Uncoupling agent

Figure 12-8. Principles of the chemiosmotic theory of oxidative phosphorylation. The main proton circuit is created by the coupling of oxidation in the respiratory chain to proton translocation from the inside to the outside of the membrane, driven by the respiratory chain complexes I, III, and IV, each of which acts as a protonpump. Q, ubiquinone C, cytochrome c F Fq, protein subunits which utilize energy from the proton gradient to promote phosphorylation. Uncoupling agents such as dinitrophenol allow leakage of H" across the membrane, thus collapsing the electrochemical proton gradient. Oligomycin specifically blocks conduction of H" through Fq.

Fujino, M., T. Arima, Y. Sato, and H. Tako. 1985. Suppression of action of some excitation-contraction (E-C) uncoupling agents and mechanism in E-C coupling in single skeletal muscle fibers of frog, R. japonica. Jour. Physiol. Soc. Japan 47 519. 

Kelly DP, Syrett PJ. 1964b. The effect of uncoupling agents on carbon dioxide fixation by a Thiobacillus. J Gen Microbiol 34 307-17. 

During the First World War, trinitrotoluene (abbreviated to TNT) was the explosive used in shells and bombs. It was noticed that many of the women who packed the explosive into shells suffered from fever and loss of body weight. In some cases the fever was fatal. The cause was not known. Some time later, it was discovered that a very similar chemical, namely dinitrophenol, was an uncoupling agent. The fever and loss of weight were due to uncoupling caused by TNT (i.e. causing a leak of protons back across the mitochondrial membrane). 

C. Uncoupling agents provide an alternate pathway to transfer protons back into the mitochondrial matrix, which dissipates the proton gradient and bypasses ATP formation by the ATPase. 

Chemical agents (such as 2,4-dmitrophenol) that are able to bind a proton and be soluble in the lipid bilayer can also act as uncoupling agents. 

Oxidative Phosphorylation. Oxidative phosphorylation, that is the production of ATP during the passage of electrons down the terminal electron transport chain, may be disrupted in two distinct ways. Compounds that divorce the process of electron transport and the phosphorylation of ADP are termed uncoupling agents. They permit NADH and succinate to be oxidised via the electron transport chain without the production of ATP and are lethal. Oxidative phosphorylation may also be inhibited directly, thus preventing the oxidation of NADH and succinate. Several products are available that exploit these modes of action. Characteristically, they have wide activity spectra that span major disciplines of pesticide use. 

The transport process is unclear. In some cases it is inhibited by uncoupling agents, suggesting that uptake is linked to the electrochemical proton gradient, while evidence has also been found for a symport mechanism involving ferrichrome and Mg2+.1201... 

Models based on the above concept have been proposed, such as the one shown in Fig. 14-8. Evidence for this model is based on studies of the hydrolysis of ATP labeled in the y phosphate with 1kO. The label is lost from the released P, to water (H18OH) because of the rapid reversibility of reactions 2 and -2. Uncoupling agents do not influence this loss, indicating is not involved in reactions 2 or -2, nor in reactions -3 or -1. 

The effect of nonfatal injuries such as a 2-hour period of bilateral hind-limb ischemia or a full-thickness scald of 20% of skin surface on the LDso of DNOC and its hyperthermic effect were evaluated in male rats (Stoner 1969). The intraperitoneal LDs° of DNOC was significantly (p<0.001) reduced from 24.8 to 26.2 mg/kg to 14 mg/kg DNOC when DNOC was given 1.5- 24 hours after either type of nonfatal injury. The authors concluded that the toxicity of DNOC was increased by previous trauma. These investigators proposed that this interaction was associated with sequential blocking of the tricarboxylic acid cycle with inhibition of citrate synthetase reaction during the early part of the response to the injury. Because DNOC acts as an uncoupler of oxidative phosphorylation, less ATP is produced. Therefore, the effects of trauma will be enhanced by an uncoupling agent such as DNOC. 

The maturation of fat body mitochondria is affected by the neuroendocrine balance in adult male B. discoidalis. Removal of the CC arrests the maturation of the respiratory enzymes at their partly developed, 5-day level (58, Figure 1). Injections of CC extracts on days 0 to 5 result in precocious, 10-day levels of respiratory activity by 5 days of age. Administration of the uncoupling agent 2,4-dinitrophenol indicates that electron transport, rather than phosphorylation, is the rate-limiting step for respiration in fat bcxiy mitochondria (59). A situation that differs from vertebrate mitochondria where phosphorylation is limiting. This indicates that the levels of cytochrome enzymes available for electron transport in fat body mitochondria determine the respiratory and ATP synthesis capacities of the tissue and influence its biosynthetic potential. 

It is possible to add uncoupling agents that prevent the formation of ATP. In such cases there is increased oxygen consumption and generation of heat. Hibernating animals may use this mechanism to their advantage. 

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