Mitochondrial oxidative phosphorylation inhibitors

Uncouplers. Uncouplers dissociate electron transport from photophosphorylation. Both noncyclic and cyclic phosphorylation are inhibited, but electron transport reactions are either unaffected or stimulated. Because uncouplers relieve the inhibition of electron transport imposed by energy transfer inhibitors, they are considered to act at a site closer to the electron transport chain than the site of phosphate uptake. In Figure 2, they are shown (site 2) as dissipating some form of conserved energy represented as on the noncyclic and cyclic ATP-gener-ating pathways. Perfluidone is the only herbicide identified to date that functions as a pure uncoupler at pH 8.0 (2). Compounds that uncouple photophosphorylation also uncouple mitochondrial oxidative phosphorylation. 

Mitochondrial electron transport system Sulfuramid, chlorfenapyr Azocyclotin, cyhexatin, Fenbutatin-oxide, propargite, Tetradifon, diafenthiuron Uncouplers of oxidative phosphorylation Inhibitors of ATP synthase... 

Hollingworth RM (2001) Inhibitors and uncouplers of mitochondrial oxidative phosphorylation. In Krieger R (ed.) Handbook of Pesticide Toxicology, 2nd edn. San Diego, CA Academic Press. 

Since sodium azide or oligomycin, inhibitors of mitochondrial oxidative phosphorylation, did not affect the Ca " release, it became generally accepted that the intracellular pool corresponds to the SR. 

It remains to be established, however, that this peculiar phosphorylating system is native to the nucleus and does not represent contamination of the nuclear fraction by intact cells or by intact or fragmented mitochondria. The arguments against mitochondrial contamination rest on several observations. Among them are the absence of cytochrome oxidase in nuclei and the difference between nuclear and mitochondrial oxidative phosphorylation with respect to their responses to inhibitors. 

Stern and Timonen [37] demonstrated that thymus nuclei contain little cytochrome oxidase, but this finding is not in agreement with observations made on liver, spleen, or pancreas nuclei prepared in 0.25 m sucrose. Although the difference between the effect of inhibitors on nuclear and mitochondrial oxidative phosphorylation excludes the possibility of contamination of nuclei by intact mitochondria, it does not eliminate the possibility of contamination by fragmented mitochondria. Inhibitors do not affect the phosphorylating particle and the intact mitochondria the same way. 

Many inhibitors of substrate oxidations, substrate transport, electron transport, and ATP synthesis are known including many well-known toxins (see Sherratt, 1981 Harold, 1986 Nicholls and Ferguson, 1992). These are not discussed here except to mention specific uncouplers of oxidative phosphorylation. Classic uncouplers such as 2,4-dinitrophenol have protonated and unprotonated forms, both of which are lipid soluble and cross the inner mitochondrial membrane discharging the proton gradient. This prevents ATP synthesis and stimulates respiration. 

Mechanistic studies have shown that TBT and certain other forms of trialkyltin have two distinct modes of toxic action in vertebrates. On the one hand they act as inhibitors of oxidative phosphorylation in mitochondria (Aldridge and Street 1964). Inhibition is associated with repression of ATP synthesis, disturbance of ion transport across the mitochondrial membrane, and swelling of the membrane. Oxidative phosphorylation is a vital process in animals and plants, and so trialkyltin compounds act as wide-ranging biocides. Another mode of action involves the inhibition of forms of cytochrome P450, which was referred to earlier in connection with metabolism. This has been demonstrated in mammals, aquatic invertebrates and fish (Morcillo et al. 2004, Oberdorster 2002). TBTO has been shown to inhibit P450 activity in cells from various tissues of mammals, including liver, kidney, and small intestine mucosa, both in vivo and in vitro (Rosenberg and Drummond 1983, Environmental Health Criteria 116). 

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). 

Other work has indicated that chlordane and heptachlor are energy transfer inhibitors as evidenced by marked decreases in oxidative phosphorylation of rat hepatic mitochondria following in vitro incubation of the mitochondria with the pesticides (Ogata et al. 1989). Interestingly, even though heptachlor epoxide is more toxic than either chlordane or heptachlor in tests of general toxicity, it was less effective in inhibiting mitochondrial respiration. 

Effects of Valinomycin on Oxidative Phosphorylation When the antibiotic valinomycin is added to actively respiring mitochondria, several things happen the yield of ATP decreases, the rate of 02 consumption increases, heat is released, and the pH gradient across the inner mitochondrial membrane increases. Does valinomycin act as an uncoupler or an inhibitor of oxidative phosphorylation Explain the experimental observations in terms of the antibiotic s ability to transfer K+ ions across the inner mitochondrial membrane. 

In soybean seedlings in-vivo supply of cadmium and lead was reported to stimulate the respiration rate (Lee et al., 1976a, b) this effect was ascribed to a demand for ATP production through oxidative phosphorylation because photophosphorylation was reduced. In vitro, several metal ions were effective inhibitors of the mitochondrial respiratory electron transport chain (Kleiner, 1974 Koeppe, 1977) Koeppe (1977) considered the inhibition of the electron transfer at the terminal NADH-oxidase to be specific for cadmium. 

Although the importance of the nuclear T3 receptor is well established, there also is evidence for the existence of non-nuclear T3 receptors. For example, Segal and Ingbar46 demonstrated an immediate T3-mediated increase in calcium accumulation in rat thymocytes, an increase that was insensitive to inhibitors of protein biosynthesis. A direct effect on oxidative phosphorylation has been suggested to result from T3 binding to saturable receptors of mitochondrial membranes. The possible roles of non-nuclear T3 receptors have been reviewed by Sterling47. 

X1B-X14), and NADP-linked isocitrate dehydrogenase SIS, S16). Numerous studies have been carried out with isolated mitochondria to identify the source of NADPH for 11 -hydroxylation by studying the effect of inhibitors and uncouplers of oxidative phosphorylation in the presence of different hydrogen donors (83, 19S, SIO, SIS, SI4-SSS). Part of this work was inconclusive since several complicating features in the metabolism of adrenocortical mitochondria were insufficiently taken into account such as secondary inhibitory effects of the substrates, inhibitors, or uncouplers used 83, SI4, SSO, SSS) the requirement of transport of substrates across the mitochondrial membrane SS3, SS4) and the possibility of intramitochondrial dismutation reactions SS3). 

ATP-ADP translocase is specifically inhibited by very low concentrations of atractyloside (a plant glycoside) or bongkrekic acid (an antibiotic from a mold). Atractyloside binds to the translocase when its nucleotide site faces the cytosol, whereas bongkrekic acid binds when this site faces the mitochondrial matrix. Oxidative phosphorylation stops soon after either inhibitor is added, showing that ATP-ADP translocase is essential. 

Additional studies [212,218,219,242,243] to quantitate the role of the adenine nucleotide translocator in the control of mitochondrial respiration have been performed utilizing inhibitor titrations with carboxyatractyloside. The results indicated that in State 4 (no ADP), no control was exerted by the translocator. However, as the rate of respiration was increased up to State 3 (excess ADP), the control strength of the carrier increased to a maximum value of 30%, at 80% of State 3 respiration. These studies indicate that the adenine nucleotide translocator cannot be considered to be the only rate-controlling step in oxidative phosphorylation. However, they do provide experimental support for a controlling role for the carrier at intermediate to maximal levels of respiration. An important corollary of these studies is that the reaction rate may be altered by a change in substrate concentration (elasticity). It is also clear that to confirm these studies quantitatively, they must be extended to intact cells. Although such studies have been more difficult, the results are compatible with the conclusion reached by Tager et al. [212]. 

Subsequent reports described a syndrome of type B lactic acidosis in patients treated with zidovudine and other nucleoside reverse transcriptase inhibitors, including stavudine, lamivudine, and didanosine which has also been attributed to mitochondrial DNA toxicity [95-106]. There are five types of DNA polymerase in human cells that catalyze the synthesis of new complementary DNA from the original DNA template (HIV encodes a reverse transcriptase DNA polymerase which uses RNA as the template). The active triphosphate metabolites of zidovudine, didanosine, and stavudine inhibit DNA polymerase gamma in mitochondria, block the elongation of mitochondrial DNA, and deplete mitochondrial DNA [91-93,101,105-108]. The link between NRTl effects on mitochondrial DNA and lactic acidosis is not entirely clear but is most likely related to disturbances of oxidative phosphorylation and impaired pyruvate metabolism leading to lactate accumulation. 

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