitamin C in cells is preferentially ascorbic acid, but that vitamin C enter cells in its oxidized form as dehydroascorbic acid which competes with glucose"

Getting late but the number of each kind of receptors on each kind of cell would be important.

Recently doctor Levy pointed me to a paper that showed the number of GLUT on breast cancer cells was many times the number of healthy breast cells, indicating a strong preferential for DHAA by cancer cells. (Will post reference here later. The number of GLUT (insulin receptors) found in the cancer cells were about 12 times normal cells)

[Steve Brown]

Thank you for running this experiment.

And from this, what do you think we can conclude?

It would seem that you showed that ascorbic acid can be in the blood at the combination/concentrations found in the blood and not necessarily react with the sodium bicarbonate.

What I can conclude from the experiment is that there is no immediate and obvious reaction of ascorbic acid with sodium bicarbonate in the blood. Therefore, any reaction that does occur is at a slow rate, allowing ascorbic acid to be present in the blood. Of course, an amount of ascorbic acid that could lower the pH of the blood below normal would be dangerous, and in that case, buffering mechanisms would neutralize as much ascorbic acid as it can. If the blood becomes too acidic, the rate of breathing will be suppressed to raise the concentration of carbon dioxide, and thereby amount of bicarbonate, in the blood.

Thank you for the help on the difference in molecules between AA and DHAA, but I still do not understand the often made statement "uptake of vitamin C by cells is preferentially for dehydroascorbic acid."

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.

Following is a link to an interesting paper titled "Dynamic Flow: A New Model for Ascorbate," by D.S. Hickey, Ph.D.; H.J. Roberts, Ph.D.; and R.F. Cathcart, M.D. On the roles of GLUT and SVCT transporters, they make the interesting point that DHA in the blood is relatively toxic, and an important role of GLUT transporters is to keep the concentration of DHA in the blood at or below 2 µM/L [micromoles per liter], well below the hundred or so µM/L for ascorbate in a healthy person. To keep the concentration of DHA at such a low level, the GLUT transporters must be very active in the tranport of vitamin C into cells, even while SVCT transports ascorbate into cells. A person who consumes too much sugar faces the additional hazard of too much DHA in the blood. Here is a quote from the article:

"Erythrocytes [red blood cells] have a high capacity to import dehydroascorbate using GLUT1 and reduce it back to ascorbate. Uptake of dehydroascorbate by erythrocytes is a protective mechanism that can lower its concentration in healthy plasma to levels lower than 2 µM/L. Once inside the red blood cell, dehydroascorbate is reduced to ascorbate. Thus we can suggest uptake of dehydroascorbate by red blood cells is an antioxidant mechanism to prevent damage in many disease states."

http://orthomolecular.org/library/jom/2 ... 4-p237.pdf

This reply to be continued in next post...

[Steve Brown]

...especially in light of that paper johnwen cited on ascorbate uptake by liver cells which doesn't even mention glucose uptake. In fact, while "sodium" dependent transporters are involved, the experimenters used ascorbic acid! I think your discussion above helps explain how SDVT work - with ascorbic acid - because of the NA+ ions in solution.

The Na+ ions come from salt. I'm not familiar with the exact chemistry of the operation of SVCT transporters, but it seems intuitive that sodium ascorbate would be more readily transported by SVCT than ascorbic acid, because it's already associated with an Na+ ion.

So far the mechanism, in my mind, from this discussion is that your statement should be modified to
"the uptake of vitamin C by GLUT transporters in cells is preferentially for dehydroascorbic acid"

I would omit the word "preferentially" because GLUT transports only DHA, not ascorbate. As such, GLUT performs the essential funtion of providing DHA for mitochondria in cells.

...and from my reading of the other paper, I propose (for discussion) that
"the uptake of vitamin C by cells is preferentially for ascorbic acid from SDVT transporters"

I would not agree with that, because it is already clear that GLUT transporters have a vital function, and the amount of DHA transported depends on the concentration of glucose in the blood. The paper by Hickey, Roberts, and Carthcart suggests that for red blood cells, the primary mechanism of transport is by means of GLUT transporters. It appears that both GLUT and SVCT tranporters have various degrees of activity in various cells.

and this...
"The uptake of vitamin C in cells is preferentially ascorbic acid, but that vitamin C enter cells in its oxidized form as dehydroascorbic acid which competes with glucose"

I would not agree with that statement, for reasons given above.

Getting late but the number of each kind of receptors on each kind of cell would be important.

Agreed.

Recently doctor Levy pointed me to a paper that showed the number of GLUT on breast cancer cells was many times the number of healthy breast cells, indicating a strong preferential for DHAA by cancer cells. (Will post reference here later. The number of GLUT (insulin receptors) found in the cancer cells were about 12 times normal cells)

It suggests to me a ravenous appetite of cancer cells for glucose, as certain kinds of cancer cells may not be able to metabolize fat. If the DHA inside the cancer cell gets reduced to ascorbate, it could prove toxic to the cancer cell.