Oxidative Stress and the CNS

Re^3: Symposium 783

On Mon Dec 14, Bernhard H.J. Juurlink wrote
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>Dr, Rice,
>��One thing that puzzles me is why neurons which have a poor ability to reduce oxidized-ascorbate (because of their relatively oxidized cytosol) should have an order of magnitude more ascorbate than astrocytes which have a very good ability to reduce oxidized-ascorbate.�Is there any evidence for ascorbate metabolic coupling between neurons and astrocytes with oxidized-ascorbate transferred from neurons to astrocytes and reduced-ascorbate transferred from astrocytes to neuron? �It is known that the glucose transporters transport oxidized-ascorbate and there would be a concentration gradient differential tending to move oxidized-ascorbate out of the neuron to the extracellular space and from there to the astrocyte. �What about the movement of reduced-ascorbate in the other direction?

>Why should there be so much ascorbate in neurons? �Does it have a function other than reducing oxidized-tocopherol? �What would this be? �BJ

Dr. Juurlink � excellent questions! �First I must clarify your statement that the cytosol of neurons is �relatively oxidized.� �Yes, compared to that of less metabolically active glia (astrocytes), this might be true. �BUT, the overall intracellular environment of both neurons and glia is reducing. �This is indicated by tissue levels of ascorbate and glutathione (GSH) which are roughly 99% in the reduced state in rapidly frozen samples (the ratio in specific subcellular organelles can differ from this, of course). �

Localization of ascorbate (10 mM) to the cytosol of neurons (at a concentration of 10 mM), therefore, puts it exactly the right place to act as an important component of the neuronal antioxidant network. �As you note, ascorbate can help maintain tocopherol in the reduced state. �But other studies have shown that it can also directly reduce a variety of reactive oxygen species, including H2O2, hydroxyl radical, superoxide, and nitric oxide. �It has been proposed that ascorbate (and GSH) are particularly important for neutralizing the highly reactive hydroxyl radical, since there are no specific enzymes for this molecule. �When cytosolic ascorbate is oxidized in this way, it will be readily re-reduced by GSH, which our data suggest is 2.5 mM in neurons. �Oxidized GSH, in turn, will be rapidly returned to its reduced state by GSH-reductase, so that redox balance is maintained. �Perhaps your data showing that a small increase in GSH levels markedly enhances cellular reducing capacity (Juurlink, this symposium) includes a positive effect on ascorbate recycling.

Consequently, I do not think shuttling of oxidized ascorbate (dehydroascorbate, DHA) between neurons and glia is either required or likely, although it is an intriguing suggestion. �

You asked what ascorbate might be doing in neurons besides acting as an antioxidant. �We have previously reported that the brains of anoxia-tolerant species, like pond turtles, have very high levels of ascorbate (Rice et al. 1995). �We have postulated that these levels are necessary to maintain redox balance and prevent oxidative damage upon reoxygenation after a hypoxic dive. �Interesting, GSH is not elevated in turtle brain tissue. �Other postulated functions for elevated ascorbate levels might include serving as an alternative metabolic substrate or as an organic osmolyte for cell volume regulation. �However no data from our laboratory or others have yet demonstrated such functions. �Ascorbate DOES have actions as a neuromodulator as reviewed by Rebec and Pierce (1994). �It is also a specific cofactor for norepinephrine synthesis. �Perhaps most importantly, it is released from brain tissue by glutamate in a dose-dependent fashion via glutamate/ascorbate heteroexchange (O�Neill, Fillenz, et al. 1984; �Cammack et al. 1991). �Nontheless, the ability of ascorbate to act as an antioxidant and scavenger of reactive oxygen species suggests that these actions comprise its most important function in the CNS.

Tue Dec 15