GRAMICIDIN AND ION TRANSPORT IN ISOLATED LIVER MITOCHONDRIA

The use of membrane vesicles in transport studies

Transport-competent plasma membrane vesicles isolated from mammalian cells provide a system to investigate mechanisms and regulation of nutrient and ion transport systems. The characteristics of membrane vesicle systems to study transport in erythrocytes, renal and epithelial membranes, Ehrlich ascites cells, and mouse fibroblasts are discussed. Studies of Na+-stimulated and Na+-independent amino acid and glucose transport in these systems are evaluated, with emphasis on experimental verification of concepts stated in the Na+ gradient hypothesis. Nucleoside, phosphate, and calcium transport systems in plasma membrane vesicles from mouse fibroblast cultures are discussed. Also, current biochemical approaches to investigate mechanisms of regulation of nutrient transport systems by hormones or cellular proliferative state are described.

The respiration of brain mitochondria and its regulation by monovalent cation transport

The Na+ and K+ permeability properties of rat brain mitochondria were determined to explain the influences of these cations upon respiration. A new procedure for isolating exceptionally intact mitochondria with minimal contamination by synaptosomes was developed for this purpose. Respiration was uncoupled by Na+ and less so by K+. Uncoupling was maximal in the presence of EDTA plus Pi and was decreased by Mg2+. Maximal uncoupler-stimulated respiration rates were inhibited by Na+ but largely unaffected by K+. The inhibition by Na+ was relatively insensitive to Mg2+. Membrane Na+ and K+ conductances as well as neutral exchanges (Na+/H+ and K+/H+ antiport activities) were determined by swelling measurements and correlated with metabolic effects of the cations. Cation conductance, i.e. electrophoretic Na+ or K+ permeation, was increased by EDTA (Na+ greater than K+) and decreased by Mg2+. Magnesium preferentially suppressed Na+ conductance so as to reverse the cation selectivity (K+ greater than Na+). Neutral cation/H+ exchange rates (Na+ greater than K+) were not influenced by chelator or Mg2+. The extent of cation-dependent uncoupling of respiration correlated best with the inner membrane conductance of the ion according to an empirical relationship derived with the model K+ conductor valinomycin. The metabolic influences of Na+ and K+ can be explained in terms of coupled flow of these ions with protons and their effect upon the H+ electrochemical gradient although alternative possibilities are discussed. These in vitro studies are compared to previous observations in situ to assess their physiological significance.

Phosphate transport and proteins with SH groups in rat liver mitochondria

Phosphate transport in rat liver mitochondria was studied by following [32P] phosphate uptake within physiological concentrations. Transport inhibition due to mersalyl and protection by mersalyl against N-ethylmaleimide measured in those conditions corresponded to earlier results obtained by the swelling technique. When mitochondria were incubated with [3H] N-ethylmaleimide in the presence of mersalyl, the radioactive labeling in proteins of particles obtained after sonication was decreased in all fractions, but three proteins were both highly alkylated and also highly protected by mersalyl (M.W. 48,000 - 36,000 - 31,000). Two of these (M.W. 36,000 and 31,000) were partially purified by ultrogel chromatography in the presence of sodium dodecyl sulfate. Furthermore, it was shown that both phosphate and nigericin diminished labeling by N-ethylmaleimide in the final supernatant fraction. Two proteins (M.W. 98,000 and 31,000) were significantly alkylated by [3H] N-ethylmaleimide and protected by phosphate and nigericin.

Comparison of the effect of linear gramicidin analogues on bacterial sporulation, membrane permeability, and ribonucleic acid polymerase

Various analogues of linear gramicidin were tested for their biological activity in restoring the normal spore phenotype of gramicidin-negative mutants of Bacillus brevis and for their ability to increase cation conductivity of black lipid membranes and to inhibit bacterial RNA polymerase. Whereas many biologically active gramicidin analogues had no effect on membrane permeability, all biologically active peptides were able to inhibit ribonucleic acid (RNA) polymerase. These observations make it unlikely that membranes are the site of action of gramicidin during bacterial sporulation, but they are consistent with the notion that gramicidin functions to control RNA synthesis during the transition from vegetative growth to sporulation (Sarkar & Paulus, 1972). The relationship between peptide structure and the ability to restore normal sporulation and inhibit RNA polymerase showed that the eight amino-terminal residues have little influence on the function of gramicidin, whereas the highly nonpolar repeating sequence D-leucyl-L-tryptophan is essential for biological activity and may represent the site of interaction with RNA polymerase.

Conformation of gramicidin A channel in phospholipid vesicles: a 13C and 19F nuclear magnetic resonance study

We have determined the conformation of the channel-forming polypeptide antibiotic gramicidin A in phosphatidylcholine vesicles by using 13C and 19F NMR spectroscopy. The models previously proposed for the conformation of the dimer channel differ in the surface localization of the NH2 and COOH termini. We have incorporated specific 13C and 19F nuclei at both the NH2, and COOH termini of gramicidin and have used 13C and 19F chemical shifts and spin lattice relaxation time measurements to determine the accessibility of these labels to three paramagnetic NMR probes--two in aqueous solution and one attached to the phosphatidylcholine fatty acid chain9 all of our results indicate that the COOH terminus of gramicidin in the channel is located near the surface of the membrane and the NH2 terminus is buried deep within the lipid bilayer. These findings strongly favor an NH2-terminal to NH2-terminal helical dimer as the major conformation for the gramicidin channel in phosphatidylcholine vesicles.

Partial purification and characterization of the phosphate transporter from bovine heart mitochondria

A highly active phosphate transporter was extracted with octylglucoside from bovine heart submitochondrial particles that were first partially depleted of other membrane components. It was then partially purified by ammonium sulfate fractionation. After reconstitution of the transporter into liposomes prepared with a crude mixture of soybean phospholipids, the Pi/OH exchange, but not the Pi/Pi exchange, was stimulated three- to fourfold by valinomycin and nigericin in the presence of K+. Both Pi/OH and Pi/Pi exchange activities were sensitive to mercurials and other SH reagents. The rutamycin-sensitive ATPase complex from mitochondria was reconstituted together with the phosphate transporter and adenine nucleotide transporter into liposomes. After inhibition of externally located ATPase, the hydrolysis of ATP was sensitive to atractyloside and mersalyl.

Mitochondrial phosphate transport and the N-ethylmaleimide binding proteins of the inner membrane

Mitochondria have been prepared from the flight muscles of mature blow-flies (Sarcophaga bullata). Phosphate transport by these mitochondria, determined by rates of passive swelling in ammonium phosphate, is sensitive to inhibition by N-ethylmaleimide. 20 nmol of N-ethylmaleimide/nmol cytochrome A inhibit the swelling by 90%. When the mitochondria are inhibited by N-[3H]ethylmaleimide, then solubilized in dodecyl sulfate/mercaptoethanol at 100 degrees C and then electrophoresed on dodecyl sulfate-polyacrylamide gels, many labeled protein bands can be detected, including a large labeled peak that has the same mobility as the tracking dye, bromophenol blue. Sonic submitochondrial particles that are prepared from the N-[3H]ethylmaleimide-labeled mitochondria, solubilized, and electrophoresed on dodecyl sulfate-polyacrylamide gels, possess only seven major labeled protein bands with no radioactive peak at the tracking dye. These labeled proteins have molecular weights of 71, 68, 64, 45, 32, 30, and approx. 10 . 10(3). The nmol N-[3H]ethylmaleimide bound to each of these proteins per nmol cytochrome A are 0.15, 0.19, 0.35, 0.45, 0.87, 0.10, and 0.17, respectively, when the mitochondria are inhibited with 21.5 mol N-[3H]ethylmaleimide/mol cytochrome a at 10 micron cytochrome A. Coty and Pedersen (1975) J. Biol. Chem. 250, 3515-3521) sensitized rat liver mitochondria to N-[3H]ethylmaleimide and identified five labeled proteins. Only the labeled 32 . 10(3) dalton and the 45 . 10(3) dalton proteins are common to both systems.

Ion movements during energy-linked mitochondrial structural changes

The structure of isolated rat liver mitochondria has been observed in the electron microscope following incubation of the mitochondria in vitro under a variety of conditions. The results show that ultrastructural changes are only associated with the energization and deenergization of isolated mitochondria if the composition of the incubation medium permits ion movements in or out of the matrix. The mechanism of energy coupling does not appear to depend on these major mitochondrial structural changes. The addition of low levels of valinomycin greatly increases the rate at which the matrix compartment swells and shrinks on energization and deenergization even at low K+ concentrations.

Biochemical studies on peptide alkaloids: induction of ion selective mitochondrial swelling

The effect of frangulanine, a cyclopeptide alkaloid isolated from Hovenia dulcis Thunb., on mitochondrial swelling was studied. Frangulanine induced mitochondrial swelling in 0.15 M KCl solution at the concentration of 6.5 muM. The alkaloid showed ion selectivity on the induction of mitochondrial swelling. Upon addition of frangulanine, mitochondria underwent swelling in 0.15 M KCl or RbCl solution but in neither NaCl nor LiCl solution.

Biological function of gramicidin: selective inhibition of RNA polymerase

This paper describes a novel biochemical effect of gramicidin, a class of peptide antibiotics produced by Bacillus brevis during the transition from vegetative growth to sporulation. Gramicidin inhibits RNA synthesis by purified RNA polymerase (nucleosidetriphosphate:RNA nucleotidyl-transferase, EC 2.7.7.6) by interfering with the binding of RNA polymerase to DNA. This effect seems to involve the destabilization of the "open" RNA polymerase-DNA complex, a mode of action consistent with the control of promoter selection. Selectivity in the inhibition of RNA synthesis by gramicidin is observed when transcription is partially blocked by low levels of actinomycin D. Since the inhibition of RNA synthesis by gramicidin is obtained in a highly purified system devoid of membranes, it must be distinct from the ionophoretic activity of the antibiotic. It is possible that this new mode of action reflects the function of gramicidin during bacterial sporulation.

Effect of long-chain fatty acids and acyl-CoA on mitochondrial permeability, transport, and energy-coupling processes

The following effects of fatty acids and acyl-CoA thioesters on energy metabolism of mitochondria can now be assumed: (1) Inhibition of adenine nucleotide translocation. This effect may increase the energy state of mitochondria respiring under state 3 conditions and decrease phosphorylation potential in the surrounding medium (the cytoplasm). (2) Increased permeability to monovalent cations. This may lead to a partial energy dissipation due to a futile recycling of K+ (or another cation), namely and energy-dependent uptake and a passive outflow. (3) True uncoupling due to increased permeability to protons. This effect probably occurs at high concentrations of fatty acids only. (4) Substrate effect. Fatty acids in the form of acyl-CoA are excellent respiratory substrates for mitochondria of most tissues. Their oxidation is coupled to the generation of high energy state of the mitochondrial membrane and, consequently, to ATP synthesis.

The uptake and extrusion of monovalent cations by isolated heart mitochondria

The factors involved in the movement of monovalent cations across the inner membrane of the isolate heart mitochondrion are reviewed. The evidence suggests that the energy-dependent uptake of K+ and Na+ which results in swelling of the matrix is an electrophoretic response to a negative internal potential. There are no clear cut indications that this electrophoretic cation movement is carrier-mediated and possible modes of entry which do not require a carrier are examined. The evidence also suggests that the monovalent cation for proton exchanger (Na+ greater than K+) present in the membrane may participate in the energy-dependent extrusion of accumulated ions. The two processes, electrophoreti c cation uptake (swelling) and exchange-dependent cation extrusion (contraction) may represent a means of controlling the volume of the mitochondrion within the functioning cell. A number of indications point to the possibility that the volume control process may be mediated by the divalent cations Ca+2 and Mg+2. Studies with mercurial reagents also implicate certain membrane thiol groups in the postulated volume control process.

Special features of Pi transport in pig heart and rat liver mitochondria as revealed by 6,6'-dithiodinicotinic acid (CPDS)

CPDS (6,6'-dithiodinicotinic acid), a non permeant thiol agent which affects several mitochondrial functions in a way different to that of mersalyl [18-19] revealed striking differences between the phosphate translocating systems of pig heart and rat liver mitochondria. Pi entry was measured either by swelling in 0.12 M ammonium phosphate or by rapid centrifugation in 32Pi medium. Pi efflux was measured after preloading of mitochondria with 32Pi, by exchange against Pi or malate; the "ATP-FCCP" system has been tested previously [19]. In pig heart mitochondria, Pi entry seems to proceed exclusively via the Pi/OH- carrier; CPDS completely inhibits this transport and the energy-linked functions. In contrast n-butyl-malonate does not affect the Pi-entry and the energy-linked functions. The Pi efflux is not affected either by CPDS or mersalyl, which do not produce a swelling in the "ATP-uncoupler system". In rat liver mitochondria, CPDS inhibits only the Pi/OH- carrier; both CPDS and n-butylmalonate are necessary to inhibit completely Pi entry. CPDS as well as mersalyl provokes a swelling in the presence of the "APT-uncoupler system". The results suggest two distinct functions of phosphate transport in both types of mitochondria.

Interaction between membrane properties and proteins synthesis in reticulocytes - a two step inhibition of protein synthesis by valinomycin

In this work we consider the differential effect of Valinomycin used at different concentrations both on the protein synthesis of reticulocytes and on 42K exchange. We demonstrate that there is a two step action of this antibiotic. At 10(-6)M and below the drug has no effect on the 42K exchange, but it stops, however reversibly, protein synthesis. At 10(-5)M the drug has a very sharp action on the 42K exchange and stops protein synthesis in an irreversible way. Ribosomal population checked by two ways, sucrose gradient and direct counting on E.M. sections shows that at low concentrations of Valinomycin (10(-8)M to 10(-6)M) there is no breakdown of the polysomes which can be detected by either one of these methods. On the contrary, after short incubation with 10(-5)M of Valinomycin the breakdown of ribosomes is very clear, as evidenced by sucrose gradient analysis. By direct ribosomes clusters counting on E.M. sections this breakdown is seen only after long incubation.