| Electron transport system inhibitors
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| Inhibitors of electron transport selectively inhibit complexes I, III, or IV, interrupting the flow of electrons through the respiratory chain. This stops proton pumping, ATP synthesis, and oxygen uptake. Several inhibitors are readily-available poisons that could be encountered in the practice of medicine.
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| page 106 |  | | page 107 |
| Figure 8.14 Proton transport by uncouplers. Uncouplers transport protons into the mitochondrion, dissipating the proton gradient. DNP is an example of an exogenous uncoupler. The uncoupling proteins (UCP) are endogenous uncouplers in the IMM and are regulated by hormones. The gradient consisting of protons and other factors constitute the mitochondrial membrane potential (MMP), which is expressed in millivolts (mV). DNP, 2,4-dinitrophenol; IMM inner mitochondrial membrane. |
| 2,4-DINITROPHENOL POISONING |
| An unresponsive 25-year-old woman is carried into the emergency room by her boyfriend, because after taking two doses of 'weight loss' pills, she complained of headache, fever, chest pain, profuse sweating, and weakness. Initial findings were: rectal temperature, 40.8 °C (105.5 °F), pulse 151 beats per minute, respiratory rate, 56 per minute, blood pressure 40/10. In 15 minutes she died and could not be resuscitated. After death, rigor mortis set in after 10 minutes and her temperature rose to 46 °C (115 °F) in 10 additional minutes. It was found that she was a body builder and that she took 'weight loss' capsules purchased from a friend, because she wanted to have a leaner body for a show. Among her personal effects, was a plastic bottle containing capsules that proved to contain 2,4-dinitrophenol. |
| Comment. Dinitrophenol (DNP) is an uncoupler of oxidative phosphorylation (see Fig. 8.14). It was first discovered to induce eight loss during World War I when it was noticed that French munitions workers who were exposed to dinitrophenol during the synthesis of dynamite (trinitrotoluene, TNT) rapidly lost weight. In the 1930s it was prescribed by physicians for weight loss and was also available over-the-counter, but because people suffered significant side effects, such as cataracts, blindness, kidney and liver damage and death, it was banned for medical use in United States after a congressional investigation. The case above was adapted from those hearings. Dinitrophenol is currently used industrially in the manufacture of dyes, explosives, herbicides, insecticides and lumber preservatives. DNP kills bacteria and fungi by uncoupling phosphorylation. Unfortunately, DNP has resurfaced as an illegal weight-loss product. It radically increases consumption of oxygen and metabolic fuels, and nearly all metabolic energy is wasted as heat. Cells die both because of excess temperature and lack of ATP. |
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| Rotenone inhibits complex I (NADH-Q reductase)
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| Rotenone, a common insecticide, and some barbiturates (e.g. amytal) inhibits complex I. Because malate and lactate are oxidized by NAD+, their oxidation will be inhibited by rotenone. However, substrates yielding FADH2 can still be oxidized, because Complex I is bypassed and electrons are donated to ubiquinone. Addition of ADP to a suspension of mitochondria supplemented with malate and phosphate (Fig. 8.15) markedly stimulates oxygen uptake as ATP synthesis occurs. Oxygen uptake is markedly inhibited by rotenone, but, when succinate is added, ATP synthesis and oxygen consumption resume until the supply of ADP is exhausted.
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| Figure 8.15 Inhibition of Complex I, such as rotenone, retard oxygen uptake by mitochondria when NADH-producing substrates are being oxidized. |
| Rotenone inhibition of Complex I causes reduction of all components prior to the point of inhibition, because they cannot be oxidized, whereas those after the point of inhibition become fully oxidized. This is known as a crossover point, and it can be determined spectrophotometrically, because light absorption by respiratory-chain components changes according to redox state. Such analyses were used to define the sequence of components in the respiratory chain.
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| Antimycin A inhibits complex III (QH2-cytochrome c reductase)
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| Figure 8.16 Inhibition of Complex III by antimycin. Antimycin A inhibits Complex III, blocking transfer of electrons from both Complex I and flavoproteins, such as Complex II. |
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The inhibition of Complex III by antimycin A prevents transfer of electrons from either complex I or FADH2-containing flavoproteins to cytochrome c. In this case, components preceding complex III become fully reduced, and those after it become oxidized. The oxygen uptake curve (Fig. 8.16) shows that the stimulation of respiration by ADP is inhibited by
antimycin A, but that the addition of succinate does not relieve the inhibition. Ascorbic acid can reduce cytochrome c, and addition of ascorbic acid restores respiration, illustrating that complex IV is unaffected by antimycin A.
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| Cyanide and carbon monoxide inhibit Complex IV
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| Figure 8.17 Inhibition of Complex IV. The inhibition of Complex IV interrupts the transfer of electrons, in the final step of electron transport. Electrons cannot be transferred to oxygen, and the synthesis of ATP is halted. |
| Azide (N3-), cyanide (CN-), and carbon monoxide (CO) inhibit Complex IV (cytochrome c oxidase) (Fig. 8.17). Because complex IV is the terminal electron transfer complex, its inhibition cannot be bypassed. All components preceding complex IV become reduced, oxygen cannot be reduced, none of the complexes can pump protons and ATP is not synthesized. Uncouplers such as DNP have no effect, because there is no proton gradient. Cyanide and carbon monoxide also
bind to hemoglobin, and it cannot carry oxygen (see Chapter 4). In these poisonings, both the ability to transport oxygen and to synthesize ATP are impaired. The administration of oxygen is used for the treatment of such poisonings. It is of interest that sodium azide is the nitrogen source for the inflation of air bags; it may pose an environmental problem if it is accidentally released, i.e. non-explosively.
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