This article is great [Yudkoff et al., 1994: (http://www.jbc.org/cgi/reprint/269/44/27414.pdf)(http://www.ncbi.nlm.nih.gov/pubmed/7961653)] and helps to shed light on that article in the last posting. Yudkoff et al. (1994) found that the flux of substrates through the tricarboxylic acid (TCA) cycle between 2-oxoglutarate and oxaloacetate (i.e. including the 2-oxoglutarate dehydrogenase and malate dehydrogenase enzymes) was 3 times faster, when glucose was included, than the flux through oxaloacetate and 2-oxoglutarate. When glucose was not present, the flux through the "mini-cycle," an "alternate TCA cycle" that Yudkoff et al. (1994) think is driven by the extremely high activity of glutamate-oxaloacetate transaminase (GOT), was 5-fold faster than the flux through the oxaloacetate-to-2-oxoglutarate section of the cycle. The authors think that when 2-oxoglutarate, such as can be derived from glutamine, via glutamate, and other alternative substrates are being oxidized (these include ketones), this GOT-driven mini-cycle is likely to become very important in the brain.
The very high activity of GOT, in relation to the activities of the TCA cycle enzymes, could become problematic when the entry of glutamine-derived glutamate is converted into 2-oxoglutarate in large amounts, such as during and after ischemia. The oxidation of 2-oxoglutarate in the TCA cycle has been estimated to increase very drastically in the brain, after ischemia (it works out to a 23 and 60-fold increase in the entry of glutamate-derived 2-oxoglutarate into the TCA cycle) [Pascual et al., 1998: (http://stroke.ahajournals.org/cgi/content/full/strokeaha;29/5/1048)(http://www.ncbi.nlm.nih.gov/pubmed/9596256)]. The glutamate dehydrogenase reaction is shown as a reversible reaction, and it is reversible. But even though the equilibrium constant of the reaction implies that glutamate-dehydrogenase-mediated glutamate formation can occur, from 2-oxoglutarate and ammonia, this only occurs at very high intramitochondrial ammonia levels [Plaitakis et al., 2001: (http://www.ncbi.nlm.nih.gov/pubmed/11746417)]. There is evidence that glutamate can be formed by glutamate dehydrogenase activity, though, when ammonia levels are very high [Waagepetersen et al., 2000: (http://www.ncbi.nlm.nih.gov/pubmed/10899921)]. Ammonia levels are high when the flux through glutaminase is increasing, such as during and after ischemia. But glutamate would also be less readily available during ischemia, and the flux through the mitochondrial GOT would favor, even more strongly during ischemia than in the absence of ischemia, the formation and efflux of aspartate from the mitochondria (Yudkoff et al., 1994). The mini-cycle and the poor capacity of glutamate dehydrogenase to contribute to glutamate formation would tend to shrink the pool of glutamate that would be available to exert feedback inhibition of glutaminase. There's evidence that a relative lack of glutamate availability can cause glutaminase activity to increase, in neurons and probably astrocytes in the brain, without being subject to feedback inhibition by glutamate [Brand and Chappell, 1974: (http://www.pubmedcentral.nih.gov/picrender.fcgi?artid=1167992&blobtype=pdf)(http://www.ncbi.nlm.nih.gov/pubmed/4375961)]. Although this is beneficial to some extent, inasmuch as glutaminase activity provides a source of glutamate that can undergo oxidation in the TCA cycle, upon its conversion into 2-oxoglutarate, this would seem to create a kind of futile cycling involving glutaminase, GOT, and then cytosolic glutamine synthetase activity. Glutamine synthetase activity consumes very large amounts of ATP, and this type of GOT-facilitated cycling of glutamine carbons could help explain the neuroprotective effects of glutamine synthetase inhibition during ischemia or other metabolic insults in the brain. Paradoxically, there is evidence that exogenous glutamine can interrupt this cycling, to some extent, by increasing the ubiquitination of glutamine synthetase by the 26S proteasomal pathway in astrocytes [Labow et al., 2001: (http://www.ncbi.nlm.nih.gov/pubmed/11533295)]. This could, to some extent, mimic the effects of glutamine synthetase inhibition (to the extent that those effects have something to do with the energetically-demanding quality of glutamine synthetase activity and are not completely a result of "astrocyte glutamine accumulation," as a phenomonen that is supposedly independent of all other variables and physiological processes and has no conceivable biological explanation) without disrupting the availability of glutamine, to both neurons and astrocytes, as a precursor to intramitochondrial glutamate. In the articles by Yudkoff and colleagues, on the interactions of ketone and glutamine metabolism, their suggestion that an increase in 3-hydroxybutyrate oxidation could produce a shift in the equilibrium of mitochondrial GOT (aspartate aminotransferase is GOT) toward glutamate formation (increasing the glutamate/aspartate ratio) is more or less the same thing as saying, in my opinion, that mild inhibition of mitochondrial GOT activity helps to preserve the glutamate pool and thereby increase GABA formation, etc. This is because the efflux of aspartate and the "mini-cycle" (Yudkoff et al., 1994), especially during conditions of low glucose availability, due to ischemia or glucose depletion due to some other cause (Yudkoff et al., 1994; Pascual et al., 1994), could conspire to drive the mitochondrial GOT reaction toward aspartate formation.
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