Amiloride-sensitive Na+-H+ antiporter in Escherichia coli

Decoupling of the catalytic and transport activities of complex I from Rhodothermus marinus by sodium/proton antiporter inhibitor

The energy transduction by complex I from Rhodothermusmarinus was addressed by studying the influence of 5-(N-ethyl-N-isopropyl)-amiloride (EIPA) on the activities of this enzyme. EIPA is an inhibitor of both Na(+)/H(+) antiporter and complex I NADH:quinone oxidoreductase activity. We performed studies of NADH:quinone oxidoreductase and H(+) and Na(+) translocation activities of complex I from R. marinus at different concentrations of EIPA, using inside-out membrane vesicles. We observed that the oxidoreductase activity and both H(+) and Na(+) transports are inhibited by EIPA. Most interestingly, the catalytic and the two transport activities showed different inhibition profiles. The transports are inhibited at concentrations of EIPA at which the catalytic activity is not affected. In this way the catalytic and transport activities were decoupled. Moreover, the inhibition of the catalytic activity was not influenced by the presence of Na(+), whereas the transport of H(+) showed different inhibition behaviors in the presence and absence of Na(+). Taken together our observations indicate that complex I from R. marinus performs energy transduction by two different processes: proton pumping and Na(+)/H(+) antiporting. The decoupling of the catalytic and transport activities suggests the involvement of an indirect coupling mechanism, possibly through conformational changes.

Biology of the 2Na+/1H+ antiporter in invertebrates

The functional expression of membrane transport proteins that are responsible for exchanging sodium and protons is a ubiquitous phenomenon. Among vertebrates the Na+/H+ antiporter occurs in plasma membranes of polarized epithelial cells and non-polarized cells such as red blood cells, muscle cells, and neurons, and in each cell type the transporter exchanges one sodium for one hydrogen ion, is inhibited by amiloride, and regulates intracellular pH and sodium concentration within tight limitations. In polarized epithelial cells this transporter occurs in two isoforms, each of which is restricted to either the brush border or basolateral cell membrane, and perform somewhat different tasks in the two locations. In prokaryotic cells, sodium/proton exchange occurs by an electrogenic 1Na+/2H+ antiporter that is coupled to a primary active proton pump and together these two proteins are capable of tightly regulating the intracellular concentrations of these cations in cells that may occur in environments of 4 M NaCl or pH 10-12. Invertebrate epithelial cells from the gills, gut, and kidney also exhibit electrogenic sodium/proton exchange, but in this instance the transport stoichiometry is 2Na+/1H+. As with vertebrate electroneutral Na+/H+ exchange, the invertebrate transporter is inhibited by amiloride, but because of the occurrence of two external monovalent cation binding sites, divalent cations are able to replace external sodium and also be transported by this system. As a result, both calcium and divalent heavy metals, such as zinc and cadmium, are transported across epithelial brush border membranes in these animals and subsequently undergo a variety of biological activities once accumulated within these cells. Absorbed epithelial calcium in the crustacean hepatopancreas may participate in organismic calcium balance during the molt cycle and accumulated heavy metals may undergo complexation reactions with intracellular anions as a detoxification mechanism. Therefore, while the basic process of sodium/proton exchange may occur in invertebrate cells, the presence of the electrogenic 2Na+/1H+ antiporter in these cells allows them to perform a wide array of functions without the need to develop and express additional specialized transport proteins. J. Exp. Zool. 289:232-244, 2001.

Mutational analysis of amiloride sensitivity of the NhaA Na+/H+ antiporter from Vibrio parahaemolyticus

The activity of the NhaA Na+/H+ antiporter of Vibrio parahaemolyticus is inhibited by amiloride. We found an amino acid sequence in the NhaA that was identical to a putative amiloride binding domain of the Na+/H+ exchanger in mammalian cells. We constructed mutant NhaAs that had amino acid substitutions in the putative amiloride binding domain by site-directed mutagenesis. These include V62L (Val62 replaced by Leu), F63Y, F64Y, and L65F. Most mutant NhaAs showed decreased sensitivity for amiloride. Among these, the F64Y mutant NhaA showed the least amiloride sensitivity, with a Ki value 7 to 10 times greater than that in the wild type. Thus, the sequence between residues V62 and L65 in NhaA, especially F64, is very important for the inhibitory effect of amiloride on the antiporter.

Inhibition of melibiose transporter by amiloride in Escherichia coli

Amiloride inhibited the active transport of melibiose via the melibiose transporter in Escherichia coli. Since amiloride is known to inhibit the Na+/H+ antiporter which is essential for the establishment of an electrochemical potential of Na+ that is the driving force for melibiose transport, we tested the effect of amiloride on the activity of the melibiose transporter itself. Amiloride inhibited the melibiose counterflow. Also, inhibition of Na+ uptake induced by melibiose influx and some inhibition of H+ uptake induced by melibiose influx were observed. These results indicate that amiloride directly inhibits the melibiose transporter, perhaps by competing with Na+. It seems that the Na+ binding site and the H+ binding site in the melibiose transporter are somehow different from each other judging from the difference in the inhibition pattern of amiloride.

Amiloride and harmaline are potent inhibitors of NhaB, a Na+/H+ antiporter from Escherichia coli

The diuretic drug amiloride is a specific inhibitor of sodium transporting proteins in several cell types. Attempts to inhibit this activity in membrane vesicles derived from various bacteria, did not yield clear results. Therefore, we tested the effect of amiloride and its derivatives on the purified Na+/H+ antiporters of E. coli reconstituted in functional form in proteoliposomes. Whereas NhaA is not inhibited by amiloride, both amiloride and harmaline are potent inhibitors of NhaB with K0.5 of 6 and 15 microM, respectively. The pattern of inhibition by amiloride derivatives is different from that reported for mammalian antiporters but similar to that reported for the Na+/H+ antiporter of D. salina [Katz, A., Kleyman, T.R. and Pick, U. (1994) Biochemistry 33, 2389-2393]. Clonidine is a poor inhibitor (K0.5 = 200 microM) while cimetidine had no effect on the antiporter up to concentration of 1 mM. These new potent inhibitors provide us with important tools for the study of the mechanism of action of NhaB.

Assessment of uncoupling by amiloride analogs

The amiloride analogs N5-methyl-N5-isobutylamiloride, N5-ethyl-N5-isopropylamiloride, and N5,N5-hexamethyleneamiloride are frequently used to investigate NaH exchange on the premise that they are highly specific inhibitors of the NaH-antiporters. We assessed the relative protonophoric activity of these compounds in reconstituted and native membrane vesicles, using acridine orange fluorescence to measure intravesicular pH. All the compounds tested were found to be potent protonophores at concentrations which are normally used to demonstrate inhibition of NaH exchange. Uncoupling was dependent on both the pH of the assay system and the total amount of lipid present. At the pH optima, which lay in a range from 7.5 to 8.5, these amiloride analogs were more potent uncouplers than the classical protonophore carbonyl cyanide m-chlorophenylhydrazone. Therefore, extreme care must be taken in the interpretation of results obtained using these or similar derivatives of amiloride.

Sodium/proton antiport is required for growth of Escherichia coli at alkaline pH

Evidence is presented indicating that Escherichia coli requires the Na+/H+ antiporter and external sodium (or lithium) ion to grow at high pH. Cells were grown in plastic tubes containing medium with a very low Na+ content (5-15 microM). Normal cells grew at pH 7 or 8 with or without added Na+, but at pH 8.5 external Na was required for growth. A mutant with low antiporter activity failed to grow at pH 8.5 with or without Na+. On the other hand, another mutant with elevated antiporter activity grew at a higher pH than normal (pH 9) in the presence of added Na+ or Li+. Amiloride, an inhibitor of the antiporter, prevented cells from growing at pH 8.5 (plus Na+), although it had no effect on growth in media of lower pH values.

Nature of the light-induced h efflux and na uptake in cyanobacteria

We investigated the nature of the light-induced, sodium-dependent acidification of the medium and the uptake of sodium by Synechococcus. The rate of acidification (net H(+) efflux) was strongly and specifically stimulated by sodium. The rates of acidification and sodium uptake were strongly affected by the pH of the medium; the optimal pH for both processes being in the alkaline pH range. Net proton efflux was severely inhibited by inhibitors of adenosine triphosphatase activity, energy transfer, and photosynthetic electron transport, but was not affected by the presence of inorganic carbon (C(i)). Light and C(i) stimulated the uptake of sodium, but the stimulation by C(i) was observed only when C(i) was present at the time sodium was provided. Amiloride, a potent inhibitor of Na(+)/H(+) antiport and Na(+) channels, stimulated the rate of acidification but inhibited the rate of sodium uptake. It is suggested that acidification might stem from the activity of a light dependent proton excreting adenosine triphosphatase, while sodium transport seems to be mediated by both Na(+)/H(+) antiport and Na(+) uniport.

Generation of a transmembrane gradient of Na+ in Methanosarcina barkeri

A transmembrane Na+ gradient was generated by Methanosarcina barkeri during methanogenesis. The intracellular Na+ concentration amounted to approximately one fifth of the extracellular one. A secondary Na+/H+ antiport system was shown to be responsible for Na+ extrusion. This system could be inhibited by amiloride. In the presence of amiloride the delta pH across the cytoplasmic membrane increased and a transmembrane Na+ gradient could neither be generated nor maintained. The possible role of Na+ in the oxidation of methanol to the level of formaldehyde is discussed.