Extracellular ATP induces a nonspecific permeability of thymocyte plasma membranes

We have previously demonstrated that extracellular ATP can give medullary thymocytes the calcium message required for the induction of their blastogenesis, without mobilization of intracellular calcium. We describe here the effects of extracellular nucleotides on membrane permeability to monovalent and divalent cations in mouse thymocytes. Among all nucleotides tested, under physiological conditions, only ATP and, to a lesser extent, 2-methylthio-ATP, adenosine 5'-O-(3-thio-triphosphate), and ADP were able to depolarize thymocyte plasma membranes and to induce Na+ and Ca2+ influxes into thymocytes; other nonhydrolysable ATP analogs were only effective in the absence of Mg2+. The ATP-induced effects were inhibited in a dose-dependent manner by Mg2+ and greatly potentiated in its absence, which suggests that the tetrabasic ATP4 is probably the active species and that a phosphotransferase activity is not involved in its effects. There ATP-mediated changes in ion fluxes result from an increase in nonspecific permeability of thymocyte membranes, probably by pore formation. These ion flux changes might be responsible for the mitogenic induction of phorbol 12-myristate 13-acetate treated medullary thymocytes. The potency order for the adenine derivatives to affect these fluxes (ATP greater than ADP much greater than AMP greater than adenosine) suggests the presence of ATP specific receptors (P2 purinergic receptors) on thymocyte plasma membranes.     

Ca2(+)-induced secretion by electropermeabilized human neutrophils. The roles of Ca2+, nucleotides and protein kinase C

Studies of stimulus-response coupling have benefitted from the availability of permeabilization techniques, whereby putative second messengers and intracellular modulators can be introduced into the cell interior. Electropermeabilization, which uses high-intensity electric fields to breach the plasma membrane, creates small pores, permitting access of solutes with molecular masses below 700 KDa. Neutrophils permeabilized by this technique, but not intact cells, discharged lysosomal constituents when exposed to micromolar levels of Ca2+. Secretion by electroporated neutrophils was significantly enhanced by the presence of Mg-ATP (0.3-1.0 mM). Contrary to expectations, it was determined that ATP was not the only nucleotide which enhanced Ca2(+)-induced secretion in the presence of Mg2+. Not only could GTP, XTP, ITP, UTP or ADP partially or completely replace ATP, but even non-hydrolyzable nucleotides such as ADP beta S ATP gamma S, and App[NH]p were effective. GTP gamma S and GDP beta S were inhibitory, while Gpp[NH]p was inactive. None of these nucleotides induced secretion on its own. In contrast, neutrophils which were permeabilized and then washed, were only slightly activated by Mg-ATP and other nucleotides; even the response to Ca2+ alone was less. This hyporesponsiveness of washed cells proved to be due to a time-dependent deactivation of the permeabilized neutrophils taking place at 4 degrees C. In an effort to assess the role for protein kinase C (PKC) in secretion in this system, we examined the effects of phorbol myristate acetate (PMA), a PKC agonist. PMA enhanced degranulation induced by Ca2+ by lowering the requirement for this divalent cation; enhancement by PMA was not dependent upon exogenous ATP. Three inhibitors of PKC with varying specificity, namely H-7, K-252a, and staurosporine, all abrogated PMA-enhanced secretion. These agents also inhibited secretion stimulated by Ca2+ plus ATP in parallel with that induced by Ca2+ plus PMA, strongly suggesting a role for PKC in modulation of degranulation by ATP. Our results show that electropermeabilized neutrophils provide a convenient, useful model for stimulus-secretion coupling. These data also suggest that the 'requirement' for Mg-ATP, which has been observed in other permeabilized cell systems, is not simply for metabolic energy or as a substrate for kinases. It is possible that these nucleotides all interact with a recently described neutrophil receptor for adenine nucleotides or with a recently postulated exocytosis-linked G-protein.     

Adenine nucleotides modulate phosphatidylcholine metabolism in aortic endothelial cells

ATP and ADP, in concentrations ranging from 1-100 microM, increased the release of [3H]choline and [3H]phosphorylcholine (P-choline) from bovine aortic endothelial cells (BAEC) prelabelled with [3H]choline. This action was detectable within 5 minutes and was maintained for at least 40 minutes. ATP and ADP were equiactive, and their action was mimicked by their phosphorothioate analogs (ATP gamma S and ADP beta S) and adenosine 5'-(beta, gamma imido) triphosphate (APPNP), but not by AMP, adenosine, and adenosine 5'-(alpha, beta methylene)triphosphate (APCPP): these results are consistent with the involvement of P2Y receptors. ATP also induced an intracellular accumulation of [3H]choline: the intracellular level of [3H]choline was increased 30 seconds after ATP addition and remained elevated for a least 20 minutes. The action of ATP on the release of choline metabolites was reproduced by bradykinin (1 microM), the tumor promoter phorbol 12-myristate 13-acetate (PMA, 50 nM), and the calcium ionophore A23187 (0.5 microM). Down-regulation of protein kinase C, following a 24-hour exposure of endothelial cells to PMA, abolished the effects of PMA and ATP on the release of choline and P-choline, whereas the response to A23187 was maintained. These results suggest that in aortic endothelial cells, ATP produces a sustained activation of a phospholipase D hydrolyzing phosphatidylcholine. The resulting accumulation of phosphatidic acid might have an important role in the modulation of endothelial cell function by adenine nucleotides. Stimulation of phospholipase D appears to involve protein kinase C, activated following the release of diacylglycerol from phosphatidylinositol bisphosphate by a phospholipase C coupled to the P2Y receptors (Pirotton et al., 1987a).     

ATP analogues induce membrane permeabilization in transformed mouse fibroblasts

The mechanism underlying ATP-induced permeabilization of transformed mouse fibroblasts was studied by using nonhydrolyzable analogues of ATP. Incubation of 3T6 cells with 0.6 mM of either ATP, 5'-adenylyl imidodiphosphate (p[NH]ppA) or adenosine 5'-[beta, gamma-methylene]triphosphate (p[CH2]ppA) resulted in an increase of 17-, 8- or 5-times, respectively, in the cell membrane permeability, measured by the efflux of normally impermeant metabolites from the cells. The induced cell permeabilization was preceded by a reduction in the membrane potential (delta psi), determined according to the distribution of the cation tetraphenylphosphonium (TPP+) between the cells and the medium. Reduction of 26, 18 and 13 mV in delta psi was exerted by 0.6 mM of either ATP, p[NH]ppA or p[CH2]ppA, respectively. In 3T3 cells the untransformed counterparts of 3T6 cells, neither reduction of delta psi, nor alterations in membrane permeability were exerted by either ATP or by its analogues. The data indicate that the dissociation of the beta, gamma-phosphate bond is not essential for membrane permeabilization by external ATP, implying that the binding of ATP to the cell surface of transformed cells is sufficient to initiate the permeabilization process. The data also suggest that delta psi is involved in the control of membrane permeability.     

Substrate-site interactions in the membrane-bound adenine-nucleotide carrier as disclosed by ADP and ATP analogs

The binding parameters of a number of ADP or ATP analogs to the adenine nucleotide carrier in mitochondria and inside-out submitochondrial particles have been explored by means of two specific inhibitors, carboxyatractyloside and bongkrekic acid. The nucleotides tested fell into two classes depending on the shape of the binding curve. Curvilinear Scatchard plots were obtained for the binding of ADP, ATP, adenosine 5'-triphospho-gamma-1-(5-sulfonic acid)naphthylamidate [gamma-AmNS)ATP) and adenylyl (beta,gamma)-methylenediphosphate (p[CH2]ppA); on the other hand, rectilinear Scatchard plots were obtained in the case of naphthoyl-ADP (N-ADP) and 8-bromo ADP (8Br-ADP) binding. The total number of binding sites for N-ADP and 8Br-ADP could be extrapolated with good accuracy to 1.3-1.5 nmol/mg protein; this value corresponds to the number of carboxyatractyloside-binding sites in heart mitochondria (Block, M.R., Pougeois, R. and Vignais, P.V. (1980) FEBS Lett. 117, 335-340). On the other hand, because of the curvilinearity of the Scatchard plots for the binding of ADP, ATP, (gamma-AmNS)ATP and p[CH2]ppA, the total number of binding sites for these nucleotides could only be approximated to a value higher than 1 nmol/mg protein, the exact value being probably equal to that found for N-ADP and 8Br-ADP binding, i.e. 1.3-1.5 nmol/mg protein. Curvilinearity of Scatchard plots was discussed in terms of negative interactions between nucleotide-binding sites located on the same face of the adenine nucleotide carrier. A possible relationship between the features of the binding plots and the transportable nature of the nucleotide is discussed. Contrary to the enhancing effect of bongkrekic acid on [14C]ADP uptake observed essentially in nucleotide-depleted heart mitochondria (Klingenberg, M., Appel, M., Babel, W. and Aquila, H. (1983) Eur. J. Biochem. 131, 647-654), binding of bongkrekic acid to nondepleted heart mitochondria was found to partially displace previously bound [14C]ADP. These opposite effects of bongkrekic acid may be explained by assuming that bongkrekic acid is able to abolish negative cooperativity between external (cytosolic) ADP-binding sites.     

Phosphoenolpyruvate carboxykinase in mouse pancreatic islets. ATP-induced changes in sensitivity to Mn2+ activation

The presence of high phosphoenolpyruvate carboxykinase (EC 4.1.1.32) activity in mouse islet cytosol has been demonstrated. The enzyme was activated by Mn2+ with a Ka of 100 X 10(-6) mol/l. The mean total activity of the Mn2+-stimulated phosphoenolpyruvate carboxykinase in islet cytosol estimated at 22 degrees C with saturating concentrations of the substrates oxaloacetate and ITP was 146 pmol/min per micrograms DNA. Km was calculated to be 6 X 10(-6) mol/l for oxaloacetate and 140 X 10(-6) mol/l for ITP. The islet phosphoenolpyruvate carboxykinase activity was not increased after starvation of the animals for 48 h. Preincubation of the cytosol at 4 degrees C with Fe2+, quinolinate, ATP, Pi, glucose 6-phosphate, fructose 1,6-bisphosphate, NAD+, NADH, oxaloacetate, ITP, cyclic AMP and Ca2+ had no effect on the enzyme activity. However, preincubation of the cytosol at 37 degrees C with ATP-Mg inhibited the Mn2+-stimulated phosphoenolpyruvate carboxykinase activity progressively with time and in a concentration-dependent manner. A similar but weaker inhibitory effect was observed with p[NH]ppA, whereas p[CH2]ppA, ADP, AMP, adenosine and Pi had no effect. It is tentatively suggested that ATP and p[NH]ppA either by adenylation or otherwise affect the interaction between islet phosphoenolpyruvate carboxykinase and the recently discovered Mr = 29000 protein modulator of the enzyme in such a way - perhaps by causing a dissociation between them - that phosphoenolpyruvate carboxykinase loses its sensitivity to Mn2+ activation.     

Activation of electropermeabilized neutrophils by adenosine 5''-[gamma-thio]triphosphate (ATP[S]). Role of phosphatases in stimulus-response coupling.

Electrically permeabilized human neutrophils were used to study the mechanism of activation of the NADPH oxidase by chemotactic factors. The respiratory burst elicited by formyl-methionyl-leucyl-phenylalanine (fMLP) was strictly dependent on the addition of ATP. The response was also supported by adenosine 5'-[gamma-thio]triphosphate (ATP[S]), but not by the non-hydrolysable analogue (p[NH]ppA). In the presence of ATP, displacement of fMLP from its receptor by antagonist peptides resulted in the abrupt termination of the O2-consumption burst. In contrast, the response persisted after displacement of fMLP when ATP[S] was present. This finding is consistent with the formation of biologically active thiophosphoproteins which are resistant to cleavage by cellular phosphatases. Accordingly, lower concentrations of ATP[S], as compared with ATP, were required to support the fMLP response. The data indicate that protein phosphatases control the extent and duration of the response in cells stimulated with chemoattractants. Unlike ATP, sub-millimolar concentrations of ATP[S] elicited a spontaneous respiratory burst in the absence of fMLP or other stimuli. This effect was inhibited by p[NH]ppA and was not observed in intact (non-permeabilized) cells, indicating interaction of ATP[S] with an intracellular adenine-nucleotide-binding site, possibly a protein kinase. These results suggest that protein kinases are active in neutrophils in the absence of exogenous stimuli, but that accumulation of the essential phosphoprotein(s) is normally prevented by the ongoing vigorous phosphatase activity. It is conceivable that control of the respiratory burst is exerted by inhibition of phosphatase activity, instead of or in addition to the more commonly postulated activation of protein kinases.     

Micromolar free calcium exposes ouabain-binding sites in digitonin-permeabilized Xenopus laevis oocytes.

As demonstrated previously, digitonin-permeabilized Xenopus oocytes have a large internal pool of sodium pumps which are inaccessible to cytosolic ouabain [Schmalzing, Kröner & Passow (1989) Biochem. J. 260, 395-399]. Access to internal ouabain-binding sites required permeabilization of inner membranes with SDS. In the present study, micromolar free Ca2+ was found to stimulate ouabain binding in the digitonin-permeabilized cells (K0.5 0.5 microM-Ca2+, h 1.9, average of seven experiments) without disrupting intracellular membranes. Sustained incubation at 9 microM-Ca2+ was as effective as SDS in inducing access to the ouabain-binding sites of the internal sodium pumps. Omission of either Mg2+ or ATP completely abolished the Ca2+ effect. Half-maximal stimulation by Ca2+ required approx. 0.4 mM-MgATP. Of a variety of nucleotides tested, none was as effective as ATP (rank order ATP greater than ADP greater than ATP[S] (adenosine 5''-[gamma-thio]triphosphate) greater than CTP greater than UTP greater than ITP = XTP greater than GTP). Pi, AMP, cyclic AMP, cyclic GMP, GTP[S] (guanosine 5''-[gamma-thio]triphosphate) and a stable ATP analogue p[NH]ppA (adenosine 5''-[beta gamma-imido]triphosphate), were ineffective. The metalloendoproteinase inhibitor carbobenzoxy-Gly-Phe-amide reduced the Ca2+ effect by some 50%. Inhibitors of chymotrypsin and the Ca2+ proteinase calpain had no effect. Ca2+ ionophores (A23187 and ionomycin) and the polycations neomycin and polymixin B blocked the Ca2+ response entirely. Neomycin also abolished a Ca2(+)-independent stimulation of ouabain binding by the wasp venom mastoparan. The requirements for increasing the accessibility of ouabain-binding sites are remarkably similar to those for exocytosis in secretory cells, suggesting that oocytes and eggs possess a Ca2(+)-regulated pathway for the plasma membrane insertion of sodium pumps.     

Glutathione status of rat thymocytes and splenocytes during the early events of their ConA proliferative responses

Glutathione plays an important role in the lymphocyte mitogenic response. We have demonstrated that 2-ME increases the ConA proliferative response of rat splenocytes and in parallel, causes an enhancement of glutathione synthesis in these cells. On the other hand, 2-ME had the same action on the glutathione level of thymocytes during the late phase of their mitogenic response, but it had no effect on the [3H]thymidine uptake of these cells. To clarify this discrepancy and the role of glutathione during the mitogenic response, we studied the glutathione status of thymus cells during the early phase of the ConA-induced proliferative response in the presence or the absence of 2-ME in parallel with that of whole spleen cells and the T cell fraction of splenocytes. During the early events of the mitogenic response, i.e., during the 24th h, we observed a normal 2 GSSG/GSH + 2 GSSG ratio in cultured cells, indicating a normal redox state, and that ConA involved an increased glutathione level in thymocytes but not in whole splenocytes and in splenic T cells. 2-ME had no effect on the glutathione level of stimulated thymocytes during the early phase of the mitogenic response. This phenomenon could be related to an absence of its effect on [3H]thymidine uptake. On the other hand, 2-ME induced an enhancement of the glutathione level and [3H]thymidine uptake in the two types of stimulated splenocytes. This study suggest that thymocytes do not have the same mechanism of glutathione synthesis induction as that which occurs in splenocytes during the ConA proliferative response. This mechanism could be related to the maturation state of the T cells.(ABSTRACT TRUNCATED AT 250 WORDS)     

ATP analogues induce membrane permeabilization in transformed mouse fibroblasts

The mechanism underlying ATP-induced permeabilization of transformed mouse fibroblasts was studied by using nonhydrolyzable analogues of ATP. Incubation of 3T6 cells with 0.6 mM of either ATP, 5′-adenylyl imidodiphosphate (p[NH]ppA) or adenosine 5′-[β,γ-methylene]triphosphate (p[CH₂]ppA) resulted in an increase of 17-, 8- or 5-times, respectively, in the cell membrane permeability, measured by the efflux of normally impermeant metabolites from the cells. The induced cell permeabilization was preceded by a reduction in the membrane potential (Δψ), determined according to the distribution of the cation tetraphenylphosphonium (TPP⁺) between the cells and the medium. Reduction of 26, 18 and 13 mV in Δψ was exerted by 0.6 mM of either ATP, p[NH]ppA or p[CH₂]ppA, respectively. In 3T3 cells the untransformed counterparts of 3T6 cells, neither reduction of Δψ, nor alterations in membrane permeability were exerted by either ATP or by its analogues. The data indicate that the dissociation of the β,γ-phosphate bond is not essential for membrane permeabilization by external ATP, implying that the binding of ATP to the cell surface of transformed cells is sufficient to initiate the permeabilization process. The data also suggest that Δψ is involved in the control of membrane permeability.     

Beta-adrenergic receptors on murine lymphocytes: density varies with cell maturity and lymphocyte subtype and is decreased after antigen administration

beta-Adrenergic receptors were assayed on intact, viable, murine splenocytes and thymocytes using the labeled adrenergic antagonists [3H]-dihydroalprenolol l-[ring propyl-3H(N)] ([3H]DHA) and 4-(3-t-butylamino-2-hydroxypropoxy)-[5,7-3H]benzimidazol-2-one ([3H]CGP 12177). The sites detected by [3H]DHA did not always possess the characteristics of beta-adrenergic receptors and were demonstrated to be stereospecific only after the addition of the binding assay. Populations of cells from C57Bl/6 inbred and CF1 outbred mice were compared. Purified T cells from C57Bl/6 mice had fewer receptors than did either whole spleen or B cells. Thymocytes from either strain had significantly fewer receptors than did the other lymphocyte populations. However, mature medullary thymocytes purified from C57Bl/6 mice had higher numbers of receptors per cell which were comparable to those of the splenic T cell. Radiation-resistant splenocytes recovered from CF1 mice 24 hr after 700 rad of irradiation possessed greatly increased numbers of receptors per cell. Immunization with sheep red blood cells caused a significant reduction in the density of receptors on splenocytes from C57Bl/6 mice. The wide variations observed in the density of beta-adrenergic receptors, possibly related to cell maturity or state of activation, seem to provide opportunities for differential modulation of cell functions by either endogenous or exogenous adrenergic agents.     

Transport protons do not participate in ATP synthesis/hydrolysis at the nucleotide binding site of the H(+)-ATPase from chloroplasts.

The H(+)-ATPase from chloroplasts CFoF1, was brought into the active, reduced state by illumination of thylakoids in the presence of thioredoxin and dithiothreitol. Uni-site ATP synthesis was initiated by the addition of 20 nM [alpha-32P]ADP, and enzyme-bound and free nucleotides were separated by a pressure column. The ratio of enzyme-bound ADP to ATP was 0.55 +/- 0.05. In a second experiment, uni-site ATP hydrolysis under energized conditions was initiated by the addition of 36 nM [alpha-32P]ATP; enzyme-bound and free nucleotides were separated by a pressure column. Both procedures were carried out under continuous illumination. The ratio of enzyme-bound ADP to ATP was 0.46 +/- 0.04. In a third experiment, uni-site ATP hydrolysis under de-energized conditions was initiated by the addition of 39 nM [alpha-32P]ATP and NH4Cl/valinomycin in the absence of illumination. Free and enzyme-bound nucleotides were separated also by a pressure column. The ratio of enzyme-bound ADP to ATP was 0.43 +/- 0.02. This ratio was always the same irrespective of whether the reaction runs in the synthesis or the hydrolysis direction. Furthermore, the ratio does not depend on the membrane energization. We conclude, therefore, that the protons are not directly involved in the reaction at the catalytic site.     

Dissociation between inositol polyphosphate production and mitogenesis in mouse thymocytes

Cortical and medullary thymocytes can be separated from each other by virtue of the fact that only cortical thymocytes bear peanut agglutinin (PNA) receptors. The mitogenic responses of subpopulations of thymocytes were studied. We have confirmed the results of Conlon et al. [(1982) J. Immun. 128, 797-801], that lectin-induced stimulation of unseparated cells, and PNA- but not PNA+ thymocytes, results in DNA synthesis. In contrast, both subpopulations, as well as unseparated cells, synthesize DNA in response to the calcium ionophore A23187 in the presence of the phorbol ester TPA, suggesting an impairment of signal transduction in PNA+ cells. However, comparable amounts of inositol phosphates were accumulated in PNA- and PNA+ thymocytes in response to Concanavalin A (Con A). We suggest that mitogenic lectins generate a third signal in addition to elevation of intracellular free calcium concentration and activation of protein kinase C. This signal is generated in PNA- but not in PNA+ thymocytes and is obligatory for lectin-induced stimulation.     

Adenosine-receptor-mediated stimulation of low-Km GTPase in guinea-pig cerebral cortex.

Inhibition of receptor-coupled adenylate cyclase by hormones is proposed to be associated with GTP hydrolysis. Since adenosine inhibits cerebral-cortical adenylate cyclase via A1 adenosine receptors, the present study attempts to verify this mechanism for A1-selective adenosine derivatives. In guinea-pig cortical membranes N6-(phenylisopropyl)adenosine (PIA) increased the Vmax. of the low-Km GTPase, with an EC50 (concentration causing 50% of maximal stimulation) of about 0.1 microM, and the stimulatory effect was competitively antagonized by 5 microM-8-phenyltheophylline. The rank order of potency of the stereoisomers of PIA and of 5-(N-ethylcarboxamido)adenosine (NECA) to stimulate GTPase correlated with their ability to inhibit adenylate cyclase activity (R-PIA greater than NECA greater than S-PIA). Competition binding studies with (-)-N6- ([125I]iodo-4-hydroxyphenylisopropyl)adenosine suggest that adenylyl imidodiphosphate (p[NH]ppA), an essential component of the GTPase assay system, is a more potent A1-receptor agonist than ATP, with an IC50 (concentration giving half-maximal displacement of radioligand binding) of 7.9 microM. On the basis of the p[NH]ppA concentration used in the GTPase assay (1.25 mM), enzyme stimulation by adenosine seems to be highly underestimated. Nevertheless, adenosine-induced GTP hydrolysis reflects an increased turnover of guanine nucleotides at the Ni regulatory site and appears to be a crucial step in the sequence of events processing the inhibitory signal to adenylate cyclase.     

Exogenous ATP enhances calcium influx in intact thymocytes

Recent observations have indicated that exogenous adenosine triphosphate (ATP) may influence lymphocyte functions such as proliferation and cytoxicity. Here we report a novel activity of extracellular ATP--it specifically increases Ca2+ uptake in murine lymphocytes. ATP added to thymocytes increases the rate of [45Ca2+] uptake by up to 20-fold. The increased rate is seen with ATP concentrations as low as 500 microM and is half-maximal at approximately 2 mM ATP. The magnitude of stimulation by ATP is dependent on Mg2+ concentration, and ATP-Mg2+ complex is probably the true activator. Of the high-energy phosphate-containing compounds tested, including deoxy-ATP, only GTP showed a modest stimulation of calcium uptake. ADP, AMP, cyclic AMP, and adenosine did not significantly increase calcium uptake. Cellular integrity as indicated by trypan blue exclusion and ethidium bromide/acridine orange staining was unaffected by ATP. Ca2+ influx is the major mode of action of ATP in raising intrathymocyte Ca2+ levels, because neither the Ca2+ efflux nor the [45Ca2+]-Ca2+ exchange was significantly altered in the presence of ATP. Verapamil, a Ca2+ channel blocking agent, could not prevent the ATP effect, suggesting that ATP may be acting by a mechanism other than the voltage-dependent Ca2+ channel. An analysis of intracellular and extracellular ATP levels by chemiluminescence assay indicated no significant ATP entry into intact lymphocytes. Also, ATP added to the medium containing thymocytes was destroyed (approximately 50% by 20 min). The nonhydrolyzable ATP analogs, AMPPCP and AMPPNP, were unable to stimulate a significant amount of Ca2+ uptake, suggesting the involvement of a cell surface phosphotransferase activity. This was supported by the demonstration of a threefold to fivefold increase in the labeling of protein and phospholipid fractions obtained from intact thymocytes exposed to [gamma 32P]ATP for 30 min. Ca2+ is believed to play an important role in a variety of lymphocyte functions, including mitogenesis and natural killer cell activity. The data herein thus provide a potential mechanism for the action of exogenous ATP on these lymphocyte functions.     

The effects of octylglucoside on the interactions of chloroplast coupling factor 1 (CF1) with adenine nucleotides

The effects of octylglucoside (OcGlc) micelles, which stimulate a Mg-specific ATPase activity in chloroplast coupling factor 1 [Pick, U. and Bassilian, S. (1982) Biochemistry, 21, 6144-6152], on the interactions of the enzyme with adenine nucleotides have been studied. 1. OcGlc specifically accelerates the binding and the release of ADP but not of ATP or adenosine 5'[beta, gamma-imido]triphosphate (AdoPP[NH]P) from the tight-sites. The binding affinity for ADP and for ATP is only slightly decreased (twofold) by the detergent. ATP competitively inhibits the binding of ADP and vice versa in the presence or absence of OcGlc. 2.OcGlc-induced inactivation of CF1-ATPase is correlated with the release of bound nucleotides. In the absence of medium nucleotides ADP X CF1 is rapidly inactivated while ATP X CF1 and AdoPP[NH]P X CF1 are slowly inactivated by OcGlc in parallel with the release of bound nucleotide. In contrast, low concentrations of either ATP or ADP in the medium effectively protect against OcGlc inactivation while AdoPP[NH]P, whose binding to CF1 is inhibited by OcGlc, is ineffective even at millimolar concentrations. The results suggest that the occupancy of the tight-sites protects the enzyme against OcGlc-induced inactivation. 3. Mg ions specifically inhibit the release of bound ADP and the OcGlc-induced inactivation of CF1. High concentrations of medium ATP and ADP (K50 = 100 microM) also inhibit the OcGlc-induced release of bound nucleotides in an EDTA medium. In contrast, in the absence of OcGlc, medium ADP and ATP accelerate the release of bound adenine nucleotides. 4. Mg-ATP in the presence of OcGlc stimulates the release of bound ADP from CF1. Bound ATP is neither released nor hydrolyzed at the tight-sites under these conditions where medium ATP is rapidly hydrolyzed. Mg-ADP stimulates the release of bound ADP only in the presence of inorganic phosphate or of phosphate analogs, e.g. arsenate, pyrophosphate or selenate. 5. It is suggested that: (a) ATP and ADP bind to the same tight-sites, but OcGlc activation specifically accelerates the exchange of bound ADP at the site. (b) CF1 contains low affinity adenine nucleotide binding sites which may be the catalytical sites and which influence the tight-sites by cooperative interactions. (c) Mg-ATP in the presence of OcGlc induces a conformational change at the catalytical site which accelerates the release of ADP from the tight-site. The implications of these results to the role of adenine nucleotides in the regulation and mechanism of ATP hydrolysis by CF1 are discussed.     

Properties of F1-ATPase from the uncD412 mutant of Escherichia coli.

Properties of purified F1-ATPase from Escherichia coli mutant strain AN484 (uncD412) have been studied in an attempt to understand why the amino acid substitution in the beta-subunit of this enzyme causes a tenfold reduction from normal MgATP hydrolysis rate. In most properties that were studied, uncD412 F1-ATPase resembled normal E. coli F1-ATPase. Both enzymes were found to contain a total of six adenine-nucleotide-binding sites, of which three were found to be non-exchangeable and three were exchangeable (catalytic) sites. Binding of the non-hydrolysable substrate analogue adenosine 5'-[beta gamma-imido]triphosphate (p[NH]ppA) to the three exchangeable sites showed apparent negative co-operativity. The binding affinities for p[NH]ppA, and also ADP, at the exchangeable sites were similar in the two enzymes. Both enzymes were inhibited by efrapeptin, aurovertin and p[NH]ppA, and were inactivated by dicyclohexylcarbodi-imide, 4-chloro-7-nitrobenzofurazan and p-fluorosulphonyl-benzoyl-5'-adenosine. Km values for CaATP and MgATP were similar in the two enzymes. uncD412 F1-ATPase was abnormally unstable at high pH, and dissociated into subunits readily with consequent loss of activity. The reason for the impairment of catalysis in uncD412 F1-ATPase cannot be stated with certainty from these studies. However we discuss the possibility that the mutation interrupts subunit interaction, thereby causing a partial impairment in the site-site co-operativity which is required for 'promotion' of catalysis in this enzyme.     

Guanine Nucleotides Modulate Cortical S2 Serotonin Receptors

Abstract

Many radiolabelled receptors coupled to intracellular adenylate cyclase activity have been found to be modulated by physiological modulators such as GTP (guanosine triphosphate) and Gpp(NH)p (guanosine-imido-diphosphate). In particular, the apparent affinity of agonists competing for the binding of ³H-antagonist-labelled receptors is reduced in the presence of GTP and Gpp(NH)p. We report herein the agonist-specific effects of GTP and Gpp(NH)p on rat brain cortical S² serotonin receptors. The agonists serotonin, 5-methoxytryptamine, bufotenine, and tryptamine display threefold lower affinities for S₂ serotonin receptors in the presence of 10^-4M GTP or Gpp(NH)p than in the absence of the nucleotides. The antagonists spiperone, cinanserin, cyproheptadine and methysergide are unaffected by the guanine nucleotides. The Hill coefficients of the agonists increase from between 0.70–0.80 to 0.90–1.00 due to guanine nucleotides. ATP, ADP, and GDP have little or no effect. This pattern of guanine nucleotide effects has been found with receptors which are modulated by a guanine nucleotide regulatory protein and may indicate that the S₂ serotonin receptor may be coupled to intracellular adenylate cyclase activity.     

Metabolism of adenine nucleotides in thymocyte cytoplasm and subcellular fractions after treatment with concanavalin A, papaverine and adenosine

Studies with rat thymocytes labeled with [14C]adenine and fractionated by digitonin treatment revealed that the cytoplasm of these cells contains about 60% of the total adenine nucleotide pool with a higher ATP/ADP ratio and metabolic activity as compared with the structural components. The incorporation of [14C]adenine and [14C]adenosine into thymocyte adenine nucleotides results in predominant labeling of cytoplasmic ATP, in which the specific radioactivity of this nucleoside triphosphate is two and three times as high as in subcellular structures. Concanavalin A decreases the ATP level in thymocytes without changing its specific radioactivity. This compound does not influence the total content and amount labeled adenine nucleotides in the structural fraction. Papaverine accelerates the catabolism of ATP, mainly in thymocyte cytoplasm and, in a lesser degree, in its structural fraction. In each fraction the papaverine-induced catabolism of ATP is localized in the compartment which is more intensively labeled with [14C]adenine than the whole fractionation ATP pool. Adenosine markedly accelerates adenine nucleotide catabolism in the cytoplasmic and structural fractions of thymocytes; however, only in the first one of them this acceleration is due to ATP elevation. Papaverine and adenosine do not directly influence either the content or specific radioactivity of adenine nucleotides of the structural fraction isolated from [14C]adenine-labeled thymocytes.     

Mechanism of action of ATP on intestinal epithelial cells. Cyclic AMP-mediated stimulation of active ion transport

ATP, ADP and AMP but not adenosine increased cyclic AMP in dispersed enterocytes prepared from guinea pig small intestine. This action of ATP was augmented by IBMX and was reproduced by App(NH)p or App(CH2)p. ATP also increased the formation of cyclic [14C]AMP in enterocytes that had been preincubated with [14C]adenine. Gpp(NH)p and NaF each caused persistent activation of adenylate cyclase in plasma membranes from enterocytes and ATP caused significant augmentation of this persistent activation. In addition to increasing cellular cyclic AMP and augmenting Gpp(NH)p and NaF-stimulated persistent activation of adenylate cyclase, ATP increased the Isc across mounted strips of small intestine and inhibited net absorption of fluid and electrolytes in segments of everted small intestine. These results indicate that intestinal epithelial cells possess a receptor that interacts with ATP and other adenine nucleotides and that receptor occupation by ATP causes activation of adenylate cyclase, increased cyclic AMP and changes in active ion transport across intestinal mucosa.