Structure-function relations of the first and fourth extracellular linkers of the type IIa Na+/Pi cotransporter: II. Substrate interaction and voltage dependency of two functionally important sites

Functionally important sites in the predicted first and fourth extracellular linkers of the type IIa Na+/Pi cotransporter (NaPi-IIa) were identified by cysteine scanning mutagenesis (Ehnes et al., 2004). Cysteine substitution or modification with impermeant and permeant methanethiosulfonate (MTS) reagents at certain sites resulted in changes to the steady-state voltage dependency of the cotransport mode (1 mM Pi, 100 mM Na+ at pH 7.4) of the mutants. At Gly-134 (ECL-1) and Met-533 (ECL-4), complementary behavior of the voltage dependency was documented with respect to the effect of cys-substitution and modification. G134C had a weak voltage dependency that became even stronger than that of the wild type (WT) after MTS incubation. M533C showed a WT-like voltage dependency that became markedly weaker after MTS incubation. To elucidate the underlying mechanism, the steady-state and presteady-state kinetics of these mutants were studied in detail. The apparent affinity constants for Pi and Na+ did not show large changes after MTS exposure. However, the dependency on external protons was changed in a complementary manner for each mutant. This suggested that cys substitution at Gly-134 or modification of Cys-533 had induced similar conformational changes to alter the proton modulation of transport kinetics. The changes in steady-state voltage dependency correlated with changes in the kinetics of presteady-state charge movements determined in the absence of Pi, which suggested that voltage-dependent transitions in the transport cycle were altered. The steady-state and presteady-state behavior was simulated using an eight-state kinetic model in which the transition rate constants of the empty carrier and translocation of the fully loaded carrier were found to be critical determinants of the transport kinetics. The simulations predict that cys substitution at Gly-134 or cys modification of Cys-533 alters the preferred orientation of the empty carrier from an inward to outward-facing conformation for hyperpolarizing voltages.     

Effect of arginine modification on kidney brush-border-membrane transport activity.

The effect of phenylglyoxylation on brush-border-membrane functions was studied with membrane vesicles from rat kidney cortex. Na+-gradient-dependent uptake of phosphate, glucose and alanine was inhibited by 65, 88 and 70% by pre-incubation of vesicles with 50 mM-phenylglyoxal for 2 min. The inhibition showed a dependency for alkaline pH. Borate co-operativity in butanedione inactivation was used to prove that inhibition was caused by arginine modification. Intravesicular volumes, alkaline phosphatase, aminopeptidase M and Na+-H+ exchange were not affected by phenylglyoxal treatment. Inhibition of phosphate uptake was studied in more detail and showed that the chemical modification introduced by phenylglyoxal inhibited the overshoot of phosphate uptake caused by the Na+ gradient, and decreased the apparent maximal velocity of the phosphate-transport system in its interaction with Na+. Phosphate uptake measured in the absence of Na+ was not affected by phenylglyoxal. Shunting of the transmembrane electrical potential with K+ and valinomycin had no effect on phenylglyoxal inhibition, proving that the alteration of transmembrane electrical potential could not be responsible for this effect. Phenylglyoxal had no ionophoric effect on the Na+ gradients studied (1-100 mM). Na+ efflux was also unaffected by phenylglyoxal treatment. Na+, harmaline and amiloride were ineffective in protecting against phenylglyoxal inhibition, suggesting that the site modified was not an Na+-binding site. These results indicate the involvement of highly reactive arginine residues in phosphate, glucose and alanine uptake.     

Covalent and non-covalent inhibitors of the phosphate transporter of sarcoplasmic reticulum.

The sarcoplasmic reticulum (SR) of skeletal muscle contains a Pi transporter which transports Pi into the lumen of the SR, increasing the level of accumulation of Ca2+ by SR by forming insoluble salts with Ca2+. Phosphonocarboxylic acids inhibit the transport of Pi by the transporter, phosphonoformic acid itself being transported into the SR increasing the level of accumulation of Ca2+. Phenylphosphonic acid also inhibits Pi transport, distinguishing the Pi transporter of SR from the Na+/Pi transporter of brush-border membranes. Oxalate transport is also inhibited by the phosphono-carboxylic acids, consistent with the suggestion that oxalate and phosphate are carried on the same transporter. The effects of maleate are, however, not inhibited, suggesting a separate carrier for the dicarboxylic acids. Acetic anhydride and phenylglyoxal inhibit the transporter, Pi providing protection against the effects of acetic anhydride, suggesting the presence of a lysine residue at the Pi binding site. ATP provides protection against the effects of acetic anhydride and phenylglyoxal, suggesting the presence of an ATP binding site on the transporter.     

Solubilization and reconstitution of the renal phosphate transporter

Proteins from brush-border membrane vesicles of rabbit kidney cortex were solubilized with 1% octylglucoside (protein to detergent ratio, 1:4 (w/w). The solubilized proteins (80.2 +/- 2.3% of the original brush-border proteins, n = 10, mean +/- S.E.) were reconstituted into artificial lipid vesicles or liposomes prepared from purified egg yolk phosphatidylcholine (80%) and cholesterol (20%). Transport of Pi into the proteoliposomes was measured by rapid filtration in the presence of a Na+ or a K+ gradient (out greater than in). In the presence of a Na+ gradient, the uptake of Pi was significantly faster than in the presence of a K+ gradient. Na+ dependency of Pi uptake was not observed when the liposomes were reconstituted with proteins extracted from brush-border membrane vesicles which had been previously treated with papain, a procedure that destroys Pi transport activity. Measurement of Pi uptake in media containing increasing amounts of sucrose indicated that Pi was transported into an intravesicular (osmotically sensitive) space, although about 70% of the Pi uptake appeared to be the result of adsorption or binding of Pi. However, this binding of Pi was not dependent upon the presence of Na+. Both Na+-dependent transport and the Na+-independent binding of Pi were inhibited by arsenate. The initial Na+-dependent Pi transport rate in control liposomes of 0.354 nmol Pi/mg protein per min was reduced to 0.108 and 0 nmol Pi/mg protein per min in the presence of 1 and 10 mM arsenate, respectively. Future studies on reconstitution of Pi transport systems must analyze and correct for the binding of Pi by the lipids used in the formation of the proteoliposomes.     

Involvement of arginine residues in the allosteric activation and inhibition ofSynechocystis PCC 6803 ADPglucose pyrophosphorylase

ADPglucose pyrophosphorylase (EC 2.7.7.27) from the cyanobacteriumSynechocystis PCC 6803 was desensitized to the effects of allosteric ligands by treatment with the arginine reagent, phenylglyoxal. Enzyme modification by phenylglyoxal resulted in inactivation when the enzyme was assayed under 3P-glycerate-activated conditions. There was little loss of the catalytic activity assayed in the absence of activator. Pi, 3P-glycerate, and pyridoxal-P were able to protect the enzyme from inactivation, whereas substrates gave minimal protection. The protective effect exhibited by Pi and 3P-glycerate was dependent on effector concentration. MgCl₂ enhanced the protection afforded by 3P-glycerate. The enzyme partially modified by phenylglyoxal was more resistant to 3P-glycerate activation and Pi inhibition than the unmodified form.V _max at saturating 3P-glycerate concentrations and the apparent affinity of the enzyme toward Pi were decreased upon phenylglyoxal modification. Incorporation of labeled phenylglyoxal into the enzyme was proportional to the loss of activity. Pi and 3P-glycerate nearly completely prevented incorporation of the reagent to the protein. Results suggest that one arginine residue per mol of enzyme subunit is involved in the binding of allosteric effector in the cyanobacterial ADPglucose pyrophosphorylase.     

Involvement of arginine residues in the allosteric activation and inhibition ofSynechocystis PCC 6803 ADPglucose pyrophosphorylase

ADPglucose pyrophosphorylase (EC 2.7.7.27) from the cyanobacteriumSynechocystis PCC 6803 was desensitized to the effects of allosteric ligands by treatment with the arginine reagent, phenylglyoxal. Enzyme modification by phenylglyoxal resulted in inactivation when the enzyme was assayed under 3P-glycerate-activated conditions. There was little loss of the catalytic activity assayed in the absence of activator. Pi, 3P-glycerate, and pyridoxal-P were able to protect the enzyme from inactivation, whereas substrates gave minimal protection. The protective effect exhibited by Pi and 3P-glycerate was dependent on effector concentration. MgCl₂ enhanced the protection afforded by 3P-glycerate. The enzyme partially modified by phenylglyoxal was more resistant to 3P-glycerate activation and Pi inhibition than the unmodified form.V _max at saturating 3P-glycerate concentrations and the apparent affinity of the enzyme toward Pi were decreased upon phenylglyoxal modification. Incorporation of labeled phenylglyoxal into the enzyme was proportional to the loss of activity. Pi and 3P-glycerate nearly completely prevented incorporation of the reagent to the protein. Results suggest that one arginine residue per mol of enzyme subunit is involved in the binding of allosteric effector in the cyanobacterial ADPglucose pyrophosphorylase.     

Modification of myo-inositol monophosphatase by the arginine-specific reagent phenylglyoxal.

myo-Inositol monophosphatase is inhibited by the arginine-specific reagent phenylglyoxal. The rate of inactivation is decreased in the presence of Pi, a competitive inhibitor of the enzyme. The effect of Pi is dependent on the presence of Mg2+, but is unaffected by Li+, an uncompetitive inhibitor. In the absence of Mg2+, the substrate, Ins(1)P, binds to the enzyme but is not converted into products, and affords only a small degree of protection against inactivation by phenylglyoxal. Li+ had no further effect under these conditions, but in the presence of Mg2+ caused a marked potentiation of the protective effect of substrate alone. In the absence of substrate, Li+ had no effect on activation by phenylglyoxal. Incorporation of 14C-labelled phenylglyoxal showed that inactivation was associated with modification of a single arginine residue per monomer in the dimeric enzyme. These findings support a mechanism in which Li+ inhibits monophosphatase by trapping a phosphorylated enzyme intermediate and preventing its hydrolysis.     

Phosphate-dependent sodium transport in S. faecalis investigated by 23Na and 31P NMR

Na+ movements in S. faecalis were studied by 23Na NMR. They proved to be dependent on phosphate concentration in the buffer during the de-energization step. K+ and H+ were also studied respectively by potentiometry and 31P NMR and were shown not to be implicated. For de-energized cells the internal phosphate concentration, on the contrary, was directly linked to the external phosphate contained in the buffer. The experiments showed a Na+/Pi dependence in this prokaryote so far known only in eukaryotes.     

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.     

Endocytosis and Na+/solute cotransport in renal epithelial cells

Endocytic uptake of [3H]sucrose and lucifer yellow, markers for fluid-phase endocytosis, was studied in cultures of the renal epithelial cell lines LLC-PK1 and OK. Endocytosis in LLC-PK1 cells was inhibited when the cells were grown in the presence of gentamicin (1 mg/ml) for 4 days or when the cells were treated with concanavalin A (1 mg/ml) for 5 h. These changes occurred without perturbation of intracellular Na+ and K+ content, indicating that the cells maintained normal ion gradients. The inhibition of endocytosis was accompanied by marked increases in the apparent Vmax for Na+-dependent cell uptake of solutes such as Pi and L-alanine. The apparent Km was unchanged. In contrast, treatment of OK cells with concanavalin A produced marked stimulation of endocytosis and inhibition of the Na+-dependent uptake of Pi and L-glutamate. These changes occurred in the absence of changes in intracellular Na+ and K+ content. Neither gentamicin nor concanavalin A had a direct effect on Na+/solute cotransport in these cell lines. The changes in Na+/Pi cotransport induced by concanavalin A in both LLC-PK1 and OK cells were blocked by keeping the cells at 4 degrees C during exposure to the lectin, suggesting that endocytosis may be part of the mechanism which mediates the changes in solute uptake. The reciprocal relationship between the changes in endocytosis and the changes in Na+/solute cotransport is consistent with the possibility that the number of Na+/solute cotransporters present in the plasma membrane may be altered by an increase or decrease in the rate of membrane internalization by endocytosis. The Vmax changes in Na+/solute cotransport provide indirect support for this conclusion.     

Characterization of a Pi transport system in cartilage matrix vesicles. Potential role in the calcification process.

The mechanisms by which calcium (Ca2+) and inorganic phosphate (Pi) accumulate into matrix vesicles (MV) have not been elucidated. In the present study the characteristics of Pi uptake into MV isolated from mildly rachitic chicken growth plate cartilage have been investigated. The results indicate that Pi accumulates into MV mainly via a Na(+)-dependent Pi transport system. In the absence of NaCl in the extravesicular medium, Pi uptake was a nonsaturable process. In the presence of 150 mM NaCl, the initial rate of Pi uptake was 4.38 +/- 1.02-fold higher than with 150 mM choline chloride (mean +/- S.E., n = 8, p less than 0.005). Other cations showed partial activity to drive Pi into MV as compared to Na+:Li+ (64.4%) greater than K+ (39.8%) greater than choline (39.0%) greater than tetramethylammonium (30.0%) greater than N-methylglucamine (26.3%). Na(+)-dependent Pi transport activity displayed saturability towards increasing extra-vesicular concentrations of Na+ and Pi. The apparent Km for Pi was 0.68 +/- 0.16 mM. The Na+ concentration producing half-maximum Pi transport activity was 106.2 +/- 11.0 mM. Kinetic analysis suggests that Na+ interacts with the Pi carrier with a stoichiometry of more than one Na+ ion with one Pi molecule. In MV isolated from normal chicken growth plate cartilage, this Na(+)-dependent Pi transport system was barely expressed. In contrast to the effect on Pi uptake by MV, the activity of alkaline phosphatase was not changed when NaCl was substituted for choline chloride in the assay medium. In addition to this observation which suggests that this enzyme is not related to the Pi transport activity described in this study, levamisole, which inhibited alkaline phosphatase activity did not affect the Na(+)-dependent uptake of Pi. Both arsenate and phosphonoformic acid, two inhibitors of the epithelial Na(+)-dependent Pi transport systems, were active inhibitors of the Na(+)-dependent Pi uptake by MV with a higher potency for phosphonoformic acid. Associated with the expression of a facilitated Na(+)-coupled Pi transport in MV, in vitro calcification assessed by 45Ca2+ uptake also showed a marked dependence on extravesicular sodium. This relationship was markedly attenuated in MV isolated from normal chicken growth plate cartilage expressing a weak Na(+)-facilitated Pi transport activity. In conclusion, a saturable Na(+)-dependent Pi carrier has been characterized which facilitates Pi transport in MV. Its potential role for Ca-Pi accumulation into MV and subsequent development of vesicular calcification followed by mineralization of the osteogenic matrix is proposed and remains to be further investigated.     

Regulation of the monovalent cation permeability of brain mitochondria

The Na+ and K+ conductances of rat brain mitochondria were estimated from rates of metabolically dependent swelling and uncoupling of respiration. These were maximal in the presence of EDTA plus Pi. Pi could not be replaced with acetate. Na+ conductance was greater than that of K+ and was therefore examined in greater detail. According to the influences of N-ethylmaleimide, internal Pi (exogenous and perhaps endogenous) promoted Na+ permeability. Treatment with the ionophore A23187 obviated the Pi requirement although EDTA was still necessary. The stimulation by EDTA with Pi or A23187 and inhibition by exogenous Mg2+ suggested endogenous polyvalent cations could also regulate Na+ conductance. The influence of these substances upon endogenous Mg2+ (and Ca2+) levels is consistent with such a role of membrane-bound Mg2+. Low levels of ruthenium red (150 pmol/mg) inhibit Na+ permeation, indicating that the number of 'sites' or 'channels' involved may be small. The Ca2+ uniport is not directly involved in Na+ flow according to its greater sensitivity to inhibition by ruthenium red.     

In vitro absorption of inorganic phosphate and other electrolytes in the carp intestine

Net movements of water, Na, Cl, K, Ca, Mg and Pi were measured in the stripped and everted intestine of the carp. A high concentrative absorbtion of Pi was observed, whereas Na and Cl were absorbed almost isotonically. The rate of Pi absorption was greater in the middle part than in the anterior or posterior part of the intestine.     

The physiological basis for altered Na+ and Cl- movements across the gills of rainbow trout (Oncorhynchus mykiss) in alkaline (pH = 9.5) water

To test the hypothesis that internal ion imbalances at high pH are caused by altered branchial ion transporting capacity and permeability, radiotracers (24Na+ and 36Cl-) were used to measure ion movements across the gills of intact rainbow trout (Oncorhynchus mykiss) during 3 d exposure to pH 9.5. At control pH (pH 8.0), the trout were in net ion balance, but by 8 h at high pH, 60%-70% reductions in Cl- influx (JClin) and Na+ influx (JNain) led to net Cl- and Na+ losses of -200 micromol kg-1 h-1. Outflux (diffusive efflux plus renal ion losses) was not initially altered. By 72 h, net Cl- balance was reestablished because of a restoration of JClin. Although JNain remained 50% lower at this time, counterbalancing reductions in Na+ outflux restored net Na+ balance. One-substrate ion-uptake kinetics analyses indicated that reduced ion influx after 8 h at pH 9.5 was caused by 50% decreases in Cl- and Na+ maximal transport rates (JClmax, JNamax), likely reflecting decreased numbers of functional transport sites. Two-substrate kinetic analyses indicated that reduced internal HCO-3 and H+ supply for respective branchial Cl-/base and Na+/acid transport systems also contributed to lower JClin and, to a lesser extent, lower JNain at pH 9.5. Recovery of JClmax after 3 d accounted for restoration of Cl- balance and likely reflected increased numbers of transport sites. In contrast, JNamax remained 33% lower after 3 d, but a lower affinity of the gills for Na+ (fourfold greater KNam) accounted for the chronic reduction in Na+ influx at pH 9.5. Thus, reestablishment of Cl- uptake capacity and counterbalancing reductions in Na+ outflux allows rainbow trout to reestablish net ion balance in alkaline waters.     

Characterization of Na+-dependent phosphate transport in cardiac sarcolemmal vesicles

Pi uptake by purified bovine cardiac sarcolemmal vesicles was stimulated by an inwardly directed Na+ gradient, but not by such gradients of K+, Rb+, Li+, and choline. When Na+ was present both inside and outside the vesicles, or when Na+ gradient was dissipated by monensin, the Na+-dependent Pi uptake increased with time, reached a peak, and then declined approaching a steady state. The initial rate of Na+-dependent Pi uptake was a saturable function of Pi concentration (Km = 0.5 mM). These findings indicate the existence of a Na+,Pi-cotransporter in the sarcolemma. The Na+-activation curve of the Pi uptake exhibited positive cooperativity, suggesting the requirement for multiple Na+ binding to the functional unit of the carrier. The initial rate of Na+-dependent Pi uptake decreased as extra-vesicular pH increased in the range of 5.5-8.7. The uptake rate increased under conditions that are known or expected to generate an inside-negative membrane potential, indicating that Pi uptake is accompanied by the uptake of positive charge. These results suggest the electrogenic cotransports of two Na+ and one H2PO4-. We conclude that this cotransporter catalyzes the secondary active transport of Pi across the cardiac plasma membrane and regulates myocardial energy metabolism. We also suggest that the cotransporter may control intracellular Na+ and thus be involved in the regulation of trans-sarcolemmal Ca2+ movement and cardiac contractility.     

Electrogenic Kinetics of a Mammalian Intestinal Type IIb Na+/Pi Cotransporter

The kinetics of a type IIb Na(+)-coupled inorganic phosphate (Pi) cotransporter (NaPi-IIb) cloned from mouse small intestine were studied using the two-electrode voltage clamp applied to Xenopus oocytes. In the steady state, mouse NaPi-IIb showed a curvilinear I-V relationship, with rate-limiting behavior only for depolarizing potentials. The Pi dose dependence was Michaelian, with an apparent affinity constant for Pi (Km(pi)) of 10 +/- 1 microM: at -60 mV. Unlike for rat NaPi-IIa, (Km(pi)) increased with membrane hyperpolarization, as reported for human NaPi-IIa, flounder NaPi-IIb and zebrafish NaPi-IIb2. The apparent affinity constant for Na(+) (Km(na)) was 23 +/- 1 mM: at -60 mV, and the Na(+) activation was cooperative with a Hill coefficient of approximately 2. Pre-steady-state currents were documented in the absence of Pi and showed a strong dependence on external Na(+). The hyperpolarizing shift of the charge distribution midpoint potential was 65 mV/log[Na]. Approximately half the moveable charge was attributable to the empty carrier. A comparison of the voltage dependence of steady-state Pi-induced current and pre-steady-state charge movement indicated that for -120 mV 相似文献   

Breakdown of Na+/K+-exchanging ATPase phosphoenzymes formed from ATP and from inorganic phosphate during Na+-ATPase activity.

The reactivity towards Na+ and K+ of Na+/K+-ATPase phosphoenzymes formed from ATP and Pi during Na+-ATPase turnover and that obtained from Pi in the absence of ATP, Na+ and K+ was studied. The phosphoenzyme formed from Pi in the absence of cycling and with no Na+ or K+ in the medium showed a biphasic time-dependent breakdown. The fast component, 96% of the total EP, had a decay rate of about 4 s(-1) in K+-free 130 mm Na+, and was 40% inhibited by 20 mm K+. The slow component, about 0.14 s(-1), was K+ insensitive. Values for the time-dependent breakdown of the phosphoenzymes obtained from ATP and from Pi during Na+-ATPase activity were indistinguishable from each other. In K+-free medium containing 130 mm Na+, the decays followed a single exponential with a rate constant of 0.45 s(-1). The addition of 20 mm K+ markedly increased the decays and made them biphasic. The fast components had a rate of approximately 220 s-1 and accounted for 92-93% of the total phosphoenzyme. The slow components decayed at a rate of about 47-53 s(-1). A second group of experiments examined the reactivity towards Na+ of the E2P forms obtained with ATP and Pi when the enzyme was cycling. In both cases, the rate of dephosphorylation was a biphasic function of [Na+]: inhibition at low [Na+], with a minimum at about 5 mm Na+, followed by recovery at higher [Na+]. Although qualitatively similar, the phosphoenzyme formed from Pi showed slightly less inhibition and more pronounced recovery. These results indicate that forward and backward phosphorylation during Na+-ATPase turnover share the same intermediates.     

Reconstitution of the partially purified renal phosphate (Pi) transporter.

Proteins from rabbit kidney brush border membranes were solubilized with 1% Nonidet P-40 (crude membrane proteins) and fractionated according to their isoelectric points (pI) by chromatofocusing. The eluate was pooled into three fractions according to the pI of the samples (1, greater than 6.8; 2, 6.8-5.4; 3, 5.4-4.0). The crude membrane proteins as well as the three fractions were reconstituted into liposomes and transport of Pi was measured by a rapid filtration technique in the presence of an inwardly directed K+ or Na+ gradient. Arsenate-inhibitable Na+-dependent transport of Pi was reconstituted into an osmotically active intravesicular space from both the crude membrane proteins and Fraction 1. In contrast, Fractions 2 and 3 were inactive. Treatment of the crude membrane proteins and the three fractions with the method for extracting phosphorin (a Pi-binding proteolipid found in brush border membranes) yielded Mn2+-dependent binding of Pi characteristic of phosphorin only in the extracts from crude membrane proteins and Fraction 1, the same fractions in which Na+-dependent transport of Pi was found in the reconstituted system. When reconstituted into liposomes, phosphorin was, however, unable to yield Na+-dependent transport of Pi. Moreover, we cannot eliminate the possibility that Na+-Pi transport can occur in the absence of phosphorin, since complete recovery of Na+-Pi transport was not achieved. However, the present data showing localization of the recovered binding and transport systems for Pi in the same protein fraction lend support to the hypothesis that phosphorin might be a constituent of the renal Pi transport system. Whether the presence of phosphorin is necessary or accessory for Na+-dependent Pi transport in intact brush border membrane vesicles or in liposomes reconstituted with crude or purified membrane proteins requires further investigation.     

Coupling of ATP synthesis to reversal of rat liver microsomal Ca2+-ATPase

The reversal of the rat liver microsomal Ca2+-ATPase transport cycle was studied. Microsomes were loaded with 45Ca2+ (approximately 30 nmol/mg of protein) in an ATP-dependent process, and the time dependency of the microsomal 45Ca2+ efflux was determined with various ADP and inorganic phosphate (Pi) concentrations. Pseudo-first-order rate constants (K'e) for 45Ca2+ efflux were determined. Although there was considerable 45Ca2+ efflux in the absence of added ADP or Pi, the addition of ADP or Pi alone had minimal effects upon the K'e; in contrast, a 2.5-fold increase in the K'e was observed in the presence of both ADP and Pi. The apparent Km values for ADP and Pi were 4 microM and 0.22 mM, respectively. Stimulation of 45Ca2+ efflux by ADP and Pi was associated with ATP synthesis. The calcium ionophore A23187 prevented ATP synthesis, which indicates that the Ca2+ gradient facilitates the coupling of ATP synthesis to Ca2+ efflux.     

Selective inhibition by ionophore A23187 of the enzyme isomerization in the catalytic cycle of Na+, K+ -ATPase

The effect of an ionophore A23187 on the purified Na+,K+-ATPase from the outer medulla of pig kidney was investigated. When the enzyme was pretreated with A23187 in the presence of Na+ and K+, the ATPase activity was inhibited almost completely. When the pretreatment was performed in the presence of Na+ and absence of K+, formation of the phosphoenzyme (EP) from ATP was only slightly retarded. The steady state level of EP thus formed was not altered, but EP decomposition was strongly inhibited. Under these conditions, the accumulated EP was sensitive to ADP and insensitive to K+. On the other hand, when the pretreatment was performed in the absence of Na+ and presence of K+, EP formation following simultaneous addition of Na+ and ATP was extremely slow, but the steady state level of EP was not substantially altered. When the pretreatment was performed in the absence of Na+ and presence of K+, EP formation from Pi was unaffected, and the EP formed was in rapid equilibrium with Pi of the medium. These results demonstrate that A23187 selectively inhibits isomerization of the enzyme between the high Na+ and low K+ affinity form and the low Na+ and high K+ affinity form in the catalytic cycle, whether or not the enzyme is phosphorylated. This inhibition is quite similar to the A23187-induced inhibition of the enzyme isomerization in the catalytic cycle of the Ca2+ -ATPase from sarcoplasmic reticulum (Hara, H., and Kanazawa, T. (1986)J. Biol. Chem.261, 16584-16590). These findings suggest that some common mechanism, which is involved in the enzyme isomerization, between these two transport ATPases is strongly disturbed by A23187.