Uncoupling of mitochondria

There are numerous in vitro and in vivo studies, in which the damaging free radical-mediated effects of iron have been demonstrated. Many such examples are cited in the following chapters. However, recent studies [170,171] showed that not only iron excess but also iron deficiency may induce free radical-mediated damage. It has been shown that iron deficiency causes the uncoupling of mitochondria that can be the origin of an increase in mitochondria superoxide release. Furthermore, a decrease in iron apparently results in the reduction of the activity of iron-containing enzymes. Thus, any disturbance in iron metabolism may lead to the initiation of free radical overproduction. 

The first defect, described in 1962 is, in fact, one of the rarest (Luft s syndrome). It arises from the uncoupling of mitochondria. The resting metabolic rate is markedly raised, there is profuse sweating, fever and generalised muscle weakness. The mitochondria of these patients have an increased permeability, not so much to protons, as in brown adipose tissue mitochondria, but to cations, such as Ca, the entry of which similarly dissipates the proton motive force. 

Phospholipids, the major hpid in cellular membranes, are a primary target of peroxidation caused by free radical release. Peroxidation of lipids in the inner mitochondrial membrane may contribute to the inhibition of electron transport and uncoupling of mitochondria, leading to inflammation and cellular necrosis. Induction of CYP2E1 and other P450 cytochromes also increases formation of other radicals and the activation of hepatocarcinogens. 

Nucleic acids are not the only biomolecules susceptible to damage by carotenoid degradation products. Degradation products of (3-carotene have been shown to induce damage to mitochondrial proteins and lipids (Siems et al., 2002), to inhibit mitochondrial respiration in isolated rat liver mitochondria, and to induce uncoupling of oxidative phosphorylation (Siems et al., 2005). Moreover, it has been demonstrated that the degradation products of (3-carotene, which include various aldehydes, are more potent inhibitors of Na-K ATPase than 4-hydroxynonenal, an aldehydic product of lipid peroxidaton (Siems et al., 2000). 

However, if the heat produced is not sufficient to maintain the temperature, despite decreased heat loss, it can be produced by specific processes. These are shivering, substrate cycling or uncoupling of ATP formation from electron transfer in mitochondria. 

Moreno-Sanchez, R. et al. (1999) Inhibition and uncoupling of oxidative phosphorylation by nonsteroidal antiinflammatory drugs study in mitochondria, submitochondrial particles, cells, and whole heart. Biochemical Pharmacology, 57 (7), 743-752. 

The initial effect of salicylate is stimulation of the depth of respiration due to increased production of carbon dioxide and greater use of oxygen. This is due to the uncoupling of oxidative phosphorylation in the mitochondria, which increases the rate of oxidation of substrates (see below). 

The basic mechanism underlying the toxicity of salicylate is the uncoupling of oxidative phosphorylation. For oxidative phosphorylation to take place, there is a requirement of a charge difference between the intermembrane space and the matrix of the mitochondria (Fig. 7.60). This is achieved when electrons move down the chain of multienzyme complexes and electron carriers (the electron transport chain), causing protons to move from the mitochondrial matrix to the intermembrane space. Consequently, a pH difference builds up, which is converted into an electrical potential across the membrane of approximately 200 mV over 8 nm. 

Uncouplers of Oxidative Phosphorylation In normal mitochondria the rate of electron transfer is tightly coupled... 

Thyroid hormones are intimately involved in regulating the basal metabolic rate. Liver tissue of animals given excess thyroxine shows an increased rate of 02 consumption and increased heat output (thermogenesis), but the ATP concentration in the tissue is normal. Different explanations have been offered for the thermogenic effect of thyroxine. One is that excess thryroxine causes uncoupling of oxidative phosphorylation in mitochondria. How could such an effect account for the observations Another explanation suggests that the thermogenesis is due to an increased rate of ATP utilization by the thyroxine-stimulated tissue. Is this a reasonable explanation Why ... 

As a consequence, in the presence of arsenate oxidation of 3-phosphoglyceraldehyde continues but ATP synthesis ceases. Arsenate is said to uncouple phosphorylation from oxidation. Arsenate can also partially replace phosphate in stimulating the respiration of mitochondria and is an uncoupler of oxidative phosphorylation (Chapter 18). Enzymes that normally act on a phosphorylated substrate will usually catalyze a slow reaction of the corresponding unphosphorylated substrate in the presence of arsenate. Apparently, the arsenate ester of the substrate forms transiently on the enzyme surface, permitting the reaction to occur. 

The pH gradient created by respiration collapses abruptly if an uncoupler is added (see fig. 14.18). In the absence of an uncoupler, the pH change caused by a brief period of respiration decays slowly, in agreement with the view that the mitochondrial inner membrane blocks the free diffusion of protons between the matrix and the intermembrane space. Mitchell and Moyle reinforced this conclusion by experiments in which they added a small amount of HC1 to a suspension of mitochondria and measured the rate at... 

Respiring mitochondria extrude protons. A weakly buffered suspension of mitochondria is provided with an oxidizable substrate and allowed to use up all the O2 in the solution. The traces show measurements of the pH of the suspension, with pH decreases plotted upward. When a small amount of O2 is added (upward-pointing arrow), respiration can occur for a few seconds. The pH of the solution decreases suddenly at this point (Exp. A). The pH change persists for a period of several minutes, long after the 02 has been used up. If an uncoupler is added, the pH returns abruptly to nearly its original level. Experiment B was done in the presence of a detergent that disrupted the inner membrane, making the membrane... 

Zl. Zetterstrom, R., and Ernster, L., Bilirubin, an uncoupler of oxidative phosphorylation in isolated mitochondria. Nature 178, 1335-1337 (1956). 

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