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Antioxidants such as ascorbate and glutathione serve to inactivate radicals and counteract spontaneous oxidation reactions, such as unwanted disulfide bonds in proteins. These systems are cycles, where a "spent" antioxidant (say, oxidized glutathione) is convert back to its reduced form by another reducing agent, usally NADPH.

My question is, how much NADPH (or reducing equivalents) does a cell devote to this antioxidant defense? What is the flux, in molar amounts per unit time, through these systems? Does it represent a large portion of the energy metabolism of cells?

I am guessing that the flux is quite large, but I have never seen any data on this. Most likely it varies a lot between cell types, but I would be happy to see any numbers. I think this is important for understanding antioxidant chemistry in cells, yet it is rarely discussed in the literature.

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Most experiments to try to get at this type of question use stable isotope tracers to measure NADPH production from the various pathways that can make it, and then subtract away the amount that's predicted to be needed for growth/anabolism (based on the observed growth rate and biomass composition). I think this paper is by far the best example:

http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4104482/.

Quote:

The overall demand for NADPH for biosynthesis is > 80% of total cytosolic NADPH production (Figure 4b), with a majority of this NADPH consumed by fatty acid synthesis. At least in transformed cells growing under aerobic conditions, most cytosolic NADPH is devoted to biosynthesis, not redox defense.

Note that these are fast growing cells without additional oxidative stress. Interestingly, what the authors find is that imposing additional oxidative stress doesn't actually change NADPH production that much, but it slows down growth as more NADPH is used for anti-oxidant defense (Extended figure 9).

For skin cells, the situation might be a bit different, at least in terms of the increase in NADPH production upon oxidative stress. You might look at this paper: http://www.cell.com/molecular-cell/fulltext/S1097-2765(15)00456-6

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  • $\begingroup$ Yes, I am aware of Rabonowitz 's paper. It doesn't contain any direct data on antioxidant fluxes afaik --- the values you refer to are rather model-based predictions. Zamboni's paper does not really address the question I believe ; it is about pentose phosphate fluxes. But perhaps this is the only data available. $\endgroup$ – Roland Aug 23 '16 at 15:27
  • $\begingroup$ So the major error in the model would come if there was a source or a sink of cytosolic NADPH that was not being considered. Probably you can judge for yourself how likely that is to be, but I for one am reasonably satisfied with the set of reactions taken into account. If anything, I would guess that an unidentified sink (with a net NADPH->NADH result, such as a lipid synthesis and degradation cycle) is more likely than an unidentified source, which would even further lower the estimate of NADPH used for antioxidant defense. $\endgroup$ – Victor Chubukov Aug 23 '16 at 15:34
  • $\begingroup$ Also I can reemphasize that to me, the major caveat is really the cell type and environment being considered -- doing this type of exercise in a real in vivo context as opposed to a cell line would be really fascinating. $\endgroup$ – Victor Chubukov Aug 23 '16 at 15:36

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