The Cytosolic Redox Potential and the Proton Gradient Across the Inner Mitochondrial Membrane

These articles are interesting [Adam-Vizi et al., 2006: (http://www.ncbi.nlm.nih.gov/pubmed/17056127); Barron et al., 1998: (http://www.ncbi.nlm.nih.gov/pubmed/9737943); Brand et al., 2005: (http://www.ncbi.nlm.nih.gov/pubmed/16076285)(http://www.pubmedcentral.nih.gov/picrender.fcgi?artid=1316271&blobtype=pdf); Veech et al., 1972: (http://www.pubmedcentral.nih.gov/picrender.fcgi?artid=1178599&blobtype=pdf)(http://www.ncbi.nlm.nih.gov/pubmed/4342558)] and, when viewed together, help to shed light on the problems that can emerge from excessive elevations in the cytosolic redox potential (RP). The cytosolic redox potential is the NADH/NAD+ ratio and correlates with the cytosolic lactate/pyruvate ratio (LPR) and the glycerol-3-phosphate (G3P)/dihydroxyacetone phosphate (DHAP) ratio. Barron et al. (1998) found that inhibiting glutamate-oxaloacetate transaminase (OAT) activity, which is one component of the malate-aspartate shuttle but is obviously not one of the actual transporters for importing malate into or exporting aspartate from the mitochondrial matrix, reduced the rate of glucose oxidation significantly and increased glycolytic activity, in association with increases in the LPR and cytosolic RP and G3P/DHAP ratio. The inhibition also increased oxygen consumption significantly. This is interesting, and the authors suggested that the inhibition of that aminotransferase enzyme, which is dependent on coenzymated vitamin B6 (PLP), had reduced the proton motive force that drives oxidative phosphorylation by some mechanism or set of mechanisms. It's not clear from the article what mechanisms could account for that inhibition, but the data don't allow one to determine that. The authors are basically showing uncoupling of the redox reactions and phosphorylation (usually referred to as just "uncoupling"), but the implication is also that there's an acidification of the intermembrane space by the increase in the cytosolic [H+], occurring in association with an elevated cytosolic redox potential and lactate/pyruvate ratio. Usually, uncoupling by drugs involves futile cycling of protons through the inner mitochondrial membrane by drugs that are weak bases or weak acids (they carry the protons back into the matrix, through the membrane, and then cross the membrane again and pick up another proton that's been transported out into the intermembrane space by the respiratory chain enzyme complexes--this can occur 500-1000 times a second).

The proton motive force (PMF) is the movement of protons from the matrix side of the inner mitochondrial membrane to the side of the membrane that faces the intermembrane space (the "cytosolic" side). Protons move out of the mitochondrial matrix and into the intermembrane space, and then the protons drive ATP synthesis by moving back into the matrix, through the proton channel of the F0 component of the F1F0-ATPase enzyme complex ("complex V"). The message of the Barron et al. (1998) article is basically that a significant mismatch between (or "uncoupling" of) glycolytic activity and glucose oxidation, such that the cytosolic NADH/NAD+ ratio is greatly elevated and is deranging the intramitochondrial NADH/NAD+ ratio, could reduce the PMF that drives oxidative phosphorylation. In my opinion, the authors' discussion of the potential diminution of the proton gradient across the inner mitochondrial membrane, which is usually less significant as a contributor to the proton motive force than the mitochondrial membrane potential is, by a significant elevation of the cytosolic NADH/NAD+ ratio per se is very important and has validity. I think that that phenomenon could explain the diminishing returns that seem to show up with the use of uridine alone, in the treatment of mitochondrial disorders. One would expect to see the same problems, in my opinion, with the use of high doses of ribose alone, in the treatment of brain injuries or neurodegenerative diseases or whatever else, given that ribose increases the cytosolic NADH/NAD+ ratios. Essentially, the activities of the respiratory chain or TCA cycle enzymes (or even the activity of the pyruvate dehydrogenase complex) would not be able to keep pace with the transporters and OAT activity of the malate-aspartate shuttle. This also has relevance to an understanding of the peripheral neuropathy that occurs with high-dose vitamin B6, given that the beneficial neuroprotective effects of vitamin B6 (and its effects on energy metabolism) are strongly dependent on its enhancement of the overall rate of the malate-aspartate shuttle enzymes and transporters. Barron et al. (1998) examined the opposite scenario (inhibition of a vitamin B6-dependent enzyme), but the mechanisms at work imply that increases in the cytosolic redox potential could diminish the proton gradient (and hence the proton motive force) across the inner mitochondrial membrane. Some other mechanisms include the inhibition of isocitrate dehydrogenase and 2-oxoglutarate dehydrogenase activities by greatly-elevated intramitochondrial NADH levels. The intramitochondrial redox potential (NADH/NAD+) will not necessarily increase, via malate influx and aspartate efflux from the mitochondrial matrix, as the cytosolic redox potential does. But that's sort of the point of the article. This posting became too complicated, and I'll have to end it here.