Protonophoric effect

Fig. 17 Mitochondrial effects of valproic acid. Valproic acid freely enters mitochondria, and thus translocates protons into the mitochondrial matrix. This protonophoric effect can slightly imcouple mitochondrial respiration, and can help trigger mitochondrial permeability transition (MPT). Inside the matrix, valproate is extensively transformed into valproyl-CoA, thus sequestering intramitochondrial CoA. The lack of CoA impairs both mitochondrial fatty acid 6-oxidation and pyruvate oxidation. Valproate is also dehydrogenated by cytochrome P450 (CYP) into 4-ene-valproate, which then forms 4-ene-valproyl-CoA and 2,4-diene-valproyl-CoA within mitochondria. The latter is an electrophilic metabolite, which may inactivate 6-oxidation enzymes...

Long-chain fatty acids also cause uncoupling. The mechanism appears complex, but seems to be distinct from either simple membrane disruption or a classical protonophoric effect, and appears to require the involvement of active transporters [9, 14, 19, 21-24). 

FIGURE 13.2. Effects of FC on respiration of rat liver MCh. Dependence of respiration rate activated by ADP (V3), calcium transport (Vca), and protonophore CCCP (Vcccp) upon concentration of FC. Substrates pymvate plus malate. 

The above reasoning was recently confirmed by several pieces of indirect evidence. It was found that in H. halobium (i) protonophorous uncouplers decrease A/in and cause a repellent effect [45], (ii) cyanide and DCCD have no effect on the photoresponse of a mutant which possesses sensory rhodopsins but no bacteriorhodopsin [48], and (iii) in a similar mutant, A l is not involved in photosensing [49]. 

Perhaps it is the Na cycle that is responsible for the effect observed by Michel and Oesterhelt[109] (see also [110]) who reported that the protonophorous uncoupler abolishes A/Ih+ at much lower concentrations than those affecting the phosphate potential. 

Heterodisulfide (CoM-S-S-HTP) reduction - coupled to primary translocation. The reaction in which ATP is synthesized during methanol reduction to CH4 could be identified with energetically competent membrane vesicles of the methanogenic strain G61. These vesicles, which are orientated more than 90% inside-out, catalyzed CH4 formation from CH3-S-C0M by reduction with H2 (Reactions 7,8) and coupled this process with the synthesis of ATP [112]. CH3-S-C0M reduction generated a ApH (inside acidic) as monitored by acridine dye quenching protonophores and ATP synthase inhibitors exerted their effects in accordance with a chemiosmotic type of ATP synthesis [113],... 

This reaction was found to be coupled with the synthesis of ATP. The effects of protonophores and the H -ATP synthase inhibitor DCCD on the rate of CO oxidation, on the electrochemical proton potential and on the ATP content of the cells indicate A/IH -driven ATP synthesis by a H -translocating DCCD-sensitive ATP synthase [233] ... 

Fromenty B, Fisch C, Berson A, Letteron P, Larrey D, Pessayre D (1990b) Dual effect of amiodarone on mitochondrial respiration. Initial protonophoric uncoupling effect followed by inhibition of the respiratory chain at the levels of complex I and complex II. J Pharmacol Exp Ther 255 1377-1384... 

It is unlikely that the effect of valinomycin is due to a conductance change which would stimulate an electrogenic pump, since protonophores like TCS have much less effect than ionophores and lipid-permeable cations do not substitute for these ionophores [58]. The effect of K -ionophores must be attributed to a requirement of the pump enzyme for internal K" ". Under certain conditions transport could be shown to be driven by a large gradient in the absence of ATP [66],... 

Many different protonophores, each with a different effectiveness, are available like 2,4-dinitrophenol (DNP) or 5-chloro-3-tert-butyl-2 -chloro-4 -nitro-salicyl-anilide (CCCP). Manipulation of the two components of the A/Ih can be effected with two potassium ionophores. Valinomycin increases the electrogenic permeability of a membrane for potassium and leads to the dissipation of the if a high concentration of potassium is present (more than 10 mM). Nigericin catalyzes an electroneutral potassium proton exchange and thus dissipates the A pH under the same conditions. In combination these two ionophores function as an uncoupler. 

However, many lipophilic carboxylic acids satisfy the log P and pKa criteria for uncoupling, but the charge on the anion cannot be delocalized beyond the carboxyl group and so they do not act as effective protonophoric uncouplers. Those that do have structural features that allow the charge to be delocalized include the anacardic acids, where the adjacent phenol stabilizes the negative charge (Fig. 13.4.4a) [101]. A similar intramolecular interaction occurs in the salicylanilide class of uncouplers, of which one of the most active is S-13 (11, Fig. 13.4.4b). 

The activity of polysulfide sulfur respiration in cells of VF. succinogenes or in proteoliposomes is decreased to 30% upon the addition of a protonophore [6]. This effect can be explained if it is assumed that the activity is limited by the amount of MMbH in the absence of a Ap, and that MMbH dissociates from... 

The mechanism(s) by which these effects are manifested has yet to be fully elucidated however, the most favored proposal is a protonophore... 

It is not inconceivable that such permeability effects are a result of an initial direct effect of herbicide binding to ATPase or an electron transport chain component and/or a protonophore action. However, most, if not all, the herbicides discussed in this section can be classed as multisite inhibitors as evidenced by their inclusion in previous chapters, particularly Chapter 1, and elsewhere in this chapter. Such a membrane partitioning would not be specific to the inner mitochondrial membrane but would occur in other... 

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