In this article [Bose et al., 2003: (http://www.jbc.org/cgi/reprint/278/40/39155)(http://www.ncbi.nlm.nih.gov/pubmed/12871940)], Bose et al. (2003) found that inorganic phosphate (Pi) increased the respiratory rate in the isolated mitochondria from pigs' skeletal muscle cells and cardiac muscle cells, and the authors used the increase in the rate of NADH generation as an important indication that the respiratory rate had increased. The increase in NADH generation occurred in the presence of uncouplers (compounds that uncouple the redox reactions of the multienzyme complexes in the electron transport chain with the generation of ATP by the F1F0-ATPase protein) [see here for discussion: (http://hardcorephysiologyfun.blogspot.com/2009/05/cytosolic-redox-potential-and-proton.html)], and Bose et al. (2003) noted that the Pi-induced increases in the rate of NADH generation were likely to have been a result of the "global" activation of various or numerous NAD(+)-dependent dehydrogenase enzymes or enzyme complexes by Pi. The authors cited research, on p. 39161, showing that Pi can activate the NADH-generating TCA cycle enzymes 2-oxoglutarate dehydrogenase, NAD-dependent isocitrate dehydrogenase (this is not the same as the NADP-dependent isocitrate dehydrogenase enzyme). The NADH is then more or less immediately oxidized to NAD+ by respiratory chain enzymes, assuming there's enough oxygen and the mitochondria have not been damaged, etc., and numerous TCA cycle dehydrogenase enzymes either bind complex I, a multienzyme respiratory chain complex that oxidizes NADH formed by TCA cycle enzymes, and channel NADH to complex I or are functionally coupled to complex I activity less directly [Sumegi and Srere, 1984: (http://www.jbc.org/cgi/reprint/259/24/15040.pdf)(http://www.ncbi.nlm.nih.gov/pubmed/6439716)]. Bose et al. (2003) also argued, on p. 39162, that the way in which Pi appears to regulate respiration, by multiple mechanisms, might mean that Pi could exert an antioxidant function
["the generation of free radicals in the mitochondria may be minimized" (Bose et al., 2003, p. 39162], but the authors also cited, on p. 39163 (reference 35), research implying that Pi could exacerbate the augmentation of the rate of free-radical formation following ischemia. I'd wonder what the concentrations used by the authors might have been, in some of those articles cited, because I've seen cell-culture studies showing effects of Pi that don't make sense to me and use supraphysiological concentrations of Pi, show proapoptotic or toxic effects of massive concentrations of Pi, or contrast, in ways that may lack physiological relevance, the effects of excesses of Pi with the supposed protective effects of various drugs, etc. That said, I do think Pi could affect mitochondrial functioning in ways that are not desirable, but it's noteworthy, as Bose et al. (2003) intimated, that ischemia and other forms of metabolic stress can cause Pi to be released during the degradation of phosphocreatine and could derange mitochondrial Pi homeostasis in ways that would be more significant than the ways in which increases in Pi availability would be likely to derange Pi homeostasis. One is unlikely to be able to "hide" from ischemia-induced, wild extremes in mitochondrial Pi influx by restricting dietary Pi, for example, because Pi depletion has the potential to exacerbate those "wild swings" in Pi availability by causing hypoxia, ATP depletion, hemolysis, rhabdomyolysis, etc., in my opinion. But it's worth noting that excesses of intracellular, free Pi could produce adverse effects on mitochondrial functioning.
Bose et al. (2003) also cited research showing that the transport of Pi across the inner mitochondrial membrane is likely to influence the pH gradient across the inner mitochondrial membrane, given that Pi transport appears to be coupled to OH(-) or H(+) transport, and that Pi is used a substrate in the phosphorylation of ADP by the F1F0-ATPase protein. I don't know if there's an enzyme-bound intermediate that's formed from Pi and that contains a hydrolyzable phosphodiester bond, etc. It looks like it wasn't known, as of 2000 [Vinogradov, 2000: (http://jeb.biologists.org/cgi/reprint/203/1/41.pdf)(http://www.ncbi.nlm.nih.gov/pubmed/10600672)]. That's remarkable. That's a terrific article, though, by Vinogradov (2000), and the most important piece of information in there is probably the statement on p. 44 that F1F0-ATPase is "activated" by a free magnesium ion, meaning free Mg(2+), and doesn't just depend on magnesium bound to adenosine nucleotides, as MgATP(2-) and MgADP(-). That could be really important, and it's basically like saying it's a catalytic magnesium ion. That could conceivably help to explain some of the supposed neuroprotective effects of magnesium and could also explain some of its apparent effects on exercise performance, etc. Magnesium also generally enhances overall glycolytic activity and creatine kinase activity, and those effects, along with its purine nucleotide-buffering effects (preventing the loss of adenosine nucleotides, etc.), could also be relevant in those contexts. Never mind that there's an incorrect assumption, in most articles and textbooks, that the intracellular magnesium concentration is high enough to bind to all of the available free ATP and ADP, etc. That's not likely to be true in 99.9 percent of people, and, in my opinion, the activities of many enzymes that contain binding sites for catalytic magnesium ions are likely to be sensitive to changes in magnesium status. Vinogradov (2000) also noted that magnesium is likely to also bind Pi, probably more loosely than magnesium binds some of its other substrates and regulatory factors. I've seen that mentioned in other articles, including the article by Bose et al. (2003), and Bose et al. (2003) cited research, on the first page of their article, describing the capacity of Pi to bind calcium and magnesium (I'm assuming they're talking about reversible binding, in the context of the regulation of intramitochondrial free calcium by its complexation with orthophosphate, etc.) and influence their effects on respiration. I'm not sure what the mechanism is by which Pi activates the TCA cycle enzymes, but I'll have to read on that. Maybe it's partially a result of allosteric effects, and maybe some of those allosteric effects are a result of Pi-induced changes in Ca(2+) binding to the enzymes or enzyme complexes, etc. I'll have to look at some of those articles.
No comments:
Post a Comment