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V Shoshan

Publications and source records attributed to V Shoshan.

18 recordsLinked to original sources
PubMed1988

Renal cortical mitochondrial transport of calcium in chronic uremia.

Calcium overload of tubular cells may occur in uremia, and may be the underlying functional abnormality in the continued deterioration of renal function in chronic renal failure. In order to study this question further, the effect of chronic uremia on the calcium transport properties and respiratory rates was examined in mitochondria (Mi) isolated from the cortex of the remnant kidneys of subtotally nephrectomized rats (SNX) and sham operated controls (C). Plasma calcium concentration was similar in both groups of rats, but a significant hyperphosphatemia was seen in SNX, 8.6 +/- 0.6 mg%, as compared to 7.2 +/- 0.2 mg% in C (P less than 0.001). Mi calcium and phosphate concentrations (nmol/mg protein) were significantly elevated in SNX, 49.9 +/- 7.9 and 35.1 +/- 4.2, respectively, in SNX compared to C, 21.2 +/- 4.2 and 21.4 +/- 2.7, respectively (P less than 0.01). Mi respiratory control ratio and ADP/O were similar in both experimental groups. Kinetic parameters for calcium uptake (Ca2+ concentrations in the medium of 1.25 to 16 microM) revealed initial velocities 1.5-fold higher in SNX Mi than in C. Mi retention of calcium in the presence of medium Ca2+ concentrations up to 500 microM was studied. Calcium retention was reduced in SNX: the Mi were unable to retain calcium at concentrations of 250 microM. The addition of ruthenium red to the medium substantially improved calcium retention by the uremic Mi. Chronic parathyroidectomy did not correct either the increased calcium uptake or the poor retention of uremic Mi.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed1983

Tetraphenylboron causes Ca2+ release in isolated sarcoplasmic reticulum and in skinned muscle fibers.

The lipophilic anion tetraphenylboron (TPB-) but not the lipophilic cation tetraphenylarsonium (TPA+) inhibited ATP-dependent Ca2+ accumulation by isolated sarcoplasmic reticulum. TPB- did not inhibit ATP hydrolysis but did induce Ca2+ release from preloaded vesicles. It did not appear to disrupt lipid bilayers or to act as a Ca2+ ionophore since it had no effect on the Ca2+ content of phospholipid vesicles. TPB- also induced Ca2+ release from sarcoplasmic reticulum in chemically skinned muscle fibers causing tension development. In contrast to other Ca2+-releasing agents such as caffeine, proton ionophores, or quercetin, the rise to peak tension was slow and tension was sustained, suggesting that Ca2+ release channels, once opened by TPB-, were held open as long as the compound was present in the membrane. Ca2+ uptake was re-established upon removal of TPB- or addition of TPA+. TPB- or TPA+ would probably distribute within the membrane, altering surface charges on both sides of the membrane. The fact that only a negatively charged ion brought about opening of Ca2+ release channels suggests that specific surface charges control Ca2+ release channels in sarcoplasmic reticulum. Although we have not been able to prove that TPB- acts exclusively on physiologically relevant Ca2+ release channels, we have shown that TPB- does not release Ca2+ from proteoliposomes reconstituted with the Ca2+ + Mg2+ ATPase. Thus TPB- does not induce Ca2+ release through channels formed by the ATPase molecule.

Animals↗
PubMed1982

Studies of Ca2+ release from sarcoplasmic reticulum.

Our studies of Ca2+ release have shown that three classes of compounds--quercetin, which inhibits Ca2+ release by reversal of the Ca2+ pump, H+ ionophores and DCCD, which affect proton gradients, and lipophilic anions, which act to oppose a separation of membrane surface charges--all have potential in probing the mechanism of Ca2+ release from sarcoplasmic reticulum.

Animals↗
PubMed1981

Quercetin interaction with the (Ca2+ + Mg2+)-ATPase of sarcoplasmic reticulum.

In sarcoplasmic reticulum vesicles or in the (Ca2+ + Mg2+)-ATPase purified from sarcoplasmic reticulum, quercetin inhibited ATP hydrolysis, Ca2+ uptake, ATP-Pi exchange, ATP synthesis coupled to Ca2+ efflux, ATP-ADP exchange, and steady state phosphorylation of the ATPase by inorganic phosphate. Steady state phosphorylation of the ATPase by ATP was not inhibited. Quercetin also inhibited ATP and ADP binding but not the binding of Ca2+. The inhibition of ATP-dependent Ca2+ transport by quercetin was reversible, and ATP, Ca2+, and dithiothreitol did not affect the inhibitory action of quercetin.

Adenosine Diphosphate↗
PubMed1981

A proton gradient controls a calcium-release channel in sarcoplasmic reticulum.

Sarcoplasmic reticulum vesicles from mammalian skeletal muscle have previously been shown to develop a proton gradient (alkaline inside) of 0.15-0.5 pH units during active Ca2+ uptake. We found that dissipation of this gradient by the proton ionophores gramicidin, nigericin, and carbonyl cyanide p-trichloromethoxyphenylhydrazone caused a rapid transient tension in skinned rabbit psoas muscle fibers. Increases, but not decreases, in medium pH of approximately 0.2 units over the range from pH 6.5 to pH 7.5 also elicited transient tensions. In isolated vesicles, physiological levels of Ca2+ (3.3 microM), inhibited pH-induced Ca2+ release. Dicyclohexylcarbodiimide blocked pH- and ionophore-induced Ca2+ release under conditions in which it could bind to sarcoplasmic reticulum proteins but did not inhibit Ca2+ uptake. We propose that a proton gradient generated across sarcoplasmic reticulum membranes during Ca2+ uptake maintains a Ca2+ release channel in a closed conformation and that dissipation of this gradient permits the Ca2+ release channel to open. We further propose that elevated myoplasmic Ca2+ also causes the Ca2+ channel to close, permitting Ca2+ uptake through Ca2+/Mg2+-ATPase to function effectively. As the proteolipids of sarcoplasmic reticulum bind dicyclohexylcarbodiimide under conditions in which Ca2+ release is blocked and as they have previously been shown to have Ca2+ ionophoric activity, we propose that the Ca2+-release channel either resides in the proteolipids or is controlled by H+ fluxes through the proteolipids.

Animals↗
PubMed1980

Quercetin interaction with the chloroplast ATPase complex.

1. Quercetin, a flavonoid which acts as an energy transfer inhibitor in photophosphorylation is shown to inhibit the P-ATP exchange activity of membrane-bound CF1 and the ATPase activity of isolated CF1. Quercetin, affects also the proton uptake in chloroplasts in a manner similar to that of dicyclohexylcarbodiimide. 2. The light-dependent proton uptake in EDTA-treated chloroplasts is stimulated by quercetin. In untreated chloroplasts quercetin has a dual effect: it enhances at pH above 7.5 while at lower pH values it decreases the extent of H+ uptake. Similar effects were obtained with dicyclohexylcarbodiimide. 3. Like quercetin, dicyclohexylcarbodiimide was also found to inhibit the ATPase activity of isolated CF1. 4. Quercetin inhibits uncoupled electron transport induced by either EDTA-treatment of chloroplasts or by addition of uncouplers. Quercetin restores H+ uptake in both types of uncoupled chloroplasts. 5. The mode of action of quercetin and dicyclohexylcarbodiimide in photophosphorylation is discussed, and interaction with both CF1 and F0 is suggested.

Adenosine Triphosphatases↗
PubMed1980

The effect of phosphate on light-induced exchange of ADP at the tight nucleotide binding site of CF1.

The effects of phosphate on the energy-dependent exchange of the tightly bound adenine nucleotides in chloroplast thylakoids has been studied. 1. Phosphate was found to have a dual effect on ADP binding; at low light intensities Pi enhanced the rate of binding and the total amount of tightly bound ADP, while at saturating light intensities, it decreased the level of bound nucleotides. Half-maximal stimulation or inhibition of ADP binding occurred at about 30 microM phosphate. 2. Phosphate inhibition of ADP binding at saturating light intensities is prevented in the presence of an ADP-regenerating system. Pi had no effect on the amount of ATP bound to chloroplast coupling factor 1 (CF1), which was lower than the amount of ADP bound. 3. Arsenate acted similarly to phosphate at low light intensities but not at saturating light intensities. 4. At low light intensities the interaction of phosphate with the membrane-bound CF1 increased the binding affinity for ADP about 8-fold. 5. Kinetic analysis of the ADP binding showed that phosphate increased the rate constant for ADP binding to the adenine nucleotide-depleted form of CF1.

Adenosine Diphosphate↗
PubMed1980

The interaction of N,N'-dicyclohexylcarbodiimide with chloroplast coupling factor 1.

1. Incubation of soluble spinach Coupling Factor 1 (CF1) with dicyclohexylcarbodiimide (DCCD) results in the inactivation of the ATPase. The DCCD inactivation is time- and concentration-dependent. Complete inactivation of the CF1-ATPase activity requires the binding of 2 mol of DCCD/mol of CF1. The binding sites of DCCD are located on the beta subunit of CF1. 2. DCCD modification of soluble CF1 eliminates one adenine nucleotide binding site which is exposed by dithiothreitol activation or by incubation with tentoxin. The inactivation of both the ATPase activity and the adenine nucleotide binding site are pH-dependent. The inactivation of both the ATPase activity and the adenine nucleotide binding site are pH-dependent. Half-maximal inhibition occurs at about pH 7.5. 3. The DCCD-modified CF1, reconstituted with EDTA-treated chloroplasts, is fully active is restoring proton uptake but not in restoring ATP synthesis or light-dependent adenine nucleotide exchange.

Adenosine Triphosphatases↗
PubMed1980

Quercetin inhibits Ca2+ uptake but not Ca2+ release by sarcoplasmic reticulum in skinned muscle fibers.

Quercetin inhibited Ca2+-dependent ATP hydrolysis, ATP-dependent Ca2+ uptake, chelator-induced [ethylene glycol bis(beta-aminoethyl ether)-N,N,N',N'-tetraacetic acid] Ca2+ release, and ATP synthesis coupled to Ca2+ release in isolated vesicles of sarcoplasmic reticulum. Use of this inhibitor permitted evaluation of whether Ca2+ release from sarcoplasmic reticulum in situ occurs through a reversal of the uptake pathway. Release of Ca2+ from the sarcoplasmic reticulum of skinned muscle fibers can be detected by the measurement of tension in the fiber. If the sarcoplasmic reticulum of these preparations is first allowed to accumulate Ca2+, tension development may be induced by the addition of Ca2+ itself or of caffeine to the bathing medium or by depolarization with Cl-. The presence of quercetin during the loading phase inhibited Ca2+ uptake by sarcoplasmic reticulum in situ. When quercetin was added together with initiators of tension development, however, the rate of tension development was enhanced 4- to 7-fold and the relaxation rate of the fibers was greatly inhibited. These results suggest that quercetin had no effect on Ca2+ release in skinned fiber; its effect on Ca2+ reuptake could account for the apparent enhancement of the release rate and for the prolonged relaxation time. These observations rule out reversal of the Ca2+ pump as the mechanism of Ca2+ release in situ.

Animals↗
PubMed1980

Ion pathways in proteins of the sarcoplasmic reticulum.

In summary, we have begun to characterize three different ion pathways in the sarcoplasmic reticulum. Ca2+-ionophoric activity has been traced to a 13,000-dalton CNBr fragment localized at the amino terminus of the ATPase molecule... The pathway involved in Ca2+ release can be distinguished from the pathway involved in Ca2+ uptake by its insensitivity to quercetin. An anion pathway is sensitive to DIDS and appears to be localized in the ATPase molecule

Amino Acid Sequence↗
PubMed1979

ATP synthesis and hydrolysis in chloroplast membranes. Differential inhibition by antibodies to chloroplast coupling factor 1.

1. Divalent antibodies against chloroplast coupling factor 1 inhibited the factor ATPase, ATP synthesis, hydrolysis and Pi-ATP exchange in chloroplasts. These antibodies also inhibited coupled electron flow rates but not the basal or uncoupled rates. 2. Several types of non-precipitating, modified antibodies prepared from the original antibody preparation strongly inhibited the ATPase and Pi-ATP exchange reaction but had little effect on ATP formation. 3. It is suggested that the inhibition of ATP synthesis by the divalent antibodies is probably due to an indirect blocking of the active site, while the inhibition of ATP-utilizing reactions by the modified antibodies is related to their effect on the transfer of ATP from a non-catalytic to a catalytic site on coupling factor 1, via an energy-dependent conformational change.

Adenosine Triphosphate↗
PubMed1978

Kinetics of nucleotide binding to chloroplast coupling factor (CF1).

Studies of the kinetics of association and dissociation of the formycin nucleotides FTP and FDP with CF1 were carried out using the enhancement of formycin fluorescence. The protein used, derived from lettuce chloroplasts by chloroform induced release, contains only 4 types of subunit and has a molecular weight of 280 000. In the presence of 1.25 mM MgCl2, 1 mol of ATP or FTP is bound to the latent enzyme, with Kd = 10(-7) or 2 . 10(-7), respectively. The fluorescence emission (lambdamax 340 nm) of FTP is enhanced 3-fold upon binding, and polarization of fluorescence is markedly increased. The fluorescence changes have been used to follow FTP binding, which behaves as a bimolecular process with k1 = 2.4 . 10(4) M-1 . s-1. FTP is displaced by ATP in a process apparently involving unimolecular dissociation of FTP with K-1 = 3 . 10(-3) S-1. The ratio of rates is comparable to the equilibrium constant and no additional steps have been observed. The protein has 3 sites for ADP binding. Rates of ADP binding are similar in magnitude to those for FTP. ADP and ATP sites are at least partly competitive with one another. The kinetics of nucleotide binding are strikingly altered upon activation of the protein as an ATPase. The rate of FTP binding increases to at least 10(6) M-1 . s-1. This suggests that activation involves lowering of the kinetic barriers to substrate and product binding-dissociation and has implications for the mechanism of energy transduction in photophosphorylation.

Adenosine Diphosphate↗