Uncouplers in mitochondria

To act as an efficient protonophoric uncoupler a weakly acidic compound must have properties that allow it, in both the uncharged protonated and the anionic deprotonated forms, to enter and cross the membrane lipid bilayer. The compound must have a suitable plQ such that on the more acidic, intermembrane side of the membrane a significant proportion is protonated, whilst on the less acidic matrix side a proportion is deprotonated. A compound that is not acidic enough may transfer a single proton across the membrane, but will not release it in the matrix, and hence cannot repeat the cyde. In contrast, too strong an acid will remain deprotonated even in the intermembrane space. For this reason 2,4-dinitrophenol (pJCa 4.04) is a stronger uncoupler than 2-nitrophenol (plQ 7.14) and picric acid (pKa 0.53) does not act as a protonophoric uncoupler in mitochondria, although all three compounds have a similar lipophilidty [88]. 

A. Salicylates - Evidence has been given that aspirin increases bleeding time and decreases blood platelet aggregation, probably by virtue of the acetoxy group. On the other hand, in oxidative phosphorylative uncoupling in mitochondria, the action is due to the salicylic acid moiety. ... 

A number of substances have been discovered in the last thirty years with a macrocyclic structure (i.e. with ten or more ring members), polar ring interior and non-polar exterior. These substances form complexes with univalent (sometimes divalent) cations, especially with alkali metal ions, with a stability that is very dependent on the individual ionic sort. They mediate transport of ions through the lipid membranes of cells and cell organelles, whence the origin of the term ion-carrier (ionophore). They ion-specifically uncouple oxidative phosphorylation in mitochondria, which led to their discovery in the 1950s. This property is also connected with their antibiotic action. Furthermore, they produce a membrane potential on both thin lipid and thick membranes. 

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. 

It was also observed in earlier studies that mitochondria not only accumulate Ca2+ as an alternative to phosphorylation of ADP (Ca2+ uptake uncouples phosphorylation from electron transport), but could also accumulate much larger amounts of Ca2+ if phosphate was also taken up, resulting in precipitation of Ca2+ within the matrix as insoluble hydroxyapatite, visible as electron-dense granules by EM. An unusual feature of these hydroxyapatite deposits is that they fail to become crystalline and remain amorphous even over protracted periods of time. Their presence in mitochondria in a number of disease conditions underlines the role for mitochondria as a sort of safety device, which can enable the cell to survive, if only for a limited period of time, situations of cytoplasmic Ca2+ overload. 

A different direction in ion-selective electrode research is based on experiments with antibiotics that uncouple oxidative phosphorylation in mitochondria [59]. These substances act as ion carriers (ionophores) and produce ion-specific potentials at bilayer lipid membranes [72]. This function led Stefanac and Simon to obtain a new type of ion-selective electrode for alkali metal ions [92] and is also important in supporting the chemi-osmotic theory of oxidative phosphorylation [69]. The range of ionophores, in view of their selectivity for other ions, was broadened by new synthetic substances [1,61]. 

They ion-selectively uncouple oxidative phosphorylation in mitochondria [115, 140], resulting in their bactericidal action [68]. 

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. 

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. 

In mitochondria, the allelochemlcals acted primarily as electron transport inhibitors. Malate oxidation was more sensitive than either succinate or NADH oxidation. No evidence for interaction with a specific membrane complex was obtained. Instead, Inhibition of substrate oxidation seems to result from alterations and perturbations produced in the inner membrane as reflected in interference with the behavior of transport processes. The compounds did not act as uncouplers or directly inhibit ATP synthesis. However, naringenin, some of the flavones, and the cinnamic acids dj inhibit the hydrolysis of ATP catalyzed by mitochondrial Mg -ATPase. 

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. 

A recently introduced insecticide/acaricide, chlorfenapyr (Figure 3.14), has been shown to disrupt the electrochemical gradient in mitochondria and, thereby, uncouple oxidative phosphorylation. There is also evidence that the compound is a pro-insecticide, being converted into the active form by mixed function oxidases within the insect or mite. 

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 ... 

Agents that uncouple oxidative phosphorylation in mitochondria uncouple photoelectron transport and ATP formation in photosynthesis. Explain. 

The stereochemical structure was clarified with the use of X-ray crystallography by Iitake et at. (75). As for mode of action of tetranactin, Ando et at. (76) observed that tetranactin is an uncoupler in cockroach mitochondria and supposed that the antibiotic caused the leakage of alkali cations such as K+ through the lipid layer of the biomembrane in mitochondria, followed by uncoupling. 

Thermogenic systems based on uncoupling proteins or on the alternative electron transport pathways found in mitochondria of certain plants represent what in many ways is the sim-... 

Some processes will not be dependent on some of the forces when the appropriate cross coefficients naturally vanish. For example, some degrees of imperfections due to parallel pathways of reaction or intrinsic uncoupling within the pathway itself may lead to leaks and slips in mitochondria. This, however, may add complexity to the phenomenological analysis, because failure of models to fit the properties of a system may be the result of unaccounted coupling. 

Using the experimentally attainable static head condition (state 4 in mitochondria) and the uncoupled oxygen flow (JoXmc, we can determine the degree of couphng q... 

---