Uncoupling Agents of Oxidative Phosphorylation

Uncoupling of oxidative phosphorylation by 2,4-dinitrophenol (2,4-DNP). The anionic form of 2,4-DNP is protonated in the intermembrane space, is lipid soluble, and crosses the inner membrane readily. In the matrix, the protonated form dissociates, abolishing the proton gradient established by substrate oxidation. The ionized form of 2,4-DNP is poorly soluble in the membrane lipids and therefore is not easily transported across the membrane (dashed arrow). It is lipophilic and capable of transporting protons from one side of the membrane to the other (a protonophore), thus abolishing the proton gradient. 

located on the inside, interacts with the hydrophilie groups. The matrix side of the inner membrane has a negative potential, so the positively charged valinomycin-K+ complex is drawn inward. The complex uncouples oxidative phosphorylation by decreasing the membrane potential. Unlike valinomycin, it exchanges H for K+ across the membrane. Its 

Structure of valinomycin (a) and its complex with K (b). Valinomycin, which consists of three identical fragments of D-valyl-L-lactyl-L-valyl-D-a-hydroxyisovaleric acid (D-Val-L-Lac-L-Val-D-Hyi), is a mobile or channel-forming ionophore and an uncoupler of oxidative phosphorylation. Note the hydrophobic exterior and the hydrophilic interior of the complex. [Structure (b) is reproduced, with permission, from B. C. Pressman Biological application of ionophores. Amu. Rev. Biochem. 45,501 (1976). 1976 by Annual Reviews Inc.] 

Structure of gramicidin A (a) and the formation of a helical pore through a lipid bilayer by assembly of two gramicidin A molecules via their formyl groups at the N termini (b). A variety of monovalent cations are transported through the static pore. [Structure (b) is reproduced with permission from Y.A. Ovchinnikov Physico-chemical basis of ion transport through biological membranes lonophores and ion channels. Eur. J. Biochem. 94,321 (1979).] 

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Another theory was offered to explain the mode of action of these compounds when it was noted that dicoumarol is a potent uncoupling agent of oxidative phosphorylation [363]. The antibacterial action of the 3-acetyl-4-hydroxycoumarin may be associated with its uncoupling action [364, 365]. Similarly, a relationship has been suggested between the anticoagulant activity of dicoumarol and related compounds and their uncoupling activity [366]. 

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.

A few caveats are in order as to what defines a lead. Firstly, a lead is more than just a compound that shows a defined level of activity in a primary screen. The screen must have been validated usually this will be by obtaining the expected responses from pharmacological standards or known drugs. Any reasons for false positives must be understood. Certain substances such as chemically reactive or unstable compounds, protein denaturants, membrane destabilizing agents or uncouplers of oxidative phosphorylation will record as active in a great variety of screens. These must be recognized and eliminated by suitable secondary procedures. 

The control of lice, mites and warbleflies continues to be achieved by the older agents. The selection pressure that caused resistance in cattle ticks and sheep blowfly has not been encountered by these other ectoparasites since their less frequent incidence has required less insecticide/acaricide usage. In addition to the OPs, rotenone (92) (the active principle of derris), an uncoupler of oxidative phosphorylation, continues to be used to combat mange. 

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. 

ANTIFUNGAL, ANTIMICROBIAL and ANTIINFLAMMATORY activity, and to be a powerful cytotoxic agent (functioning by DNA intercalation and uncoupling of oxidative phosphorylation). It is an ENZYME INHIBITOR (alanine aminotransferase and human plasma diamine oxidase). It shows antiplaque activity, and has been used in toothpastes and oral rinses. Causes temporary change in intraocular pressure. Sanomigran pizotifen. 

This chapter summarizes the discovery and development of chlorfenapyr, a potent uncoupler of oxidative phosphorylation as an insect control agent. This compound shows activity against a broad spectmm of crop and urban pests while having relatively little impact on beneficial insects. 

The role played by exchange reactions in oxidative phosphorylation is not known these reactions could be the steps of the coupling mechanism, but in reverse. Some indirect evidence suggests that they are indeed involved in oxidative phosphorylation. First, as already mentioned, these exchange reactions are affected by agents uncoupling oxidative phosphorylation. Their response to the uncouplers, however, does not correlate with the effect of these compounds on the efficiency of oxidative phosphorylation itself. Secondly, these reactions are influenced by the carriers state of oxidation. 

When they further observed that the normal nucleus contains a high proportion of mono-, di-, and trinucleotides of adenine, they claimed to have provided direct proof of their theory by demonstrating that the mono-or dinucleotides in the nucleus may be converted to ATP when oxygen is present. (The nucleotides can be extracted from the nucleus with acetate buffer at pH 5.1.) This conversion certainly suggested the existence of an intranuclear process of oxidative phosphorylation. As in mitochondria, oxidative phosphorylation in the nucleus is inhibited by uncouplers or agents blocking the electron transport chain. Nuclear oxidative phosphorylation is blocked by cyanide, azide, and antimycin A, or by dinitrophenol but, in contrast to mitochondria, it is resistant to Janus green, methylene blue, carbon monoxide, Dicumarol, and calcium. 

Various substances inhibit the formation of ATP without interrupting the transport of electrons. This phenomenon is called uncoupling of oxidative phosphorylation." The most frequently used substance is dinitrophenol. Dicumarol, the ants o-nist of vitamin K, also is an uncoupling agent. A biologic regulation of couplii is probable the biologic uncoupler, however, is still not known. 

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. 

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