Anionic uncouplers

Mild ATP depletion due to anionic uncouplers can be further aggravated by MPT. This has been shown with carbonylcyanide-p-trifluoromethoxyphenyl hydra-zone (Costantini et al. 1996 Catisti and Vercesi 1999), diclofenac (Masubuchi et al. 2002), and nimesulide, at least when the latter is incubated without albumin, which otherwise sequesters nimesulide in the medium and prevents cellular toxicity (Berson et al. 2006). Likewise, the anionic uncouplers salicylic acid and valproic acid facilitate MPT in isolated mitochondria incubated with calcium (Trost and Lemasters 1996). 

A relationship correlating the weak acid uncouplers activity with their A %-, pAi°, and has been presented on the basis of protonophoric theory of uncoupling activity, in which the concentration of anionic ionophore (A ) within a biomembrane is supposed to be controlled by the ionic partition of A at the biomembrane solution interface according to Eq. (28) [19]. The biomembrane solution interface could be polarized or electrogenic [37]. Experimental results on the activities of uncouplers on rat liver mitochondria [30] have been explained reasonably [19,24]. 

Mitochondria do three things oxidize substrates, consume oxygen, and make ATP. Uncouplers prevent the synthesis of ATP but do not inhibit oxygen consumption or substrate oxidation. Uncouplers work by destroying the pH gradient. The classic uncoupler is dinitrophenol (DNP). This phenol is a relatively strong acid and exists as the phenol and the phenolate anion. 

The relationship between the different conductances of the glutamate transporter is not well-understood. The structural basis for these conductances and whether the anions and glutamate permeate the same pore of the transporter protein is not known. Sonders and Amara (40) proposed two models a single pathway for substrate and the uncoupled anion movement (Fig. 3A), or multiple permeation pathways in a single transporter molecule (Fig. 3B) (40). This idea will be discussed further in relation to what is known about the quaternary structure of glutamate transporters. 

Metabolic acidosis follows, and an increased anion gap results from accumulation of lactate as well as excretion of bicarbonate by the kidney to compensate for respiratory alkalosis. Arterial blood gas testing often reveals this mixed respiratory alkalosis and metabolic acidosis. Body temperature may be elevated owing to uncoupling of oxidative phosphorylation. Severe hyperthermia may occur in serious cases. Vomiting and hyperpnea as well as hyperthermia contribute to fluid loss and dehydration. With very severe poisoning, profound metabolic acidosis, seizures, coma. 

The fact that the anions of the uncouplers are large, often aromatic, and therefore soluble in the lipid bilayer supports this interpretation the protonated uncouplers can diffuse into the mitochondria and the anion can diffuse back out. Mitochondria can also be uncoupled by a combination of ionophores, e.g., a mixture of valinomycin (Fig. 8-22), which carries K+ into the mitochondria, plus nigericin, which catalyzes an exchange of K+ (out) for H+ (in).172... 

Many compounds that uncouple electron transport from phosphorylation, like 2,4-dinitrophenol, are weak acids. Their anions are nucleophiles. According to the scheme of Fig. 18-12, they could degrade a high energy intermediate, such as Y B, by a nucleophilic attack on Y to give an inactive but rapidly hydrolyzed... 

Tire uncoupling protein resembles the ATP/ADP and phosphate anion carriers (Table 18-8), 1 which all have similar sizes and function as homodimers. Each monomeric subunit has a triply repeated 100-residue sequence, each repeat forming two transmembrane helices. Most mitochondrial transporters carry anions, and UCP1 will transport Cl. h/1 However, the relationship of chloride transport to its real function is unclear. Does the protein transport H+... 

---