Electroneutral exchange

The H,K-ATPase, expressed in the parietal cells of the stomach, transports H+ ion from cytoplasm to lumen in exchange for extracytoplasmic K+ ion in an electroneutral exchange using the energy of ATP hydrolysis. 

Electrogenic antiport, ADP uptake favored by the membrane potential can also mediate electroneutral exchange of ADPout for... 

Transfer of citrate through the inner membrane of MCh is provided by a tricarboxylate transporter (m.w. 32.5 kD), which also catalyzes transport of treo-Ds-isocitrate, cis-aconitate and other tricarboxylates (LaNoue and School-werth, 1979 Kaplan et al, 1990). This is electroneutral exchange for either another tricarboxylate or dicarboxylate (e.g. malate or succinate), or for phosphoenolpyruvate. Formation of glutathione-citryl thioester is irreversibly inhibited by (-)erythrofluorocitrate (IC50 = 25 pmol FC/mg protein), which makes a stable adduct with the synthase (Kun et al, 1977). However, the block of citrate transport... 

The activity of this transporter which catalyzes net movements of adenine nucleotides is mentioned here in connection with the electrogenic adenine nucleotide exchange carrier, although it is likely to be a proton-compensated electroneutral exchanger. No counter ion has been identified, nor have compensating proton movements been reported in conjunction with the net nucleotide transport. Also, no influence of ApH on steady state total nucleotide levels has been reported. 

Other electroneutral exchange transporters. Although it may be tempting to discount the Sluse studies due to the kinetic complexity of the data obtained and obvious technical problems, other workers studying exchange transporters have concluded that substrate binding sites on either side of the membrane are independent of each other and that the ratio is not a constant. Variation of the... 

When the resting parietal cell is stimulated by acid secretagogues, the tubulovesicles are transformed into the secretory canaliculus. The parietal cell has the largest mitochondrial content of any mammalian cell (—34% of cell volume) and the ATP generated by this is mainly used for acid secretion. Hydrolysis of ATP results in a conformational change in the protein that mediates the electroneutral exchange of intracellular and extracellular K+. The pump is activated only when it is associated with a potassium chloride pathway in the canalicular membrane (Fig. 3.6). This allows potassium chloride efflux into the extra-cytoplasmic space and thus results in the secretion of HCl at the expense of ATP... 

The last mediator of gastric secretion in the parietal cell is an H+,K+-ATPase (proton or acid pump) which is a member of the phosphorylating class of ion transport ATPases. Hydrolysis of ATP results in ion transport. This chemical reaction induces a conformational change in the protein that allows an electroneutral exchange of cytoplasmic H+ for K+. The pump is activated when associated with a potassium chloride pathway in the canalicular membrane which allows potassium chloride efflux into the extracytoplasmic space, and thus results in secretion of hydrochloric acid at the expense of ATP breakdown. The activity of the pump is determined by the access of K+ on this surface on the pump. In the absence of K+, the cycle stops at the level of the phosphoenzyme [137]. 

Carbamoyl phosphate reacts with ornithine to form citrulline (see Fig. 38.12). The high- energy phosphate bond of carbamoyl phosphate provides the energy required for this reaction, which occurs in mitochondria and is catalyzed by ornithine transcarbamoylase. The product citrulline is transported across the mitochondrial membranes in exchange for cytoplasmic ornithine and enters the cytosol. The carrier for this transport reaction catalyzes an electroneutral exchange of the two compounds. 

Rb efflux [66,68], Calculation of the ratio between Rb efflux and -influx measured under identical circumstances, leads to a value close to one. These findings are all in agreement with the assumption that (K -I- H )-ATPase catalyses an electroneutral exchange process. 

Transport of phosphate, pyruvate and glutamate via their respective translocators is electroneutral and H -coupled. Although in Fig. 1 their transport has been written as cotransport of the anion with H (symport), it is also possible that the actual transport mechanism involves an exchange of the anion with OH (antiport). Experimentally, these two mechanisms cannot be distinguished. The translocators for a-oxoglutarate and malate (the latter is also called the dicarboxylate translocator) also mediate electroneutral exchanges. 

The tricarboxylate translocator mediates an electroneutral exchange between citrate (or isocitrate) and malate. In this case H is involved since citrate must be converted to citrate ". This translocator also transports phosphoenolpyruvate,... 

Both neutral and carboxylic ionophores have been extensively employed as tools for in vitro studies of biological systems (for reviews see refs. 17 and 83). However, because their electroneutral exchange-diffusion mode of transport does not perturb membrane potential, only the carboxylic subclass of ionophores is sufficiently tolerated by intact animals to produce well defined pharmacological responses. 

Waldman BC, Rudnick G. UDP-GlcNAc transport across the Golgi membrane electroneutral exchange for dianionic UMP. Biochemistry 1990 29 44-52. 

Upon illumination chromatophores accumulate Ca (Michels, Konings, 1978) and Na (Hellingwerf et al., 1982). Sodium is transported via an electroneutral exchange system against protons. The system functions optimally at pH 8 and is inactive below pH 7. 

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